35DM8

[reelingenious]

Does anyone have the mechanical advance curve for above dizzy, so i can compare with 35D8...........or are they the same?
Al

[DaveEFI]

I'd guess the advance curve is engine, rather than dizzy, dependant.

[reelingenious]

The advance curve is entirely a product of the Dizzy, controlled by bob weights and and their restraining springs. This is otherwise known as centrifugal advance and is related to engine speed. The amount of this advance, and when in the rev range, is what I am trying to find without putting it on a rolling road.

[DaveEFI]

Think you have missed the point The advance curve is matched to the engine. The type of dizzy doesn't matter. In other words, you could have the same curve in both a DM and DML dizzy - or different curves on two different DMs.

I have the BL manuals for the SD1, and there are several different curves used with the same dizzy depending on engine spec. And the country the car was designed for too - for emissions regs.

[reelingenious]

Take your point Dave thanks,
But I have a factory produced curve or the 35D8 used on the 3.5 MGV8. The engine I am using is a 3.9, so what I'm really asking, and perhaps there isn't a definitive answer, is, "is there a significant difference in the ideal advance curve for 3.5 and 3.9". I realise that desired performance characteristics will greatly affect the ideal way the dizzy gets to maximum advance, and indeed what that figure is. Since many of the MG V8's were taken off the LR production line, one assumes!!!!????? they were better set up for climbing up the side of a Welsh mountain or pulling up tree stumps! Maybe I should go for 123 or another dizzy with adjustable advance curves. BUT without spending time on a rolling road, choosing a curve becomes mostly guesswork. But a dizzy, or at least weights and springs, to suit a sporting 3.9 would be a good start at least............Phew!!...sorry!

[sidecar]

How you set the dizzy up depends a lot on what carb you are running, if you are running and Edelbrock 500 for example you will have problems running a decent curve if you want to retain the vacuum advance. I actually think that Rover and Lucas did not really get their sums right anyway because the curve that they run even with SU carbs is really poor. To be more specific the static/idle figure that they specify is really low and this has been done because the vac system is not working correctly. (It comes in too soon so to get round the issue they just ran hardly and static advance).

If you ignore the vac system then you want to be running around 12-14 degrees at idle, 34 degrees all in at a round 2750-3000 RPM. The all in figure is more or less set in stone by the fact that many tuners have run these engines on dynos and that is what the 3.9 lump needs give or take a couple of degrees. The idle figure seems to change more from one engine to another and I think that this is down to the camshaft more than anything. For example the two stage III 4.6 lumps that I have worked on ran best with 20 degrees at idle.

The above figures would be right even if you do run a vac advance system but you would only want it to add more advance at high levels of manifold vacuum. It must not add any advance just as the throttle is being cracked open when say driving out of a bend in the road. That is the problem I found when trying to run with the vac system on my Eddy carb. Whilst at highish revs when coming out of a bend the bob weights have added say 30 odd degrees, the vac system would then add another 14 degrees and the engine would kick back which is really bad for it.

I've moved on from bob weights now, I now run a locked out dizzy with a programmable MSD unit.


You can re-curve a dizzy if you are handy with files and prepared to spend quite some time testing stuff out.

I wrote this ages ago, it might help....

http://how-to-build-a-pilgrim-sumo.wiki ... by-members

The gumph I wrote is on modifying the Lucas dizzy under the other stuff.

[reelingenious]

Thanks sidecar, really interesting stuff.