Ive heard that using either of these rods should result in about the same torque but the hp will be lower on the 5.565 engine. Anyone know how much to expect? Say the 5.7 puts out 400 hp, what would be expected assuming the same exact setup only 5.565 rods and pistons to make the same compression.

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Mark Lock
- 86 IROC-Z - My car, mods and ET's
- -= IROC Online =- - The site dedicated to the IROC-Z

[This message has been edited by lock (edited October 10, 2000).]

 

I don't think you will notice any difference in hp. For one thing there arn't too many pistons that have identical 5.565 and 5.7 versions and for that reason, a dyno test of identical motors will be difficult to find.

 

My short rod stroker (before I finally blew up the engine) ran high 11's at 115-117mph.

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Stephen's racing page

87 IROC-Z Pro ET Bracket Race Car and knocking on the SuperPro ET class
383 stroker (carbed) with double hump cast iron heads and pump gas
LS6 Big Block buildup now in progress

Best results before the engine blew up
Best ET on a time slip: 11.857
Best corrected ET: 11.163
Best MPH on a time slip: 117.87
Best corrected MPH: 126.10
Best 60 foot: 1.662

Racing at 3500 feet elevation but most race days it's over 5000 feet density altitude!

Member of the Calgary Drag Racing Association

 

on a dyno not alot will show up. that little bit of a difference would show mostly in light throttle driving due to the lighter piston weight of the 5.7 rod engine allowing it to rev more freely. other than that not much difference.

 

Ditto to all comments. Back in 1992 Joe Sherman did a dyno flog with a 406 comparing 5.565 and 5.7" rods. Identical pistons, except for a slight weight difference and of course pin height location. All other engine factors remained the same. I don't have the article with me now,,, best I remember the 5.565 rod made a few less horses,, but it peaked 500 rpm less and fell off the curve quicker. My own personal experience with rod ratios is that you can use a smaller cam with a longer rod and get the same torque and power with more vacuum. However, there is little difference and the 5.565 combo is much easier to assemble.

 

You guys are missing the main points here. The pros and cons of different length connecting rods have little to nothing to do with weight. Any weight shaved by shorter (lighter) rods is made up for the extra piston material to position the rod further down. And the opposite for long rods. Sure the rod weighs slightly more but the piston is also slightly lighter because the wrist pin is so far up. So it has nothing to do with weight.

The reason you use different length pistons is to achieve greater dwell time at TDC. The longer the rod, the longer time the piston will dwell and the more the piston will stay at the best part of the power stroke, being the highest compression stage.

The other obvious reason is that when you build a stroker motor with 5.7 rods, your R/S ratio goes to crap and the 6.0" rods will somewhat correct for that but still won't bring you back to as good as the stock 350 as far as durability and fast-revving potential.

 

I asked this same question to Speedomotive and got this response.

"Dyno work I've seen showed the short rod at 401 hp with dart heads at 9.5-1. The 5.7 gets 423 at 10.0-1. "

Since they didnt get the same compression, I wonder how much of a difference that makes in these numbers.

 

Believe me some of us totally understand the long rod theory. While folks that do not understand the principle probably would not mind an in depth discussion about it,,, I do believe the question was that would be the horsepower difference. No disrespect meant,, but I don’t think half the folks out there (every where) cares a rats about how it’s achieved – extended dwell vs piston speed, cam phasing to take advantage of the different cylinder filling capabilities, being able to run a higher compression ratio with the longer rods, ring package location, yada, yada, yada. Most folks are looking for what will it do in terms of horses/torque. Simply,, shorter rod,, more torque at a lower rpm,,, longer rod,, more horses at a higher rpm,, everything else constant, that’s from my person experience in building a number of both combinations,, and seeing a number of dyno pulls. True perfectly matched components (not the same) I’d lean towards the longer rod combo, and did on the race car since 4340 rods and JE pistons are basically the same money regardless of length/pin height. However, for a “budget” cast crank, stock rod 5.565 vs 5.7 (ARP bolts a must), TRW forged or cast pistons comparison,,, I’d take the money difference in the long rod combo and put it in a better set of heads (or port work) on the short rod set up,, and the short rod will pack a long rod lunch,, every time.

As far as compression ratio power difference, a 9.5 – 10:1 jump would theoretically give you 6 to 7 horses. I assume they gave you the peak horses rating,, did they give you the rpm at which they were rated? How about torque? There is a lot more to it than just the power at peaks. I like to take an average of the horses and torque when comparing dyno pulls. Just curious,, what did they quote you,, as far as the money difference? I wonder if they used the same sized ring thickness and placement? I would guess that they're probably different.

 

I dont have the power range. They do show a graph of the "mighty mouse" package on their site though. The 5.565 kit will be $150 cheaper from what I gather on their site. I cant imagine my setup getting over 6000 rpm so Ill probably end up taking the 5.565 rod route to save the bucks.

 

You wnt to keep the stroke to rod ratio 1.5 and higher. There will be less piston to side well friction. Divide the rod length by the stroke.

 

You wnt to keep the stroke to rod ratio 1.5 and higher. There will be less piston to side well friction. Divide the rod length by the stroke.

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'91 Formula
383CI Supercharged
5-speed
3.42
12.34@117.04
on Drag Radials

 

BadSS very neatly summed it up. The differences from rod length are so tiny on a street motor that IMHO it is a waste of money to go there.

On the other hand, if you're in a cubic-inch-limited class, or a "cam-lift rule" class, that sort of thing makes sense. It's like how you have to think if you're running with the Super Stock guys: if you're in that class and for example you run a HV oil pump, you will lose, period. But if you're in a "claimer" class, you'd have to be an idiot to spend something on the order of $40-50 per HP.

BTW my money is where my mouth is: I'm running a 400 with a stock (5.56") bottom end on the street, and seem to be doing fairly well with it. I doubt that the extra $$$$ for something else would have made enough of an improvement to be considered money well spent. Like Bad says, take that money and put it in the heads, where you'll get some real-world, measurable, immediate, butt-dyno results.

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"So many Mustangs, so little time..."

 

well lock, just picked up my short block today. machine work finally done. my personal choice was 6 inch rods and it looks as if it will be all worth it soon. tomorrow, assembly.

 

Good stuff Zippy! I just purchased a T56 so my rebuild will be on a budget. I wasnt expecting to rebuild this quickly but my 350 is smoking like crazy now .

 

So there's no difference in length and there's a trade off in weight?

What should one consider when purchasing connecting rods?

Can you get a light piston and light rod if you are not gonna blow or spray?

What (relatively speaking) is light, medium, and heavy?

Thanks,
Jason