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+--- Thread: MK2 Ruby: Very high compression pressure (/showthread.php?tid=9999)
MK2 Ruby: Very high compression pressure - Ilmoro - 15-05-2025
My Ruby engine was rebuilt about 2000 miles ago (not by me) and is +60 with HC head. It runs OK but sounds harsh particularly when climbing. It rapidly runs out of steam on hills and will not exceed 45mph on the flat. Timing is correct, auto advance functions correctly and brakes are not binding.
I carried out a compression test fully expecting to find unbalanced pressure or maybe some leaky valves. I was surprised and initially delighted to find all 4 cylinders gave 150psi, but now am not so sure. Does this pressure indicate a (too) high compression ratio. Could this be the cause of the harsh running and lack of performance and if lowering the CR could rectify this, how should I go about it: Are thicker gaskets available?
I would be grateful for any thoughts, experience and advice on this.
Thanks
Ian
RE: MK2 Ruby: Very high compression pressure - AustinWood - 16-05-2025
Are you sure that the auto advance is working?
RE: MK2 Ruby: Very high compression pressure - John Cornforth - 16-05-2025
Hi
The overbore will have increased the CR by maybe 5%, i.e. not much.
Compression gauges are probably only accurate to +/- 5% so might be slighly over reading.
The head may have been skimmed, which will also up the CR. You could measure this externally.
In the old days a severe build up of carbon would have been a reason, but with modern fuel and oil it's less likely
I am not aware of any thicker head gasket options. In desperation you could try using two standard HC gaskets but it will reduce heat flow from the head.
However, I wouldn't immediately think that a raised CR would lead to a serious drop in performance. Is the exhaust free flowing, i.e. not choked up ?
RE: MK2 Ruby: Very high compression pressure - Chris KC - 16-05-2025
As Austin said, I'd definitely double-check the ignition. How did you set the timing?
Is it on the standard Zenith carb?
A Ruby ought to be able to pull 50mph on the flat.
The HC head is a sound choice but people will skim more and more metal off them - the only way to really be sure is to measure it. Personally I wouldn't recommend a CR above 6.5 or so for a road car (as advised by Bill Williams many yonks ago). In my feckless youth I tried various heads with CR up to about 10:1 and they certainly make the car noisier; some went fast and some didn't.