Advance and Retard

[lewton]

Have had an ACT twin plenum fitted to my precat,and since it has been fitted car has not been running as crisply as I would have expected.Bit hesitant and tends to bog down at around 2500 revs.

After poking around under the bonnet have noticed that there is no vacuum pipe from distributor to plenum and no union on plenum to fix one too.Vacuum unit on distributor seems to be blanked off.

Has anyone come across this before? And could this be my problem?

Any advice would be appreciated

[clive f]

I`m sure I`ve seen rob from v8d say on here that the vacuum pipe doesn`t need to be connected to the plenum, and I`ve tried it myself in the past and it didn`t make any difference to the running of the engine, so I would think your problem may lay elsewhere.

are you still running with the AFM, perhaps that is causing the problem if you now have a twin plenum?

perhaps you should contact ACT?

Edited by clive f on Tuesday 11th August 08:45

[rev-erend]

Clive is right .. after Rob mentioned the marginal benefits of blocking the vac pipe - I did that on mine.

It seemed to give a worth while benefit in that it accelerated more crisply probably to the small detriment of fuel consumption .. but that was not something I worried about.

If your car has not been remapped since that new twin inlet system was fitted - then that is your problem.

[Hoover.]

Advance = Griff .... Retard = Chimp

Edited by Hoover. on Wednesday 12th August 17:03

[rev-erend]

Hoover. said:

Advance = Griff .... Retard = Chimp

Edited by Hoover. on Wednesday 12th August 17:03

And for some weird reason the Chimp out sold the better looking car by 3 to 1.. weird.

[eliot]

If the vacuum advance serves no purpose why is an electronic system mapped for load and RPM then? Removing the vacuum pipe is akin to removing the load axis from a map.

If you look at this map - removing the vacuum signal would be the same as only having the top (100kpa) line:

i.e. just your centrifugal advance.

Edited by eliot on Wednesday 12th August 21:28

[GreenV8S]

rev-erend said:

Clive is right .. after Rob mentioned the marginal benefits of blocking the vac pipe - I did that on mine.

It seemed to give a worth while benefit in that it accelerated more crisply probably to the small detriment of fuel consumption .. but that was not something I worried about.

I don't believe it would produce any performance benefits. The vac advance has no effect under full load so disconnecting/disabling it would also have no effect.

By retarding the ignition (removing the effects of vacuum advance) the engine will be less efficient (higher fuel consumption) under light load conditions and run hotter, but the increased air flow at idle might help smooth out uneven running at idle if you have a wild cam.

Other than helping smooth out the idle (if that's a problem) I can't see any reason to do away with the vacuum advance and I can see plenty of reasons not to.

[Hoover.]

rev-erend said:

Hoover. said:

Advance = Griff .... Retard = Chimp

Edited by Hoover. on Wednesday 12th August 17:03

And for some weird reason the Chimp out sold the better looking car by 3 to 1.. weird.

arghh but look at my residuals

[Uncle Fester]

That’s because they were, well, cheap as Chimps.

The Griff as the real performance car in the range cost more.

Once TVR offered a 500 Chimp as a direct comparison it sold far less than the 500 Griff.

If you could afford the real thing most people accepted no substitute.

[v8 racing]

eliot said:

If the vacuum advance serves no purpose why is an electronic system mapped for load and RPM then? Removing the vacuum pipe is akin to removing the load axis from a map.

If you look at this map - removing the vacuum signal would be the same as only having the top (100kpa) line:

i.e. just your centrifugal advance.

Edited by eliot on Wednesday 12th August 21:28

Its not quite the same, the problem with the vacum advance is it comes into too early, on a mildly tuned or standard car this is fine and makes the car run very well, the problem is on large ported heads with long duration cams such as the 500's, from around 1200rpm the vacum along with the mechanical advance can pull as much as 45 degrees, this ends up with very very little throttle opening to acheive town driving hence the shunting, if you could set the vacum to come in around 1700 rpm that would be sweet

[rev-erend]

GreenV8S said:

rev-erend said:

Clive is right .. after Rob mentioned the marginal benefits of blocking the vac pipe - I did that on mine.

It seemed to give a worth while benefit in that it accelerated more crisply probably to the small detriment of fuel consumption .. but that was not something I worried about.

I don't believe it would produce any performance benefits. The vac advance has no effect under full load so disconnecting/disabling it would also have no effect.

By retarding the ignition (removing the effects of vacuum advance) the engine will be less efficient (higher fuel consumption) under light load conditions and run hotter, but the increased air flow at idle might help smooth out uneven running at idle if you have a wild cam.

Other than helping smooth out the idle (if that's a problem) I can't see any reason to do away with the vacuum advance and I can see plenty of reasons not to.

Pete - like you I was highly skeptical of it .. but as Rob had suggested it and it only takes 2 mins to do I thought I'd try it.

The acceleration after felt .. well very raw and quicker. Don't forget only the vac assistance was removed - the advance retard mechanish is still retained.

For info - mine was a Mech 404 cam .. so very wild.

Found the original post :

http://www.pistonheads.co.uk/gassing/topic.asp?h=0...

Edited by rev-erend on Thursday 13th August 09:23

[GreenV8S]

v8 racing said:

Its not quite the same, the problem with the vacum advance is it comes into too early, on a mildly tuned or standard car this is fine and makes the car run very well, the problem is on large ported heads with long duration cams such as the 500's, from around 1200rpm the vacum along with the mechanical advance can pull as much as 45 degrees, this ends up with very very little throttle opening to acheive town driving hence the shunting, if you could set the vacum to come in around 1700 rpm that would be sweet

Just to add to the confusion, engines with a long cam tend to produce less manifold depression and sometimes need a modified vac cannister in order to get it to apply enough vac advance at low rpm.

[eliot]

rev-erend said:

The acceleration after felt .. well very raw and quicker. Don't forget only the vac assistance was removed - the advance retard mechanish is still retained.

Flat out acceleration with or without vacuum advance shouldn't make any difference. This is because there is no or very little vacuum in the plenum at wide open throttle - therefore the vacuum advance doesn't move (i.e. the same as having it disconnected)