Crank case volume...

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?

I have some comments!

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

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

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

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.

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!

Keean,

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

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)

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.

Keean,

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

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

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.

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.

D

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...

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="">

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!

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)?

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.

D