Vacuum Advance?

[lesliehedley]

Here's one for the more knowledgeable amongst us. I have a 400SE and it has a distributor with a vacuum advance pipe connected to the plenum chamber just beside the throttle butterfly. I know some 400SEs had vacuum advance and some didn't. I am assuming the advance weights in the distributor will be slightly different dependant on whether or not there is a vacuum advance mechanism. But on my car, I have all the vacuum advance equipment, but the plenum housing isn't drilled through, so there isn't any vacuum being drawn. If I have a distributor which is set up expecting vacuum and there isn't any, is this going to affect how the car picks up under acceleration etc.? Should I change the weights, or drill the hole through the plenum housing?

Thanks

Leslie

[greenv8s]

Here's one for the more knowledgeable amongst us. I have a 400SE and it has a distributor with a vacuum advance pipe connected to the plenum chamber just beside the throttle butterfly. I know some 400SEs had vacuum advance and some didn't. I am assuming the advance weights in the distributor will be slightly different dependant on whether or not there is a vacuum advance mechanism. But on my car, I have all the vacuum advance equipment, but the plenum housing isn't drilled through, so there isn't any vacuum being drawn. If I have a distributor which is set up expecting vacuum and there isn't any, is this going to affect how the car picks up under acceleration etc.? Should I change the weights, or drill the hole through the plenum housing?

Thanks

Leslie

The vac advance is just there to improve part-throttle economy. The car will work fine with it connected to fresh air, just with slightly worse economy on a cruise. The rest of the dizzy will be unchanged, whether it's fitted or not - the advance weights and so on will be the same. Make sure the vac advance unit is firmly attached to the dizzy, but personally if it's disconnected I wouldn't go out of my way to reconnect it.

Cheers,
Peter Humphries (and a green V8S)

[2 sheds]

And most work best without anyway. due to lower octane fuel these days.
Tim

[joospeed]

and just to add to the controversy I'm going to advise running with the vac advance .. it's the only way the ignition can sense engine load, you'll get much better part throttle economy and may improve part throttle response also. The hole needs to be just on the outside of the throttle butterfly when the throttle is closed so it's uncovered just as the throttle opens .. some wedges with larger throttle butterflies have to have the hole redrilled since to get the base idle correct especially with wild cams you need the butterfly more open than normal in the first place.
You don't know which to do now do you? .. the joys of asking on an open forum!!

[shpub]

With even more controversy... some of the Wedges had modified weights and some didn't. Very musc on a case by case basis as well. My 390SE engine did because the thing pinked without them. If everything is running fine might be better to leave well alone.

[350matt]

Plumb it up, you can always just cap off the manifold tapping if it doesn't work. If it works then Bonus

Matt

[2 sheds]

joospeed said: and just to add to the controversy I'm going to advise running with the vac advance .. it's the only way the ignition can sense engine load, you'll get much better part throttle economy and may improve part throttle response also. The hole needs to be just on the outside of the throttle butterfly when the throttle is closed so it's uncovered just as the throttle opens .. some wedges with larger throttle butterflies have to have the hole redrilled since to get the base idle correct especially with wild cams you need the butterfly more open than normal in the first place.
You don't know which to do now do you? .. the joys of asking on an open forum!!

I can't argue with this theory, its not really controversial just giving the full picture, but in my experience i would say the majority run better without vacuum advance, the answer is to try both.
Tim

[Brm Brm]

Top lesson learned in 10 years of Wedge ownership -"If it aint broke - dont fix it!" My SEAC has the vacuum advance disconnected and, importantly, has never had any problems with pinking with either LRP or Optimax at 8 degrees advance in any weather. Whilst part throttle economy may benefit from a plumbed in advance, does it really matter? - we are after all running large capacity V8s.

[2 sheds]

Brm Brm said: Top lesson learned in 10 years of Wedge ownership -"If it aint broke - dont fix it!" My SEAC has the vacuum advance disconnected and, importantly, has never had any problems with pinking with either LRP or Optimax at 8 degrees advance in any weather. Whilst part throttle economy may benefit from a plumbed in advance, does it really matter? - we are after all running large capacity V8s.

Brm Brm, it used to have it connected but was horrible and pinked its nuts off, and was disconnected by Rob at V8 Developments, and this transformed it, but yours is running close to 11.5:1 comp ratio. with a modified dizzy.
Tim

[danny hoffman]

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Can you explain Joolz as I may have this problem.

Cheers

Danny

[ANDYM]

I appologise for stating the obvious but is the hole blocked? On my 400 I couldnt tell until i put a light in the throttle opening it is only a tiny hole and may have picked up something.

Just a thought

Andy.

[lesliehedley]

Thanks for the replies. I've checked and the hole is definitely not blocked. It just hasn't been drilled through. I've had my wedge over 4 years now and it has always pinked, so I think I'll give it a go with the vacuum advance. If it's no better I can always block it off again. Thanks again to all those who replied.

Leslie

[2 sheds]

lesliehedley said: Thanks for the replies. I've checked and the hole is definitely not blocked. It just hasn't been drilled through. I've had my wedge over 4 years now and it has always pinked, so I think I'll give it a go with the vacuum advance. If it's no better I can always block it off again. Thanks again to all those who replied.

Leslie

Also check advance @ 4000 should be about 28 degrees, if still pinking the distributor weights may need modifying, some of the 390/400s had high comp ratio/ spec and need modification to prevent pinking, to avoid retarding the timing too much.
Tim

[wedg1e]

Hmmmm..... interesting. My 390 hasn't enough graduations on the timing scale for me to know what max advance is, but I'd be surprised if it was as much as 28 degrees! I run about 8 degrees BTDC at idle, no vac, which doesn't change much with the vac connected. I have one of those restrictors in the vac line as well, which Joolz advised me some time ago is to prevent the advance coming on during some part of the range.... 'opposite lock', I think he said...
My car doesn't pink as it is; I can provoke it by fiddling with the distributor setting, which also alters the whole engine response! Doh... I can never decide what settings to go for. This year, MUST do a rolling road session... I believe we have such things oop north these days: apparently it's a canvas belt drive that wraps around the tyres, with some big chunks of ocean liner that are progressively added to load the engine. Indication is by punched cards, Nixie tubes and/ or mercury columns....

Ian

[joospeed]

teehee ian

28 degrees is small fry for rover v8 advance .. 28 is about what you run with a mechanical dizzy advance curve, it gives fair top end power without having it pinking it's nuts off in the midrange - remember bob weights is very much a compromise, you need to be able to wind the advance up again at high revs to make proper top end power .. 33 degrees is typical, 35 on high capacity limited breathing engines to maintain power at very high revs. Had a 5l chim gain 16bhp just by mapping on an aftermarket ignition - plus gained better economy and part throttle too. With someone like mark adams mapping the fuelling and mapped ignition you'll be getting the most out of your engine everywhere.
Joolz
:bigbelieverinmappedignition:

[2 sheds]

Try the Omex mapped ignition, absolutely amazing. I think you can purchase them and get fitted through mark, i'll be adding them to my range soon.
Tim