rod length selection?

i read a theory earlier and just wanted to run it by you guys. o.k here is a example of what i was wondering. if you take for example a 350 chevy engine if you lose the longer 6.0 rod you will end up with more torque but if you use the shorter 5.7 rod you can end up with higher horsepower and be able to rev higher.would this be true? can you guys give me a few of the rules when choosing rod length? i believe i will be going with a ford 302 stroked to 347 but was just wondering about this theory. is it BS or truth?

Some truth, some misunderstanding. No BS. Optimum rod length is a rod that will result in a rod/stroke ratio between 1.6 and 1.8. Example: 5.7" rod, 3.48" stroke, r/s ratio is 1.637:1. That's a 350. 327 is better. 1.75:1. A 350 with the 6" rod will yield a r/s ratio of 1.72:1.
Smokey Yunik (RIP, one of the best engine builders EVER) said to always put as long a rod in there as you can fit. When we build a circle track 350, we use 6" rods. When we build a 400 SB, even a "stock" one, we recommend to install at least the 5.7 rod from the 350. I disagree that a shorter rod will allow an engine to rev higher.
What the actual result is, the longer rod will offer a lower rod angle (in relaton to the crank throw), reducing side load on the cylinder wall, and the pressure against the weakest part of the rod. It also allows the piston to "dwell" at the top and bottom of the bore for a slightly greater amount of time, improving filling on the intake stroke, and higher cylinder pressure on the power stroke. Where the idea a shorter rod makes more horsepower came from, I have no idea. Not so.

We have seen a significant number of failures with the 347 strokers. The rod/stroke ratio for those with the 5.4" rod is a mere 1.54:1. Not too good.... For a street engine, that makes more power than the 302, in the same block, I recommend the 331 or 332 (depends on supplier, which it's called, but essentially the same) kit. That's a 3.25" stroke. Same as 327 Chevy, actually. With a good set of heads, those things can really make some high-end power. It has a more desirable rod/stroke ratio of 1.66:1.

Didn't mean to load you up with information, but it's a fairly complex issue. I hope this helps.

Where the idea a shorter rod makes more horsepower came from, I have no idea. Not so.
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The shorter rods increase the speed the piston travels down the bore . It does 2 things the faster the air travels, on the intake stroke ,the higher the Volumetric Effientcy. The rod angel on the short rod also has more leverage on the crank during the firing cycle. Piston speed and lighter rods do increase horse power duablity is a different issue. Compare a pontiacs torque per cubic inch verses any other engine. The long rod makes the pulling power you need for the street. High RPM screamers are short lived but if light weight is the only issue then the cost to run will go up as in any aplication.If you can fit a sprotsman block in your car you will build a longer life more driveable faster engine.

mokicruz,
Okay, I can understand what you're saying about the leverage, but the speed is another matter.
A given stroke will have the same "mean piston speed", no matter the length of the rod, while the rod length WILL have an effect on the accelleration and decelleration of the piston as it approaches TDC and BDC. With a longer rod, the piston will "dwell" longer at both TDC and BDC. Given that, and given the "mean" (average) speed is the same, the accelleration and decelleration speeds will be quicker with the longer rod.
As for the Pontiac, I'm glad you use that as an example, as I have a BUNCH of experience building stockers and racers. We ALWAYS use the 6.8" rod in the Injun for any serious performance or race application. We have found it to be the "cure all" for the bottom end and RPM problems. I doubt the length has as much to do with it as the rod bearing, but it still helps. We have several 406 CID versions (3.75" stroke) that rev well past 7,000 and live. Many are in street cars with MANY miles, plus the racing. The "strokers" usually peak power right at 6,000. Ten years ago, that was unheard of for a street-friendly, "pump gas" Pontiac.
The major factor in the production of low-end and mid-range torque the Injun is so famous for, comes from the design of the intake port. Consider the volume of air/fuel being drawn through (200-plus CFM on a stock head) with such a small cross sectional area (1.8 square inches). That indicates a very efficient design. To achieve the same flow at a given lift, a small block Chevy head will have over 2 square inches. The chamber must also get a little applause, as it is, short of the Hemi, the nicest chamber out there, in a 2-valve V8. Actually, for low-speed and driving, it's better than Hemi. For high nitro (NOT nitrous) loads, nope, it takes the number 2 position behind Hemi.
I got sidetracked... Sorry... If you know of any published information that may add to my understanding of these issues, please, let me know.

Jim

http://www.iskycams.com/ART/techinfo/ncrank1.pdf

This is printed info by isky that will show how far down the piston is at the same crank degrees. You might find it interesting.

Mokicruz,
Thanks for the e-mail and site info. Looks like I'll be doing some fairly heavy reading in the next few days... LOL Might have to dust off the ol' calculator, as well!

Jim

I found it reading up on the ford F1 engine long time ago. The rod lenght on that engine made the difference between also ran status or a winner. The old 1000hp BMW with thier head design and shortened rods also paid huge dividends. It alway made me curious if the fuel is 80% burned in the first 2" the piston travels how something that minute could effect torque that much? Is there more to the combustion chamber design on a pontiac with High velocity intake runners that make that engine so torque efficent? Bruce Fulper knows the secret but he's not telling. Look at the motorcycle engines Horse Power now they Are amazing with 1.52 rod ratios. You see alot of broken crank cases but at 13,5000 redlines you can expect that. I've over wound my ninja several times maybe to 16,000 and it stills hanging with 19,000 miles.

First, Flopper only SAYS he knows. There are a couple of guys WAY past him in Pontiac performance, they just don't have a high profile. Bruce has been known to "fudge" numbers, too. He gets "air time" in the Chevy mags. The Pontiac mags don't talk to him anymore... I don't mean to say he knows nothing and has contributed nothing. He has simply alienated the Pontiac "world", and is now on the outside, looking in. NOBODY knows everything, and EVERYONE can learn from anyone. He doesn't agree with the former, and only the latter if he's the one teaching. Very abrasive guy. I met him once, at his home. I grew up in that town, so I thought I'd drop in to see who was making a living building Pontiacs in San Bernardino. He was putting fuel injection on a high-dollar race car, in the dirt in his front yard (yes, his "shop" was in his garage). He's moved since then, to a small shop, but the point is, he tried to put forth the image of a high class race shop, all the while running a "cottage" industry from his home.
For the record, Jim Butler, Jeff and Mark Kaufmann, Ken Keefer, Old Man Taylor, Marty Palbikyn, Pete McCarthy, John Angeles, are all better sources of accurate and selfless information. And Jim Hand knows more than all of them combined! Jim and I have an ongoing dispute over cams and static compression versus dynamic compression, but we get along quite well. His book is a big hit, having sold out the first printing in less than 4 months, and the second has already hit the shelves. We're all quite proud of it, and are quite happy with the response. Bruce is conspicuous by his absence. So is Mr. Nunzi. Enough Pontiac gossip.

The Pontiac chamber in the factory head is TOO efficient. That is, it burns too quickly. A Pontiac with the same level of tune as a Chevy in a similar CID range, will require about 4 degrees less total timing than the Chevy. In fact, if you give it too much timing, the head gaskets will not live. We've tried every form of gasket and every form of fastener. Over 36-37 degrees total, they tend to start eating the gaskets. The same Chevy will like as much as 40.
An example I can recall, to illustrate the chamber's features, is the nitro cars. The last non-Hemis that were competitive at the top fuel level (in funny cars) were Pontiac. A Pontiac factory chamber can stand as much as 65% nitro. Most factory Chevy heads will go away over 50%.
The fully machined wedge is real pretty. The earlier heads had "closed" chambers, which actually produced a bit more torque than the "open" ones, but the open chambers provide much cleaner burn (emmission friendly). The open one also is a bit less shrouded around the intake valve.
The Edelbrock head uses a conventional chamber, not unlike those found in their Chevy offerings. The KRE head has a modern HSC (high swirl combustion, not "heart shaped chamber") chamber and a more centered spark plug. It also uses the conventional "D-port" exhaust flange, instead of the round port like the E-head. I doubt either of these are suited to fuel class racing like the factory stuff.
FYI, there's a new head (clean sheet of paper new) coming in the next couple of months. Known as the "Tiger", it is claimed to flow 400 CFM right out of the box, and incorporates Pontiac-style chambers. We're quite excited about it. Could wreak havoc among the BBC and BBD crowds.
We shall see!

Jim