Sorry I had to drag this into a new thread.

I picked up this 400 SBC motor. It's suppose to be .020 over 400. The pistons are flat top 4 valve reliefs. I'm sure the deck is ) and head gasket will be the standard stuff.

I have been told a few times that with 64 CC heads the compression will be 11.5 and with 72CC head it will be 10.5.

How did my 383 have 2 relief flat tops and 65CC heads and come out to 10.8:1 compression? did that 1 cc make that much difference?

exuse me if i'm not making much sense either. I'm under the weather.

 

Depends on the amount of relief in the piston. flat top 2 valve reliefs are generally 4-7cc with most I've seen around 5. My 383 pistons were 5cc reliefs. That gave about 11 to 1.

4 valve reliefs could be 7-12cc. If the deck is 0 and the gasket is .040" or so, compression could be anywhere from 11.35 to 10.75 to 1. If the reliefs are smaller than 7cc, then your back up in the 11.62 to 1 range.

The bore makes the difference in the compression between a 383 and a 400 and its a big difference.

 

Oh. Thats what I was missing.

Anyway to measure the CC's? I could probably look at the Summit and find these in there catalog.

 

use a buret and drop water into the reliefs

Or possibly I think you can fill with clay and carefully take out that clay piece, and drop it in a graduated cylinder filled with water. Measure displacement in CC's. 1 mililiter of water = 1 cubic centimeter

 

You would need a deck sealing plate and a measureing burea(spelling?) to check the CC's. - the 4.1+ bore size of a 400 based engine compared to the 4.0ish of a 350 based engine makes for a substancial difference in compression.

The typical .041ish thick felpro gaskets drop off a notable amount of compression though, as will a larger cam. - Static compression is not nearly as important as dynamic compression. Last I knew Keith Black's website had some great calculators for figuring static and dynamic compression; just input your engine's info and it equates everything for you.

 

I got tree'd on that first part.

 

Compression Distance (in) 1.560 in.
Piston Head Volume (cc) +6.00cc


Copied that from the Summit site. Pretty sure its the numbers for these pistons. Does that help narrow down the compression for this motor?
.020 400 with 64CC heads. .041 gasket,

I have mixed answers between 11.1 and 11.6

 

I get 11.45:1 with .041" gasket assuming 0 deck and 4.200" bore on the gasket.

 

How thick of a gasket can you go? I would love to see closer to 11:1. I feel funny about that much compression in a 93 octane car.

 

That I'm not sure. You'd need .056-0.057" gasket to reach 11 to 1 on that setup and I wouldnt go that high. I'd run no more than .050, but to be honest, i think it will be fine on pump gas 93 at 11.45 to 1. Good cooling, not to aggressive timing, and colder plug should fight off any detonation. Problem will be getting the distributor timing curve right. EFI you can do whatever timing you want anywhere. That comes in handy in situations like this.

 

Yeah. Maybe i'll find a middle ground gasket. like a .045-.048.

 

don't run too much gasket, you need proper quench in the motor. I run 11.2:1 on pump gas with no issues, in the past we've run 12:1 motors on the same swill with colder plugs and 34 total timing on the street, big cam and loose stall.

 

Good advice in here.

I'd just run the typical fel-pro performance .041 gasket. A little softer timing and colder spark plug will run just fine on pump swil, and leave you room to utilize some occassional good fuel when dinking around at the track.

In my 355, I ran the heaviest springs and least advance cams in the dizzy, meaning it had most of it's 34* all the time. That was with a -7 plug.