rod/stroke question
rod/stroke question
(OP)
Would it be correct to assume that if two engines of the same displacement and same basic configuration each had different rod/stroke ratios, that they could both be optimized to produce the same power levels? In other words, if care was taken to account for the different breathing characteristics that each rod/stroke combination produced, would both engines produce the same power in the end - or is one fundamentally superior over another? For sake of argument, let's say the target for peak power was 7500 rpm and the two ratios were 1.6:1 and the other was 2.1:1 r/s.
Thanks
200v
Thanks
200v





RE: rod/stroke question
RE: rod/stroke question
Cheers
Greg Locock
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RE: rod/stroke question
"It is essential to use the longest connecting rod you can possibly fit in the engine."
"All the dynomometer testing I have done indicates a racing engine always produces more torque and horsepower when the rod ratio is increased beyond 1.75. In fact, in any conventional engine with stroke length of approximately 2.5 to 3.5 inches, I believe there will be a notable increase in performance as the rod ratio is increased - all the way up to ratios as high as 3.0:1 ! "
Smokey Yunick, in 1983 "Smokey Yunick's Power Secrets" page 36.
Primary applications were Gen 1 small block Chevy, with stock 9.020 inch deck height, circle track, Nascar and his Indy engine, making power at 7500 and maybe a bit higher.
RE: rod/stroke question
RE: rod/stroke question
- Steve
RE: rod/stroke question
RE: rod/stroke question
Thanks for the input. I was thinking changing rod length and deck height to keep bore and stroke the same.
Based on the fact that the piston speed away from tdc is going to be different for the different r/s ratios, and that this would result in a different demand on the cylinder head/intake manifolding, would it be safe to assume that the top end would be configured differently to optimize power for each case? Would the heads, cam, and intake porting need to change if a r/s ratio changed in order to have both engines produce the same power?
I understand the differences that rod/stroke ratios bring in terms of piston accelerations, piston sideloading, engine lifetime, engine weight, aero. I'm trying to get a handle on whether one r/s ratio will always make more power than another based only on this variable assuming everything else is optimized.
Thanks all
RE: rod/stroke question
RE: rod/stroke question
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: rod/stroke question
Does anyone know of test data using the same engine and just changing the cylinders and rods to test the kind of r/s ratio changes being asked about? I've seen test data on 383 Chevy engines between the different rod lengths and that changes the r/s ratio a bit but nowhere close to the numbers the OP has thrown out.
RE: rod/stroke question
How would we expect the tuning parameters to change to make each design optimal for peak power. Specifically, how would ignition timing, intake port length and diameter, cam duration, and cam timing tend to vary (if at all) between the two ratios?
Thanks
RE: rod/stroke question
RE: rod/stroke question
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: rod/stroke question
With the smaller ratio:
- Ignition advance timing should be less as the piston will achieve «final» compression later;
- Exhaust valve should open earlier as piston goes down faster;
- Exhaust valve should close earlier as piston goes away from TDC faster;
- Intake valve should open later as piston start going up later;
- Intake valve should close later as piston arrives near TDC later;
- I think port length & diameter should be unaffected as the strength of the pressure waves should be similar and the valve durations should be relatively the same, only with an offset.
Here's a graphic representation of piston position from HPWizard.com:
RE: rod/stroke question
RE: rod/stroke question
if you plot out the piston position vs crank angle with a small change in rod length the difference is remarkably small.
99% of people would be better off focusing their attention to getting the right cam in the first place.
RE: rod/stroke question
The cost benefit ratio of increasing deck height to get longer rods is unlikely to be favourable however.
RE: rod/stroke question
RE: rod/stroke question
RE: rod/stroke question