View Full Version : Crank case volume...

05-09-2003, 05:08 AM
Some people have said that modifing the crank case volume to be a certain size ratio is important...

Taking the helmholtz resonance equation:

n=speed-of-sound/(2*Pi) * sqrt (port-area / (port-length * crank-volume))

Now measurements from a cylinder give:

average-port-area = 0.000728 square-meters
(from port height of 8mm widths 20mm,18mm,15mm,18mm,20mm)

average-port-length = 0.044 meters (average of 32mm and 55mm for inside and outside curve lengths)

speed-of-sound = 340 m/s (at 20 degrees above ambient)

so calculating the resonant crank volume for say 10000 RPM = 166 Hz

0.0165/((166*2*Pi)/340)^2 = 0.00174 cubic meters = 1740 cc

Now, the volume inside the crank case (minus the volume taken up by the crank + 1/2 cylinder volume)) is approx. 350cc !!!

if we work backwards from the given crank volume, that gives a frequency of 372Hz = 22323 RPM...

It would seem the crank volume is too small!!

But, thinking about the port opening, the port is only open for 1/3 of the time (I think the tranfer port is at around 120 degrees ATDC) this would mean the tuning to get the pulse back before port closure would effectively be 7441 RPM. Applying this idea to the volume calculation:

for 10000RPM = equivalent 30000RPM = 500Hz
the volume required would be approx 190cc

From this it would seem that adding material into the crank volume is going to shift the resonance from 7500RPM up to 10000RPM ... Traditionally you want the resonance at peak toque.

Firstly, does anyone have any more accurate measurements of transfer port area, port length, crank volume...

Any comments?

05-09-2003, 08:12 AM
I have some comments!

All the above = F***ing boring me to death!!

05-09-2003, 10:16 AM
"All the above = F***ing boring me to death!!"

:lol: :lol: :lol: :lol: :rollin:

05-09-2003, 10:53 AM
I wasn't going to say anything as some people here are educated beyond their intelligence. There is absolutely no crankcase resonance frequency period. The mass and thickness of the cases, water jackets and the like, absorb sound frequency. The intake charge is so fast at high RPM it doesn't matter anyway if there was any. BTW, the crankcase volume is unequal and too big. Nuff said.

05-09-2003, 12:09 PM
Hmm... somebody seems to be fibbing

This is a quote from foxter on this very board!!!

foxter (Posts 54 : 12/29/02 10:58:45 am)

The stuff you want to use is Devcon-"Aluminum Casting Repair Putty", I get mine from McMaster-Carr, they have a website. You must make sure you rough up the area's where you will be applying this stuff, I use a scribe or awl and gouge it good. The volume should be 1.5 to 1.7. It would take me quite a while to explain everything else, but if you have worked on 2-strokes before and understand the concept and design, you should be able to figure it out.

If you look at the ratios above you will see the value calculated for the ratio is a little over 1.5, and is probably a slight under estimate...

If resonanance was not an issue then probably the smaller the better, to give the exhaust the biggest pressure wave you can!

05-09-2003, 12:20 PM

So why do you think foxster is lying? All he is saying is that the crankcase volumes are unequal and that the bikes performance would improve by reducing crankcase volume. Increasing cranckcase compression is a very common thing, especially on Yamaha TZ250s. You are developing mathematical formulas that may make some abstract sense, but are not connected to the real fundamentals concerning power output. In other words, your assumptions are a little off base. I tried to provide some input on ignition curves, but you only wanted to apply this information to exhaust gas temp...you were missing the big picture.

The guy was trying to give you some good information. Why do you want to turn that around?

Rocky Stargel
Team Stargel Aprilia

05-09-2003, 12:35 PM
I am sorry if thats the way it seemed, I am not interested in insulting anyone, only on having a genuine technical debate. I should not have jumped to conclusions... Its just that I have read this board for a long time, and remebered several threads from ages ago discussing these kind of issues...

Foxter distictly refers to a ratio of 1.5-1.7, he does not say "as small as possible" - I was concerned that his reply inferred that crank volume was irrellavent to power considerations.

If you doubt that resonance in the inlet can have any affect on power, you should read up about Honda's VTEC system, which includes variable resonance air intakes.

The formula above is not something I just made up, it is taught on engine design courses, and is well established in the perfomance engine design field... (Infact it is a simpler form of the more genereal electrical impedance/capacitance model of engine fluid dynamics)

05-09-2003, 01:05 PM
I hope I haven't offended people too much to continue this discussion...

The engine cases are aluminium and will reflect sound in the 500Hz range perfectly well. Infact wood will reflect the sound (not as well) but well enough to make your bass-reflex hifi speakers work (its the same principle).

If the intake charge is so fast that resonance is unimportant, how come the exhaust (which is even faster when it leaves the cylinder) is affected by resonances.

It doesn't matter if the crank volumes are unequal, it will just mean each cylinder will have a slightly different resonance... resulting in a slightly broader, and less peaky response.

Yes, the crankcase is too big (at approx 350cc) this will resonate with the transfers as about 7500RPM ... but by adding material to the crankcases, it should be possible to raise this towards 10000RPM, however it is unlikely that the volume can be reduced all the way to 190cc.

05-09-2003, 01:11 PM

I'm not questioning the validity of your formulas. If Foxster thought crank volume was irrellevent to power considerations, why would he go to the trouble of epoxying the cases? I dont think you are a bad guy. All I am saying is to listen to someones opinion if they seem to be a good source of first hand knowlesge.

If I asked the Aprilia race department for information on something and then argued with their response based on a complex set of mathematical calculations, they would probably no be apt to provide more information to me in the future. The loss would be mine, not theirs. They have more experience with it than I do...

Have a nice day,

Rocky Stargel
Team Stargel Aprilia

05-09-2003, 01:32 PM
Hey, okay - I see your point, we are talking about the difference between practical experiance, and theoretical design... I defer to greater experiance...

It also happens that the theory kind of says the same thing anyway (the smaller the crank volume the better - providing you don't go below 190cc)

So, how do people feel about intake tuning... This must make an appreciable difference, or Suzuki would not have bothered with those two long loopy pipes to boost mid range performance.

What I was thinking was the airbox and carb also forms a tuned system, has anybody tried a dual resonant intake - using an extra cable from the power valve actuator to open a butterfly valve in the intake manifold, changing its resonance with RPM, you could tune the airbox to say 6000RPM with the valve open, and 10000RPM with the valve shut... What to you think of this?

Here is a link to a description of this kind of system on a Porsche leo.worldonline.es/jaumep...rioeng.htm (http://leo.worldonline.es/jaumepor/angles/tecnica/varioeng.htm)

Obviously in this case it would be stuffing more air into the crank volume before the reed-valves shut, rather than directly into the cylinder, but the principle should be the same.

05-09-2003, 03:13 PM
Keean, I apologize if you were offended by any remarks. You must understand that the people that view this site do not Quantum mechanics degrees. Average Joes' don't relate the complex math to anything. From what I read of your figures, they are correct but, what you don't take into consideration are all the other variables.
I have several friends I confer with that are Mechanical engineers and Engine specialists. We have been testing and developing engine models for a long time. What I do on this forum is try to explain things as simple as possible. Thank you Mr. Stargel for your remarks. As for the Resonance frequency that is refered to, it is of less importance compared to intake velocity or volume. The intake pulse, as it is more commonly refered to, should be as small as possible. Better throttle response and more linear power. The so called resonance frequency has not much effect on a very short intake. And inside the crankcase, velocity is more important, and I don't believe that the air moving in the crankcase is all that affected by sound frequency. If that is what you are refering to. We measure the intake tract length and volume when we build a pipe. There are more figures we use to build pipes than we use anywhere else on an engine. I have built pipes that were very "tight" in their dimensions and eventually exploded apart as a result of overrev. These are good pipes, just overreved once too often. Mr. Stargel, racing an Aprilia, will understand this. I do not want to offend anyone here, just get tired of some of the extraordinary explanations or claims.


05-10-2003, 05:58 AM
People who don't want to read the maths don't have too, It's not like I'm forcing them to...

You say the resonance frequency is less important than intake velosity and volume. This is true, but the point of the resonance is that it can slightly increase the volume of air taken in ... (ie when the reed valves shut the air left in the crank case will be at slightly higher pressure)

I agree that under normal circumstances the intake-pulse should be as small as possible for linear throttle response. But If you arrange a resonant intake it will boost power at a specific RPM just like a tuned exhaust, but loose power over the rest of the range. The idea with the variable resonance intake is exactly the same as a power valve... Just tuning the intake to one frequency is like trying to ride a bike with the powervalves locked open - and would obviously not be as good as a simple resonance free intake... As for a short intake, a quater-wave-pipe for 333Hz would be 25cm, not huge, but wave-pipes don't give a very big boost - you really need a resonant cavity (a helmholtz style resonator) for any gain...

I am not out to make any extraordinary claims, I will freely admit that the power to be gained will be small (5hp possibly) and I am not tying to produce super-tight resonances that explode ... (really, the pipes explode?) Infact I am more interested in boosting mid-range power than top end over-rev.

By the way for anyone interested the easyest way to improve over-rev on a standard pipe is to modify the powervalve... By removing the lip on the last (fixed) section of the powervalve and giving it a sooth slope to blend in with the port edge and the next valve stage, you remove a disruption that deliberately slows exhaust exit velosity from the cylinder - limiting over-rev. I have used the modified valves for some time, and can say that they will over-rev to 13,500 RPM on standard pipes with practically no other modification. The engine has done at least 5000 miles and everything still looks in good condition inside...

Cannon Fodder MOO
05-10-2003, 09:15 AM
I like tha maths! :rollin:

I can be a bit dumb at times though <img src=http://www.ezboard.com/intl/aenglish/images/emoticons/embarassed.gif ALT=":o">

this stuff is interesting when I can apply it. i ain't even acquired an RS yet (falco still in shop) but I'm happy thinkiing I shouldn't rev the damn thing much past 12 000 RPM if I want it too last. That was after looking at the mean piston speed "equation", if you call that an equation :lol: .

Anyways I figured that crank volume was worth a look, but only because I thought that the smaller the crank volume the more pressure build up, thus faster induction of the intake mixture. It's been stated that the crank is too big, does that mean a dumbass like me will see some pics of a modified crankcase??

Also can a crankcase be to small?? When would this occur? Why? If the crankcase is too small will there be a waste due to intake gas loss through the exhaust port? Will there be an insufficient volume of air/fuel mix available?

Hell I ain't going too look at the exhaust formulas atm cos they look like they need a bit of time.

oh and I'm going to try and port map an RD250 barrel for a quick education, maybe I'll learn something then!

05-10-2003, 01:34 PM
Hmm... I wouldn't practice on your only set of barrels... actually I remember you saying you had/we're getting a spare set.

I was thinking about foxters comments - about blowing up exhausts, and although he didn;t say it presumably engines... All this leads me to be very conservative when applying modifications - just the bare minimum, It would be nice to be able to test more radical ideas without the expense or danger of a 250 engine exploading... So I thought it would be a good idea to practice on model aeroplane engines - these are tiny 2 stoke engines complete with expansion chambers, costing from 40 pounds, and with capacities of about 2cc, revving up to 18,000 RPM, and producing half a horsepower...

What do people think about the idea of testing ideas on a scale model - would this be useful of just a waste of time (it would certainly be less noisy)?

05-10-2003, 07:48 PM
Keean, I have not blown up an engine because of a pipe or because of porting or carb mods. I have torched a crank because of cylinder head volume, it was my mistake, in too much of a hurry. I have run dozens of engines, from motocross and roadracing to street stuff. 2 and 4 stroke alike. The pipe that I refer to is for my RSR125 aprilia roadrace bike. The closing cone keeps blowing apart. I shortened the pipe 20mm during testing. Lengthening it 10 mm has cured the problem. The engine makes 51 bhp at 13,000 rpm.


05-11-2003, 10:40 AM
It seems everyone agrees smaller is better for crank volume, presumably after the 'pulse' of compressed mixture when the transfers open, the exhaust drags extra mixture through the reed valves, this would suggest that by static analysis you cant really have a crank case that is too small in a reed valved engine, but you could have one too small with a rotary-disc-valve (when there might not be enough fresh mixture in the crank to provide a full charge to the cylinder).

Infact modeling the pressure sound wave inside the crank suggests that the reverse negative pressure wave is dissapated by the reed valves opening to supply extra fuel... This would suggest that crank case resonance could play a part in rotary-disc-valved engines, but is unlikely to have an effect on reed valved engines...

However is seems that the airbox/carb system will experiance a negative pressure pulse just after the transfers open, and perhaps could be tuned to return a reflected pulse just before reed valves close around TDC.
This would be arround 240 degrees of crank rotation or 0.75 of one rotation. For 10000 RPM that would be a frequency of around 222Hz. This would boost power around 10000RPM, but loose power elsewhere. There are several different things that can be done to stop the power loss at RPMs other than 10,000, such as multiple tunings, or destruction of the tuned cavity using a valve to open/join sections together. This could be achieved simply by taking an extra cable from the power-valve servo, running an aditional servo. I still think there may be some extra power to be gained by this... I think it would be fairly easy to produce a set of tubes to change the tuning to different RPMs, then dyno the bike to see the affect. The trick would then be to combine two different intake lengths such that one makes extra power above 6000RPM (for example), but looses power below, and the other that makes power below 6000RPM and looses it above - combining these with a valve which is opened by the powervalve actuator at 6000RPM, would result in better power accross the range (although it may not boost max HP)...

05-11-2003, 01:51 PM

What are the specs on your 125 (cases, rod, cylinder, carb, ignition, exhaust)?

Rocky Stargel
Team Stargel Aprilia

05-15-2003, 03:28 PM
Rocky, I apologize for not responding earlier, I have ridden a clapped out nighthawk 650 to Vegas for my stepdaughters birthday. To answer our question, I have mapped the cylinder, unforutnately I have suffer 2 computer crashes and have lost all my programs and data for the mapping of the cylinder. I haven't done a crankcase volume measurement yet. I tried to recover anything from the other harddrives but with no luck. I haven't even touched the bike since last year, been busy with many other things at the moment. I can tell you there is alot of epoxy in the cases and the cylinder. It was Simone Sanna's factory bike. As for Keean, you really need to get a close look at an NSR500 or a YZR500. They use 28mm and 34mm carbs, depending on the track length and configuration. You have a lot of knowledge of numbers and equations, but there is no substitute for testing. A friend of mine wrote a 500 page term paper on engine theory for the University of Michigan, to get his masters degree in mechanical engineering. He has worked for Artic Cat for years now as the head engineer in the engine department. He says hands on is the best teacher, I agree with him.


05-16-2003, 05:36 AM
I agree that there is no substitute for testing, but I enjoy trying to understand the processes that go on. To me it is useless to be able to say the volume must be 13.393cc (for example) if you have no idea why that is true... Okay if your goal is to produce the fastest bike that is great - but that is not what I am after. If somebody says that idea is great and fits with my experiance perfectly (or I conduct a test which shows this), I am happy. If somebody says "no, thats wrong" I am equally happy because it means I have to find a new-idea, or way of modeling to explain the observed behaviour. This is just the way I enjoy working - devise a theory that explains how it works (a mathematical model) - test that theory against reality - improve model, repeat until good enough...

In the end you might end up with a few simple rules (keep the crank volume as small as possible) or lots of very complex ones (expansion chamber shape, ignition timing)...

A friend of mine is a mechanical-engineer who designs parts for formula1 cars - he designs theoretically and tests using computer software, other more experianced people tell him they can do better by hand, and change the parts... the parts break and they end up using his original design. He has cut the weight of several components - and earned the respect of the more experianced designers... I guess different people work in different ways.

By the way I would love to see the insides of an NSR or YZR, any photos or measurements would be great! although I don't really see Honda letting anyone poke around their pits or get close enough to take photos or get measurements... I was at a formula3 race, where they have open pits (unlike formula one where its invitation only) and one of the teams had a spare engine out, anyway I was explaining to a friend the basic engine parts - like thats the water-pump etc... anyway as soon as somebody realised I new anything at all about what I was looking at they immediately covered the engine - obviously worried I might see something I could use to my advantage...

05-18-2003, 10:14 AM
keean one important parameter is missing

chamber can creat suction from case (from transfers ) at certain RPM so unless you find a way to calculate the waves strenght and timing and RPM it occures you cant determine what perefect crankcase volume should be

it comon practice to stuff the case but that might be a way to widen the band with certain chamber

05-18-2003, 10:43 AM
I think you are saying the expansion chamber negative pressure wave will pull mixture from the transfers, and hence from the crank case... Yes I talked about this a couple of posts back - and my conclusion was it would just pull fresh mixture through the reed valves... In other words Foxter was spot on when he said there was no crank-case resonance, at least in the RS250. I think there will be a resonance (or ideal volume) for rotary-disc-valved engines as the exhaust "pull" cannot just open the valve as it is timed off the crank rotation...

However this does mean the airbox and intake are acousticaly connected to the exhaust expansion chamber in reed valved engines.

So at the moment I would summerise like this:
for the RS250 (and other reed valved engines) the smaller the crank volume the better. Gains may be possible by using a variable length intake or other variable resonance system infront of the reed valves. Infact it appears the smaller the crank volume the tighter the coupling between the expansion chamber and the intake/airbox...

For rotary disc valved engines I think crank case resonance could play a part in engine tuning, and that it is definitely possible to have a crank volume that is too small. Also the intake is less important as the disc-valve effectively breaks any acoustic coupling between the expansion chamber and intake.

05-18-2003, 04:06 PM
Hi there,

Just reading through and seen that you have an rs125 making 51BHP@1300rpm

Thats fantasitic I thought considering a highly tuned Honda RS125 makes about 48 tops.

I'm not exactly sure of the differences in the stock and race rs125's [Aprilia].

Could you go through with me what kind of setup they have ?
Carb size, Compression ratio, bore&stroke[i hear the CC is the same but bore and stoke slightly different]

Please let me know as i've always wondered.


05-19-2003, 10:32 AM
Madhatter, I noticed the photo you have there of the 250 bottom end is backwards. As for the RSR125, the bore and stroke are 54x54.5 and the compression is 16 to 1 theoretical. To answer Keean, the exhaust return pulse is quite complex. Having experimented with different pipes and materials, it is hard to say which is the best material. Each has an advantage and disadvantage. But, the most important thing to understand is the compression of air. A 2-stroke is nothing more than an airpump which runs itself. If the crankcase becomes too small, the intake charge will be weak and the velocity of the air to fill the cylinder will be slow. If the volume is to large, the same effect happens, too much area for the piston to come down on to charge the cylinder. If you can get the air in the intake moving fast enough, like a bull through a china shop, the cylinder charge will be great. I built a Honda RS125 once for a gentleman and the engine "seeped" oil from the crankcases. Along the parting seam on the bottom of the engine. The engine made so much crankcase pressure it would force a small amount of oil through the case halves. It did not leak air, just pushed out oil. I checked the cases on a Numerex inspection table and also lapped them and they were perfect. It still would seep the oil. The bike was very fast and made alot of power. The owner never had it dyno tuned but he was every bit as fast as my Aprilia down a straightaway. He actually finished second place in a 250 race against TZ250's. And yes there were more than 2 bikes in the race.


05-19-2003, 01:01 PM
Foxster i sew Pedrosa honda is dead even with top aprilias so looks like rotary valve has no advantage ?

also i wonder if hondas use wet clutch like a normal rs125 ?

and is this 51HP on the crank or on the sprocket or on the rear wheel ? because i noticed warthals dyno reads too high when stock bike dynoed compared to Dynojet dynos

wild claims made by gilera 56hp for thier 125 which wasnt any faster than other 125 last year -- and how is this possible with unleaded gas ?
and looks like this year there is no more gilera ? what happened to them ?

05-20-2003, 10:11 AM

What is the spec on your RS125R? What ignition, cylinders, carbs and exhausts are you running?

Just curious,

Rocky Stargel
Team Stargel Aprilia

05-20-2003, 12:41 PM
Now thats odd... I was just reading about 4 stroke engines using one-way valves and exhaust suckers to lower pressure in the crankcases (people claim that lower air density in the crankcases results in less turbulant-air drag on the crankshaft) - this results in oil seeping into the crankcase (probably slightly more useful)...

Anyway lets say I was going to pack my crankcase - I would say you need to keep the airflow from reed-valve to under piston, and from there to transfers as unobstructed, as possible ... meanwhile packing any space elsewhere in the crankcase with putty... The only problem is there doesn't seem to be that much space! You obviously dont want to add weight to the piston or other moving parts... and I would be reluctant to put padding in the recesses for the crank counterweights, as the clearance looks quite tight anyway. About the only places I can see that are suitable are:

1) remove reed stops and use putty to act as a stop for the reed and use up some unflowed volume behind the reeds.

2) perhaps reshape the intakes behind the reed valves where they open onto the crankshaft, keeping the area more constant.

This is an idea I would particularly like feedback on, seeing as the max flow will be deternined by the part of the intake with the smallest area, would it make sense to limit the area of the intakes in the crankcase to 34 square-mil (the same as the carbs) as it looks to me the area (inside the reed valves) is more like 100 square-mil?

05-22-2003, 07:09 AM
To start with, I would still like comments of the previous post about where to put epoxy as I am not conviced by this yet...

And now the contravercial bit: I have just read an article by a snow mobile (2 stroke) engine designer, In his opinion larger crank-cases are better! He believes that pumping losses due to primary compression outweigh the gain of squirting the mixture through the transfers, because the suction from the exhaust is so large with modern exhausts... Now whilst this kind of makes sence - I am not sure what effect this would have on scavenging, surely the mixture needs to exit the transfers with some velocity so that its momentum makes it travel up the cylinder wall, rather than just being sucked towards the exhaust port?

My gut feeling is that his theory might have some value at high RPM, but surely having low primary compression will hurt low RPM?

Maybe Foxter was right and the effect of crank volume is so small that you just won't notice no matter how big the crank-volume is... But then why would anyone stuff their crankcase at all?

05-22-2003, 11:34 AM
First, iiph, the hondas use a dry clutch on the GP machines. The bike is dynoed for rear wheel horsepower, gilera and derbi might be measuring it at the output shaft or from the crank.
Rocky, my carb is the 39.5mm Dellorto, the cylinder is a 97' rotax, I will spec it out probably later next month when I have time. The pipe is my own Titanium piece, I made 3 of them of different spec for different tracks. I looked at Rossi's bike, the last one he raced in 1999 and it was funny how the upper pipe is so different from a 125's upper pipe, maybe because of the rpm difference or maybe the carb difference.
Keean, snowmobiles use a clutch like a scooter, allowing the engine to rev substantially before taking off, so the engine is always in a higher rpm range than a 2-stroke motorcycle. I tried to explain this to Dr. Rob Tuluie, on his Tul-aris, he uses a 800cc twin snowmobile engine. When I built the pipes for it, the snowmobile engine shop that built the engine for him gave me specs for a snowmobile pipe. I told him they wouldn't work and the engine would lose power. He lost 5-8 bhp with the pipes. They had too much volume and were too long.
What must be remembered is that motorcycle manufacturers build their bikes, other than factory racing use machines, for the average Joe, and thus must build them safe to avoid lawsuits. But they do have a lot of room for improvement, you just have to figure it out.


05-22-2003, 12:08 PM
Hmm... So would I be right in thinking stuffing your crank case will help peak torque but not necessarily peak HP (is this what tuner Graham File means when he refers to 'usable power'?)

This is probably what I want, as I would like to boost the power a bit accross the range, not necessarily boost max HP that much... I've got some Ti Putty, so I mighr give it a go (I have a spare for most engine parts so if something gets damaged I can replace them)...

Now on the subject of power accross the range - it occurs to me that the powervalves dont make the best use of the stepper motor! In theory the stepper motor could move the PV one step at a time, instead of just using 3 positions. What I am wondering is if with the powervalve closed the exhaust port is restricted (making the gas exit faster) - if it were set somewhere between position 1 and position 2 would the tuning of the pipe be somewhere between the two tunings as well... If this were true, and a sensor that can detect the pressure pulse in the exhaust header can be found, it would be possible to design a new PV controller that adjusts the PV such that the return pulse arrives just as the piston closes the exhaust port much more often than the traditional controller... What do you think of this idea ... If it seems worthwhile I could build such a controller (providing a sensor to detect the exhaust pulse can be found...)

05-22-2003, 01:53 PM
Hi there,

Are you joking about my picture ?

Hope you didn't mistake the RS125 With the 250 engine.


(thats a 125 engine in my pic, maybe some peple would take a quick look and mistake the reed housing for another cylinder ?


Tell me you where joking !



05-22-2003, 02:00 PM
"Rocky, my carb is the 39.5mm Dellorto, the cylinder is a 97' rotax, I will spec it out probably later next month when I have time. The pipe is my own Titanium piece, I made 3 of them of different spec for different tracks. I looked at Rossi's bike, the last one he raced in 1999 and it was funny how the upper pipe is so different from a 125's upper pipe, maybe because of the rpm difference or maybe the carb difference."


So you are using small carbs. Does your cylinder have electronic powervalve? What head insert? What number is on your ignition ECU? What ignition map were you using to get the 51hp? When did you see the pipe on Rossi's 99 250? If it was a 'show' bike, it was not the pipes he was racing with. On the 250s, Aprilia is up to 60 different pipe types now.


Rocky Stargel
Team Stargel Aprilia

05-28-2003, 05:50 PM
Keean, Ive been away for a while, but better late than never, so..............

I have had a look at your ideas here and I can't help thinking that you are a little off the track here.

What young Foxster here says is bang on, maybe you should brush up un your 2 stroke theory again, intake velocity, air volume and compression ratio. When foxtser says 1.5 - 1.7 what do you understand this to mean?? the exhausts overiding/creating all you have said and the scavanging is responsible for packing more charge in. if you apply your "resonance"stuff to the exhaust you will be better off. remember the piston sucks air into the crancase, COMPRESSES it (closing the reed) and it is transfered to the top by the piston coming down charging the transfers. timing and transfer volume ratio etc also have more influence than crank volume (don't confuse with compression ratio !!) as do carb size and intake length. I strongly doubt you could get another 5bhp with what you are saying.

Please find out more on rotary valves b4 you theorise on them and just finally I don't believe that the Porche 911 Carrera actually comes out with a 2 stroke motor so it dont matter what its got.
no offence, just calling it how i see it. :D

05-29-2003, 04:20 AM
Well I guess a lot of stuff has been covered, and my ideas have changed a little as people have given input...
(mainly my opinions about how large an effect certain canges might have)

I realise intake velocity, air volume, and compression ratio are primary effects - I am looking at more subtle second order effects. When discussing the crank volume as a ratio I take that to be the ratio of the crank volume to the cylinder volume...

I dont understand your point about the exhausts... As far as I understand it, the exhaust resonance is largest single affect in the powerband... As for the 'resonance stuff' well, firstly it is not mine - secondly exhaust designers already use it, as do intake designers...

Your point about crank volume seems a little off - There are two stages of compression in a 2stroke - primary compression and secondary compression. Primary compression is what happens in the crank-case, secondary compression is what happens in the cylinder. The primary compression ratio depends on the crank volume compared to the swept volume of the cylinder.
'charging the transfers' is a misleading term as it doesn't really describe what is happening. Actually the crank-volume and transfers are a connected space, as the cylinder descends, the pressure inside the entire crank volume, not just in the transfers increases... When the piston uncovers the transfer ports, as long as the pressure in the crank-case is greater than the pressure in the cylinder, mixture is sucked into the cylinder. In the powerband, the exhuast is also sucking, and pulls mixture through the cylinder into the expansion chamber...

I dont know what you mean about rotary-disc valves - I am completely familiar with how they work... A disc is attached to the crank, with a slot. As the disc rotates the slot opens and closes the crank intake, but unlike a reed valve it is controlled by crank rotation not by the pressure in the crank-case. It is obvious from this that once the disc valve is closed no more mixture can enter the crank - so if the crank volume is too small there wont be enough mixture to fill the cylinder. With a reed valve, the exhaust can cause the reed valves to open during the transfer phase, sucking mixture from the intakes - this cannot happen with a disc valve because it would cause primary compression loss at low RPM when the exhaust is not sucking much...

Just because the example I gave was from a porche (4 stroke) does not mean that the effect does not happen in a 2 stroke - they are both internal combustion engines and have a great deal in common.

With a 4stroke the principle is that a resonant intake causes pressure pulses in the intake pipe, if timed correctly, these pulses arrive at the cylinder valve just as it closes packing more mixture into the cylinder ...

With a 2stroke the pulses are the same but rather than packing more mixture directly into the cylinder, it packs more mixture into the crank-case, just before the piston starts to descend and closes the reed valves...

One final point: scavenging is the clearing out of exhaust gases, not packing more charge in. There is some overlap though as the better scavenged a cylinder is the more space for fresh mixture.

05-30-2003, 05:36 AM
Oky-doky.............., what you have reflected here seems to differ a bit from where you have started, but my point is the exhaust pulse from the motor into the pipe not only sucks out the "scavenged" charge but also returns it on its way back (from cone shape at other end of belly pan) this return pulse is much wider than outgoing and forces excess charge back in. this effectively ends and overides ANY secondary resonance effects that there ever may be. the return pulse getting out of time with the outgoing is what cocks the whole thing up and the power valve is one way of beating it, water injection is another. yes compresion acts equaly on all of the crancase and this pressure must seal the reed (as per rotary), also exhaust pulse and timing mean that reed bleed does not occur at usable RPM. reed flutter is exactly what you don't want. there are multi stage reeds available that already mirror what you have suggested with dual intakes. Aprilia played about with direct port injection on the 500 trying to mirror the DITECH, but with not 100% success. a rotary valve is like a slice of pie, if it aint enough you can make it bigger. Volume, Velocity, compression. (timed port area)

OK,....... imagine the return wave from the size and power of an exhaust against an alledged crancase resonance (volume/volume) and consider the energy behind each. my moneys on the exhaust.

"this cannot happen with a disc valve because it would cause primary compression loss at low RPM when the exhaust is not sucking much..." not sure where ur goin with dat one. the reed is also either open or closed, there also is a lag, or pressure required to actually open a reed. a rotary is just a nice big round hole. Please consider................:D

05-30-2003, 08:21 AM
Ill start with the last bit: There are two phases to the exhaust suck, and the return pulse. When the exhaust sucks, it can lower the pressure in the crank case so much that is pulls extra mixture through the reed valves -I think this is good because it allows more charge into the cylinder and expansion chamber so that eventually more charge ends up in the cylinder - I dont see your point about the timing as the exhaust suck starts when the pulse enters the widening cone of the expansion chamber, which would be roughly half way between exaust port opening and the return pulse arriving - this is going to vary with RPM, but seeing as the transfers are open for a good while, (120 degrees or so) it is very likely to suck straight through the cylinder and on into the crankcase. Whether it sucks any fresh mixture through the reeds will depend on the strength of this suction, relative to the volume of air in the crankcase - so this kind of depends on the volume of the expansion chamber compared to the volume of the crankcase, cylinder and exhaust header.

Maybe with a disc valve you could have it open earlier (ie soon after the transfers open whilst the piston is still descending) to allow the exhaust to suck fresh mixture, but I would have thought this will have adverse affects on performance if the exhaust is out of time (eg low RPM) when it could push mixture back through the crank into the intake - this cannot happen with pressure activated reed valves... I guess what I am saying is it very difficault to get the disc timing right at all RPM...(unless you have some kind of variable geometry disc) - I was not at any time suggesting one is better than the other - although Honda use reed-valves! My point was that because the valves behave differently which air volume has the largest effect changes: for reed valves the air intake/carb/airbox has the greatest affect - for disc valves the crank volume is more important, although if I am wrong about the timing of the disc-valve opening and closing then it could also be the intakes...

Any crank resonance would increase the velosity of charge entering the cylinder when the transfers open... so it has nothing to do with the exhaust return pulse... Infact it works in a similar way !!!

To explain: simple exhuast designs treat the pulse in the exhaust as travelling at constant speed, and behaving like a wave on a pond. But air is compressible and has 'springiness'. This means the wavefront compresses the air infront of it like a spring... so you effectively have a mass of air (the exhaust pulse) pressing on a spring (the air already in the expansion chamber - as the stinger has a smaller diameter not all of it can escape in time). Just like when a metal weight bounces off a spring, the spriginess of the air pushes back in its own time...If the timing of the mass/spring system is different from the exhaust timing it will slightly lessen the force of the return pulse... If the timing is the same it will slightly enhance it...

With a crankcase the same applies, Air piles into the crankcase from the inlet, and the mass spring effect will make it push back a split second later... (imagine a spring going into the crankcase through the inlet and bouncing out the transfers - the timing of the spring's bounce is the same as the resonant frequency of the volume of air in the crankcases) if this push coincides with the transfers opening the air will spring into the cylinder with a little extra force than just that due to compression... This may seem to be insignificant compared to the exhaust return pulse, but it will improve scavenging efficiancy by forcing the fresh charge higher in the cylinder. All these effects are real, and demonstratable using typical school physics equipment. My point is this is happening in the engine anyway - just at some random RPM, so as a designer it surely must be better to choose the RPM it occurs at (to either give a boost to peak power, boost peak torque, or flatten the curve to give better power over a wider range). People have said that these effects are small and you are better off designing with volume/velosity - but this may not be an either or situation: you can take the requirements of volume/velosity , and the resonance requirements - which will give you a simultanious-equation which when solved will give transfer and crank dimensions that satisfy all requirements...

What I am most interested in is how much effect they actually have... For money is no object racing, you are going to want every possible (even small) improvement... However for those of us on a budget it is more of a performance-over-cost question...

The multi-stage reed valves are more to do with flowing air better at high and low RPM, a soft reed will open with less pressure, and so works better at low RPM, but the softer and less springy a reed is, the longer it takes to close when the suction from the crank stops. This means it does not close fast enough to seal properly at hight RPM. A stiff reed will close quickly, but is much harder for low speed air to open at low RPMs. Reeds do not change the timing of pressure pulses in the intake/airbox - only changing the length of the intakes or changing the volume of the airbox would do that...

06-02-2003, 05:44 PM
As its been pelting down with rain i have had a little too much time on my hands so here we go, i appologise deeply to you all.............:rolleyes:

Resonance Effect
F = Vs / 2 * the square root of A / Vc (L + 1/2 the square root of A)
Vs is the sonic speed (about 1100 ft/sec)
A is the cross-sectional area of the inlet
L is the inlet pipe length
Vc is the crankcase volume

Crankcase Volume
Primary compression ratio =
Case Volume @ TDC / Case Volume at BDC
CRp = V1 + V2 / V1
CRp is the primary compression ratio
V1 is crankcase volume @ BDC
V2 is piston displacement

You had me baffled, I'll admit, but I never actually bothered to check the formula you were using, but I take it the first one is what it SHOULD be. i have also included the 2nd for you as you appear to be into that sort of thing.

most people spend a life time with the basics without mastering them, me included. ;)

06-03-2003, 01:45 PM
Nothing to appologise for, as I for one thought all the comments were useful... There is by no means a simple solution. As foxter pointed out - intake pulses (resonance in the intakes) can be a bad thing as they will make throttle response uneven. If you tune for one frequency it detracts from others... I am sure current designs are very well suited for the purpose they are intended. For example changing any tuning parameter (not just crank-resonance) to improve peak-horsepower will detract from drivability. The only way I can see of improving this situation is to add electronic control - for example the intake area could be varied using a servo valve, and/or a second chamber attached to the crank volume could be opened with a servo valve. The question with this is how much effort for what kind of gain... The limiting factor for doing these kind of things seems to be the SAPC box... It would be nice to have a programmable ECU (linkable with a PC to allow software to be uploaded) with multiple sensor input channels (for things like crank position, combustion pressure sensors, and exhaust pressure sensors) and several assignable servo contol outputs. It would seem sensible to start with exhaust valve positions - as there would be more to gain - before even thinking about the intakes...

As for the formulas, the one I was using for resonance was metric, hence there are no "fiddle factors", also I was using different volumes, the main crank volume, and the transfers as the pipe. The formula given above is imperial, and uses the complete crank volume and the intake pipe... Both of these are valid, it would just change which resonance is being tuned - I think you could also tune both at the same time... Just goes to show the interdependance of all the parts in an engine...