Vacuum Advance

[Colin Fairhurst]

I have been googling the subject of retaining the ignition vacuum advance on slightly modified engines for road use. It would appear that this facility is very often omitted on racing engines that are operated flat out on full load most of the time.

In the case of road use when the throttle is only partially open most of the time and when on tick-over it appears that the reduced amount of fuel/air entering the cylinders means that the 'flame speed' is slower and needs an earlier spark (advanced) to give better engine efficiency and fuel economy hence the vacuum advance which only occurs in these light-load periods.

I also learned that during the 1970s car manufacturers moved the vacuum connection from the inlet manifold to the inside of the carburettor just in front of the throttle butterfly in order to reduce the exhaust emissions on tick-over and slight throttle opening to meet the legislation at the time. This being at the expense of engine and fuel efficiency.

I then started to think that it could be a benefit to move the vacuum connection back to the manifold on cars of the 1970s and early 80s say, for example, on my wife's 1975 Spitfire 1500.

I would appreciate any thoughts about this.

I haven't given this subject a lot of thought in the past and have just followed the manuals etc. to make sure that I had everything set up properly.

 

Regards, Colin

 

[ed_h]

Colin--

 

I've had the same thought process as you have. When the spark is advanced at idle, the engine normally speeds up, giving more RPMs at the same throttle setting. The engine clearly "wants" some advance at idle. The reasoning seems sound: idle is a relatively high vacuum state, and the rarefied mixture will burn slower. So it seems that an advanced spark is exactly what we need.

 

There are a number of sites I've been referred to when I bring this up. Most of them admonish us to not mess with the "proper" ported vacuum connection for advance, but none that I've seen include much convincing scientific support for that warning.

 

I don't have a running car right now that I can try it on, but I'll probably investigate it further when I do.

 

One caveat I've thought of: In a car designed without any vacuum advance at all (like my '74 TR6), adding vacuum advance on top of the existing centrifugal advance could result in too much advance under certain conditions. In cars like that, possibly the maximum centrifugal advance may need to be reduced.

 

Ed

[TriumphV8]

There is a lot of discussion on the centrifugal advance about the last degree

of advance in some ares to be added or not.

The vacuum advance, what in my view is the more relevant, is neglected.

We can ad easily 20 degrees of vacuum advance in some engine states

and these are the states we drive much more than at full throttle.

 

We should not think about that in two different advances, centrifugal and vacuum,

but in a 3D mapping providing at each state of the engine the proper advance.

 

This vacuum pickup at the carbs is done because we do not want that

advance at idle although we have a lot of vacuum than.

The reason is pollution control.

So we take the throttle position into account leaving vacuum advance

unattended when plate is fully closed.

 

It is a lot of work to provide a good 3D mapping with a modern electronic

like Megasquirt but its a pain to do that manually with the two mechanical systems.

Another problem there is that a hotter cam gives another/less vacuum making the

original vacuum advance system somewhat useless.

For that case different vacuum boxes are provided, giving different amounts of advance

and reacting at different vacuums but as I said its expensive and a pain to set up.

 

I would recommend a modern system to those people who want to improve

instead of fiddeling with the original.

The first step might be the 123tune but a proper electronic is better

like Alden, Megajolt or Megasquirt where only ignition advance can be used.

 

The result is a more crispy engine, less fuel consumption

and driving at part throttle same speed with less pedal pressing.

The fuel consumption of a TR6 can be reduced more than 0.5l/100Km

compared to no part throttle advance.

[oldtuckunder]

Remember when playing with vacuum advance that the vacuum taken at the carb (two possible different places) and that at the manifold are way different at the same rpm's/throttle position and that the correct vacuum capsule on the distributor needs selecting to match the vacuum source.

 

Alan

[John L]

Would the 'Aldon Amethyst Mappable Ignition - Vacuum' system be a good addon. The weights are fixed closed in the distributor, and the advance curve setup on a computer, has anybody tried this system yet?

Would be interested to know what you guys think about this box of tricks.

John

[Colin Fairhurst]

Colin--

 

I've had the same thought process as you have. When the spark is advanced at idle, the engine normally speeds up, giving more RPMs at the same throttle setting. The engine clearly "wants" some advance at idle. The reasoning seems sound: idle is a relatively high vacuum state, and the rarefied mixture will burn slower. So it seems that an advanced spark is exactly what we need.

 

There are a number of sites I've been referred to when I bring this up. Most of them admonish us to not mess with the "proper" ported vacuum connection for advance, but none that I've seen include much convincing scientific support for that warning.

 

I don't have a running car right now that I can try it on, but I'll probably investigate it further when I do.

 

One caveat I've thought of: In a car designed without any vacuum advance at all (like my '74 TR6), adding vacuum advance on top of the existing centrifugal advance could result in too much advance under certain conditions. In cars like that, possibly the maximum centrifugal advance may need to be reduced.

 

Ed

 

Ed,

I think that the problem with reducing the centrifugal advance to compensate for adding vacuum advance is that you will lose some advance at full throttle when there is no vacuum.

It might be worth trying to find out if your TR6 was designed to be without vacuum advance as a crude way of getting the engine to burn off the undesirable emissions in the exhaust manifold at the expense of engine and fuel efficiency. In this case adding vacuum advance could improve things.

One thing that puzzles me is that the SU HS4s on my TR2 engine (which I have recently changed for a slightly modified TR3 engine with HIF6s) have got the vacuum advance connection into one of the carbs. and not the manifold. There must be a reason for this and I don't think it had anything to do with emissions in 1955.

Can anybody explain this please?

 

Colin

[stuart]

They all have the connection into the carb its just that the position was changed later on to be on the butterfly from in the main body on the earlier carbs.

Stuart.