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Offline Bobcat Racer

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Too much compression?!
« on: July 01, 2019, 01:43:10 PM »
Hi Everyone,
A fellow Pinto racer was kind enough to lend me a set of 40mm DCOE Webers and both my current race motor and spare motor have D-port heads that wouldn't work with the intake.  Time to mix and match with the parts I had available.  I took a 78K mile, 8 plug head motor, pulled the head and replaced it with an oval port head shaved .100 with a fresh valve job and a stock Ranger roller cam and followers.  Put it all together on my test stand and it ran very well so I decided to test compression as I keep a sheet on all of the motors I test before going into the car.  It ranged from 205-210 psi per cylinder which shocked me a bit.  My other race motor never showed more than 185-190 psi and the head is shaved .118.  It should be noted that the engine ran quite well on 87 octane pump gas but there was also no load ever placed on the engine.

I'm going to try another compression gauge but did I make an error in having the head milled?  I know the combustion chambers in the 8 plug heads are opened up a bit more than the oval port or 4 plug D-port heads but not enough that it would make the compression that high.  Any help would be greatly appreciated because the race is two months away.

Cheers,
Greg

Offline Wittsend

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Re: Too much compression?!
« Reply #1 on: July 01, 2019, 05:00:32 PM »
There are two measurements of compression:

Static compression is a calculated number regarding cylinder sweep, piston surface volume, how deep the piston is in the cylinder, gasket thickness and combustion chamber size. Flat top pistons make this easier but some even calculate right down to the small gap between the first ring and the cylinder wall.

Dynamic compression factors valve timing as well.

One of the reasons a "Race Engine" gets higher compression is to compensate for longer cam duration and/or overlap.  There are calculators on line that will tell you both types but you do need to know the cam timing, bore and stroke numbers and combustion chamber size.  This will be helpful as there are general guidelines and if your combination is way outside the numbers there is cause for concern.  Otherwise if it works - use it. You may have to increase octane of lower the ignition timing.

Building an engine is a "system" intended to have appropriately matched parts for the purpose.  Sometimes you get fortunate straying (slightly) outside the norm and other times you get undesirable results. If your other engine had a different (performance) cam it may account for the lower compression readings. But not knowing the combustion chamber size differences before and after any milling of either head is an unknown.