I know 6.0" rods will stay at TDC longer and help with better combustion but is there any other advantages? I heard that a 6" rod will make more torque and a flatter curve, any truth to this? I figure with a 383 I dont have to worry about torque too much. The reason I ask is beacsue I have 5.7 rods now, I would just need higher comp. pistons. Would it be worth to buy new 6" rods. The motor would go in my daily driver, not too crazy of a cam and probably some nitrous use.

Thanks in Advance
-Mark

 

Doesn't really matter too much.

If you have the 5.7's stay with them. The shorter rods in a 383 will help give you a wider ring pack and usually a beefier piston.

We have had some good ole threads about this.....

I think Mindgame and I talked about this in LT1 tech too, might want to search there or in advanced tech about it.

Bret

 

The shorter rods should help your 383 live longer in the higher rpm range.

 

My favorite is the 5.850 rod but there aren't as many pistons made for it. The longer the rod the lighter the piston usually too but you want a certain amount of piston top per appliction. NA is different than heavy NOS for example.

 

I know the long threads about rod length, Ive read most of them. I just wanted to see opinions on my situation and if the cost outweighed the benefits.

-Mark

 

There is not a nickle's worth of differance in either. None you can measure or feel.

 

Yeah can't remember, I think 5.7 is easier on the sides of the cylinder walls one being that the piston has more support area in the bore, but 6" would prolly make more power/torque, it stays at TDC longer meaning the cam cam will be open more so in relation to where the piston is so say 5% down the bore the valve will start to open instead of 7% down the bore, but it all depends on the cam though, you have to build the cam around that, as with a 6" rod you may end up where the piston at 75% down the bore "round figure of where piston is traveling the fastest" the valve may have either not be open as much or may have already started closing, in regards to a 5.7" rod, so you have to figure out a different cam for a different rod in theory.

 

From what i read they both make the same average power/torque... it was just distibuted differently.

 

So I guess you are disagreeing with:

 

CUT AND PASTE FROM ONE OF MY OLD POSTS, this should be a sticky

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

 

Good Post.....

We have beat this horse to death around here and it's basically over very small amounts of HP. A Cup team or PS team would love to find 5hp from a rod change, but the average joe will never see the difference.

Bret

 

Thanks for all the insight guys Looks like I will just keep my 5.7 rods

-Mark

 

I think this whole thing started with Smokey Yunick. He was trying to get the last few hp out a combo optimized in every other way in the context of some fairly strict rules. Has very little to do with anything other than an all out race motor.

Rich

 

I think you're right. That's where I first learned about rod length and ratio.
(His book has a permanent home in my bathroom, next to the throne.)

At the risk of vastly oversimplifying the subject, the theory is that the longer ("slower") rod will (among other things), allow the pressure generated by the fuel/air explosion a longer period of time to impart it's force to the piston and by extension the rods & crank.

I would say that a completely NA engine should use the longest rods that are practical. (Notice I say practical, not possible.)
If you are using any power adder, then it may be desireable to use a shorter ("faster") rod to add to detonation resistance.

My personal opinion, which I will express with my checkbook, is that a long rod and a very tight quench is the best of both worlds.

Having said that, there were several people on this thread who said it doesn't make all that much difference; and most of them have very fast cars.

Specific advice...? If you already have the 5.7's, and have no burning desire to go long, then use them and spend the extra money on something else.

 

a nice quote i remembered reading from David Reher that i looked up:

"We also wanted to point out some of the common myths and misconceptions about high-performance motors. For example, I've seen dozens of magazine articles on supposedly "magic" connecting rod ratios. If you believe these stories, you would think that the ratio of the connecting rod length to the crankshaft stroke is vitally important to performance. Well, in my view, the most important thing about a connecting rod is whether or not the bolts are torqued!

If I had to make a list of the ten most important specifications in a racing engine, connecting rod length would rank about fiftieth. Back in the days when Buddy Morrison and I built dozens of small-block Modified motors, we earnestly believed that an engine needed a 1.9:1 rod/stroke ratio. Today every Pro Stock team uses blocks with super-short deck heights, and we couldn't care less about the rod ratio. A short deck height improves the alignment between the intake manifold runners and the cylinder head intake ports, and helps to stabilize the valvetrain. These are much more important considerations than the rod-to-stroke ratio. There's no magic - a rod's function is to connect the piston to the crankshaft. Period."