Balancing Vs. Rod Length SB's - Team Camaro Tech
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post #1 of 8 (permalink) Old Nov 7th, 05, 08:33 AM Thread Starter
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Balancing Vs. Rod Length SB's

Conn Rods 5.700 vs. 6.000
There's a tremendous amount of really misleading information posted up here from time to time pertaining to balancing units and rod length selection. The rod length has very little effect on the overall operation and may be even helpful to the shop doing the balancing. A heavier "bobweight" means less drilling (less labor) for the shop. Normally you would think because the 6.000" rod is heavier that the job will be easier due in part to a lighter (shorter) piston. It doesn't work out this way. If you choose the right crankshaft, I'm talking 4340 and sold as an "internal" unit most will now make the bobweight with no "fill", regardless of the rod length. Eagle's 4340's are down to an
"1850" target. READ THIS CAREFULLY: I have 2 400 SB's just off the balancer (3 weeks ago). Both units are 406" (4.155"), ESP SIR(LW) rods, and Ross
"flat-tops". The first has the 5.700" (535 Grams) rod and the piston #90008 (455 grams), the other has the 6.000" rods and #90007 (406 Grams) pistons. The bobweight for the 5.700's is 1604 (using 92 gram Ferrea pins) and for the 6.000's is 1649 (also with the 92 gram pins). You can "see" that the 5.700" rods would be an advantage in this case, both to help in balancing and also for "lighter" internal parts. All the cranks we use are "profiled" for the 5.700" rods, regardless of the rod we use. There are a couple of advantages to the "longer" rod, however. First and most important, it takes some loading off the bores, which in a stock G.M. block is a definite plus, and second, the "dwell" time produces a much "cleaner" burn when it "fires". One other issue while I'm here pertaining to the choice of blocks used to build around. If you take all the components (the ones you're going to use) and choose any "aftermarket" unit, whether it's G.M., Dart, or World, you'll see up to an add'l 20 HP over a stock G.M. 400 casting. This we've tested most recently. Thanks, Gary in N.Y.
P.S. The pistons are sold from Ross with 120 gram pins, but we don't use them, if you choose to keep them you'd add 28 grams to both bobweights. Rick D., if you catch this post, pay close attention to it, this is all fact.
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post #2 of 8 (permalink) Old Nov 7th, 05, 08:56 AM
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Re: Balancing Vs. Rod Length SB's

Good stuff,..but how would a block add 20 hp?
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post #3 of 8 (permalink) Old Nov 7th, 05, 01:25 PM
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Re: Balancing Vs. Rod Length SB's

Thanks, Gary!

Rick Dorion
69 RS Conv therapy program, Autogear M22, 8-pt cage, with a new 410! SOLD
New therapy program - 68 Coupe. Will be survivor exterior, modern underpinnings! SOLD
67 Belair with perfect floors, pinchwelds and firewall. Hmmm!
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post #4 of 8 (permalink) Old Nov 7th, 05, 04:14 PM
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Re: Balancing Vs. Rod Length SB's

Thanks for the info GOSFAST. Does an aftermarket block have any HP improvements over a good 4 bolt .030 350 block?

67 Camaro [email protected], [email protected], 1.455 60ft 421sbc
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post #5 of 8 (permalink) Old Nov 7th, 05, 05:44 PM
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Smile Re: Balancing Vs. Rod Length SB's

The cylinders and top-end are flat more rigid! therefore the cylinders stay rounder and thus the rings seal better!

Right Gary?


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post #6 of 8 (permalink) Old Nov 7th, 05, 06:50 PM
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Re: Balancing Vs. Rod Length SB's

By going to and aftermarket block you reduce cylinder flexing witch 400's in my opion do when you go over .030 bore.

If you look at blocks used in Nascar And Pro Stock they are made from a compacked grafite material.The cylinder walls in this type block are very strong.This is were you pick up the extra 20 Hp as gary said by reducing cylinder wall flex.
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post #7 of 8 (permalink) Old Nov 8th, 05, 05:42 AM Thread Starter
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Re: Balancing Vs. Rod Length SB's

With all the testing with various units on the dyno we are able to sort through more details than usual. We see alloy blocks "consume" up to 40 HP and see just the opposite, as I said above, with aftermarket "iron" units (up to +20 HP). We feel it has to do with the ability of the block to "retain" more heat from the charge, due to thicker walls, which in turn produces higher the higher HP gains. The theory goes back to what we know about "iron" heads, that they produce more power, than alloys, with lower C.R.'s. I guess you could say due to the "ridgidity" of the block, but only because it's cast up much heavier, and therefore has the ability to retain more heat during the "firing" cycle. One test fresh in my mind, we used a 400 G.M. casting (421" with no concrete) and 23 degree alloy heads and produced slightly under 720 HP, when the parts were transferred to the "Dart" casting, we saw 737 HP, with no other changes. Thanks, Gary in N.Y.
P.S. Don't forget what this post was about originally, rod length, whether you choose a 5.700" or 6.000", has almost no effect on determining internally or externally balancing a unit. "Sharp" engine builders can figure the parts ahead of time to see a potential problem. If anyone doubts what's here, as for balancing, you can get the weights from the vendors and see for yourself. Remember "lite-weight" parts rev quicker and last longer (usually).
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post #8 of 8 (permalink) Old Nov 8th, 05, 06:30 AM
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Thumbs up Re: Balancing Vs. Rod Length SB's

I don't know the "lubricity" differences between the so-called, "compacted graphite",( I assume), Nodular-iron blocks vs good old say 30,000 or 40,000 psi Gray-iron, but I have a feeling that the Gray-iron will be "slicker" than the Nodular will just b/c of the iron carbide and left-over carbon flakes being more prevailant in the material vs the carbon "balls" or nodulars in the Nodular-iron.

But I suppose the strength of the Nodular over the Gray-iron more than makes up for this??

One other point is I do figure Gray-iron will have a more porous cylinder wall surface finish that will alllow trapping ever so slightly more oil for the rings vs the Nodular-iron, but again the Nodular-iron's cylinder wall surface should be harder so the rings will still burnish and slide fine.

Anyway, like Gary said, BTTT, good thoughts and comments on picking STRONG, lightweight rotationg assembly parts to help take so reciprical load of the motor at high speed! Definately is a "free" power producer as long as the parts are strong enough like Gary says, imho.

Good info here!!!!

Please carry-on...


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