Compression Test = Compression Ratio
Thread Starter
Safety Car
Joined: Nov 2008
Posts: 3,717
Likes: 551
From: Seattle Washington
Compression Test = Compression Ratio
First off I know that this is not the way to do it, and that the only real way to to figure out the true compression ratio is to CC the Heads and Cylinder with the Piston at TDC.
But that being said I am investigating the Engine in my next project. As the Car needs a lot of work I'm trying not to go into the Engine right away. With exception of one Cylinder at 205 psi all the rest were between 170 and 195 not to bad of a spread. The Engine has had 800 miles put on it in the last 28 Years, and I would assume long periods of not running periods so a rusted valve seat or to can be exspected. I know nothing of the internals other than the Car had the body off the Frame and the Engine rebuilt prior to 1985. The 427 Engine has a 512 Block, 074 Heads and is basicly a L-88 on the outside. The Cam is way to mild and or the Lobes are worn as it idles pretty good.
So back to my original question, with a 185 psi average, what compression ratio could I figure it might have? So who out there has a simaler 427, and knows there compresson Ratio and what there Compression Test is ? Of coarse the other way to look at it is that if one Cylinder has 205 psi they all should, and that the Engine is not in very good condition. Thanx...
But that being said I am investigating the Engine in my next project. As the Car needs a lot of work I'm trying not to go into the Engine right away. With exception of one Cylinder at 205 psi all the rest were between 170 and 195 not to bad of a spread. The Engine has had 800 miles put on it in the last 28 Years, and I would assume long periods of not running periods so a rusted valve seat or to can be exspected. I know nothing of the internals other than the Car had the body off the Frame and the Engine rebuilt prior to 1985. The 427 Engine has a 512 Block, 074 Heads and is basicly a L-88 on the outside. The Cam is way to mild and or the Lobes are worn as it idles pretty good.
So back to my original question, with a 185 psi average, what compression ratio could I figure it might have? So who out there has a simaler 427, and knows there compresson Ratio and what there Compression Test is ? Of coarse the other way to look at it is that if one Cylinder has 205 psi they all should, and that the Engine is not in very good condition. Thanx...
Race Director
Joined: May 2005
Posts: 10,674
Likes: 122
From: Millington Illinois
St. Jude Donor '08-'09
There is no way to tell compression ratio by what your compression test shows. A 10 to 1 engine could show 150 PSI or 220 PSI depending on the cam specs. The only way to tell is take the heads off, measure everything and try to get a part number off the piston.
You need head chamber size, bore, stroke, piston dome or dish CC, gasket thickness, gasket bore.
Edit: 512 block can be 427 or 454.
You need head chamber size, bore, stroke, piston dome or dish CC, gasket thickness, gasket bore.
Edit: 512 block can be 427 or 454.
Last edited by 63mako; Mar 13, 2013 at 04:17 PM.
Melting Slicks
Joined: Oct 2002
Posts: 2,317
Likes: 111
From: St Louis MO
There is no way to know what the static compression ratio is unless one knows when the intake valve closes. Otherwise, there is testing equipment used by racing organizations for engine tech when a record is broken (such a NCCC), which when hooked up into the spark plug hole with the pushrods disconnected, can determine the displacement when the engine is cranked; and then, the combustion space volume by sonic resonance when the piston is at TDC. And finally, by mathematical calculations, the compression ratio can be determined.
Thread Starter
Safety Car
Joined: Nov 2008
Posts: 3,717
Likes: 551
From: Seattle Washington
I cut the Oil Filter and there was some Aluminum ( Bearing material) but not bad. I am considering removeing the Pan and inspecting a Bearing or to. If so do any of the Piston Manufactures stamp a number on the bottom side ? As in could I tell whats in it from the bottom?
