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Bruce Sutherland
United Kingdom
1580 Posts |
 Posted - 24/03/2008 : 21:19:38
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Yes, I've completed a search on this subject, without a satifactory answer, so:
What is the optimum compression ratio for a standard naturally aspirated twin 1" SU OMs on a PB engine with a standard camshaft and 'spot on' valve timing and running modern unleaded fuel for 'spirited' road use?
The original 6.8:1 seems too low: Blower mentions 7.25:1 but for tials use only - why?
Gurus to the fore, please.
Bruce. |
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ags
United Kingdom
275 Posts |
Posted - 25/03/2008 : 00:01:47
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Hi Bob,
I am not sure whether I count myself as a guru, but I ran my PB on a static ratio of 7.8 to 8:1 for about ten years on both road and track. The ratio varied according to the state of other modifications within the combustion chambers but was always carefully checked as part of the pre-season preparation by the oil into head on bench method. I was also careful to ensure that the ratio did not vary between cylinders by anything within my limits of measurement. As you will know, if you have read my earlier missives, I was very careful with valve timing at any head off opportunity and carburettor and ignition settings before any important meeting. I also made checks on compression pressures, especially of course when I suspected something was amiss, but these did tend to be rather lower than "book" values. Typical measurements would be 140 to 145 psi (wet or dry) with about 5 - 10 psi between cylinders. My reference said about 175 psi would be expected for an 8:1 ratio with valve overlap as for our cars.
I am sure that this ratio was safe on Seventies' and Eighties' fuels (four star not 100 or 110 octane) at probably a genuine 98 octane. Your guess is probably better than mine about today's 95 or 97 mixtures, but I do not intend to alter this set up when I return to the road. Sodt's law says that I may well be proven wrong about this decision.
My best guess about the low value in Blower, which does reflect Thirties' experience, is that it was the result of running on Thirties' fuels. My reading around this subject suggests that all road fuels then were in the 80 octane bracket, possibly even the low 80s, but certainly much lower than is expected today.
But :- You pays your money and you takes your choice. Having said which I am sure that the Achilles' heel of our engines is the cooling system, and pre-ignition has killed many engines better cooled than ours. So if you aim for high values make sure that your radiator is in perfect order and take NO CHANCES AT ALL of running lean mixtures or over retarded ignition. My water pump used to come into and go out of the setup quite frequently, but eventually was dispensed with on the grounds that I preferred the horsepower or so that it cost, and in any case was only a noticeable improvement in static or very slow traffic on extremely hot days.
More technical ramblings from,
Andrew Smith MMM571 |
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Bruce Sutherland
United Kingdom
1580 Posts |
Posted - 25/03/2008 : 22:44:56
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Hi Andrew,
Many thanks for your lucid advice - guess I'll go for somewhere between 7.5 and 8.0:1 then.
Bruce. |
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