Distrbutor? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

ClassicRedneck05 · 01-05-2007, 02:20 AM

Can anyone explain to me the purpose of the Vaccume Advance on a distb? and exactly what it does for the engine?

motopatzo · 01-05-2007, 02:43 AM

At part throttle, your engine is more efficient when the timing is slightly advanced. Your centrifugal weights are just starting to open and adding vacuum advance, makes the engine burn the fuel better thus increasing mpg's.

bryanw1968 · 01-05-2007, 11:48 AM

It is also a way to not have a bunch of innitial timing in the motor at idle. Like mentioned above, as soon as you start giving it gas the vacuum drops because the motor is under a load. Once that happens the vacuum advance module starts to give your distributor more timing to get all that weight moving. MOST vacuum advance distributors have an adjustable module to make it give more or less timing. They usually take an allen wrench to adjust it. Check out the pick below.

68speedalert · 01-05-2007, 02:23 PM

In addition to what they already said, the vacuum and centrifugal advance are to ADVANCE the timing when RPM's raise.

To get the best performance and fuel economy, you need to get the most timing advance WITHOUT causing spark knock (pinging).

With a SBC engine, total advance (base+centrifugal+vacuum) it's about 40 degrees BTDC (before top dead center) and 2500-3000 rpm.

Not sure about the RPM spec.

Too much base advance will make it impossible for the starter to start the engine.

*Billla* · 01-05-2007, 02:42 PM

I'll give yet another spin

- all good perspectives. The best fuel economy and power is gained with correct advance at a given RPM and load. Mechanical advance addresses the need for advance based on RPM, vacuum advance is responsible for advance based on load. Vacuum advance provides advance in addition to mechanical advance under part-throttle conditions; at cruise an SBC will accept up to 52 (not 40) degrees total advance (!).

Setting vacuum advance for a particular application is both a science and a bit of an art

Most street-driven applications work well with starting the advance at about 8" Hg and providing about 16 degrees of (crankshaft)advance assuming mechanical advance is limited to 36 degrees and is all-in by 3000 RPM. Initial advance should end up between 12-14 degrees or the mechanical advance should be bushed or otherwise restricted to get within that range.

*Billla* · 01-05-2007, 02:48 PM

Quote:

Originally Posted by bryanw1968

It is also a way to not have a bunch of innitial timing in the motor at idle.

This is taken care of to a certain extent by having the vacuum advance connected to ported vacuum vs. manifold vacuum on a street engine.

68speedalert · 01-05-2007, 03:19 PM

Quote:

Originally Posted by Billla

This is taken care of to a certain extent by having the vacuum advance connected to ported vacuum vs. manifold vacuum on a street engine.

Makes no difference for starting, which is why high base timing is an issue.

You definately have aced me on your specs. I was quoting from what I THOUGHT I remembered, don't get to play with mechanical type advances much anymore.

Good post. You get rep points for that one!!!!

bryanw1968 · 01-05-2007, 03:26 PM

I run a full mechanical advance on my SBC. I sometimes wish I had both mechanical and vacuum again.

The port that the vacuum advance hose is supposed to be hooked up to is the port that does not see vacuum until after part throttle is achieved. If you have it hooked up wrong you'll be adding timing in through the distributor at idle.

68speedalert · 01-05-2007, 03:32 PM

What's wrong with that? Many engines (all makes and models) had that from the factory?

bryanw1968 · 01-05-2007, 03:36 PM

Quote:

Originally Posted by 68speedalert

What's wrong with that? Many engines (all makes and models) had that from the factory?

I had an HEI with the mech/vac setup and like it. But, I found a used MSD Pro Billet dist, blaster coil and 6AL box for $200.00 and I couldn't pass it up. The race version of the MSD distributor is mechanical advance only. It's not that I don't like vacuum, it's just the way this worked out. Oh, and I get worse gas mileage

*Billla* · 01-05-2007, 03:41 PM

Quote:

Originally Posted by bryanw1968

I run a full mechanical advance on my SBC. I sometimes wish I had both mechanical and vacuum again.

The port that the vacuum advance hose is supposed to be hooked up to is the port that does not see vacuum until after part throttle is achieved. If you have it hooked up wrong you'll be adding timing in through the distributor at idle.

I wouldn't drive a street engine without a vacuum advance; done it and have the gas bills and fouled plugs to prove it

Hooking the vacuum advance to the manifold vacuum isn't a problem; it just occasionally leads to an uneven idle depending on the cam. Some high-perfomance engines from the factory had manifold vacuum to provide advance at idle to smooth it out.

*Billla* · 01-05-2007, 03:44 PM

Quote:

Originally Posted by bryanw1968

I had an HEI with the mech/vac setup and like it. But, I found a used MSD Pro Billet dist, blaster coil and 6AL box for $200.00 and I couldn't pass it up. The race version of the MSD distributor is mechanical advance only. It's not that I don't like vacuum, it's just the way this worked out. Oh, and I get worse gas mileage

Yeppers; sometimes the great deals just don't work out the way we expect. I thought MSD had some sort of external box that you could connect to the 6 series to retard spark based on vacuum - kind of the opposite of boost

I haven't looked, but I remember something there...

bryanw1968 · 01-05-2007, 04:04 PM

Quote:

Originally Posted by Billla

Yeppers; sometimes the great deals just don't work out the way we expect. I thought MSD had some sort of external box that you could connect to the 6 series to retard spark based on vacuum - kind of the opposite of boost

I haven't looked, but I remember something there...

I'm not sure if it has anything to do with vacuum but I think this is what you're talking about. I am thinking about getting one.

*Billla* · 01-05-2007, 04:05 PM

Quote:

Originally Posted by bryanw1968

I'm not sure if it has anything to do with vacuum but I think this is what you're talking about. I am thinking about getting one.

Might be time to just sell your bling distributor instead

bryanw1968 · 01-05-2007, 04:49 PM

Quote:

Originally Posted by Billla

Might be time to just sell your bling distributor instead

No way man! That bling is serious bidniss. And, I like having the coil and the module outside the distributor. The HEI setup has too many eggs in one basket

01-05-2007, 02:20 AM	#1
ClassicRedneck05 Senior Member Join Date: Dec 2005 Location: Humble, Texas Posts: 262	Distrbutor? Can anyone explain to me the purpose of the Vaccume Advance on a distb? and exactly what it does for the engine?

01-05-2007, 02:43 AM	#2
motopatzo Registered User Join Date: Oct 2006 Location: maple ridge, b.c., canada Posts: 190	Re: Distrbutor? At part throttle, your engine is more efficient when the timing is slightly advanced. Your centrifugal weights are just starting to open and adding vacuum advance, makes the engine burn the fuel better thus increasing mpg's.

01-05-2007, 11:48 AM	#3
bryanw1968 Senior Member Join Date: Jul 2006 Location: Leander, Texas Posts: 850	Re: Distrbutor? It is also a way to not have a bunch of innitial timing in the motor at idle. Like mentioned above, as soon as you start giving it gas the vacuum drops because the motor is under a load. Once that happens the vacuum advance module starts to give your distributor more timing to get all that weight moving. MOST vacuum advance distributors have an adjustable module to make it give more or less timing. They usually take an allen wrench to adjust it. Check out the pick below. Attached Images

01-05-2007, 02:23 PM	#4
68speedalert Semi-Senior Member Join Date: Feb 2003 Location: Washington, MO Posts: 390	Re: Distrbutor? In addition to what they already said, the vacuum and centrifugal advance are to ADVANCE the timing when RPM's raise. To get the best performance and fuel economy, you need to get the most timing advance WITHOUT causing spark knock (pinging). With a SBC engine, total advance (base+centrifugal+vacuum) it's about 40 degrees BTDC (before top dead center) and 2500-3000 rpm. Not sure about the RPM spec. Too much base advance will make it impossible for the starter to start the engine. __________________ '68 50th Anniversery LWB Custom Fleetside, '77 305 v-8, turbo 350, factory speed alert (still works), '71 drivetrain w/front discs. Some call it ugly yellow, others call it Ochre '83 Jeep CJ-7, <80,000 miles, original owner '04 Chevy Suburban, 4x4, 2500, 6.0 ltr, Predator Diablo programmer '95 GMC K1500 Extended cab

01-05-2007, 02:42 PM	#5
Billla Account Suspended Join Date: Sep 2005 Location: Monroe, WA Posts: 3,814	Re: Distrbutor? I'll give yet another spin - all good perspectives. The best fuel economy and power is gained with correct advance at a given RPM and load. Mechanical advance addresses the need for advance based on RPM, vacuum advance is responsible for advance based on load. Vacuum advance provides advance in addition to mechanical advance under part-throttle conditions; at cruise an SBC will accept up to 52 (not 40) degrees total advance (!). Setting vacuum advance for a particular application is both a science and a bit of an art Most street-driven applications work well with starting the advance at about 8" Hg and providing about 16 degrees of (crankshaft)advance assuming mechanical advance is limited to 36 degrees and is all-in by 3000 RPM. Initial advance should end up between 12-14 degrees or the mechanical advance should be bushed or otherwise restricted to get within that range. Last edited by Billla; 01-05-2007 at 02:45 PM.

01-05-2007, 03:26 PM	#8
bryanw1968 Senior Member Join Date: Jul 2006 Location: Leander, Texas Posts: 850	Re: Distrbutor? I run a full mechanical advance on my SBC. I sometimes wish I had both mechanical and vacuum again. The port that the vacuum advance hose is supposed to be hooked up to is the port that does not see vacuum until after part throttle is achieved. If you have it hooked up wrong you'll be adding timing in through the distributor at idle.

01-05-2007, 03:32 PM	#9
68speedalert Semi-Senior Member Join Date: Feb 2003 Location: Washington, MO Posts: 390	Re: Distrbutor? What's wrong with that? Many engines (all makes and models) had that from the factory? __________________ '68 50th Anniversery LWB Custom Fleetside, '77 305 v-8, turbo 350, factory speed alert (still works), '71 drivetrain w/front discs. Some call it ugly yellow, others call it Ochre '83 Jeep CJ-7, <80,000 miles, original owner '04 Chevy Suburban, 4x4, 2500, 6.0 ltr, Predator Diablo programmer '95 GMC K1500 Extended cab



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