Compression test results

[Dom]

Hi,

Just did my first compression test. I understand I should be looking for not much more than 10% variance.
For those who know best on here, do mine look okay?

Left block - highest cylinder 166psi & lowest cylinder 157psi
Right block - highest cylinder 157psi & lowest cylinder 143psi

Thanking in advance
Dom

[martin]

Looks good Dom, did you take them dry or wet ? M.

[Dom]

Hi Martin,
They were all dry. I may try them wet too.
I was wondering if there was any problem with them in general and if the left bank being lower than the right is okay. Each consecutive cylinder was within 10% all along but the entire left bank was lower than the right.
Thanks for your reply
Dom

[flying farmer]

Nothing wrong with those if done dry.

Difference in compression between cylinder banks is probably due to an extra skim on the high compression side, again nothing to worry about.

Neil

[Dom]

Ah, its all so clear when I understand! Thanks Neil.

[Westfaling]

Sorry to revive an aging thread but is there any definitive info on compression ratio versus compression pressures for the Stag. I'm aware it differs for different engines with different cam timing/overlap but for standard Stag cams is there any data anywhere?
My target CR for my rebuild was 9.3:1 and I've got 170-175 PSI readings (all dry) - who knows if the gauge is accurate!
I'm 10 PSI low on one bank -as Neil says must be less head skim. I'm tempted to get the head re-skimmed... but by how much?
For comparison my MGB rebuild was targeted at 9.75:1 and I'm getting 185 PSI readings- same gauge.
Nick

[scraggs]

The best method, but too late for you as you've put the heads on, is to use the perspex method. As you probably know, you measure the volume of the head combustion volume and the swept volume of the cylinder. This then will give you an accurate comp ratio. As for converting psi to comp ratio, there are too many variables. Like you said valve overlap etc( even with a standard cam),is it rebored, skimmed, etc all play a big part.

[Westfaling]

I did do some CCing of the heads to come up with my predicted CR. Obviously I stuffed up as one head must have been skimmed more. I'm using the thick (60Thou) head gaskets and was thinking of swapping the low side to a regular gasket but I think that will overdo it!
Nick

[flying farmer]

Personally I would leave well alone if the engine is running well.

I have been looking at my David Vizard tuning book and in there he has a table showing the predicted increase in power from increasing compression ratio.

The expected increase from a compression of 9:1 being raised to 10:1 (probably three or four times the difference you have between cylinder banks) is a mere 1.7%. Bearing in mind you could halve this figure as only one bank is affected you are looking at about 0.85% which probably equates to about 1bhp.

If you have to take it apart again at some point in the future that is the time to take another look at it.

Neil

[Westfaling]

I'm just a bit concerned about uneven crank loading - maybe overly so. I may have to pull the engine anyway as I'm chasing a nasty rattle at part load above 2500rpm. I've exhausted chains & water pump,stethoscoped everything, boroscoped the cylinders, done compression checks, I'm going to try a different distributor in case it's a timing issue as a last resort if that doesn't fix it I may be into a strip down.
Nick

[Dave M]

I'm just a bit concerned about uneven crank loading - maybe overly so. I may have to pull the engine anyway as I'm chasing a nasty rattle at part load above 2500rpm. I've exhausted chains & water pump,stethoscoped everything, boroscoped the cylinders, done compression checks, I'm going to try a different distributor in case it's a timing issue as a last resort if that doesn't fix it I may be into a strip down.
Nick

Ive been thinking along these line, If the heads are not matched what effect if any will this have on the smoothness of the engine?

[Motorsport Micky]

With the minor differences in compressions mentioned absolutely nothing. This engine isn't a 750cc sewing machine that revs it's nuts off where any minor differential differences in the engine build and dimensional mismatches have a marked effect.
This is a lump of an engine where the original manufacturing tolerances back in the 70s would have made you ashamed. Unless you propose to blueprint the engine and do a forensic strip and start by decking the blocks ensuring they run parallel to the crank axis and then measure and correct everything from there working through the engine and heads I can't see
what benefit you can gain. The minor discrepancies will not make a discernible difference to the engine, and other than agreeing that a crank and internal balance including flywheel and clutch should make the driving experience smoother, that is as far as I would go.

Micky