rod ratio vers eng acceleration

[L89 chevelle]

Dave I would like to here your input on this. Is longer better or shorter better. Witch one accelorates faster in an engine and is there a trade off having one over the other.

I herd 1.66 was a magic number?

Your thoughts??

[rookie]

Boy,this issue has created a lot of controversy over the years.
Actually over the last couple of days for me, but that’s another story.
David is going to love this thread.

[automotivebreath]

Ever thought about how rod ratio effects squish action? Boy do I have a
one track mind!

[rookie]

Originally Posted by automotivebreath

Ever thought about how rod ratio effects squish action? Boy do I have a
one track mind!

You said it not me
I think I know I shouldn't.
But I think it yanks the piston away from TDC quicker, so it seams like on the start of the intake srtoke it would create more turbulance.

[tommurphy73]

Very interesting topic.

I have always thought that with a short rod it would be possible to run very high compression ratio without running into detonation problems. From my limited understanding of detonation it appears that if the piston dwells too long at TDC which happens with long rods then it is more prone to detonation. During the time spent pretty much stopped at TDC the piston is pushing straight down on the crankshaft trying to bend the crank rather than turning it.

With the short rod it would not dwell so long and the piston would move from TDC more quickly. Also the crank and rod angles would be more beneficial for making power as soon as the piston starts moving from TDC.

As David Vizard has explained in many articles if Higher compression ratio is used then the pressure decay on the power stroke is much faster than if low compression is used. This may give bigger gains if a short rod is used with high compression.

I had not considered squish and turbulence but it would seem that both should be better with a faster moving piston.

Interesting article here http://www.stahlheaders.com/Lit_Rod%20Length.htm

I am planning on building an engine with a short rod but need some encouragement

Regards
Tom

[L89 chevelle]

Yea I understand dwell time but I am looking at engine acceleration.

Q. does a 396/427 accelerate faster then a 454/540?? all with same rod length.

Is there a better rod / stroke ratio for doing this?

I have been told drag cars shoot for 1.66 Why??

[DavidVizard-GFN]

This is a quick answer to Tom, L89, Rookie, and HB.
Fella's, I have to tell you that you are dipping into one of the most misunderstood and misquoted subjects in all of automotive techdom.
I have a rocket scientist friend (actually several) but this one in particular wrote a 500 page book on the geometry of cranks rods and pistons. Now you would think that something this long would have said it all. He brought over a review copy of this mathematical encyclopedia of things that rotate and reciprocate. About a month later we are having lunch and having waded through a nightmarish heap of mathematics I asked "what are your thoughts on---- ". A dismayed look slowly crept over his face ‘Oh S---, I never thought of that aspect’. The point is that I think crank, rod and piston geometry is something of a Pandora ’s Box. Any time you think you are getting to the bottom of it you find another box and open it only to find more mysteries to solve than previously.

But back to the immediate point here. Each of you guys has brought up some valid points but they are not being looked at in the entire context of the prevailing situation As it stands right now I am concerned about opening a Pandora’s box on this subject. With that in mind I need to come up with a game plan so we all understand what’s needed here and equally important – why. This is going to be one of those giant article deals about 200 pages long if I let it. My plan here so I retain some sort of sanity is to start feeding you guys the stuff you need to know on a priority basis. I.e. the most important things first.

I’ll make a start here by saying that the safest bet in terms of rod length for a typical Detroit V8 is stay toward the long end of the spectrum. All this stuff about dwell at each end of the stroke affecting factors such as detonation are third order effects at best. A long rod will run with less mechanical noise and less piston friction. When you have a handle on the magnitude of the consequences of changing rod/stroke ratio’s then feel free to do so. Starting as soon as I can get to it we will look at what rod length does in terms of a Question and answer story where I summarize each of your questions and give a to-the-point answer.

As of now stick to those longer rods.
DV

[Cammer]

Rod thingy, connects crank thingy to piston thingy!

Pretty much covers it!

[tommurphy73]

David,

Are you saying that there may be some revelations at the end of this?

In the meantime could you recommend any further reading on the subject.

Regards
Tom

[shrinker]

Hello All; I have done exactly changing the rod ratio thing. We only changed the rod ratio (and corrected the pin height) went from a 6" rod to a 5.7" rod in an injected methanol SBC and it made absolutely no difference to the performance. I don't think the question is as simple as just which rod length is best. There are far more important factors to get corrected before you worry about rod ratios. I realize that the engine this was done on was probably not close enough to optimum to realize a rod ratio factor. But its still a good subject.