54 mm TB swap just my 2 cents

[TuBa]

IMHO, for the dual spark motors the 51mm TB's are the best option UNLESS you have a big bore kit, decked cylinders and heads, well dialed in valve timing...etc...

Can you please g a bit into detalis with this: the 51's are the best option for this engine from whatpoint of view?
I do not want to start a discussion but frommy butomerter the therottle response is amazing with the 54 TB but as I stated at the begining it might be just a placebo.
By the way does anybody has a Dynorun with 54 TB ???

[Aladinsaneuk]

I am also interested in this

Everyone I know who has done this on the rotax 1000 has rated it SO LONG AS THE THROTTLE BODIES have been done properly
Perhaps the qualifying comment should be:

Only do this if you fettle

[SoulDaddy]

Micah, what else would backfiring through the intake be caused by? I have 51s that do that, more so when the engine has been run a while than when cold. Very unstable at lower throttle settings - won't maintain a constant rpm, missing, etc.

Sorry for the jack but it was a good segue.

thx

[Micah / AF1 Racing]

SoulDaddy, you have a vacuum leak somewhere.

As far as dyno charts go I can show a few if this thread keeps alive till I get back to work. Even going from 51-57mm on a stock motor made no gains on my personal bike and actually lost a small amount of top end. What you "feel" though is totally different since we ride bikes and not dynos. The dyno numbers are basically the same no matter which set of TB's is installed because dyno tests are typically only done at 100% throttle settings. At least dyno charts that get printed out and bragged about. Why do they feel so much different. Math, simply math in the end.

The difference in mass airflow between say 0 and 5% throttle is MASSIVELY different between 51-57mm TB's. The effect is WAY more air bypass at a given small throttle setting on a larger TB set, we feel this as increased power because we are now making more power at say 10% TP than we made before at say 20% TP with the old 51mm TB's. The wide open mass air demand is determined less by TB size on the V990 than by valve size and port shape and velocity. The 51mm TB's have considerably high air velocity through the TB and down the intake ports than the same port using a set of 57mm TB's at heavy throttle settings. This results in better trapping efficiency in many cases from the smaller set of TB's.

I know it is weird math, especially from a guy with three sets of TB's for sale right now...

[Aladinsaneuk]

so

with respect to that, the key then is too get the right velocity stacks?

in my own - limited - way, I have experimented with different shapes/sizes of stacks to see the effects

The stacks I am currently using, similar shape to the standard ones but slightly lower in height seem to be good to me

and no - and this applies to every one, the spares i may or may not have are NOT available..... work in progress

[Micah / AF1 Racing]

All MY spares ARE available...see last couple of pages of "What's in the AF1 Shop" thread....part out time!

[TuBa]

SoulDaddy, you have a vacuum leak somewhere.

As far as dyno charts go I can show a few if this thread keeps alive till I get back to work. Even going from 51-57mm on a stock motor made no gains on my personal bike and actually lost a small amount of top end. What you "feel" though is totally different since we ride bikes and not dynos. The dyno numbers are basically the same no matter which set of TB's is installed because dyno tests are typically only done at 100% throttle settings. At least dyno charts that get printed out and bragged about. Why do they feel so much different. Math, simply math in the end.

The difference in mass airflow between say 0 and 5% throttle is MASSIVELY different between 51-57mm TB's. The effect is WAY more air bypass at a given small throttle setting on a larger TB set, we feel this as increased power because we are now making more power at say 10% TP than we made before at say 20% TP with the old 51mm TB's. The wide open mass air demand is determined less by TB size on the V990 than by valve size and port shape and velocity. The 51mm TB's have considerably high air velocity through the TB and down the intake ports than the same port using a set of 57mm TB's at heavy throttle settings. This results in better trapping efficiency in many cases from the smaller set of TB's.

I know it is weird math, especially from a guy with three sets of TB's for sale right now...

Your math sounds fine and would explain de impresive efect that has on the throttle... if I loose 2 HP at 10000 RPM but I have this effect on 5000 RPM, than up to my standards, the 54 are the best on my type of engine with the short 54 Velocity stacks

[Aladinsaneuk]

nod

the velocity stacks may well be the key to this

[Gabro]

those are the stack I used on my 54's:

[D-Rider]

SoulDaddy, you have a vacuum leak somewhere.

As far as dyno charts go I can show a few if this thread keeps alive till I get back to work. Even going from 51-57mm on a stock motor made no gains on my personal bike and actually lost a small amount of top end. What you "feel" though is totally different since we ride bikes and not dynos. The dyno numbers are basically the same no matter which set of TB's is installed because dyno tests are typically only done at 100% throttle settings. At least dyno charts that get printed out and bragged about. Why do they feel so much different. Math, simply math in the end.

The difference in mass airflow between say 0 and 5% throttle is MASSIVELY different between 51-57mm TB's. The effect is WAY more air bypass at a given small throttle setting on a larger TB set, we feel this as increased power because we are now making more power at say 10% TP than we made before at say 20% TP with the old 51mm TB's. The wide open mass air demand is determined less by TB size on the V990 than by valve size and port shape and velocity. The 51mm TB's have considerably high air velocity through the TB and down the intake ports than the same port using a set of 57mm TB's at heavy throttle settings. This results in better trapping efficiency in many cases from the smaller set of TB's.

I know it is weird math, especially from a guy with three sets of TB's for sale right now...

So, essentially what you are saying is that moving to 54s is an expensive solution to achieve much the same effect as a quick action throttle - and although with the throttles fully open, you have a greater potential volume of fuel/air available, this is of little or no benefit due to the restriction of the valves.

[Gabro]

So, essentially what you are saying is that moving to 54s is an expensive solution to achieve much the same effect as a quick action throttle - and although with the throttles fully open, you have a greater potential volume of fuel/air available, this is of little or no benefit due to the restriction of the valves.

it's mostly correct, but throttle response is very different -better- than a quickthrottle, and if good mapped the motor responds very good.

54's works way better than 57's on twin spark motor.

[D-Rider]

Soooo ....
If I want a good throttle response but I also want good fine control at small throttle openings (lower speed control on greasy roads including urban roundabouts etc), am I better starting with 51s or 54s and then getting the fuelling mapped to optimise for whichever TBs (and the rest of the induction & exhaust path) that I have gone with?

[Micah / AF1 Racing]

I like the stock 51's for ONE reason. The plates or butterflies are fitted correctly! I have yet to see an overbore job with the same level of precision that is equal to OEM. If you want bigger go to a set of 57's (OEM only). These are my findings from years of testing other people's mods.

[HisNibbs]

Your math sounds fine and would explain de impresive efect that has on the throttle... if I loose 2 HP at 10000 RPM but I have this effect on 5000 RPM, than up to my standards, the 54 are the best on my type of engine with the short 54 Velocity stacks

I think TuBa you may have missed the point. The extra BHP you feel low down is because effectively you have the throttles more open for the same twist of the wrist. What Micah indicates is that actual power at full throttle through the rev range is not much different.

I would expect the only power benefit of the increase in bore if there was one, to be at very high rpm and hence flow rates. Inlet tract length though, will change where in the rev range peak tourque is generated.

[TuBa]

The bike is amazing with the 54mm ... but considering that I have to sell the kit , my opinion does not worth even the 2 cents now...

http://www.apriliaforum.com/forums/s...32#post3140032

I am sad to do this considering the time it took me to do it but I hope it will find a decent guy to enjoy it and use it a the full capability.
TuBa