Thread: 400 compresion

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what would compression be with hypertec flat top pistons be on 400 sbc with standard heads an rods na should i get them dished at machine shop want to run pump gas

Anywhere between 9.5 and 10 to one, you will have to give camshaft specs to get a more accurate awnser. And to awnser your question weither it runs on pump gas or not. What octane pump gas as a matter of fact ?

By you want to run pump gas do you mean 87 octane or anything available at the pump? I'm running 9.26:1 in a truck and can still burn 87 octane without any knocking/detonation under any conditions. I'm running Vortec heads, though. The old generation heads are going to be more prone to detonation on pump gas.

Originally Posted by maineevent

what would compression be with hypertec flat top pistons be on 400 sbc with standard heads an rods na should i get them dished at machine shop want to run pump gas

Has the block been decked? Heads flat milled? Heads angle milled? What head gasket? What piston part number? Did you cc the heads?
P.S.The rod length has nothing to do with comp. ratio.

The rod length does have something to do with the compression ratio, but you need to know the compression height of the piston as well. You need to know the stroke, rod length, compression height, and deck height to calculate the deck clearance to get a proper compression ratio.

Originally Posted by 76GMC1500

The rod length does have something to do with the compression ratio, but you need to know the compression height of the piston as well. You need to know the stroke, rod length, compression height, and deck height to calculate the deck clearance to get a proper compression ratio.

What I believe he was refering to was longer rods = a higher comp. ratio.
His post was rather hard to understand.

Originally Posted by maineevent

what would compression be with hypertec flat top pistons be on 400 sbc with standard heads an rods na should i get them dished at machine shop want to run pump gas

To answer your question, a 400 bored 0.030" with flat-top pistons with valve reliefs equalling 7 cc's, stock 76 cc heads, 0.040" thick gasket and the piston down in the bore 0.040" at top dead center (standard rebuild procedure) will yield 9.24:1 static compression ratio. Use a very short cam of no more than 220 degrees duration at 0.050" tappet lift.

thanks for the last reply would that work to get my 400 hp or should i go with vortec heads the machine shop has them for cheap would the vortec combo work with 87- 93 octane

The Vortec heads will support more power than the stock heads. I can't really say how much. If you go with the Vortecs, you'll have to use dished pistons to get the compression ratio down low enough to use pump gas because they have a 64 cc chamber compared to the 76 cc chambers you have now. Maybe you could start another thread titled "Desktop dyno" and ask someone to plug the numbers in for you. By the way, don't be too concerned with horsepower. Torque is what moves the car.

I had to run a -12 cc dish on a 350 with the Vortec heads to get 9.26:1 compression. You'll probably have to run significantly more dish. Using the Vortecs and a reasonable cam, you'll make an honest 350-375 horsepower with 400-450 ftlbs of torque at the crank on 87 octane.

I have a question about compression myself. I took a compession test on my 406 the other day (i was bored) and i was wondering what 180 psi means in terms of compression ratio

well i have the flat top pistons and intake buy the 5,7 rods the ones i have reconditioned so the stock heads wont get me to 400 hp nore support it with out getting real expensive reason i ask is their only 50 bucks.with a 3 angle valve job an bigger valves 2.02 1.60 would they get it done with the pistons i got and this performer eps intake

also their are alot of 350 an 305 heads at my uncles shop just a thought