Joined: Jan 2022 Posts: 80 Threads: 21
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Location: cambridge
Car type: A7 special 34
Our recently rebuilt mildly hot trials engine has both performed and bedded in well. As a quick health check yesterday I measured compression and we are getting about 105 psi on all 4 bores. We are running a 37 high compression head with the block blended over a bit into the bores. I have a couple of other heads that I want to try on the car but was wondering where to settle in terms of compression ratio/pressure. I need to measure the combustion chamber volume to see if I can skim a bit off to slightly raise the compression but what are most people getting on hotter engines and where to stop? Obviously for trials we are looking for low end torque not high revs power.
Joined: Aug 2017 Posts: 325 Threads: 13
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There is an old saying that one should "leave well alone". If you are getting 105psi on all four that is good.
I would be more concerned if there was an appreciable difference between them. The more you raise the compression, the more stress you put on the engine and often that leads to unreliability.
Generally the best high torque engines have fairly low compression.
Just my four Penneth.
Joined: Aug 2017 Posts: 3,473 Threads: 108
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Location: Darkest Bedfordshire
I agree with Ray, for me that's a fine result and I'd leave it alone.
Joined: Aug 2017 Posts: 950 Threads: 18
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Location: North Yorkshire
I agree. Your compression ratio is somewhere near 7:1 which is about as high as I would go on a road engine. If the 37 head is working okay and not oiling plugs I would relax and enjoy it!
Joined: Aug 2017 Posts: 1,387 Threads: 35
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Location: Cheshire
Car type: Race Ulster, 1926 Special, 1927 Chummy, 1930 Box
I never really found any head that was better than the ‘37 one. Side valve compression ratio is a difficult subject closely related to volumetric efficiency. You can have one or the other but it’s difficult to get both.
Alan Fairless
