Vacuum advance!
Discussion
I have also asked this question in a topic associated thread on TVRCC but I think it has gone without notice, so I will ask here.
Topic: Slightly rough running in High St.
I had undrstood when I bought the 500 that it was going to be a bit lumpy at low speeds and I get exactly what I was told and I can manage OK with a bit of 'clutch feathering'.
I have heard/read on here that some owners have disconnected the vacuum advance pipe from the plenum with an apparent improvement in the smoothness.
My question is, what is actually happening to the timing through the rev band with the vacuum disconnected and are there any detrimental effects in the longer term?
Cheers, Paul
Topic: Slightly rough running in High St.
I had undrstood when I bought the 500 that it was going to be a bit lumpy at low speeds and I get exactly what I was told and I can manage OK with a bit of 'clutch feathering'.
I have heard/read on here that some owners have disconnected the vacuum advance pipe from the plenum with an apparent improvement in the smoothness.
My question is, what is actually happening to the timing through the rev band with the vacuum disconnected and are there any detrimental effects in the longer term?
Cheers, Paul
HarryW said:I think that the advance curve on the particular system that we have on these cars,isnt designed for our engine in the TVR state of tune,thus it isn't ideal and is often considered best left out.
I've always understood that VAC advance was there to help with light throttle openings and hence manners , I know some engine builders remove it as a matter of course too. .
llanelli-rob said:
I cured the very same problem on my precat 4.3 by retarding the timing from 12 to about 8deg BTDC now runs very very smooth at town speeds,it was running like a bitch and was very uncomfortable!
A dizzy will always be a compromise between low speed manners and absolute power. One of the beauties of mapped ignition is you can have your cake and eat it
.Dutchone said:
My question is, what is actually happening to the timing through the rev band with the vacuum disconnected and are there any detrimental effects in the longer term?
Cheers, Paul
AFAIK the vac advance operates to advance the timing at part throttle - whatever the revs. ie the amount of VACUUM advance is dependent on the vacuum, not the engine speed. This gives better fuel economy at cruise. There is a delay unit (one way valve) in the vac pipe which delays the vac from reaching the vac unit on the distributor. This means that transient vacuum eg when opening/closing the throttle doesn't immediately alter the timing.
So it ought only affect part throttle cruise fuel economy.
However it will also presumably affect tickover timing (although the timing should be set with the vac disconnected) and possibly therefore tickover smoothness.
I also harbour an unpopular theory that disconnecting it means insufficient advance at cruise could lead to gradual fouling of the plugs, which I'd like to think might explain my occasional misfire.
The vac advance is indeed there to improve fuel economy in cruise conditions only as mentioned earlier, however as mentioned by mongoose they are just not suitible to the majority of tivs, any big valve/hc's and all the 500's or any of them with long duration cams or ported cylinder heads will run much better with the vac disconnected,
The main reason being when ever you slow the airspeed down into an engine which you always will do with ported cylinder heads and long duration cams, you always get fuel drop out/poor cylinder filling/poor swirl/poor combustion mix/ lower compresion ratio and so on... basically you end up with a poor engine!!! SO by disconecting the vac what you are actually doing is!!!! lets start by saying that at a typical cruise around 2500 rpm 15% throttle opening your engine is making 150 ftib of torque, if you disconect the vac but apply the same throttle your engine will only be running at 2000 rpm and make around 130 ftlb of torque, but if you want to get back to your 2500 rpm you will need more throttle opening around 20% now to acheive the 2500 rpm cruise, so now the throttle is open more you have got much more airspeed into the engine and your engine is now making 180ftlb of torque, because of better cylinder filling /you have a higher compresion ratio/better swirl etc... agreed you will suffer a little on fuel economy but it will drive a heck of a lot nicer GUARANTEED!! Lets face it how many race cars do you see with vac advance units?? and a tiv is generaly what i would call a mild race engine! sorry if i havent explained this well but i am crap at explaining things!!
The main reason being when ever you slow the airspeed down into an engine which you always will do with ported cylinder heads and long duration cams, you always get fuel drop out/poor cylinder filling/poor swirl/poor combustion mix/ lower compresion ratio and so on... basically you end up with a poor engine!!! SO by disconecting the vac what you are actually doing is!!!! lets start by saying that at a typical cruise around 2500 rpm 15% throttle opening your engine is making 150 ftib of torque, if you disconect the vac but apply the same throttle your engine will only be running at 2000 rpm and make around 130 ftlb of torque, but if you want to get back to your 2500 rpm you will need more throttle opening around 20% now to acheive the 2500 rpm cruise, so now the throttle is open more you have got much more airspeed into the engine and your engine is now making 180ftlb of torque, because of better cylinder filling /you have a higher compresion ratio/better swirl etc... agreed you will suffer a little on fuel economy but it will drive a heck of a lot nicer GUARANTEED!! Lets face it how many race cars do you see with vac advance units?? and a tiv is generaly what i would call a mild race engine! sorry if i havent explained this well but i am crap at explaining things!!
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