I know this has been brought up before, but never have I heard the pros and cons of both. I am considering the long rods for a 383 that will spin around 6000RPM and hopefully 450 horsepower.

The only positive reason I have heard to keep the 5.7s is that the piston is traveling at a higher velocity around TDC and BDC.
Is this Factor going to outweigh the reduced sidewall loading of the 6.0s? How does the piston velocity at TDC affect power and airflow?

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L98, stock 3.27s, Hooker shorty headers, custom 2.5inch Y-pipe, no cat, 3inch 2chamber flowmaster, JET AFPR, Ported MAF, Best ET: 13.86 @100mph. 1.99 60'
17 inch ROH "ZS" wheels. 17x8.5(front) and 17x9.5 (rear). And just recently T56!

 

Joe,
Back before the site took a nosedive, I did a long-winded expose on this very topic. Here is the short version:

1)Although the reduced side-loading of the piston is nice on a long-rod motor, it definitely suffers pumping losses / velocity at low valve lifts (close to TDC ). This costs horsepower - but just exactly how much I have yet to determine. Two top area bracket racers have sworn to me that they will never build another 6" rodded SBC.

2)Long rod motors resist detonation, are easier on piston skirts, and will generally be easier on rod bolts and wristpins too.

3)Long rod motors force the engine builder to use a piston with an oil ring support over the wrist pin - since the relocation of the rod upward on the piston has moved the wristpin into the oil rings' domain.

For reasons #1 and #3, I'm building a 5.7 rodded 406 this spring.

Check this out................

Big block Chevys have been running a 1.53 rod/stroke ratio for 30 years now (6.135 rod swinging on a 4" stroke). These Ford crushers pump enough velocity at low speed to allow them to breathe well at low RPM.....and get this....
THROUGH 310cc HEADS.

I'll tell you that among the general engine shop population and the hobbiest crowd, well, I'm in the minority. Most people are singing the praises of the long rod motors at the top of their lungs.

And the enthusiast magazines are leading the chorus !

But among the die-hard bracket guys and pro builders - it's a toss up.

For your application, limited to 6,000 RPM, you can't lose with either. I would urge you to think about the heads and cam you're planning on using as an integral part of the long/short rod decision. 190cc and up heads coupled with a 240+ duration cam may match better with a short rod - going for max HP.

Conversely, a 180cc or less head may match well with a 224* cam, dual-plane intake, and 1-5/8 primary tube header in a long rod application such as the 383.

Food for thought. Well, I'll probably get pasted by the long rod crowd, but hey.....

That's what internet bench racing's all about, huh ? Hope this helps and good luck.

BOR


[This message has been edited by Box of Rocks (edited September 27, 2000).]

 

Did you see this thread https://www.thirdgen.org/messageforu...ML/001387.html

I have a 5.7 rod 400 and it works just fine for me.

 

My take on it is this: Between the 5.7 and 6" rods its like BOR says, a toss-up. If you already have the 5.7s laying around from another engine project doing nothing then don't waste your hard earned money just to get another .3" of rod length. Especially not on a sub-6000 RPM engine.

But if you're shelling out for a new set of rods anyway then step up to the 6"ers- they are not much more that a set of 5.7s.

That's how I'd look at it if I was considering the build.


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94 Firebird Formula M6- No options but Z rated tires. No mods over $10. 13.5@105.
79 Malibu "beater" w/junkyard 400 SBC, tweaked QJet 4bbl, finally a decent set of heads, a few other tricks. A maddening 13.000001 @ 108 on the motor with little traction. No nitrous runs with the new heads yet.
Grandma's old 78 Malibu (33K miles!) soon to have the powertrain from the 79 put in it.

"One of the last remaining QJet tuners on Planet Earth!"

 

depends on what you mean by worth it. there are cheaper modifications, but if your looking for that little extra without having to do alot different, it's worth it. it's my personal choice and is with pretty much all serious race engines.

 

Box of Rocks: I agree with you 100%. I am a dealer for JE Pistons, and I had a lengthy conversation with the head of engineering today. FWIW, I am building a turbocharged 406" engine. Here's what I learned: While 6" rods DO help side-thrust problems, the lowered ringland is not favored by JE for a boosted application. They like a lot of meat at the crown of the piston without lowering the rings. 5.7" rods help side-thrust better than stock 5.565" rods anyway. Any horsepower gains over the 5.7" rods is minimal at best. 5.7" rods also allow the use of off-the-shelf turbo pistons from JE, which I get at cost!

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1989 Formula, 383", DFI, Mini-Ram, S-Trim Vortech, blah, blah...
Best ET: 10.796 @ 125.8 mph

TURBO 406 PROJECT STARTED!!!

Other expensive hobby: assault weapons/shooting sports

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Accel EMIC/DFI Tuning
Turbosystems & Custom EFI

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