[DeTomaso] 351 C Compression Question

[Will Kooiman]

I bought a burette to check my CR.

I dropped a valve in #3.  I dropped the heads off at a shop with new valves.
I asked them to replace the valves, and pressure check the head.  The head
didn't leak.  The damage was mostly cosmetic.  They replaced the valves, but
they also shaved the heads.

I CC'd the heads.  I'm going from memory, but I believe one head was about
60CC average and the other was closer to 62CC.  I dropped the heads off at a
race shop, told them what happened, and asked him to make it right.  He got
the valve stem heights the same, plus he got the CC volume at 62ish +/- .5.
They did a great job.  They gave me a list of the CC volumes, but it's in
the garage (i.e. I'm summarizing).

One interesting note.  They said that they could shave the valve face to
make up small differences in CC volume.  I never thought about that.

Next came the block.  I installed the pistons and measured the deck height
at all 4 corners (put the piston at TDC, put a solid strap across the bore,
feeler gauge across the middle of the piston).  The pistons were
"in-the-hole", 0.005, 0.008, 0.010, 0.010 for #1, #4, #5, #8.  Love that
symmetry.  I dropped the block off at the shop, told them how much to take
off, and had them deck it to zero.  When I got it back, I checked it again.
It was spot-on.

I also used the burette to measure the volume above the piston.

So what does 0.010 mean to the CR?  If I recall correctly, it moved the
compression ration by about 0.2 to 1.

Will.

-----Original Message-----
From: detomaso-bounces at realbig.com [mailto:detomaso-bounces at realbig.com] On
Behalf Of JDeRyke at aol.com
Sent: Thursday, December 11, 2008 2:11 AM
To: tborcich at msn.com; detomaso at realbig.com
Subject: Re: [DeTomaso] 351 C Compression Question

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



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