Camgrinder;
You pointed out that...
Quote:
|
If you have an engine and change only the stroke, and keep the compression ratio etc the same the powerband will move down.
|
This is a common conclusion that many people make and somehow relate it to rod/stroke length or just the magic of a longer stroke. My response (and conclusion based on analysing a number of engine combinations) is that it has nothing to do with the stroke. When you "change only the stroke", the reality is that you're not changing only the stroke, but you are changing the stroke
and the capacity. If you try to draw a larger volume through the same sized intake system, it will max out sooner -- thus moving your power band down.
To accurately predict what you are describing, you would need two engines, one with a larger bore and a shorter stroke, and the other with a smaller bore and a longer stroke,
but both engines of the same capacity. Using the same camshaft and induction systems, valve size, etc., I maintain that in general you will get the same performance from both engines. Surprisingly, there are relatively few occasions where this has occurred. One obvious case in the 911 world is the 2.5 liter ST engines which were built both with 66 mm cranks and 70.4 mm cranks (and corresponding changes in bore to maintain a 2.5 liter capacity). Near as I can tell they both had essentially the same peak torque engine speeds and peak HP engine speeds. I have yet been able to find anyone who can provide dyno charts or published torque and HP numbers of a similar back-to-back test that supports that long stroke = more torque and lower rev range conclusions.
Disclaimers: I'm assuming reasonably normal piston speeds here, if the piston speeds are very high for the short stroke, engine, then the long stroke engine will perform differently. I've also heard that engines with a higher rod-stroke ratio tend to work better on induction constrained engines since the piston "lingers" at TDC longer allowing the exhaust inertia to draw in more intake charge during overlap prior to vacuum caused by the piston's intake stroke sucking this charge back into the cylinder. I'd be interested in seeing someone post a comparison of piston travel to rod length, but that's a subject for another thread.
Points not being argued...
John, I agree with your 2nd and 3rd paragraphs. The points that you are making tend to be more precise then I was presenting. I was just trying to distill the subject down -- which necessitates consolidating some details. Your point about intake valve closing point is essentially included in my intake duration formula since the intake valve's opening point is largerly defined by TDC and the amount of overlap desired. The same applies to the exhaust valve opening which is included in my (corrected) peak HP equation, except in that case it is the closing point that is defined by TDC and overlap. I'm not disagreeing that there isn't more subtlity to the subject, just that this piece was intended as primer, not a thesis. Besides it's not my intention to give away all of the cam grinder's secrets. Just raise the level of the overall conversation.