rod/stroke question

[200v]

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

 

[ivymike]

You mean keeping bore and stroke the same, and changing both rod length and deck height? I'm going to go ahead and say that you could probably get them pretty close, but the one with the longer rod would be the heavier engine.

 

[GregLocock]

Practically the low deck design is preferred for racing and so on, due to the reduced weight and stiffer block and better aero. There's slightly less side force on the piston with the longer rod, offset by increased rod mass.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Tmoose]

"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.

 

[ivymike]

so back to the original question - are we keeping the same deck height and changing bore/stroke/rod to maintain displacement, or are we keeping bore/stroke and changing rod length and deck height?

 

[SomptingGuy]

Rod length will affect piston motion (especially secondary out of balance forces). Not convinced it will affect power though, assuming extremes aren't in the frame.

- Steve

 

[Tmoose]

OP 200V has not been back since June 5, the day before anyone responded.

 

[200v]

Ivymike,

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

 

[ivymike]

I really don't see the rod length having an important effect within the relevant range of geometry. There may be subtle differences, but I couldn't say which one would come out better. Perhaps some else will have another view?

 

[Lou Scannon]

For any given application type and cylinder displacement, representative best-in-class engine designs tend to settle on a happy compromise among bore, stroke & rod length within a fairly narrow range. Of course these parameters & their ratios differ vastly among the wide range of different applications, from 2-stroke low speed marine, e.g., to F1 or moto GP. Many factors bear on this compromise, and it goes beyond my expertise to list them all. Certainly breathing, external size & geometry, internal stresses, friction, and total mass all play important roles, not to mention cost.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[LionelHutz]

When keeping the same engine configuration, the biggest advantage to using a longer rod is that the piston can be lighter.

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.

 

[200v]

If there are no fundamental reasons why one ratio would be able to produce more power than another, then how about this queston:

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

 

[FreeBug]

IIRC, the thinking behind Smokey's "long-rod making more power" business was all about increased piston dwell at TDC, in order to generate greater combustion pressure.

 

[Lou Scannon]

Disregarding mechanical optimization for a moment, it really boils down to the indicator diagram, all 4 parts of the cycle. The portion of the diagram that has the most influence on the overall IMEP is of course the heat release. For a given amount of charge mass and combustion efficiency, the phasing of the heat release wrt crank angle is very influential. Smokey was definitely onto something; not increased combustion pressure per se, but causing more of the heat release to occur near TDC, which obviously will show up as greater peak pressure, all else being equal. Along with rod ratio, you could play similar games by off-setting the piston pin wrt the crankshaft as well. Of course with higher pressures & more heat release near TDC you get higher peak temperatures, hence the knock margin is probably negatively impacted, but there are many other variables playing a role here so I wouldn't generalize that this would necessarily be the case.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[JackAction]

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?

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:

 

[200v]

thanks all for the very helpful information.

 

[inline6]

i think rod ratio is pretty much pointless in most instances. for a given application with fixed deck height the amount you can vary the rod length by design is usually small without resorting to reducing the cubic inches.

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.

 

[Komodo86]

It's beneficial to use the longest rod you reasonably can, i.e use the shortest compression height on the piston and fit the longest rod you can in the gap.

The cost benefit ratio of increasing deck height to get longer rods is unlikely to be favourable however.

 

[inline6]

if you are at the limit of comp height on the piston id be increasing the stroke before i worry about the damn rods. that will make a real difference....as opposed to a difference where you'd be lucky to be able to tell the difference on the dyno

 

[Pancholin]

Almost everything in life is a compromise and conrod length is no exception. Longer = more time trying to push the crank out the bottom---shorter = more time trying to push the piston through the cylinder wall.