MG T-ABC

I am curious to know if people here know about tapping manifold vacuum to run their engines using a distributor with vacuum advance? It just seems like something that would be a reasonable adaptation. Or would it only be a good idea if the engine rpm's were lower at cruising speed thanks to a modified rear end ratio?

I just don't understand why the engine seems to work so well using what most consider a backwards advance, as found with our mechanical centrifugal RPM based advance, giving almost no advance at idle and maximum advance at red line. Wouldn't it run better with vacuum advance?

well.....it has only been working perfect for over 70 years!! Like so many things on the TC!

The vacuum advance is used to rtatrd the ignition on overun for better fuel consumption. Remember you only get high vacuum on the overrun.

As Dirk says, if it ain't broke, don't fix it.

Yes, thanks, I understand where you are both coming from, and I understand the difference between mechanical vs vacuum advance. I said the original setup does work well, and if you take the "if it aint broke" adage too seriously you become OP originality police. I'm sure that original air cleaner worked dandy, but nobody seems to care much for it these days, and they like long lasting synthetic bushings, etc, etc. Thing about tinkering with a more modern distributor setup is it can be reverted back to original without a trace, very easy. Dirk says why change something that has been "working perfectly" for 70 years? That's my point, who says it been working perfectly? Thing about mechanical advance is they work fine on engines running rich, especially at high RPM's, which equates to cruising speed in this car.

Now Norman's definition of vacuum advance being about retarding ignition is only in a sense correct, as what it is actually doing is simply not supplying so much advance at more RPM's, and more advance at idle, etc. If you want to call less advance "retard" it is a bit confusing, as the ignition is never actually retarded. I understand your points of view about this, but my question regards an area of performance. It is not about fuel mileage for me on this, though it surely could increase mileage by being able to safely run a leaner mixture, rather its about maybe increasing engine performance enough to be noticed.

I am sure people have tried it before, or do it. I have not run across any articles on the subject.

The point is that it won't increase performance, it will give better MPG but I thought our aim was to use all the fosil fuel left....

well, you can't please them all, all I can say my 1946 TC ran perfectly in the 30+ years I have had her and I have driven her hard over most of the Alpine mountainpasses in Europe and they are no chickenfeed!

While not an expert it seems to me there is some confusion. As I understand it, a standard vacuum advance set-up, plumbed from downstream of the carb butterflies, will always add to the mechanical advance when it operates. I do not see how it would ever retard the spark in any sense - relative or absolute. I also do not understand what is meant by the standard centrifugal system giving "backward advance". The mechanical system simply advances the spark timing as the revs increase. This is necessary as there is less time available to complete the combustion as the revs increase and so it must be started earlier

Vacuum advance will operate only when there is significant manifold vacuum. This occurs under two main conditions - cruising under light engine load (flat or down hill terrain or on overrun) and at idle. In both these conditions the fuel mixture tends to weaken, the mixture takes longer to ignite/burn and so more advance can give more complete combustion. The advantage is lower fuel consumption at cruise and less "soot" in the combustion chambers and less heat generation in the uncooled exhaust manifold at idle (with less propensity to overheat the fuel in the carb bowls).

I agree that the standard set up works very well in terms of engine performance when "getting on with it". Vacuum advance would not change that - positively or negatively. Its contribution is more subtle.

By "retard" I mean it holds the advance back. The maximum vacuum is on the overrun when the engine is turning over above tickover and the throttle(s) are closed.

XPAGnut, I think, is talking about "backward advance" meaning mechanical advance only distributors have nearly no advance at idle, whereas vacuum ones have also no advance from the mechanical aspect, but do have quite a bit of advance from the vacuum aspect. And mechanical advance only distributors have full advance at higher RPM, weather they need it or not, while vacuum advance distributors only have some advance at higher RPM, unless in overrun, when they get the full advance. If that is the case, XPAGnut could be questioning if the mechanical advance, being solely limited by RPM, might not be hurting the engine in certain situations of use. To that I would suggest that MG knew about how improper advance was harmful versus when it was just wasteful of fuel, and they set things up to sometimes be wasteful, but never to be harmful. With a vacuum advance distributor (partly mechanical advance) you can set it up to be neither harmful nor wasteful. I think what Norman said is correct, its just wasted fuel.

I must admit that I was presuming a distributor with both mechanical and vacuum advance, which I understand is the usual design if vacuum is employed.

well, it is getting very academic now, the only vacuum I had om my TC was not my distributor but was me holding my breath going down the mountainpasses, hoping the Alfin brakedrums would do their job........They did!

"well, it is getting very academic now". OK then, Dirk, tell us about your nifty luggage rack that doesn't obstruct rear vision.
btw, I was in Villars last year thanking God I was in an AWD rather than 7794 (w/alfins)! You're nuts!

Bob Brennan
S. Freeport, ME

@ Bob- the good thing about that nifty luggage rack ( made by Paul Ireland- Google) is that not only it ,leaves your rear vision clear but also brings down the rear gravity point way down, not bad when taking hairpin turns at speed... Filling up the tank also no more hassle now and.....mother could pack 5 more pairs of shoes and me some more spares along on the trip because of that:-))

"Probably the best thing happened to me was going nuts. Nobody knew who I was untill that happened ( J.Piersall)"

Yes, I was thinking when Dirk goes down the mountain in the TC is when the vacuum advance would have been adventageous. Overrun and idle. I was wondering about the XPAG propensity to crack cranks and ravage rod bearings, and that maybe proper advance at all times could make the engine last longer. But as someone pointed out the engine was designed well, so when most power being produced timing is proper, and it rarely overheats idling unless a few degrees not advanced enough. Does seem like less stress on the rods/crank with proper advance even on overrun and idle, but not as important as when making near full power. So I guess the answer is likely it would make the engine last longer and less prone to breaking cranks, and it would increase fuel mileage.

I don't think it would make any seat of the pants difference. You can customize the mechanical advance to give whatever you need, so idle is correct as is high RPM running. Varying advance by throttle opening, in my opinion, would not increase performance or longevity. It might make it slightly more efficient, but not at open throttle where efficiency matters more. Plus mechanical advance is very reliable. I'd say there is good reason why you don't see vacuum advance on these cars, even on the really high performance stuff. But I also don't see how it would hurt anything other than looking a bit out of place. If you try it, let us know how it works out.

For better performance I suggest that your original distributor is rebuilt with an advance curve matched to modern fuels. You keep the originality as well.

TC3382 / MG 7305

Just for the record my Cosworth BDR engine which produces 220 BHP from 1700 cc has no vacuum advance.

Production cars up to around 1970 had mechanical only advance.