I rebuilt my motor (355) and used all the stock stroke specs. Even the rods. But now I see people with 355's running 6" rods. What is the difference?

Benji

 

Yeah i also wana now

 

The longer the rod is the longer the piston stays near TDC. Allowing combustion pressures to build more before the piston starts down. It helps to make a more flat torque curve adn build bottom end torque. Circle track racers usually use the longest rod they can.

 

you also get less side wall loading when the piston moves up and down in the bore. this helps stroker motors more, but works the same on 350s. its nothing too spectacular really. just remember to plan for it next time you build if you have the budget for it.

 

There is not a nickels worth of difference in a 5.7 & 6.0 rod. You can't see it on the dyno or in the wear factor (well maybe in 50,000 miles) You have less piston with a 6.0 and that's about all.
A difference can be seen with a comparison of 5.7 & something like a 6.250 but that combo will not fit a standard deck SBC but I have done the comparison on a Ford from 5.995 to 6.350
It is mostly how the manufacturer packages the kit and how the counterweights on the crank are made and the 6.0 is easier to internal balance. It also depends a lot on your heads as to what length rod to run and remember long rod are for highRPM's(8-9000) and short rod for lower RPM's(5-6000) basicly.
Build your combo around your heads and intended use.
Either will work but with a given head one or the other might have an advantage. Ya just have to do the math and see.

 

Thank you for clearing everything up. I kind of kicked myself for not going with 6" rods, but I guess I will never really know the difference.

Benji

 

I disagree. You now know... .... on the street, there is virtually nil difference to experience. As Larry noted, and I agree; in extreme cases, when coupled with complementing components (which is tough and costly to achieve, even in racing environment), in a racing application, gain can be achieved. In a street engine, on the street, this is something more suited for BENCH racing.

 

i disagree. the 6" rod as a better angle on the crankshaft on the down stroke, the piston is at the top allitle longer(as far as degrees are concered) and lets the crank rotate a few more degrees alowing better leverage on the crank while forcing it down, so it increases torqe, im not saying you get a 100 foot lbs or nothing but the torqe does increase and also as mentioned above, it make the torqe curve slightly flater

 

Believe what you want. Best I remember there was a 3# increase in torque in the mid range and less HP. Sooooo if you want to run them fine,like I said there ain't a nickels worth of difference in them.
Do the math and you will see that it will take more than .300 and a bunch of RPM's to see a difference. If you destroked it and ran a 7" rod and a large set of heads turned it 10,000RPM's you would be into something.

 

I was gunna do that, but I heard to use shorter rods if you go with a FI route... (the shorter, the less chance of bending them)

 

Heres a little tidbit from ISKY CAMS. These people paved the way for the aftermarket we have today and have probally spent more time on a dyno than anyone else in the universe.

"Rod Lengths/Ratios: Much ado about almost nothing.

Why do people change connecting rod lengths or alter their rod length to stroke ratios? I know why, they think they are changing them. They expect to gain (usually based upon the hype of some magazine article or the sales pitch of someone in the parts business) Torque or Horsepower here or there in rather significant "chunks". Well, they will experience some gains and losses here or there in torque and or H.P., but unfortunately these "chunks" everyone talks about are more like "chips".

To hear the hype about running a longer Rod and making more Torque @ low to mid RPM or mid to high RPM (yes, it is, believe it or not actually pitched both ways) you'd think that there must be a tremendous potential for gain, otherwise, why would anyone even bother? Good question. Let's begin with the basics. The manufacture's (Chevy, Ford, Chrysler etc.) employ automotive engineers and designers to do their best (especially today) in creating engine packages that are both powerful and efficient. They of course, must also consider longevity, for what good would come form designing an engine with say 5% more power at a price of one half the life factor? Obviously none. You usually don't get something for nothing - everything usually has its price. For example: I can design a cam with tremendous high RPM/H.P. potential, but it would be silly of me (not to mention the height of arrogance) to criticize the engineer who designed the stock camshaft. For this engine when I know how poorly this cam would perform at the lower operating RPM range in which this engineer was concerned with as his design objective!

Yet, I read of and hear about people who do this all the time with Rod lengths. They actually speak of the automotive engine designer responsible for running "such a short Rod" as a "stupid SOB." Well, folks I am here to tell you that those who spew such garbage should be ashamed of themselves - and not just because the original designer had different design criteria and objectives. I may shock some of you, but in your wildest dreams you are never going to achieve the level of power increase by changing your connecting rod lengths that you would, say in increasing compression ratio, cam duration or cylinder head flow capacity. To illustrate my point, take a look at the chart below. I have illustrated the crank angles and relative piston positions of today's most popular racing engine, the 3.48" stroke small block 350 V8 Chevy in standard 5.7", 6.00", 6.125" and 6.250" long rod lengths in 5 degree increments. Notice the infinitesimal (look it up in the dictionary) change in piston position for a given crank angle with the 4 different length rods. Not much here folks, but "oh, there must be a big difference in piston velocity, right?" Wrong! Again it's a marginal difference (check the source yourself - its performance calculator).

To hear all this hype about rod lengths I'm sure you were prepared for a nice 30, 40, or 50 HP increase, weren't you? Well its more like a 5-7 HP increase at best, and guess what? It comes at a price. The longer the rod, the closer your wrist pin boss will be to your ring lands. In extreme situations, 6.125" & 6.250" lengths for example, both ring and piston life are affected. The rings get a double whammy affect. First, with the pin boss crowding the rings, the normally designed space between the lands must be reduced to accommodate the higher wrist pin boss. Second, the rings wobble more and lose the seal of their fine edge as the piston rocks. A longer Rod influences the piston to dwell a bit longer at TDC than a shorter rod would and conversely, to dwell somewhat less at BDC. This is another area where people often get the information backwards.

In fact, this may surprise you, but I know of a gentleman who runs a 5.5" Rod in a 350 Small Block Chevy who makes more horsepower (we're talking top end here) than he would with a longer rod. Why? Because with a longer dwell time at BDC the short rod will actually allow you a slightly later intake closing point (about 1 or 2 degrees) in terms of crank angle, with the same piston rise in the cylinder. So in terms of the engines sensitivity to "reversion" with the shorter rod lengths you can run about 2-4 degrees more duration (1-2 degrees on both the opening & closing sides) without suffering this adverse affect! So much for the belief that longer rod's always enhance top end power!

Now to the subject of rod to stroke ratios. People are always looking for the "magic number" here - as if like Pythagoras they could possibly discover a mathematical relationship which would secure them a place in history. Rod to stroke ratios are for the most part the naturally occurring result of other engine design criteria. In other-words, much like with ignition timing (spark advance) they are what they are. In regards to the later, the actual number is not as important as finding the right point for a given engine. Why worry for example that a Chrysler "hemi" needs less spark advance that a Chevrolet "wedge" combustion chamber? The number in and of itself is not important and it is much the same with rod to stroke ratios. Unless you want to completely redesign the engine (including your block deck height etc.) leave your rod lengths alone. Let's not forget after all, most of us are not racing at the Indy 500 but rather are hot rodding stock blocks.

Only professional engine builders who have exhausted every other possible avenue of performance should ever consider a rod length change and even they should exercise care so as not to get caught up in the hype. "

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

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

 

MANY fall for the idea, they can benefit from emulating specific racing combos used for specific purposes, or believe same, by purchasing special purpose products they see advertised with decals, on race vehicles. And....do you think the race sponsors and contingency sources don't know this? Ah, the power of various forms of marketing.

 

I agree with whats been said above. in streetable power levels you wont see much if any difference in power. theres only a few justifiable reasons i've seen for running a long rod. and that is the long rods lead to shorter pistons for a given deck hieght which usually equates to a lighter piston. the only other real reason i've ever found for running them is to pick up a little piston skirt to counterwieght clearance in extremely large stroke situations (non sbc). and the situations i'm talking about didnt see the first mile on the street.

 

I'm glad everyone has stated their oppinions here. I expected someone to ask me why the hell I didn't go with a 6" rod. Now I know the pros and cons. Thanks everyone.

Benji