[DeTomaso] 351 C Compression Question

[JDeRyke at aol.com]

In a message dated 12/10/08 4:04:34 PM, tborcich at msn.com writes:

> When I look at the 4V open chamber head in pictures, it appears to be 
> shallower than the 4V Closed/Quenched Chamber Head, but maybe that an optical 
> illusion...
> 
It is. In fact, the combustion chambers are virtually the same on both types 
of head, except for those two side-pieces in the 'closed-chamber' ones. I've 
thougt for years about bolting two filler pieces into open chambers as 
substitutes, but alloy heads already have the small chambers, with better ports raised 
up and repositioned, better cooling and light weight. Thats why they say 
that, if you have to pay for the work it takes to make stock closed-chamber heads 
work as well as alloy heads, you've already spent more than alloy heads cost.

> I have not compared the two heads side by side. The Open chamber is 
> 76-78cc's and the Closed/Quenched chamber is 62cc's...does that 15cc's+/- 
> really account for that much drop in the ratio? 
> 
Sure does. Really careful builders even check the small volume left between 
the piston top and the bore above the 1st ring, the thickness of the head 
gasket etc. Compression is torque and most guys don't want to leave any on the 
table.

> With Open Chamber heads and a flat top piston is there a way to read psi 
> from a compression test to determine what the actual, or approximation 
> on compression ratio is? I frankly don't want to pull a running motor apart and spend 
> time changing heads when I could be focusing on so many other areas. 
> 
Not really. In a given engine, std end-gap rings and a short-duration cam 
will give a different compression pressure than the identical engine with tighter 
rings and a performance cam.   Absolutely everyone does it by Archimedes' 
Principle, using a 100-cc chemical burette and a plastic chamber cap. There are 
approximation-charts around, based on other people's measurements on their 
engines. Generally, if your engine shows 200 psi compression pressiure (measured 
the 'correct' way, you have a strong engine.   The 'correct way to do a 
compression test is:
1. open the throttles all the way and block them open so no airflow 
restriction.
2. remove all spark plugs so no compression drag. 
3. rotate the crank three (3.0) turns as heard by the starter laboring. This 
guarantees a single compression stroke.
4. no extra oil added into the cylinder before the test.

I know its a drag to rework an engine thats ready to go, but pulling the 
heads and getting basic info on your combination will never be easier than it is 
right now while its clean and on the stand. SEveral of the new gaskets will be 
re0useable- I've wiped off head gaskets and re-used them with no ill effects. 
Write everything down in an 'as-built' book for later reference. Ys never 
know....
Good luck- J Deryke



**************
Make your life easier with all your friends, email, and 
favorite sites in one place.  Try it now. (http://www.aol.com/?optin=new-dp&
icid=aolcom40vanity&ncid=emlcntaolcom00000010)