Long rod 6L for boost?
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From: Sutherlin OR
Long rod 6L for boost? I'm putting together a budget build 6L for my DD STS truck. I generally run 7-10psi of boost on this truck. I had someone suggest that I make a long rod motor which would help it with low end torque (which is why I'm going from the 4.8 to the 6)and more reliable higher rpm capability.
The plan is to use stock 4.8 rods (6.275") with off the self 402/408 forged pistons on the stock 6L crank and block. This combination would put the piston "in the hole" about .039. With the 76cc 317 heads ans a flat top 2cc piston and .040 head gaskets, that would give me just over 9 to 1 if I didn't deck the block. What do you guys think.
The plan is to use stock 4.8 rods (6.275") with off the self 402/408 forged pistons on the stock 6L crank and block. This combination would put the piston "in the hole" about .039. With the 76cc 317 heads ans a flat top 2cc piston and .040 head gaskets, that would give me just over 9 to 1 if I didn't deck the block. What do you guys think.
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From: Norn Iron
That would require a custom made set of pistons, becasue nobody is gona make the piston you want, with the 0.944 pin that factory rods use.
Will it be worth the hassle ? over a normal length 6.125" rod, which isnt exactly short by any means.
Will it be worth the hassle ? over a normal length 6.125" rod, which isnt exactly short by any means.
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From: Sutherlin OR
Originally Posted by stevieturbo
That would require a custom made set of pistons, becasue nobody is gona make the piston you want, with the 0.944 pin that factory rods use.
Will it be worth the hassle ? over a normal length 6.125" rod, which isnt exactly short by any means.
Will it be worth the hassle ? over a normal length 6.125" rod, which isnt exactly short by any means.
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From: Sutherlin OR
This motor is being built for truck duty, not track duty BTW, I'm looking to make the most power under the curve as possible, not necessarily peak power. I want a pulling motor, 2000-5500rpm. I'd rather run more boost with less timing and rpm to make power where I need it.
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Originally Posted by kbracing96
What size pins do off the shelf 408 pistons use?
For years all I have ever read and heard is a shorter rod will increase low end tq. The piston accelerates off of TDC quicker due to the relation of the angle of the rod with the crank. If you aren't building a competition purpose built engine the only real advantages i'd see with going with a longer rod is to reduce cyl wall side load. The sbc 350 (3.48 stroke, 5.7" rod) has a stroke to rod ratio of 1.637 the 6.0 (3.622 stroke, 6.098 rod) has a rod ratio of 1.68. The 6.0, with stock rods, has a better rod to crank angle than the sbc 350 does and it has 40 years of proven use of being a work horse and a great race engine.
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From what I understand, with a longer rod engine, the piston speed is reduced significantly. When you reduce piston speed, it obviously doesn't accellerate as much which reduces the force on the rod and rod bolts. This would allow greater rpm capability. As stated above, side loading is reduced which will help the engine live longer. Having the piston dwell at TDC longer helps increase cylinder pressure too.
My experiences have shown that a 355 with a 6.0" rod will make more power/tq than an otherwise identical 355 with a 5.7".
So how much more would a set of pistons cost with a different pin size? I know the more popular piston manufacturers make pistons for use with stock length rods. So they have the tooling and programs for the 4.8's pin size.
My experiences have shown that a 355 with a 6.0" rod will make more power/tq than an otherwise identical 355 with a 5.7".
So how much more would a set of pistons cost with a different pin size? I know the more popular piston manufacturers make pistons for use with stock length rods. So they have the tooling and programs for the 4.8's pin size.
Originally Posted by TurboGibbs
I have never seen an aftermarket gen3 rod btw.
Then you must be Blind or Deaf...
http://www.thunderracing.com/catalog...vid=3&pcid=120
Ryan K.
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[QUOTE=vanillagorilla]From what I understand, with a longer rod engine, the piston speed is reduced significantly. When you reduce piston speed, it obviously doesn't accellerate as much which reduces the force on the rod and rod bolts. This would allow greater rpm capability. As stated above, side loading is reduced which will help the engine live longer. Having the piston dwell at TDC longer helps increase cylinder pressure too.
QUOTE]
no offense but thats wrong, the movement of the piston is directly related to the stroke of the crank. the rod is only a medium between the stroke of the crank and the piston. regardless of rod size, the piston will always move the same distance over the same time (velocity) at any given rpm. your philosophy would allow the displacement to be changed simply because you changed the length of the rod.
QUOTE]
no offense but thats wrong, the movement of the piston is directly related to the stroke of the crank. the rod is only a medium between the stroke of the crank and the piston. regardless of rod size, the piston will always move the same distance over the same time (velocity) at any given rpm. your philosophy would allow the displacement to be changed simply because you changed the length of the rod.
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From: Norn Iron
Originally Posted by Forced370GTO
no offense but thats wrong, the movement of the piston is directly related to the stroke of the crank. the rod is only a medium between the stroke of the crank and the piston. regardless of rod size, the piston will always move the same distance over the same time (velocity) at any given rpm. your philosophy would allow the displacement to be changed simply because you changed the length of the rod.
Overall distance/time may be the same, but pistons velocitys at ( or acceleration if you want ) at and around TDC and BDC will be massively reduced with a longer rod.
Piston velocity doesnt just measure the average over the entire stroke, as it changes throughout its travel, and those changes are dictated obviously by rpm, but also by rod length.
Originally Posted by Ryan K
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Originally Posted by stevieturbo
not quite true.
Overall distance/time may be the same, but pistons velocitys at ( or acceleration if you want ) at and around TDC and BDC will be massively reduced with a longer rod.
Piston velocity doesnt just measure the average over the entire stroke, as it changes throughout its travel, and those changes are dictated obviously by rpm, but also by rod length.
Overall distance/time may be the same, but pistons velocitys at ( or acceleration if you want ) at and around TDC and BDC will be massively reduced with a longer rod.
Piston velocity doesnt just measure the average over the entire stroke, as it changes throughout its travel, and those changes are dictated obviously by rpm, but also by rod length.
When I said a shorter rod is supposed to impove low end tq I am refering to off-idle primarily.
Last edited by TurboGibbs; Sep 12, 2006 at 09:21 PM.
I'll keep you all posted. 
When I built my 388 big bore, I used a stock crank, off the shelf SBC 6.300" forged rods, and 4.130" bore forged pistons (that were designed to go with a 4" stroke crank/6.125" rod. It worked out to a perfect 0 deck height setup.
Positives are that the acceleration of the piston at TDC/BDC are slower, the higher pin/shorter pistons are lighter, and the overall system is smoother turning. It works REALLY well at high RPM (7000+RPM)
Downsides are because the acceleration at TDC/BDC are slower, low RPM "intake signal" is even worse than it otherwise would be. The piston spends less time at max velocity so there is less pressure differential to pull intake charges in. Now I have to keep in mind that I have 2.08/1.60 valve heads with big ports and a 244/249 112LSA solid cam so the engine would be "slugish" down low no matter what. "HOW MUCH" is the "engine" I'm not sure.
IF I were building this engine again, I would have offset ground the crank to build a 396 and installed a small bearing SBC 6.250" rod (also an off the shelf rod), not because the rod is "too long" but because the smaller bearing would free up a bit more power and work better at higher speeds. The reason I would offset grind vs. just turning the crank down is at that point, CIs are free.
Positives are that the acceleration of the piston at TDC/BDC are slower, the higher pin/shorter pistons are lighter, and the overall system is smoother turning. It works REALLY well at high RPM (7000+RPM)
Downsides are because the acceleration at TDC/BDC are slower, low RPM "intake signal" is even worse than it otherwise would be. The piston spends less time at max velocity so there is less pressure differential to pull intake charges in. Now I have to keep in mind that I have 2.08/1.60 valve heads with big ports and a 244/249 112LSA solid cam so the engine would be "slugish" down low no matter what. "HOW MUCH" is the "engine" I'm not sure.
IF I were building this engine again, I would have offset ground the crank to build a 396 and installed a small bearing SBC 6.250" rod (also an off the shelf rod), not because the rod is "too long" but because the smaller bearing would free up a bit more power and work better at higher speeds. The reason I would offset grind vs. just turning the crank down is at that point, CIs are free.
Check out this thread. I don't know if the engine was ever built but...
https://ls1tech.com/forums/showthrea...t=6.0+long+rod
https://ls1tech.com/forums/showthrea...t=6.0+long+rod