Rod Ratios

[colin]

Who knows anything about rod ratios in aircooled engines.

From the small amount of reading I've done so far, I've worked out a few things (I think)

Rod ratio is the ratio of the length of the conrod/stroke of the crankshaft

Standard
Conrod length (mm) - 136.144
Stroke (mm) - 69
136.144/69 = 1.97

A write up by the late Mark Herbert states that a lower RR will give an engine with more torque, but you shouldn't really go any lower than 1.67, This equates to a 5.4in conrod on an 82mm crankshaft.

What are the upper and lower limits for RR?

Does anybody have real data from experimenting with different ratios in the same engine or know a website with this sort of info on it?

Thanks,

 

[56oval]

Ignore anything you read on the internet on rod ratio r.e an a/c vw motor, ive run 5.3" (porsche) rod on an 82mm crank for years and they say it shouldnt be done. In the world of hi performance most rods up to an 82mm stroke will come in at 5.4 or 5.5" the tec heads will tell you to go with a 5.5 on an 82, i run 5.5's, ive seen plenty of 5.4's used as well, big cranks 5.7" rods normally chevy journal.

Dont get hung up on it, if its a stock stroke stick with a stock length 5.4" rod i beam or h beam config, stroker up to 82 either 5.4 or 5.5. 5.4 means les barrel shimming, but in some cases may mean more case to skirt clearance.

Do not get hung up on RR, its a headache you dont need, stick to what is out there.

 

[colin]

Thanks for the input Ben.

Contacted Dave, and the advice is to stick with some sort of 5.4s and that seems to be the most common size, so I'll stick with whats tried and tested.

Out of curiosity, Does the ratio change the characteristics of the engine much?

 

[56oval]

The only way to tell you is by doing the following, building a motor with a specific deck on a 5.4" rod. then hit the dyno.

then build that same engine but swap the rods out for 5.5s which will mean a wider engine, barrel shimming and a new set of cut to length pushrods.

set that motor up with identical deck, and reassemble it with 100% the same parts and config, in theory the 5.5" rod will make more torque as the piston dwell is increased at tdc.

in essence thats the theory, but i wouldnt say you wouldnt see even as much as 10ftlbs torque increase.

Until Dave does it, its down to yet more internet fondling and being convinced by theories that have never been backed dup on a dyno. So stick to the basic known combos that work without the luxury of melting your brain!!

heres an example that will melt your head.

my old engine 135bhp, 150flbs tq. 5.3" rod 2276

new engine

185bhp 160ftlbs torque. 5.5" rod 2276

you cant compare them, different rod lengths, cam lift, heads craburation compression, the only way to really know, is to do a back to back test, and in this instance, it cost a lot of time and money.

 

[Dirty1]

Correct me Ben if I'm wrong but that seems a minimal difference when it comes to RR an probably more suited to fine tuning race engines for maximum out put.

With all the various combos available in bore an stroke for VWs wouldn't short vs long stroke motors have more of an impact on the type of performance produced with short stroke motors spinning up quicker an being revier, an long stroke motors being torqueier, more controllable power. I know that's how it is on bikes but is it the same on flat4's an should your piston/rod combo be determined by the type of performance you require?

 

[caddy]

Correct me Ben if I'm wrong but that seems a minimal difference when it comes to RR an probably more suited to fine tuning race engines for maximum out put.

With all the various combos available in bore an stroke for VWs wouldn't short vs long stroke motors have more of an impact on the type of performance produced with short stroke motors spinning up quicker an being revier, an long stroke motors being torqueier, more controllable power. I know that's how it is on bikes but is it the same on flat4's an should your piston/rod combo be determined by the type of performance you require?

rod length does not affect the stroke of the engine . that's done by the crankshaft.

 

[Dirty1]

Ahhh, I get it now, just put my brain back in to gear.
But what about the combination of bore x stroke, as in long vs short stroke. Does it effect the power characteristic much?

I'm used to bikes where you can get a long rod kit an raise the cylinder an run an offset pin to alter the stroke. Dad will give me a slap LOL.

 

[kvinyl]

Ahhh...long rod...stroke...slap...

:grin:

 

[Guest]

;lol3;

 

[Roadcow]

It's just geometry and piston vs. rod angles . Stick with what Ben and Dave have to say about it.

 

[colin]

I'll certainly be sticking with whats been tried and proven (5.4s for me), But I was interested to see if the rod length actually affected the engine all that much or was it just all talk.

So the general consensus is that it's nothing to worry about, Only a headache?