Effect on exhaust port size and power !
Posted: June 15th, 2008, 12:00 pm
Ok I have had a bit of an experiment recently with two engines. Both have the exact same specs accept two things.
Both have filled in inlet ports (made smaller) but the other identical engine has the exhaust ports welded up to be made smaller and also runs a smaller exhaust as well.
Now both bikes make max power at around 9000 rpm making 153-155hp and then power falls right off unlike holding on like a stock sized port. The one with smaller exhaust ports and exhaust makes much better power all through the lower and upper mid range by 7 to 13hp in places. The one with just the filled in inlet ports only hardly betters another bike in the mid range with totally unfilled ports.
Both the smaller port bikes are making the same peak hp as a stock sized port bike but some 1500rpm earlier.
I hear so many people say smaller exhausts make better bottom and mid power because of back pressure and others say it makes no difference. My question is what is true and what is not.
Any thoughts welcome.
I can post these two dynocharts but im not fully sure how to just need to figure it out as they are in adobe format.
Both have filled in inlet ports (made smaller) but the other identical engine has the exhaust ports welded up to be made smaller and also runs a smaller exhaust as well.
Now both bikes make max power at around 9000 rpm making 153-155hp and then power falls right off unlike holding on like a stock sized port. The one with smaller exhaust ports and exhaust makes much better power all through the lower and upper mid range by 7 to 13hp in places. The one with just the filled in inlet ports only hardly betters another bike in the mid range with totally unfilled ports.
Both the smaller port bikes are making the same peak hp as a stock sized port bike but some 1500rpm earlier.
I hear so many people say smaller exhausts make better bottom and mid power because of back pressure and others say it makes no difference. My question is what is true and what is not.
Any thoughts welcome.
I can post these two dynocharts but im not fully sure how to just need to figure it out as they are in adobe format.