Thread: Changing compression ona built motor

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Originally Posted by HotRodN

Tech,
If I were to decide to go with 5.7" Rods,,, what changes,,, compression length?? I assume I'd go from 1.130" to 1.430" given the .300" difference. I also assume that torque would be slightly lower than with 6" rods.

Yes, piston compression height for the 5.7" rod will be from 1.425" to about 1.433", depending on manufacturer. Choose the piston you want to use and add the compression height of the piston to the rod length and stroke radius, just like above. That will tell you how much to cut the block decks.
My choice might be the Keith Black KB253+30. Same piston crown design as I selected for the 6" rod except the compression height is for a 5.7" rod.
https://www.uempistons.com/index.php...26e4bc2877d034
I did not choose the 6" rods for any torque advantage because I doubt there is any, I chose them for compatibility with the internal balance crankshaft. If you go with 5.7" rods, you cannot use an internal balance crank, you must use the external balance crank and balance it with a SBC 400 damper and flexplate/flywheel. The 400 damper and flexplate have an eccentric weight built into them. Again, budget a little extra cash for finish-balancing at the machine shop. It might be spot-on from the maker, but maybe not. 1.433" + 5.7" + 1.875" = 9.008", so you could cut the block decks to 9.008" for a zero deck and 0.041" squish.

Originally Posted by HotRodN

I noticed you have Hypereutectic pistons chosen,,, would forged pistons be a slightly better choice or,,, do the hyper's perform well enough to not worry about the more expensive forged. Keep in mind this is not going to be a drag car but I will step on it occasionally for the adrenaline rush LOL.

If it were my money going into this motor, I'd choose hypers. Now, if I were going to use a blower or spray, then I'd step up to forged. If you keep the motor out of detonation, even cast pistons will work well in most cases, but I like to use hypers for a little insurance.

Originally Posted by HotRodN

The thought was if it was right from the factory it should be right now.

You've already figured out that the block might not be dead nuts from the factory or maybe someone took a cut on the decks who didn't know what they were doing, so I think you're a little smarter than the guys at the machine shop.
NEVER ASSUME ANYTHING. CHECK EVERYTHING.
Doesn't your machine shop have a precision square? Use it with feeler gauge blades to check for 90 degree square between the banks, front, middle and rear.

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