Vacuum advance - is it needed?

Julian West · 02-11-06, 02:42 PM

Am looking for some opinions, can anyone help?

I'm running a SBF SVO 302 with a Holley 670 & MSD dizzy. My question is whether or not the vacuum advance will be needed?

Am I right in thinking the Vacuum advance is only effective during idle & low RPM?

Any advice/opinions thankfully received!

David York · 02-11-06, 03:11 PM

You can remove the vacume advance and convert to full mechanical advance. If yo do this you should have the distributor tuned and a curve put in it to match your aplication. It will run more consistently under power with Mechanical advance but your emissions and Mileage will suffer acordingly.

Jim c · 02-11-06, 05:33 PM

Julian
The vacuum advance works of idle & while driving on lower throttle openings.
On throttle openings where the vacuum drops the vacuum unit will give less or no advance.
They use the vacuum advance as it will give more timing at low load , small throttle openings.
Mechanical dizzys work on RPM only.

Mainly economy will be effected ( by how much who knows).
If you run 8 throttle bodys you would have to plug it anyway.(pulsing vacuum signal).

MSD has a great spark , the advantage from this will probly outway the vacuum unit.

Jim

JCoop · 02-12-06, 12:23 PM

>>If you run 8 throttle bodys you would have to plug it anyway.(pulsing vacuum signal).<<

Will you elaborate a bit? I just got a 4 X 48IDA Weber setup but currently have the MSD vacuum dizzy.

Thanks
Ray

Sorry for the hijack but still related... [img]/ubbthreads/images/graemlins/blush.gif[/img]

Paul Thompson 'Hooligan' · 02-12-06, 02:48 PM

Hi Ray

If the vacuum pipe were connected to a single runner, you end up with quite a noticeable pulsing effect of the vacuum and hence the timing. Often, especially on tbi, the vacuum points are connected to a common block which itself has a small volume to it, to smooth the pulses. This can then connect to distributor and or MAP sensor providing each with a smoother vacuum reference signal. The more radical the cam, the worse the pulsing will be, although for the efi setup, it can usually be filtered out at lower rpm levels where it is most apparent.

Julian West · 02-12-06, 03:25 PM

OK, thanks for the advice. Jim C, good point about the MSD spark - I hadn't thought of that. I hadn't thought about economy either, it's not high on my list of priorities!

Looks like I can live without the Vaccum advance after all then.

Regards to all,

Howard Jones · 02-12-06, 10:09 PM

I have a billet small cap MSD no vac advance with 6AL box. SVO 345HP 302, 9.5 to 1 CR, alum X heads, B cam, rpm performer eldenbrock, 750 holley doublepumper with mechanical secondaries.

I run the advance curve all in (35degrees) at 2600 rpms. static advance works out to about 12 degrees BTDC.

Car starts every time first crank with very little or no choke. Makes very good low end <4000rpms and pulls well up to 6K where I have it limited.

I use Calif. 91 octane unleaded.

I wanted to keep it simple to tune with mechanical everything. Works good and I can run the car about 230 miles at 80mph, 3.44 final, on a load of gas, approx 14 gals, without running it down to fumes. Not real good but I wasn't going for fuel economy when I built a GT40.

Peter D · 02-13-06, 02:45 AM

Ray, you might want to look at what the various intake manifolds offer in terms of a vacuum chamber. My 8-stack setup is on a DC&O manifold & it has quite a large chamber built in under the centre section. A bleed hole runs into it from each intake port & it has a couple of vacuum take-off ports on top at the back. I think that TWM have a similar setup under their manifold.

We originally had all vacuum devices running off one of these take-off ports, but found that the ECU was getting unreliable readings when the power brakes got a bit of a workout. This problem was solved by moving the power brake line to the second port on the manifold.

Kind Regards,

Peter D.

Julian West · 02-13-06, 08:36 AM

Howard,

Thanks for the info. Is your dizzy an MSD 8579 ? I'm currentley running an 8479, which I beleive is the same as the 8579, but is an inch taller as it has the vacuuum advance.

FWIW If I swap to the smaller dizzy, I loose the Vacuum advance, but gain an inch of chassis clearance.

Regards,

Howard Jones · 02-13-06, 01:11 PM

Julian, Yes mine is a 8579. Remember you must use a 6,7,8,... series ignition box with this distributor.

It does fit very nicely into limited room at the front of the engine.

JCoop · 02-13-06, 01:34 PM

Thanks, Peter.

That was going to be my next ?. I haven't closely examined the kit as I'm still reading the book and waiting for Oliver's geat tuning DVD. I may just use the takeoff for the brakes and T off for the little bit that the heater switch needs; then disable the dizzy vacuum and see if it will idle well like that.

Cheeers
Ray