I'd just like to get everyone's opinion on how much difference rod length makes when building an engine. I know that for various SBC combos you can use 5.7", 5.85" or 6.00" rods, but I'm wondering if using any one gives a noticeable difference in performance characteristics (assuming that everything else is as similar as you can make it)...

Thanks guys.

 

This is one of the topics you might want to try a "search" on. It's been discussed several times, and I think its fair to say that the general consensus is that for anything other than an all out race engine, it really doesn't make enough difference to worry about. Selecting a rod length to give you the compression height required by your application (e.g. - nitrous and blower motors) is probably a more valid basis for selecting the rod length.

 

whats the stroke going to be?

i just ordered 5.85" billet olivers today and i'm running a 3.80" stroke. was going to run a 6" but the pin height with the 5.85 is going to last longer. might be giving up a few hp but reliability is more important to me.

are you just runnin NA or nitrous?

 

You'll get a lot of guys that will come in and say "the best ratio is 8 rods to 1 crank". Rod length CAN play an important role, however. Check out this article on running 11:1 on pump gas (87 Octane!).

http://purplesagetradingpost.com/sum...%20engine.html

 

Rods are a good thing to hook a piston to the crank!!!!
From 5.500"-6.125" ya won't be able to tell the differance or see it on a engine dyno or with the old B-O-M.

A quote from David Reher of Reher Morrison Race engines:

Why Rod Length Doesn’t Matter


I’ve read as many articles and books as the next guy when it comes to connecting-rod length. Essentially, there are a couple of accepted theories Some state that that short rods are better, others that long rods are. Neither might be correct. Not only has Reher-Morrison plotted points on a graph using three big-block Chevy rod combinations (stock--6.135-inch; +0.250--6.385-inch and +0.400--6.535-inch) to show piston movement through crankshaft movement, and also built a test engine to prove the theory. To be honest, when the plotted graph points are overlaid on the same page, the traces are so close, it’s virtually impossible to differentiate between them. At 10-degrees ATDC (a point where the most pressure is present in the cylinder on the power stroke), the difference between the shortest rod combination and the longest rod is a mere 0.0004-inch (that’s not a misprint; it’s four ten-thousandths of an inch).

A small block test engine showed basically the same results, only this time, it was on a running dyno mule. Contrary to what some experts have touted, the tests (which were very comprehensive) showed zero gain with long rods, particularly past 1500 rpm. In simple terms, the connecting rod links the piston pin to the crank. There’s nothing more to it, so don’t waste your time working with weird rod length combinations.

 

I agree with that!

 

Rod length discussions = much ado about nothing.

Rich

 

In my personal opinion, the main issue with rod length as it relates to US, is piston dwell.

At the risk of oversimplification; when you make the rod longer, the piston tends to move slower when it's near BDC and TDC. When you make the rod shorter, it moves faster.

Here's how that works in a practical sense:

Ordinary gasoline burning engine with a not particularly aggressive dynamic compression ratio? May as well go long on the rod and give the burn more time to work on the piston.

Pushing the edge on dynamic compression, either via high static compression or forced induction? Probably be a good idea to go shorter.

Planning on running nitrous? Short is probably the way to go.

Planning on building an alcohol burning engine? Longer rods will help you take advantage of the combustion characteristics iof alcohol.


One thing is for sure...

I personally wouldn't bother replacing the rods just to have longer or shorter ones. The appropriate time for the discussion is when you are replacing them anyway and are deciding on length, and then the decision is probably better decided on the particular characteristics of YOUR engine than on trying to get every last bit of power.

In a roundabout way I guess I'm trying to say that the question of rod length is NOT seperate from the rest of the engine. It's part of a complete package, not a discrete element.

 

I agree with the above, which does not (though it may appear to) contradict my earlier post. It's just that these effects are small. Maybe to the point of bring inconsequential. That said, I like short rods for blower motors on pump gas.

Rich

 

I agree that it doesn't contradict. (Not that I was trying to.)

I used to be of the school of thought that you should always go with the longest rod possible, in large part because Smokey said it.

But then I saw evidence that it wasn't quite that simple. What hasn't changed for me is the validity of the secondary effects. By that I mean the effects that the longer rods have on the engine; other than the direct production of more power.

I have come to believe that what Smokey observed and interpreted as the direct production of more power was REALLY more about the race environment being well suited to the secondary effects of a longer rod.

(Edited for spelling typo.)

 

Most of the time as an engine builder I use the longest rod possible to get the lightest but still strong enough piston I can. However I also must run a piston that can deal with whatever abuse is gonna be heaped on top of it as well. If I need more ring land area or a bigger dish or an aluminum rod then sometimes the pin needs to be moved down and the rod shortened the same.

I still have yet to hear a truly accomplished engine builder that had done extensive scientific testing on rod length think that rod length mattered much at all to tell you the truth. I can count about 5 Pro-Stock engine builders and one F-1 engine builder on that list.

I was writing a book on engine design and theory and had sent questionaires to several guys like this and asked them to list the top ten indicators of potential power in an unfamiliar engine. NO one in 7 so far has even listed rod ratio. I guess that says something.

 

I have been working on a JAGUAR V-12 engine, 5.3 liters, very short stroke 2.75", but the rod measures 6" center to center, now I wish I knew why they did that type of engineering, cant seem to get info about it. just tossing in this bit of info on rod length from a long time racing company. gives ya somthing to think about. RON

 

Probably stuck with that block height or they would probably lower it at such a short stroke.

 

For the FLYWHEEL numbers those are I am not particularly impressed. It makes good power but nothing to get all that exited about or attribute to the rods.
Consider the stock b-body LT1 was 10:1 with IRON heads and tuned for 87 octane, that is nowhere near the ragged edge of detonation either and quench isn't particularly good which could help allow more compression without detonation. Conventional "wisdom" says aluminum allows/requires one point more compression. Trade off the b-body LT1s reverse flow for that motors aluminum heads and likely better quench, I don't think that compression on 87 is that big a deal on a well assembled motor. All that article proves is thst people have an unfounded believe they need lots of octane.

 

You still don't see many engines running 11:1 on 87, especially carbed ones. With modern AFR 195s (not the small valved 190s in that old article) and a real cam, not that sissy one they used, I'm sure it would be a pretty solid performer. I'm not knowledgeable enough to claim that the long rods are a vital part of that build, but I just put that out there for guys like you to check out.