What is your Compression ??

Ok what compresion are you guys running before boost? Most people i know say about 8.5 to 1 and im trying to come up with a off the shelf piston for a 400 bb. I planned on building another 451 for this but i cant get the compression lowered enough any ideas??? The heads are 75cc

i had 8.8 with open chamber 346 heads,polished chambers[95cc].042 gasket,.060 below deck 446.

my motor is not assembeled yet but its calculated to be 8.1:1 with some cut down TRW forged pistons in a edelbrock headed 340 that will have a lysholm twinscrew pushing about 15psi of boost on pumpfuel. just hope it will work good i dont have a much excperiens with blown motors

Matt,

I am a little on the high side. about 9.4:1 with a pretty thick MLS gasket. With original B-1's and a low deck, its pretty hard to come down much further than that...

i"m running 8.98:1 with premium gas, and 10-12 lbs of intercoled boost

12 : 1 on methanol

well that just shouldnt even count lol. i will be 8.1-8.3:1 when mine gets built

8 to 1,5lbs on pump gas,,,,,,,,for now

7.55:1 with 8lbs boost. But I have cast pistons so I am trying to play it safe.

I have been told 8.5:1 for pump gas and 9:1-9.5:1 for race gas

8.6:1 15psi with pump gas and w/m.

8:1 pump gas 10psi right now

8.6:1, have been up to 20 psi with 93 octane.

7.8 @ 30 psi with 110

I'm learning about boost everyday and one thing i found out is not all boost is the same.

A centri blower does not need the compression cut like a roots/screw type charger due to the efficiency and RPM the boost comes in at.

I am going to be building a forged bottom end for my 32V Cobra and the recomendation is to stick with thr stock 9.85 compression or even go to 10:1. That is with an intercooled D1SC. Compression for use with a Kenne Bell PD blower would be much different.

Plenty of people are running 8-10# boost on stock 9.85 bottom ends. Thats hyper pistons and powdered metal rods.

15:1 with just under 80psi boost


ya, i know i dont really qualify for this question.

Just measured up everything up tonight to double check....

70cc combustion chambers on Indy heads
27cc dishes in ross pistons
.015 down hole
.045 thick copper gaskets (just a tad for sealant thickness) with 4.06 bore
4.03 cylinder bore
4" stroke

comes out to 8.6 to 1....
going to be on E-85 and pushing lots of boost.

You can get away with more comp in a centrifigual, do to lower heat and boost curve as said.
But I have found that depending on the heads and combustion chambers they need a tick more comp. For example some of the engine I have out there with indy heads need 9.5 to 10 even on pump ( depending on boost) otherwise they just take a ton of timing.