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MG MGB Technical - Mechanical advance for stage 2 engine

What would be the best mechanical advance for a 25d dizzy to run a stage 2 18v 1860cc engine with a fast road cam?

Would it be 10deg or 15deg. I've read internet and written opinions favouring both in differing situations. Can someone explain in simple detail, has to be simple for me.

Thanks in advance.

Iwan
Iwan Jones

The answer is you need to get it on a rolling road and set it properly, no one can tell you what it should be, they can only tell you what it might be. Cr makes a big difference to advance needed especially mid range. Higher rpms and it makes a difference how the head breathes, poor breathing, more advance.

Guesstimates, you should be in the range of the following but no one can tell you what it should be, no good going down the middle for luck either.
10-15 at idle
18-25 at 2ish
26-34 at 3ish
26-38 above 4

The above illustrates the beauty of multiple advance curve systems or mapeable ignition.


Do you mean 10 and 15 crank degrees or 10 and 15 dizzy degrees ie 20 and 30 crank degrees?

Peter
Peter Burgess Tuning

"Would it be 10deg or 15deg" could also refer to static timing in crank degrees, albeit 15 degrees would be high for a standard MGB engine.

Other than that it is as Peter says i.e. you have to find what works best on your engine with your fuel. A pal has a Moss supercharger which included the distributor i.e. with a curve specified by them for this installation, but has been experimenting with an Aldon Amethyst mappable system and has improved the performance noticeably by Seat of Pants. Significantly cheaper than some alternatives.
Paul Hunt

Seat of Pants report: I could not have obtained my advance curve using conventional wights/springs dizzy. With a conventional dizzy you can only effectively raise or lower the whole advance curve by rotating it. (or you guess with springs/weights)

The mappable Amethyst can be set at 500rpm intervals. It also has the electronic equivalent of "rotating the dizzy".

www.aldon-amethyst
Michael Beswick

Hi,

Thanks for the replies I meant 10deg and 15deg a shown on the cam weights in the dizzy.

Any more info?

Ta

Iwan
Iwan Jones

15 degrees would give 30 degrees at the crank so in all probability too much. 10 degrees would give 20 at the crank, better starting point but it needs to be right between the starting point and maximum too.

Peter
Peter Burgess Tuning

"With a conventional dizzy you can only effectively raise or lower the whole advance curve by rotating it. (or you guess with springs/weights)"
Not so. With a conventional dizzy it's quite straight forward to modify the curve (though not as quick as flicking a switch). All that needs to be changed are the springs, with a good selection and experience it's a doddle. (I'm not rubbishing electronic means, just pointing out that it can be done well with old-tech stuff as well)

Peter's point about needing the right advance right through the rev range can't be overstressed (it's suprising just how many focus on only or two points) and I completely concur with Peter's range. Whether you end up at the low end or high end will depend on the combination of camshaft and compression ratio (and fuel octane of course). For a starting point be conservative and go for the lower end of the range.

One thing you have to watch for is lash in the distributor drive. Most commonly this is from wear on the camshaft gear that drives the oil pump and distributor - quite common on old camshafts and a fault that can get transmitted into reprofiled units if the cores aren't checked properly. The end result is lack of stability or even scatter in the ignition timing, along with additional advance to what's in the distributor. In bad cases it can produce a 10-15 degree step in the advance curve somehwere around 2-2500 RPM.
So it's much easier to check this before things go together.
Paul Walbran

The numbers on the weights (or stop-plate as on some versions) give the max centrifugal advance as Peter says, but this would be added to the static advance and hence can give typically 26 to 30 total dynamic advance. If you use vacuum as well this is additional, and comes into play at part throttle.

There is much less flexibility in changing mechanical advance with springs and weights (which Michael said you can do) then there is in being able to set each 500 rpm independently. A change in springs affects the whole curve and can only ever be an approximation of the ideal, unless you are very lucky or do an awful lot of dismantling and mantling. The weights or stop-plate only affects max additional centrifugal advance.
Paul Hunt

In my experience the curve of the Lucas 41234/41391 is best for an 1860 stage 2 engine - the curve is

Static timing 6 degrees BTDC

0 to 1 at 600
2 to 6 at 1200
10 to 14 at 2200
20 to 24 at 3600
26 to 30 at 4500
28 to 32 at 6000

So add the static to the curve figures to get total mechanical advance.
Chris at Octarine Services

Hi,

So, it is right to say that the curve itself can be modified with the springs; not just the position or angles of the kinks/elbows in the curve. This is how I've interpreted it.

I realise there might be a great deal of trial and error in this method.

So with 10deg weights what strength springs (both soft and hard ones would suit the modified engine outlined above?)

Also, I've read that 10 deg weights with no vacuum advance is best.

Ta,

Iwan



Iwan Jones

We are trying to tell you this cannot be done on paper. The best way is to get it on a rolling road with say a 101BY1 Aldon dizzy, run it up, see the effects of altering the timing on bhp and, if necessary, go back to Aldon with info and they can change the curve to suit.
You have a large bore engine and long stroke, both of which I feel may require less advance to avoid detonation than say an 1840 unit.

Peter
Peter Burgess Tuning

Or convert to a Ford EDIS ignition setup and map the curve with your laptop on a rolling road ....
Chris at Octarine Services

I think the magic words are Roliing Road Chris :)

Peter
Peter Burgess Tuning

A mechanical curve usually has one kink or knee, flattening out when the second spring, which is initially loose on its pins, finally comes into play. The stronger the spring the flatter the line on the graph.

Having vacuum advance certainly complicates matters at anything less than very large throttle openings, as it gives additional advance that varies with throttle opening as well as the centrifugal advance that varies with rpm, meaning you have a 3D map to configure. Vacuum advance gives you improved economy when cruising and part-throttle acceleration so obviously benefits cars that are mainly used on the road. It's usually removed for competition use as cruising economy is irrelevant and it makes setting-up easier. This gives rise to the availability of vacuum-less 'competition' distributors, with the inference that it's going to make an engine more powerful, which it rarely does.
Paul Hunt

Hi,

Thanks for all the comments. Just reading round the subject at the mo. Looks like heading down the Aldon/rolling road is best.

What's this ford edis set up?

Ta Once again.
Iwan Jones

EDIS discussion on my tech forum - http://ultra.octarine-services.co.uk/view_forum.php?id=4
Chris at Octarine Services

I'd highly recommend a rolling road set up with Peter Burgess - excellent value and results

he may still have a standard or programmable 123 dissy he can sell you as he used to be a stockist if not he's set up one you'd bought elsewhere on his rolling road

I've got a standard 123 dissy and am very pleased with it throughout the rev range especially after Peter's rr set up

I'd previously been to two other rr tuners with my present car and Peter's was by far the best
Nigel Atkins

This thread was discussed between 12/11/2013 and 16/11/2013

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