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Too much compression?!

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Bobcat Racer:
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.


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.


--- Quote from: Bobcat Racer on July 01, 2019, 01:43:10 PM ---head shaved .100 with ... a stock Ranger roller cam

--- End quote ---
There's your "problem."

The Ranger roller (RR) cam is as mild as mild gets and has no overlap so the dynamic compression is much closer to the static ratio. 10.5 or 11:1+ on a RR is probably not going to be happy even on 91.

RR cams are popular roller conversions with us turbo 2.3s because they have practically no overlap. It's also popular to retard them significantly because....the y're a truck cam and nose-over in the low 5000s.

 LOL, an original post a year and two days ago. No report back on how the "race in two months" went.

I wonder how "self retarding" the set up is given the large head cut. Something has to compensate for the slack in the timing belt.

Yeah, I knew it was old and replied anyway, mainly out of fascination that a "race" engine would be equipped with a stock roller cam.  ???


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