Compression ratio?

M

multi

New Member
Hello all ,
Planning to change heads on a 3,9 cr 8,13 to 1 with 36 cc combustion chamber, with a set of
4,6 heads with 28 cc combustion chamber,
Tin head gaskets on both ,
Is there somebody who has calculated what the
Compression ratio will be?
Best Regards Peter
 
Using the 8.13 and 36cc chamber we get a total unswept volume of 69.28cc and piston crown etc volume of 33.28cc. If using tin gaskets in both cases no need to consider that. One cylinder = 494cc in a 3.9, so cr = 494+61 /61 = 9.1 . In my experience of 3.5 engines 10.5 CR is a bit peaky (ie poor bottom end - good for an auto, and 9.25 CR has a better bottom but still revs nicely. I have a pair of these heads (Casting HRC2479) and found there are some sharp corners in the transition from the valve bowl to the port than can be easily smoothed.
 
jp928 said:
Using the 8.13 and 36cc chamber we get a total unswept volume of 69.28cc and piston crown etc volume of 33.28cc. If using tin gaskets in both cases no need to consider that. One cylinder = 494cc in a 3.9, so cr = 494+61 /61 = 9.1 . In my experience of 3.5 engines 10.5 CR is a bit peaky (ie poor bottom end - good for an auto, and 9.25 CR has a better bottom but still revs nicely. I have a pair of these heads (Casting HRC2479) and found there are some sharp corners in the transition from the valve bowl to the port than can be easily smoothed.
Click to expand...
John,

I am going to disagree on one count having driven a 10.5 : 1 engine from 1985 till 2007 as an auto. I agree in that it does have a poor bottom end, but not good for an auto, much better with a manual box. The 9.25 : 1 engine was much better with the auto.

When I had the 4.6 fitted in 2007, the research revealed that a low compression 8.35 : 1 engine was far better for an auto whereas high compression engines delivered better performance with a manual box. Dyno testing had shown that the low comp engines delivered more power and torque than the high comp engines up to about 3000 rpm after which the high comp engines had the advantage. For an auto where we would spend most of our time not exceeding 3000 rpm, the low comp engine is perfect.

Ron.
 
Ron,
I bow to your superior experience and knowledge in this. I had been told that the poor bottom end of a 10.5CR engine is easily disguised with an auto box. My experience of 8.13CR is they are almost unstallable, but dont rev very well.
 
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