Rod length effect on torque and horsepower, can short rods be better?

[Pablo]

I've been re-reading Smokey Yunick's book "Power Secrets". The material is a bit dated but the man is a genius.

I was going over the section on rod length where Smokey argues long and hard to use the longest rod that you can.

To briefly summarize, he points out that with a longer rod the piston dwells near TDC longer. He argues that this allows combustion gasses to work on the piston longer. He also says there is a mechanical advantage in that the crank arm swings over further while the piston is closer to TDC.

This got me thinking.

With a longer rod you are imparting a force at a much more obtuse angle to the lever arm of the crank.
With a shorter rod the angle becomes less and you are imparting the force closer to perpendicular which would in turn give you more leverage.

A simple way of demonstrating this is putting a degree wheel on the crank, moving it to 6 ATDC and placing a weight on the piston. Then you would measure the resulting torque required to hold the crank at 6 ATDC. With the long rod I can almost guarantee that the torque required to hold the crank would be less than with the short rod.

The losses in this system would be the increased friction from the piston on the cylinder wall, and decreased dwell time which would mean that combustion pressure would fall more quickly than in the long rod scenario. The one other potential problem is the possibly increased detonation sensitivity due to the sharper pressure rise in the chamber. Maybe reduced timing lead is all you would need..and in that, perhaps another factor that would improve power (sort of like increasing the compression ratio)

In Smokey's case I can see that at 7000 rpm N/A, the friction generated by the piston wall would become a greater proportion of time that the combustion energy would have to overcome.. so as RPM comes up the power gained from increased leverage diminishes relative to added friction.

But in our engines that run at relatively low rpm, is this a factor? Is there power to be gained from a shorter rod? I can conceivably see how you would increase low rpm torque at perhaps the expense of high rpm HP. But at what rpm does the system start to lose more than it gained? Obviously the rest of the system's tuned rpm range is playing a big factor.

Any gurus out there want to comment on this?

 

[bison]

There is a lot of debate on the subject. Shorter rods have higher velocities which could help on filling the cylinder better on the intake stroke. Imo a lot of this applies more to engines that are run on the edge and in a very narrow power band. The big bore, long rod, short stroke engine has a higher detonation limit than the long stroke, shorter rod and smaller bore though. There are a lot of things involved with all these scenarios. The biggest increases on turbocharged engines ive seen are when the exhaust pressure in between the engine and turbo is reduced as much as possible. Usually this requires a shot of nitrous to spool a big exhaust wheel in a big exhaust housing. Id look into reducing the exhaust pressures before i worried about the rod length.

 

[mikestertwo]

I did this kind of experimenting years ago with big block Oldsmobiles using 425 rods in a 455. Don't remember the exact rod lenths. I think the 455's had a 6.735" rod and the 425's used a 7" rod. Using the longer rod moves the torque curve up in the rpm range which gives you more horsepower. The car ran extremely well, 11's at over 120mph with a single 850 double pumper carb in a 3750 lb. car. I don't think shortening the rod length in our motors will help anything. Will most likely slow the motor down. It will develop more torque at a lower rpm but will sacrifice top end horsepower. Our motors make plenty of low end torque just the way they are.

 

[Alky V6]

Cylinder wall loading of a turbocharged engine should always be an important consideration. With cylinder pressures easily twice that of a n/a application, the integrity of the cylinder wall needs to be high on the list. And judging from information that tells us that rod length isn't real important with our engines, I would lean towards making sure my cylinder wall loading was minimal. Especially, if you plan to use nitrous.

 

[TNTBuick]

Shorter rod length significantly increases wear on the thrust side of the cylinder wall. For reliability and longevitiy, use the longest rod possible, even in a n/a engine. I agree with DonWG.

 

[novaderrik]

i thought that Smokey, in his later years, came to the conclusion that the job of the rod was to connect the piston to the crank, and that the right length was the length that got the job done.
or maybe that was someone else.
on a slight side track here- but Smokey got a patent for an engine in the 60's or 70's that used a turbocharger to not only compress the air/fuel mixture, but also super heat it to get more power with less fuel. this goes totally contrary to the idea of intercooling a turbo motor to make more power.
the guy definitely had some neat ideas on how things work and what can be done to make them work better.

 

[KrisW]

My old Buick Power Source Manual says, "As more Buick V6 competition engines are built, it is increasingly evident that rod length has little or nothing to do with horsepower or torque; so you should consider using the longest rod practical to minimize cylinder wall loading."

That being said, Smokey Yunick is no dummy. I wouldn't say he's wrong; it's just that the shorter rod isn't the answer to the Buick V6 question..

 

[Pablo]

My old Buick Power Source Manual says, "As more Buick V6 competition engines are built, it is increasingly evident that rod length has little or nothing to do with horsepower or torque; so you should consider using the longest rod practical to minimize cylinder wall loading."

That being said, Smokey Yunick is no dummy. I wouldn't say he's wrong; it's just that the shorter rod isn't the answer to the Buick V6 question..

Smokey didn't say to use a shorter rod. He advocates the longest rod possible. It was myself that asked the question as to whether or not that was realistically necessary

So it sounds like cylinder wall loading is what is the main consideration.

I was talking about this to a buddy of mine who is pretty good at physics/trig, etc and with his help we worked out the difference in angle of the piston rod to crank arm on a theoretical engine with a 3.5" stroke and a 6 inch rod vs a 5.7 inch rod. We worked out the math for a few different crank angles It was pretty surprising, the difference was pretty small. Smaller than you would think. I was gonna have him work out the % difference in force on the cylinder wall on that engine too but he had to run. I'll see if I can talk to him again. I really oughta go back to school to learn this stuff

 

[KrisW]

Sorry! That's what I meant. I was rocking the baby to sleep at the same time and typing one handed. Sometimes my train of thought goes off the tracks!

Since you have been brainstorming, do you know the stock length of the 3.8 connecting rods? Are they 6"?

 

[Lynn D. Brown]

Rod length

Basically, I don't think it's all that complicated. Increasing the stroke or rod length increases the cubes of the engine, and torque increases accordingly.

 

[novaderrik]

Basically, I don't think it's all that complicated. Increasing the stroke or rod length increases the cubes of the engine, and torque increases accordingly.

rod length has nothing to do with the displacement of the engine- that is dictated by the cylinder bore and the crankshaft stroke. all the connecting rod does is connect the piston to the crank- hence the name "connecting rod".

as for rod angles not being significant on a Buick V6- it is no more or less critical than ANY other internal combustion engine ever built- be it a Buick V6, a Chevy V8, or the 5hp Briggs on your push lawnmower. an engine is an engine, and the same rules apply to all of them, but the "ideal" rod length will vary from engine to engine. but there is a theoretical "ideal" rod/stroke ratio to shoot for, but i forget what the number is. i seem to recall that a 6" rod in a 3.48 stroke 350 chev engine, for example, is pretty damn close to it, tho.