Crankcase Compression

[rkm]

Im having conflicting opinions on the effect of crankcase compression on pipe tuning.

I lightened my crank and opened up a lot volume in the crankcase. Ive gained considerable midrange acceleration but I cant seem to get my topend back.

Am I gonna have to shorten the header to get this topend back?

 

[kevbo]

Crankcase volume changes can affect carb tune but wont change the pipe tune. Have you lost peak rpm? Sometimes the top end feels a little flat after doing response enhancing mods like adding compression and reducing flywheel weight.

 

[egbrig]

Volume is one of the factors that come into play otherwise there would have been one pipe. Yam 701 b/s 81x68, SD 718 b/s 81x68 two different pipes and yes I know that the SD is rotory valve inducted. The carb tune is effected by the change in volume because of the effect volume has on the pipe. The thing about CC is that you have to balance the pipe/ CC volume / and rpm range. Different pipes have different results with the same volume. Try filling in dead space in the cases. This could be a much longer post but I don't have the will to type it out.

 

[rkm]

I lost 500 rpm, Ill do some more testing.

 

[Foximus]

What is your actual plotted compression? Have any idea? 1.5:1 is a generally accepted well performing ratio.

 

[rkm]

Hmm let me find the chunks i cut off the crank and Ill get back to ya on that

 

[egbrig]

500 rpm missing? You have something else going on. Is the crank true? Is it so far out of balance that its vibrating itself to death? So many things it could be. Start checking the simple things first, pipe (water leaking into it) ignition timing?

 

[Foximus]

If you know your machined depth and what amount of radius you took off it should not be that hard to calculate the change.

 

[rkm]

Thats 500 rpm off 9000, I need to GPS and check my speed vs. rpm.
Thats not that much. More variance than that in pipe testing