Help choosing 400 pistons and cam.

doing a dirt cheap rebuild on a 400 big block. would like to run 87 octane if possible, but would like to see compression in the mid 8s with stockish smogger heads. IS there a piston out there that will do this? Never going to be a drag motor. Trying to replicate 383 Magnum performance with a little less compression.

This engine is going in a 4x4 truck. thinking MP P4452783 or maybe a Comp XE256H.

Look for stock replacement pistons from Badger or Federal-Mougal.
The trick here is to pay attention to the pistons height. If you can not find a slug slightly taller, have the machinist deck the block or heads a bit. You'll have to crunch numbers with him to finalize the exact comp ratio.

Here's the math:

4.342 bore 3.38 stroke turns into 400.4
Each cylinder = 50.05 = 820.5cc

Divide by r-1 to get volume above the piston for compression ratio r.

820.5 / 7.5 = 109.4cc

Assume 88cc head, 5cc gasket steel shim

16.4cc needed.

(16.4 / 820.5) x 3.38 = 0.068 Piston top that far down in the hole.

9.98 - 6.358 - (3.38 / 2) - 0.068 = 1.864
That's the compression height to give you 8.5:1

Both F-M and Silvolite have pistons with comp height or 1.812 or 1.813 giving you 7.72:1.

The KB 240 piston has a 1.908 comp[ height with a 8cc valve notches, gives their calc 8.9 CR with 88cc head. Campbellenterprises has them on sale for about $450 a set.

By the time you do all the milling and buy your pistons, minimum $225 for NOS on the 'bay, you have about paid for the KBs.


Otherwise you're SOL.

R.

Dog days, helpful as always!

There are some 440 Cast pistons(cheap) with approx. 1.912 Compression height. A 440 with .040 overbore = .018 overbore in a 400. The only thing that might be an issue is the weight. That'd be .022 below deck and as a flat top (no reliefs)would make about 8.99:1 Comp. Just some more food for thought.

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There are some 440 Cast pistons(cheap) with approx. 1.912 Compression height. A 440 with .040 overbore = .018 overbore in a 400. The only thing that might be an issue is the weight. That'd be .022 below deck and as a flat top (no reliefs)would make about 8.99:1 Comp. Just some more food for thought.

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I did this very thing a dozen years ago. I used a .060 over 440 piston. I believe it was a 72-78 replacement piston. I don't know the weight difference but I didn't rebalance (I know I should have)and it hasn't shaken itself apart yet and it's pretty much my daily driver.