Does anyone know the compression ratio on a 1976 400 out of a car. I know the compression ratio changed a lot throughout the years.

 

Mid 8s at best. Probably closer to 8.
Looks like it bottomed out in '78 at 7.33.

In addition to a low CR, it has a sunken piston issue which contributes to detonation etc.

https://web.archive.org/web/20120702...history03.html

 

Interesting history there. Tom Monroe's book on page 37 and again on page 38 says 1972 through 1980 400s have 8.4:1 compression ratio ... the 1971 is shown at 9.0:1 though. The '71s had the flat top pistons, no dish I think. He shows 351Ms and the 400s all with 78.4 cc head volumes.

351Ms have just 8.0:1 cr. and are very easy on starters.

If one does the math, the volume of the head gasket hole around the cylinder and the volume from piston top surface to block deck at TDC need to be included, I don't have those numbers. If not included, exagerated high cr numbers will show up, but we know it's lower as there is a dish in the piston and valve reliefs ... and pistons stop before reaching deck edges, and all that adds to the volume above a piston at TDC.

If a combustion chamber and cylinder at bottum dead center has a total volume of say 56 cubic inches and the piston sweeps 50 cubic inches so that only 6 cubic inches volume remain at top dead center, then that cylinder's compression ratio is 9.3333333:1 all day long as that is the result of 56/6 ... that is a 50 cubic inch single cylinder's swept volume, and 8 of them is 400 cubic inches. 56 cubic inches are being squeazed to just 6 cubic inches. Since the piston stops before the rings pass the head gasket, the volumes include that volume existing within the head gasket. If valves leak or the head gasket leaks or rings allow blow by, compression pressure will drop, but the compression ratio remains. You can manipulate compression pressure with valve events, but the ratio is unchanged.

 

Actual measured compression ratio is probably 7.low. The problem is my 400 will knock at the slightest provocation and I'm at altitude with Sealed Power stock replacement pistons. I'm saving my $ for a set of Tim Meyers' step head [pistons that add squish to the woeful Ford open chamber.

R.

 

I would go through the distributor and set it up so it doesn't ping. First try running it without the vacuum hooked up to it. If that works then you know the vacuum advance is going to need cut back. If it still does it then the mechanical curve needs adjusted by either stronger springs or limiting the total timing.