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Old 08-15-2010, 09:07 PM
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SBC 400 -- 5.7 or 6" rods..???

Althought I have built more than 20 "street stock" motors..this is my first shot at a "performance" motor. I have a 4 bolt main 400SBC block, .030 over. I got it with a set of SCAT 6.0" rods. I plan on a SCAT 9000 crank for the build, & have a set of new ProComp 64cc heads & an aluminum intake w/750 carb. Desktop dyno says with the cam selection it should run around 500HP w/480FP.This is a high torque / low RPM build street motor (5200 - 5500 RPM max) My question is this...everyone tells me to build it with the 6" rods..not 5.7"...but noone can tell me "Why..??" What is the reason for the longer rods & associated pistons for them..?? (This will be running right at 10:1 compression)
Any help or education provided will be greatly appreciated...I learned a long time ago to ask all those "stupid" questions....Thanks again for your help.

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Old 08-15-2010, 09:17 PM
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About all the longer rod does is allow the piston to be lighter, as it is shorter. There is a very small advantage to the longer rod in higher rpm enviroments and with restricted head flow #'s as they will very slightly help cylinder filling, but those criteria are beyond the scope of what you are wanting to do RPM-wise. At the level you want to go, there is no real difference between the two, use whichever you want to.
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Old 08-16-2010, 12:49 PM
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X2 and about those ProComp heads what are they. are they the newer 210cc heads or the older 190cc heads. ProComp has had major issues with poor quality in their 190cc heads and they have been flow tested and had about the performance of a set of stock GM vortec heads. i have heard that they are working at getting better quality and i haven't heard anything bad about the 210cc heads. a word of advise would be no matter what runner size they are i would strip them bare, have them checke out and cleaned up and then rebuild them with quality parts as this has been were they have had serious issues.

and issue that just happened with a good friend of mine about 3 months ago was he bought a crate motor with the ProComp 190cc complete heads on it and in less than six months he snaped a rocker stud. and i know this guy he doesn't go around abusing this motor i've never even seen him take it above 3000rpm.

and to be honest getting 500hp out of a 400 by only turing it 5200-5500rpm wont be too easy, possible but not easy, especially with the ProComp heads
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Old 08-16-2010, 01:06 PM
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With a 5500 rpm redline you need to drop your compression to about 9.5:1.
At that rpm level 5.7" stock rods will be better. Over that the long rod is better. There are a lot of threads on rod length- but there's a lot of bad info in them. I suggest you map it all out for yourself and let your reasoning show the way.
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Old 08-16-2010, 03:43 PM
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400's with the short 5.565" Rod do have a problem with side loading or thrust wear of cylinders. A 6" Rod gives you about the same Rod to Stroke ratio as the 350 has which is much better. The 5.7" Rod gives you more Piston choices tho'.
Rod to Stroke Ratio>> http://www.chevymania.com/chevymania/tech/rod.htm
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Old 08-16-2010, 03:57 PM
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One other consideration is how the pin hole and the oil ring groove interface. With the shorter rods (5.7) you might not need to deal with "bridging rings" across the pin bore, or special side plugs at the ends of the wrist pin(s). But of course, it is your choice. OH, with the 6.0s, you might be able to balance internally.
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Old 08-16-2010, 04:15 PM
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I'm running the Internal balance 400/3.75" Scat 9000 Crank w/6.0" Scat ProMod Rods, Wiseco PT019H3 20cc D-Dish Pistons, ProTopline Lightning Aluminum 64cc Heads. I worried about oil control using the spacers with the Pin in the bottom oil Ring land when I was building it but, haven't had any problems so far.
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Old 08-16-2010, 06:36 PM
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Take a piston with a 5.7 rod and put it in an engine, put a dial indicator on the piston n turn the crank with a degree wheel mounted on the crank read the dial indicator as the piston moves from .001 btdc to .001 atdc, the degree wheel and crank will move through about 3 or 4 degrees of rotation.Then do the identical experiment with a 6 inch rod and the appropiate piston,the degree wheel will move 9 to 10 degrees over the same range of piston movement,this is important during the transition from the compression stroke to the power stroke, by increasing the piston dwell you allow the pressure to build higher while the minimun cavity exists in the chamber, this higher pressure level translates into more effort on the piston during the early potion of the power stroke.
Crank arm also swings further before the combustion cavity begins to open, this allows the pressure of combustion to be more effectively transmitted to the crank arm while the pressure is the highest.
If I could put a foot long rod in an engine I would do it!
Of course this cant be done in a sbc or any other gas eng. that i know of.
And there is my 2 cents worth on long rods.It works,just make sure you initiate ign properly and tune cam,intake,and exhaust for this.
RPM doesnt matter.Always worked for me!!
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Old 08-17-2010, 01:11 AM
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So far as I know, the best known early advocate of long rod engines was legendary engine builder Smokey Yunick. Smokey's reasoning tracks exactly what TopWrench explained that a longer rod yields a longer Dwell at the top of the stroke when compared to a shorter rod. Persuaded by Smokey years ago, I have for decades run long rods in a series of SBCs, both carbed and blown.
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Old 08-17-2010, 02:30 AM
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Quote:
Originally Posted by 471A
So far as I know, the best known early advocate of long rod engines was legendary engine builder Smokey Yunick. Smokey's reasoning tracks exactly what TopWrench explained that a longer rod yields a longer Dwell at the top of the stroke when compared to a shorter rod. Persuaded by Smokey years ago, I have for decades run long rods in a series of SBCs, both carbed and blown.
I have seen just as much evidence from both Reher & Morrison and Iskendarian Camshafts that it makes no difference in hp, if it does make a difference it is covering up a deficiencey in either head flow, incorrect camshaft spec, or intake restriction. Isky has a real nice set of charts and graphs showing piston position in the bore vs rod length at every degree of crank rotation and the longer dwell time theory is completely busted. It is such a tiny amount that differences in ring gap and oil ring drag have a bigger effect.

If you really get into searching this subject it is impossible to fine A-B-A testing of long versus short rods, Something else always gets changed at the same time so results are not verifiable.

Smokey was ahead of his time, but some of his theories have been passed by due to better methods of engine testing and computer modeling revealing things in the equation that Smokey didn't even know was there because he had no way to test or quantify it. It has been said today that head flow deficiencies in his day are what lead to his conclusions on long rods, there were no performance heads in his time, he was working with factory stuff like fuelie or 292 turbo heads(the precurser to Iron Bowtie) and we now know these can't be made big enough.

Just something for you guys to chew on, don't beleive everything the HotRod mags try to push down your throat.
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Old 08-17-2010, 07:29 AM
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You cant prove a negative,Ralph Johnson,(Smokey's right hand man) went to work for Crane cams he was the head of r/d at Crane,I believe you can still call him.
So when it is said that Smoke didnt have the current technology and that he was years ahead of his time,those 2 staments,in a big way,contardict each other,this long rod theory is a matter of physics and thermodynamics and it works for me.
And as far as head flow go,I can tell you Smoke had the biggest rootes blower flow bench I've ever seen,the numbers he got in '85/86' were incredible!

Reverse flow cooling was invented and developed by Smoke.
The experiment I mentioned above is absosutely correct and that should speak for itself .
One thing about it is ,throwing a long rod in an engine without tuning the engine for this is at best futile,kinda like puttin a wild cam in a stock engine,wont work.
Think about when piston dwell is increased what you can do with.
1 l.s.a.
2 piston/combustion chamber shape
3 valve size
4 rocker arm ratio
5 Induction and exhaust system
Rod lenght is tied to the stroke lenght of the crank and the ratio beteew the two is most definetly a significant factor.
On a dyno the engine will always produce more torque and horsepower when the rod ratio is increased,I dont know how the experiment you mentioned was conducted, I just know what i personally have seen and studied and long rods definetly work!
One catch, engine HAS to be tuned for long rods so you cant just change the rod lenght on the same engine and expect to see results.
69cst wanted to know why and i just explained it to him,thats all.
So maybe insted of just saying somebody tried to prove it wrong,please tell me in thermodynamic and entropy terms.why not?
And you can,with me anyway go in this rabbit hole as deep as you want.
And it is a deep one.

Last edited by topwrench; 08-17-2010 at 07:41 AM.
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Old 08-17-2010, 08:03 AM
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Quote:
Originally Posted by topwrench
You cant prove a negative,Ralph Johnson,(Smokey's right hand man) went to work for Crane cams he was the head of r/d at Crane,I believe you can still call him.
So when it is said that Smoke didnt have the current technology and that he was years ahead of his time,those 2 staments,in a big way,contardict each other,this long rod theory is a matter of physics and thermodynamics and it works for me.
And as far as head flow go,I can tell you Smoke had the biggest rootes blower flow bench I've ever seen,the numbers he got in '85/86' were incredible!

Reverse flow cooling was invented and developed by Smoke.
The experiment I mentioned above is absosutely correct and that should speak for itself .
One thing about it is ,throwing a long rod in an engine without tuning the engine for this is at best futile,kinda like puttin a wild cam in a stock engine,wont work.
Think about when piston dwell is increased what you can do with.
1 l.s.a.
2 piston/combustion chamber shape
3 valve size
4 rocker arm ratio
5 Induction and exhaust system
Rod lenght is tied to the stroke lenght of the crank and the ratio beteew the two is most definetly a significant factor.
On a dyno the engine will always produce more torque and horsepower when the rod ratio is increased,I dont know how the experiment you mentioned was conducted, I just know what i personally have seen and studied and long rods definetly work!
One catch, engine HAS to be tuned for long rods so you cant just change the rod lenght on the same engine and expect to see results.
69cst wanted to know why and i just explained it to him,thats all.
So maybe insted of just saying somebody tried to prove it wrong,please tell me in thermodynamic and entropy terms.why not?
And you can,with me anyway go in this rabbit hole as deep as you want.
And it is a deep one.

It has a lot less to do with thermodynamics than you think. If anything it would fall under fluid or gas flow dynamics. And piston dwell at TDC is only a small part of it. You're making a decision based on 10 degrees of a firing cycle and there are 710 other degrees that have an impact.

Short and long rods both have their place. For a lower RPM engine (5500 RPM redline in this OP's question) with moderately sized heads short rods will do better- albeit not a very large difference between the two. The biggest advantage in this case is simply oil ring support (no pin through the ring pack), it'll also allow you more room to place the top and second ring.

Long rods would work well in a 383 trying to spin to 6500 RPM's with stock Vortec heads. In this case the 5.7" rods will give slightly better throttle response, slightly better torque, and (MOST IMPORTANTLY) increased durability over both 5.565" and 6" rods.
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Old 08-17-2010, 05:05 PM
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Hey guys relax! Please, the guy asked why,I told him why,Im sure he can do his own research,maybe Ill see ya all at the track one day,thats the place for winners and losers, this is the place for help and advise,until of course the green flag drops,then we can all try to win!!!
Have a good day n take a midol everything's ok here.
There is so many t'dynamics in a running engine,lots of them,especially in a TURBO,now thats a good xample of entropy/t'dynamics at work!!
so everything's o.k. Right? the guy asked ,n I explained to him best way i could! now he knows what happens! Why didnt you explain it to him? He only asked a very smart question!
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Old 08-17-2010, 05:41 PM
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There are so many factors rod length affects that most people don't begin to understand it. It requires research on the OP's behalf. The simplest thing is to jus give him the answer he needs- under 5500 rpm 5.7 rods will be the best option.
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Old 08-17-2010, 07:15 PM
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So do they start working at 5501 rpm?
Rods are always 6 inches long, they never get any shorter.
I have seen both on a dyno many times and I know.
1 what cam to use
2 What intake to use
3 what xhaust to use to get sec.
4 the 6 inch rods i use on my 350s the pins are below the oil rings
6 try 1000 h.p.out of 208 c.i.d. sbc running on alky n take a wag how long
those rods were
Ok so I gave him enough info he can make up his own mind.
But I did give him scientific information not heresay.
If I could use foot long rods in a gas eng I would do it!
So good night, thing about racin is skill of crew chief n eng builder, that's the way it used to be and Im glad n happy you have a diff. opinion.Makes all unique and that's where most, if not all of the engine innovations and inventions have come from.
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