19 replies to this topic

[Don't Panic]

Ok before i picked up my new car, the previous owner had it rolling road tuned for me (because he's a nice guy and there's no specialists near me)

This gave me 68BHP @ 6000RPM on the rollers (up from 61) However the tuning guy removed the vacuum advance pipe

this is written on the test report
" IGN Timing-high 32% full advance without vac"

I don't know what this means or what the vac does, anyone care to enlighten me?

Thanx

P.S. Haynes manual ordered.

[kada1980]

Basically as the engine speed is increased the ignition needs to spark sooner. You set the dissy up at idle rpm which will be some figure stated as BTDC (not sure on a series), The vacuum is taken from the intake manifold and as the rpm increases so does the vacuum which then advances the spark (all done inside the dissy) but some dissys have a centrifugal advance and if thats so you shouldn't have the vacuum advance connected as you will be over advancing the engine.

It appears that yor timing has been setup with the vac advance and needs setting up with out it connected, i'm suprised the garage didn't do it for you its only a 5 min job

HTH

Karl

[Guess-Works.com]

Nearly right.. .

The vacuum advance is an economy feature, ALL dizzys use a cetrifugal advance which is provided by springs and weights in the body... what the Vacuum does is increase the advance proportionally to the vacuum in the inlet manifold, which again is proportional to the throttle opening..

When on fully throttle there is little or no vaccum in the inlet manifold, so no vacuum advance is applied, when at part throttle/cruise there is a vaccum present so the normal advance provided by the dizzy is advance even further by the vacuum mechanism, which improves the echonomy.

The reason a car is tuned (and timing set) with the vac disconnected is so that the ignition is not artificially retarded by incosistencies in the vacuum...

Also you will notice that performance dizzy's very rarely have vacuum advances, this is because there spend most of the time on full throttle therefore the vac is useless.

[Don't Panic]

Cheers for the help, but does

" IGN Timing-high: 32% full advance without vac"

mean that the timing has been setup without the vac, and therefore does not need it, or shall i reconnect it?

[cowboy]

yes reconnect the vacuum adv pipe, as guessworks said it will help with the economy better mpg in other words

[Sprocket]

Nearly right.. .

The vacuum advance is an economy feature, ALL dizzys use a cetrifugal advance which is provided by springs and weights in the body... what the Vacuum does is increase the advance proportionally to the vacuum in the inlet manifold, which again is proportional to the throttle opening..

When on fully throttle there is little or no vaccum in the inlet manifold, so no vacuum advance is applied, when at part throttle/cruise there is a vaccum present so the normal advance provided by the dizzy is advance even further by the vacuum mechanism, which improves the echonomy.

The reason a car is tuned (and timing set) with the vac disconnected is so that the ignition is not artificially retarded by incosistencies in the vacuum...

Also you will notice that performance dizzy's very rarely have vacuum advances, this is because there spend most of the time on full throttle therefore the vac is useless.

Or when there is too much total advance, this has nothing to do with performance or economy other than the engine doesn't run right. Which is probibly why it was stated on the report and why the vacuum was removed. so reconnecting it will again over advance the ignition. Theres usualy a reason a RR operator removes the vac and its not always for performance. Its merely to get the engine running somewhere near right with a distributer that does not match the engine because of the modifications carried out have taken the engine so far away from standard.

I'd suspect the bloke before you has done what I did and changed the carb, where the original was of throttle edge depression and the new one is manifold depression. All a bit complicated, but causes this over advance.

[Don't Panic]

The manifold and filter has been changed, also fitted with electronic ignition, im not sure about the carb itself, I will assume the guy at the rolling road knows what he is doing, so i'll leave the vac off. The only problem i have with the engine is it judders a bit when cold and it can stall even on full choke, but i think this just needs the idle adjusted, I'll sort this when i have the right manuals (and TMF cd)

[Guess-Works.com]

RR power figures should be taken at full throttle where a vacuum is not present, therefore will not affect total advance...

[Sprocket]

RR power figures should be taken at full throttle where a vacuum is not present, therefore will not affect total advance...

[Dusky]

Nearly right.. .

The vacuum advance is an economy feature, ALL dizzys use a cetrifugal advance which is provided by springs and weights in the body... what the Vacuum does is increase the advance proportionally to the vacuum in the inlet manifold, which again is proportional to the throttle opening..

When on fully throttle there is little or no vaccum in the inlet manifold, so no vacuum advance is applied, when at part throttle/cruise there is a vaccum present so the normal advance provided by the dizzy is advance even further by the vacuum mechanism, which improves the echonomy.

The reason a car is tuned (and timing set) with the vac disconnected is so that the ignition is not artificially retarded by incosistencies in the vacuum...

Also you will notice that performance dizzy's very rarely have vacuum advances, this is because there spend most of the time on full throttle therefore the vac is useless.

What I am wondering for : do they use more static advance on race engine s to compensate dor the lack of vacuum advance? Or is or just 'unplugged'?

[KernowCooper]

Old topic but the answers are in the FAQs Aldon advance curves you can see there the Red curve needs more static to get the final required degrees to produce power, no vacuum advance required for race engines as there is little vacuum on a wide open throttle.

[Spider]

 

What I am wondering for : do they use more static advance on race engine s to compensate dor the lack of vacuum advance? Or is or just 'unplugged'?

 

 

For most classes of racing, you can usually use a higher octane fuel, this usually allows you to dial in more advance. Also for street use, the settings are usually a couple of degrees on the conservitive side, more to allow for variations in fuel octane that one may purchase, for the fuel as it ages and for hotter air intake temps.

[Jedi Master]

My 998 was RR tuned at Minisport and it has the vacuum pipe removed. Stage 1 kit, Piper 255 cam, HIF38 carb. Runs like a dream, pulls like a train, revs and revs like a bumble bee!

[KernowCooper]

For that state of tune I'd reconnect the vacuum advance and take advantage of the extra torque and mpg available with vacuum advance and suggest the RR operator gets Vizards and Des Hammil books and read them carefully.

 

Road use vacuum advance

Race use no vacuum advance

[Jedi Master]

For that state of tune I'd reconnect the vacuum advance and take advantage of the extra torque and mpg available with vacuum advance and suggest the RR operator gets Vizards and Des Hammil books and read them carefully.
 
Road use vacuum advance
Race use no vacuum advance

Ok, so just simply reconnect it? It won't need any retune?