Vacume advance disconnected

[chris2002hk]

Since Ive had my car the Vac advance pipe has being missing, the hole taped up at the plenium, and the dizzy end is left open. I cant figure what the reason would be for this, presumbly its best to fit a new tube to improve MPG?

[900T-R]

Yes and no. Indeed, running a bit more advance under vacuum is good for fuel efficiency - however, the vacuum advance unit is said not to react quickly enough to going from light throttle to WOT and thus will momentarily induce pinking. To counteract this, you'd need to lessen overall ignition advance and be back at square one as far as fuel efficiency is concerned.

Either on your car the vacuum advance unit has been taken out the loop because there was something wrong with it, or the prior owner/their mechanic has found that they ran into detonation when getting on the throttle - take your pick.

[dog man]

Didnt they all come with the one way valve originally to stop the vacuum adv working because of concerns over pinking?

[rigga]

No the one way valve was to stop fuel vapour being drawn back through from the plenum to the vac unit and destroying the diaphram ...

[haircutmike]

Either way it must be better to run the original set up shirley?

[900T-R]

haircutmike said:

Either way it must be better to run the original set up shirley?

Well, that's the controversy - I have different readings from TVR folks whose opinion I respect, hence the above.

[haircutmike]

900T-R said:

Well, that's the controversy - I have different readings from TVR folks whose opinion I respect, hence the above.

Minor thread hijack, sorry O/P!

Is this a teeny performance mod enabling a bit more advance or is best left well alone?

For instance, my timing was advanced untill it just started pinking under load when hot, (track conditions) and then backed off untill it didn't.
I assume you're talking about initial pick up not prolonged throttle under load?

[900T-R]

Yes, initial pick up. I'd not disconnect the vacuum advance on a std car that functions well, but OTOH the carbon twin and triple throttle plenum set ups from ACT don't feature a vacuum hose connection for the ignition vacuum advance unit anymore as 'we found it's a waste of time'. However, Mark Adams does seem to see the value in having the vacuum advance unit working as intended...

Of course, mapped ignition would be the way to go, anyway...

[Simon says]

Absolutely killed the low speed off cam low speed running on my 400 HC when the advance capsule failed more to do with the fact the engine is over cammed and needs all the help it can get I expect if the motor shows no ill effect from using it I would always leave it connected constant throttle fuel economy will suffer as a result if you don't just my take you may find its been capped because it as failed though

Edited by Simon says on Wednesday 18th May 09:21

[SILICONEKID340HP]

My old 4l was set up more advanced with no vac pipe connected ,more power higher up the rev range,lower mpg and rubbish low down drivability ..if it`s set up this way when you reconnect the vac pipe it may pink so the timming will need retarding...

[darkcat]

my advance unit was duff when i tested it last summer, fitted a new one and definately noticed better fuel economy and a bit more grunt high in the rev range.

[blitzracing]

rigga said:

No the one way valve was to stop fuel vapour being drawn back through from the plenum to the vac unit and destroying the diaphram ...

There must be some sort of air inlet somewhere to allow the diaphram to return to its resting position when the vac' drops then?

[taylormj4]

I thought vacuum advance was to help with acceleration, i.e. when the vacuum is transfered to the dizzy, rather than helping to improve performance ? Doesn't it oppose the centrifugal advance system ?
Matt

[Simon says]

blitzracing said:

rigga said:

No the one way valve was to stop fuel vapour being drawn back through from the plenum to the vac unit and destroying the diaphram ...

There must be some sort of air inlet somewhere to allow the diaphram to return to its resting position when the vac' drops then?

some of those valve looking units are nothing more than sandwiched fabric to stop or absorb the vapours getting to the vacuum canister diaphragm unless they are one way valves but then its as Mark/Blitz just stated and can't be Taylormj4 don't performance and acceleration merge?

[spend]

taylormj4 said:

I thought vacuum advance was to help with acceleration, i.e. when the vacuum is transfered to the dizzy, rather than helping to improve performance ? Doesn't it oppose the centrifugal advance system ?
Matt

When accelerating throttle is normally open, engine vacuum rises when sucking against a closed throttle.

Positioning (vs butterfly) and orifice size are quite critical

[Milky400]

spend said:

When accelerating throttle is normally open, engine vacuum rises when sucking against a closed throttle.

Positioning (vs butterfly) and orifice size are quite critical

Am i the only one slightly turned on by that paragraph

[SILICONEKID340HP]

You can get more power by diconnecting the vac pipe by advancing the timming at 3000rpm ,this gives you more advance than the standard setup .The problem is low end drivability and slightly lower MPG.This is why people go for the Malory dizzy or after market ECU.

[M@H]

Milky400 said:

Am i the only one slightly turned on by that paragraph

Yes

[taylormj4]

spend said:

When accelerating throttle is normally open, engine vacuum rises when sucking against a closed throttle.

Positioning (vs butterfly) and orifice size are quite critical

When accelerating throttle is 'always' open surely,lol. Engine vacuum is high against a closed throttle as you said but I thought the vac advance was connected to the AFM side of the butterfly, i.e. when you open the throttle you reduce the air pressure at the vac hose. Is the vac hose attached to the engine side of the butterfly ?

[Simon says]

Is the vac hose attached to the engine side of the butterfly ?
[/quote] Yes and small throttle openings will still create enough vacuum for the capsule to play