How Much Advance?

[david.g.chapman]

My +2 twinc engine is running fine at the moment - but I was wondering how much extra power I might get if I advanced the timing beyond the setting I have. Some details:

Compression ratio: an ex Federal 9.5 to 1 head with a light skim - maybe 9.8 to 1.
Stromberg carbs with low balance pipe.
New pistons with large valve cutouts, so only about 160psi across the cylinders.
CPL2 cams and slightly richer needles than standard.
Static timing 12 degrees rising to about 25 degrees total at 4000 rpm. I have the correct distributor but I have a feeling that the stromberg engine needs more advance (30 degrees +). Maybe my balance weights need attention.
Even with past carbon build up I have not heard a single pink from the engine.

Has anyone used the approach of advancing until the car pinks in top from a foot down 30 mph acceleration run, and then backing off a bit? What settings resulted?

Dave Chapman.

[rgh0]

While the Weber engine is happy with around 25 to 28 degrees max advance achieved by 4000 rpm I beleive the stromberg engines need the advance to keep climbing up to around 30 to 32 degrees at 6000 rpm. The Stromberg domestic non emission curve in the Elan manual shows this and I believe this is the best curve for a Stromberg engine.

The reason for the extra advance at higher revs is the poorer breathing at high revs of the Siamesed ports in the Stromberg head versus the Weber head.

cheers
Rohan

[alan71]

That?s very interesting Rohan. I don?t have Strombergs but I have often wondered why different carburettors would require a different advance curve. I never thought about the Siamese ports.

Alan

[rgh0]

For a fixed combustion chamber geometry burn speed and thus advance required is proportional to charge density and charge turbulence and fuel type and this sets the advance required at any rpm.

Lower charge density due to poorer volumetric efficiency of the Stromberg head especially at high revs requires more advance all other things being the same.

cheers
Rohan

[david.g.chapman]

Thanks for that.

I will have a look at the advance springs & weights, as I am not keen on putting more static advance on the engine. I have a 40953 distributor picked up at Donnington last year, and I believe one advance weight with a light spring dominates up to a certain rpm, and the second weight with a heavier spring then dominates. It's the second weight I am suspicious of.

Dave Chapman.

[redskatejbf]

David,

I have my original Stromberg carb. distrib. (from new) its marked 23D4-40953A the bob weights are marked 12deg., are yours? they may have been replaced with others with less degrees. With 12 deg. weights I am told give a total of 24 deg. plus your static. Hopefully I do not have my rectumm out the window on that statement, I am sure someone will correct me if I am wrong. If you also have the correct springs with the correct tension, then you should get the correct rev. / timing curve.
Regards John.

[ppnelan]

Yes, that'll be 12? maximum advance at the distributor which is 24? at the crank, plus the 10-12? static advance = should be plenty...

Looking at Miles' book, the soft spring controls advance up to 2000rpm. The stiffer one only controls the final 8? or so of advance = 4? or so in the distributor - you can feel how little movement there is against the stiff spring when you twist the shaft...

Matthew