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MG MGB Technical - Engine Timing Options & Why Bother?

Yes, its me again - and I have a new question!

I've read a lot of partial explanations for changing one's engine timing from the stock settings to various considerably higher settings, all with the intended goal of improving performance. I'm not sure what is meant by "improving performance" - mileage, power, or what.

Some writings left me with the impression one sets their initial/static timing to 30+ degrees at idle. Others have variations on this. BUT a recent article in MGB Driver gave all the details.

So I did the following to my 72B (18V585Z L1946):

I confirmed original timing at 10BTDC (not sure if its suppose to be 8)at 1000 rpms w/vacuum disconnected.

Next I (think I) followed the steps in article; manually turn engine until pulley timing mark lines up with 20 on timing mark indicator(engine off), make a new mark on pulley at TDC (thus giving one a new starting point that is +20 degrees), disconnect vacuum, start engine, increase rpms to 4500 and set timing at 32 degrees.

So I did this, car appears to run the same (except I had to turn idle down 300-500 rpm due to increased idle) - which means starts at slightest turn of starter and goes up and down hills and around corners, without ping or pop or pang!

My questions are:
1. Why disconnect vacuum to set timing at 4500 rpms? I don't think there is a lot of vacuum at that point.

Following this adjustment I re-checked my static timing, which was now around 16-18.

2. If my overall net static timing change was an addition 6-8 degrees advance at idle, why didn't I just set the static to 16-18 instead of setting it at 4500 rpms at 32 degrees?

3. Why am I messing with mother nature in the first place by tinkering with my timing anyway? What increased performance am I in search of?

I won't be racing this car, I won't be doing anything that requires "perfect performance" - I just want easy starting (which I had) and better mileage (which I hope to address by re-adjusting valves and going through the detailed process of tinkering with adjusting needle valves and balancing carbs).

4. Should I stop reading all these helpful hints?

Thanks again for bearing with me on my initial journey into the world of MGs.

BobA
R.W Anderson

1. Why disconnect vacuum to set timing at 4500 rpms? I don't think there is a lot of vacuum at that point.

> There is significant vacuum, at least when sitting still; probably very little on the road as the throttle would be wide open. The point is to eliminate any effect of the vac advance so you can determine/set the true mechanical advance controlled timing.

Following this adjustment I re-checked my static timing, which was now around 16-18.

2. If my overall net static timing change was an addition 6-8 degrees advance at idle, why didn't I just set the static to 16-18 instead of setting it at 4500 rpms at 32 degrees?

> Like above, you are trying to get the critical end-point correct, accounting for the unknown state of the distributor and everything driving it.

3. Why am I messing with mother nature in the first place by tinkering with my timing anyway? What increased performance am I in search of?

I won't be racing this car, I won't be doing anything that requires "perfect performance" - I just want easy starting (which I had) and better mileage (which I hope to address by re-adjusting valves and going through the detailed process of tinkering with adjusting needle valves and balancing carbs).

> "Perfect performance" is whatever you define it as; the more you know, the more perfect it gets.
6-8degrees can result in a BIG mileage improvement, and a significant power gain too. If the additional advance from the vac unit is correct, the mileage goes up way more - that's what the vac advance is for.

4. Should I stop reading all these helpful hints?

> No, read more! But you have to study it until you understand it AND can sort out the BS, of which there is a great quantity.

FRM
FR Millmore

RW. In addition to what Fletcher has noted, you should also check out the advance curve for your distributor, then, use those figures to check the timing (vacuum advance disconnected) at the various engine rpms noted. That will give you an idea of the overall condition of the mechanical advance system. Simply setting the overall advance at a high engine rpm will not do this. You run into the very real potential to be over advanced at lower engine rpms if the mechanical advance springs are weak. Since excessive mechanical advance can cause engine damage, this is worth checking out.

As to the amount of vacuum present at higher engine rpms, the effect will vary depending on the vacuum source being used. A manifold vacuum source, which I believe your car has, will show a high manifold vacuum at any steady engine speed. More than sufficient to bring the vacuum advance system into operation.

Les
Les Bengtson

*Manifold* vacuum should be disconnected to make any centrifugal timing measurements, and also at much more than idle on carb vacuum systems. Manifold vacuum will show high vacuum at idle, carb vacuum zero. When running, the carb only has to be open a few degrees for the two to read the same.

The vacuum at 4500 rpm will differ according to whether the engine is of load or on. On load i.e. driving yes there will be little vacuum left as the throttle will be open quite a bit. But off load it doesn't take much throttle to achieve 4500, and there will still be quite a bit of vacuum left. Which is why vacuum needs to be disconnected (and plugged at carb or manifold) to measure *centrifugal* advance.

Unless you are going to spend some time on a rolling road, and even then that won't really help if you have a vacuum advance distributor, about the only thing you can do is compare the curve of your distributor with what your engine *should* have. If way off you could have the wrong distributor, or the centrifugal advance springs could be tired as Les says. If it's about right all you can do then is to set the timing to give best performance without pinking at any combination of load, revs or throttle opening. This is usually the highest advance it will take on high compression engines, which may not be the case for low compression.

There almost certainly is scope for improving North American spec engines after 1967, as these were increasing configured for emissions and not performance, unlike UK engines which incidentally only had about 4 engines and distributors for the whole of production, unlike North America which had no less than 17! However I'm doubtful about the usefulness of the so-called Eurospec distributor, as this was for high compression engines and from 1971 North American engies were low compression.
Paul Hunt 2

At high road speeds regardless of steady state, the manifold vacuum will be lower than that at steady state at a low road speed, or steady state with no load. Manifold pressure (from which engine vacuum is derived, i.e. it is the difference of ambiant pressure and manifold pressure) is more a function of mass flow of air required than the engine speed. And of course, horsepower is a function of mass flow of air.
Dave Braun

Timing aB is simple. Disconnect the vacuum advance permanently. Don't forget to blank off the carb or you will get a weak mixture.

Assuming you are on un-leaded fuel & your points are set to .017": Rev the engine to 3000 rpm and set the timing to 30 degrees advance. That's it.

I always take the vaccuum mechanism off the distributor and lock the mechanism. With the early dizzie, you can do a cunning mod which retains the vernier adjustment.

R L Tinkler

>Disconnect the vacuum advance permanently.

Why? You're depriving yourself of part throttle power & fuel efficiency. Vacuum advance is a Good Thing.
Rob Edwards

I agree that vacumn advance is agod thing. As well as the advantage of much better fuel economy the B series engine may need it to work well over all of it's envelope. A recent case in "Enjoying MG" showed this. Without vacumn the car would not both idle and give good high RPM power. The "performance" vacumless dizzy was sold on e-bay to racer.
Stan Best

RL. No, timing of the MGB is not a simple thing, especially on the North American Specification cars.

First, as has been noted, removing the vacuum advance system from the circuit results in worse fuel economy, something important to all of us. The vacuum advance also allows for a better idle. Only on a race/rally car, intended to be operated at high engine rpms most of the time, do you not need a working vacuum advance.

Second, the "set it at 30 degrees of advance at 3,000 rpm" only applies to the earlier distributors used with the high compression engines. The low compression engines require more advance than the HC engines did and distributors for them had a different mechanical advance curve giving more ignition advance. From about 1971 onwards, the NA spec cars used the low compression engines and your system may not provide the proper advance.

In addition, the concept of setting the ignition advance to the maximum advance at the engine speed listed as producing that advance, is only applicable if the distributor is in excellent condition. I have seen a number of distributors having weak springs, allowing the mechanical advance to come on too soon, and one example having the advance weights which did not move. In either event, setting the ignition timing to the correct overall advance at the maximum specified engine rpm will result in the ignition timing being over advanced at lower engine rpms, resulting in possible engine damage.

The mechanical advance curve for a specific distributor needs to be know. Then, after setting the timing, it should be checked at all of the engine rpms noted on the advance specification and notes made as to what the actual advance is. If the actual advance is significantly different, more than two degrees, from the required standard, more investigation is in order.

Les
Les Bengtson

I agree that if you want economy, keep the vacuum advance but if you want performance, get rid of it.It's not unusual for the diaphragm to leak. Every MGB I have had to sort out running on problems has turned out to have air leaks in the vacuum advance/retard system!
R L Tinkler

> if you want performance, get rid of it.

I wouldn't say that, either. At part throttle, you'll get better throttle response and drivability, and probably a little more power -- i.o.w, better performance with than without. At full throttle, there's no vacuum so the vacuum advance is "turned off" (which is good, because at full throttle you engine can take less advance than it can at part throttle).

As for problems with leaking diaphragms, well, I think it's safe to say that many running problems can be fixed by repairing broken stuff! ;-)
Rob Edwards

'Getting more performance with the vacuum disconnected' is a fallacy that stems from the description of vacuum-less distributors as 'performance' distributors. The description comes from the usage, not the other way round. Performance applications use vacuum-less distributors for two reasons - the minor one is that cruising economy is hardly an issue and removing it removes a potential source of problems, but the major one concerns setting-up. With a vacuum-less distributor you 'simply' set the curve to give maximum performance at full throttle through the rev range, i.e. it is a simple 2-dimensional 'map'. Add vacuum advance to the equation and you get a 3-dimensional map. Not even modern ECUs go that far, settling for a set of 2-dimensional maps in many cases. With road cars it is a trade-off between performance and economy, and especially with the limitations of mechanical advance mechanisms (both centrifugal and vacuum) is only a crude approximation at best. You may well be able to get better performance by fitting a vacuum-less distributor with a special curve, although at the expense of loss of cruising economy as has been said. But simply disabling the vacuum on the *standard* distributor is a nonsense - not only do you get the aforementioned loss of cruising economy, but a significant loss of part-throttle acceleration as well. I've been experimenting with various vacuum capsules, and also have the ability to switch vacuum advance off, and the loss of performance *without* vacuum advance is immediately noticeable.
Paul Hunt 2

This thread was discussed between 22/06/2007 and 01/07/2007

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