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centrifugal advance curve and vacuum advance on my TR4A

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Hi there,

from another post I want to transfer my point of view about the centrifugal and vacuum advance.

If I'm wrong please veto.

My experience with different engines, cars and motorcycles is:

every engine runs best on all revs (also at tickover) on full advance (engine specific), while it better starts on late.

So in my opinion don't overdo with finding the correct centrifugal curve, this is "digital" and about the way I drive my pre war Nortons with manual avance:

late to start them, full advance from tickover to max revs, some deg. less on heavy load,

some deg. more on higher revs (a vacuum advance unit would be nice), back to late ignition when you stop the engine for the next start.

For the TR4A it could work like this, would be worth to test it?


Very interesting is the vacuum advance unit to get some more degrees for more driving fun and agility.

For experimantal use I drove my TR4A with HS6 with and without the vacuum pipe some years ago - and I don't want to miss the unit anymore.

Shure most of you know, the vacuum for the distributor advance unit is caused by Venturi

by the airflow passing the trottle butterfly, it is NOT AT ALL the vacuum in the inlet manifold.

Next is the vacuum pickup on my old HS6


The next photo is from the engine side, the butterly is some degrees open for the photo.

The bore ends direkt above the butterfly, at tickover it is mainly closed and the vacuum advance unit does not work.


This is why the pickup is not the vacuum in the inlet manifold, indeed in my opinion in this position it is an indication for the airflow.


Two questions come up: what vacuum is picked up there, and what is the reaction of the vacuum advance unit.


If you have a close look on the vacuum unit (take a fine wire brush) you see stamped there 2.6.3 for the TR4A and 3.7.8 for the TR6 (not PI I have been told)

2 is 2 inch mercury column, the low pressure the unit start to adjust the vac advance

6 is 6 inch mercury column, the low pressure the vacuum unit does not longer increase the vac advance

3 is about the maximum vac advance in degrees on the distributor, this is 6° (x2) on the crankshaft

From the workshop manual you get details, and the curves looks about like this, for the TR6 there is a variation


For my TR4A this means there is a adjustment of 2° more advance at 0.1 bar low ressure (0.9 bar absolute, 0.0 bar is vacuum).

Maximum vacuum advance is about 5.5° at 0.2 bar low pressure, added to the 24° from the centrifugal advance - not that much....

From tests I drove some years ago I know the pressure on the pickup.

0.0 - 0.05 bar low pressure, no vaccum adjustment:

tick over at the garage, playing with the engine at the garage up to 2,000 revs/min without load,

over-run downhill with no gas, the moment when you are shifting from gear to gear,

up to 0,1 bar low pressure, vacuum advance starts adding some degrees:

playing with the engine at the garage up to 3,000 revs/min without load, cruising 30 - 40 km/h in town,

massiv acceleration strating from moderate driving, driving easy / without much load up to 2,000 revs

0.1 - 0.2 bar low pressure, the TR4A vacuum advance is already on its max degrees:

moderate ecceleration, driving easy / without load up to 3,000 revs/min

0.3 - 0.35 bar low pressure, the TR4A vacuum advance is still on its max degrees:

easy and fast driving on country roads an on the Autobahn, indication for shifting to a higher gear

0.4 bar low pressure or more:


What I didn't test is to drive with low revs and high load uphill....

What do you think about?

Ciao, Marco



Edited by Z320
Added missing letters / words for better reading
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Hey Marco,

Of course the answer is 'it depends'... in this case on compression ratio, valve size&lift, fuel quality, exhaust backpressure etc etc. The more 'free breathing' an engine, the more sensitive it is to ignition timing.

To generate most power, the cylinder pressure should be at its maximum about 30 degrees AfterTDC when the crank pin has most mechanical advantage on the crank. Because the speed of flame propagation in the cylinder is highly dependent on the pressure of the charge, its necessary to 'light the flame' earlier when cylinder pressures are low. Low cylinder pressures happen when the flow of air during induction stroke is restricted (small valve/lift, closed throttle etc)...

The position of the vacuum advance port is the 'ported vacuum vs manifold vacuum' debate you can google

The 123ignitions are an eye-opener on how important ignition timing control is and how good an old TR/Jag/BMW engine can run, the great thing about the programmable ones is that you can play with all of the parameters you discuss above and quickly determine what works best for your particular engine & fuel. I strongly believe that a lot of the starting and slow running/pickup issues talked about here are actually ignition-related...either due to wear/condition of the dizzy or simple timing.

I believe a modern programmable ignition is simply better than what lucas or bosch could deliver back-in-the-day using springs, weights and vacuum capsules. I have a 1974 BMW2002 turbo - the first european turbo...renowned for frightening turbo lag...it has a simple bosch distributor with vacuum retard ... Changing to a 123ignition this engine can be 'tamed' and made much more progressive and ultimately more powerful.

Most workshop manuals say something like 'before attempting to adjust  carburettors, make sure ignition system is in good condition'... for very good reason.

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Hi Marco,

I have just  had my distributor reconditioned by Martin Jay (The Distributor Doctor) and he reset the centrifugal advance to his recommended curve.

My engine is virtually the same as a TR4a with the addition of a tubular exhaust manifold and a fast road Newman PH1 cam.  For the curve he has provided, he recommends setting the static timing at 12 - 14 deg advance but the slope of the line is much more gradual with a peak at 20 deg, giving an overall maximum advance of 32 deg.

I haven't had a chance to try it on the road propoerly yet - that will come at the weekend on the drive to Malvern - but my initial impression is a very smooth idle and a good pick up from low revs.

The vacuum unit is a 2-6-3.

If you would like to see the curve, I can clean up my existing spreadsheet graph and post it here.

Rgds Ian

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Hi Ian,

yes please, post here.

With my Pertronix electronic braker I can't find out what I have static advance,

but it must be less because I have about 15 deg. at idle.

Not long drive tested, only test drives to get the engine warm, but I'm hopeful the engine idle will be good.

A new member of our Stammtisch offered me to test his 123, it's out of his TR4A again because he is not pleased and swapped back to classic contact braker.

Ciao, Marco

Edited by Z320
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Hi Marco,

I have attached the graph below.

I have the same problem as you in that I have a Pertronix set up so I can't set the static timing but I have about 12 deg. at idle of 800 rpm with the vacuum disconnected.  Note all of the curves are centrifugal advance only.

Rgds Ian

Edited by Ian Vincent
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Hi Ian,

thank you, but please delete the data sheet if it is Martin Jay's literary property?

If i is your own creation please let it in the forum, it is welcome!

Cheers, Marco

Edited by Z320
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