Mdixon300f100":3q9k8pfa said:
So I was wondering what the displacement change does for VE in regard to the cylinder head flow. I understand it moves the powerban up the rpm range about 1k, but what about VE at lower rpm? Is there significant change in port velocity that hinders low (say 2500) rpm operation? I hear a lot of ratings in hp/ci, but is there a direct relationship between hp/ci/rpm and VE?
In general decreasing the displacement only, leaving all else as is, the smaller displacement engine will have to go to a higher rpm to create the same port flow and VE so the power band moves upward.
However looking at the 300 six we see that the port is undersized for the displacement as shown by the increase in torque off idle as the valve duration increases.
That makes it possible for the 240 to have a low rpm crossover point where it has a slightly better VE than the 300 but the 300 will have more torque over the entire rpm range simply from the fact that it has 25% more displacement than the 240.
When the intake system allows port reversion at lower rpm so there is a well defined power band (instead of flat torque curve), reducing displacement significantly reduces the low rpm VE and torque and while the power band moves upward it also becomes shorter.
One of my customers back in the late 1970s wanted more torque for his 1974 Z1 Kawasaki.
So we had his engine cases sleeved from 66mm to 76mm changing the engine displacement from 903cc to 1197cc.
We kept the stock cam and 28mm carbs and no port work to the head. The engine already had headers.
The 903 normally starts pulling hard at 5000 rpm and you shift at 8000 rpm.
When we finished there wasn't any power band to speak of, just a flat torque curve that would lift the front end off the pavement in first gear from 2500 rpm like it was a dirt bike.
In 1980 I did another 1197cc engine in a 1975 Z1 with a fully ported big valve head, "Hot" street cam, larger carbs and headers.
Now the engine started pulling at 4000 rpm and another hard pull at 7000 rpm, shift at 9500 rpm.
The owner only weighed 135 lbs. and he would run the 1/4 in 10.2 sec in full street trim including turn signals.
He promptly set out to annihilate the new Suzuki 1100 owners that were just hitting the road.
Excuse me for reminiscing the good old days.
