A 401 built on the production line wasn't really 10.25:1 compression even tho it was advertised that way....They were closer to about 9.75:1.
The biggest culprit was the deck clearance, it was usually over 0.045"! Cylinder heads are generally around 127cc.
Unfortunately, these production variances also meant the engine didn't produce the 'advertised' 325 HP.
Those that drag raced in the NHRA stock classes back then had to modify the engine for max power but still keep it within 'stock' specifications. The mods included shaving the deck surface to the min clearance spec 0f 0.010" and shaving the heads to a min volume of 123.7cc.
You can find the min specs here:
http://www.nhra.com/competition/blueprints.aspxFor a performance rebuild, it's preferred to keep the piston about 0.035" from the cylinder head. This is referred to the 'quench' distance or 'squish'.
What it means is the area between the flat outside edges of the piston and head surface are 0.035" apart at TDC. It doesn't refer to any areas inside the machined combustion chamber.
A small quench spec is desirable because as piston approaches TDC, it pushes the air/fuel charge out of those remote areas away from the spark plug (edge of cylinder) and into the machined combustion chamber. This results in a more complete burn and more power. It also helps prevent detonation as it pushes the air/fuel out of 'dead' areas where it might've been ignited by a hot spot.... you don't want anything other than the spark plug igniting the charge.
On a production 401, with piston 0.045" down and a 0.015" steel head gasket, you're at 0.060" quench distance, far from optimum! Use a thick composition head gasket and you are making your 'new' engine perform worse than the original!
For a performance rebuild, you need to consider several key items to keep clearances near design specs:
- What pistons are you using? Many replacements will increase deck clearance even more than stock!
- Head gasket type- a stock steel 0.015" or a composition that is 0.040" thick?
- What octane fuel will you be using?
- And very important, what camshaft?
Don't be too concerned with the static compression ratio of 10.25:1. You need to determine the 'Dynamic' compression which can be thought of as the 'running' compression, which is hugely dependent on the camshaft.
This Pat Kelly article on Dynamic compression is one of the standards:
http://cochise.uia.net/pkelley2/DynamicCR.htmlThe best way to get the proper quench distance and correct compression ratio for your chosen cam and desired build is to start with the proper pistons. Unfortunately, the generic cast pistons now available are worse than the stock ones.
But there are custom made forged pistons available that will be made to fit your block correctly. They will cost about $400 more than the cast ones, but you will need to decide if you want a performance engine or a stock one. They are made by Racetek/Autotek and are purchased through Tom Telesco.
What is your build plan concerning performance level, cost, and cam choice?
