Vacuum or mechanical advance distributors?

[CowtownCobra]

I have one of each but can't decide which to use. I do mostly cruising and touring but I also do some some open track events (2-3 a year) also.

Advantages/disadvantages?

Thanks

[mylesdw]

I think the choice is: with or without vacuum advance. You need the mechanical advance or the performance would be REALLY bad.

Vacuum advance makes no difference to WOT performance but it effects fuel economy at part throttle. When cruising around you will use less petrol with vacuum advance hooked up.

My car has no vacuum advance and I intend to change that one day. If you have the choice now, take the vacuum.

Hope this helps

[kountzecobra]

I would have to agree. Mileage and part throttle cruising will suffer with no vacuum advance.

[vettestr]

Guys have given great info as usual. I think tweaking the distrib. is one of the most overlooked items. It is almost a free performance modification and can be done at home with average tools. Computer controlled cars for the most part do not let you alter much but engines love timing. I run all the initial lead I can and limit max to 39 or 40 degrees on my engines.
Setting curve can be done the easiest with a dial type light but even masking tape works in a pinch. a spring and weight set cost about 10 bucks .

[Mr.Fixit]

Forget the vacuum advance, these cars don't want or need it.

Setup the distributor so you have about 20 degrees of mechanical advance, and try to get that all in by 3000 rpm.

[mylesdw]

Mr Fixit is right, no engine needs vacuum advance, that is why race cars run without it.

For road use however, where most of the time is spend at part throttle rather than WOT there a significant benefits to fuel economy and clean burning. If the car has to pass an emissions check where they check unburnt hydrocarbons with the engine running at no load, a bit of extra advance will help.

There are no disadvantages to running vacuum advance.

[Mr.Fixit]

Too much total timing during part throttle is a disadvantage.

[scottj]

If I wasn't running vacuum advance (i don't) I wouldn't run any advance at all.
I run a locked distributor (no advance at all) combined with a Stealth carb calibrated for the engine. This is the only way I can run as much cam as I run on the street and get good idle and low rpm drivability.
I have found no disadvantages.

[mr0077]

But a properly setup vacuum advance won't give TOO MUCH total timing, it just gives MORE total during light throttle to more completely burn the (leaner) mixture, giving more efficiency and better mileage (and lower emissions)...

[Mr.Fixit]

total = initial + mechanical + vacuum

They sell advjustable vac pods so you can limit the amount of advance the vacuum pod gives you, why you might wonder?

You have say 14 initial, plus the 20 mechanical by 2500, and you are cruising at part throttle so the vac unit adds another 10, now you have too much total timing and your motor is pinging at a light throttle cruise.


The slight improvement in gas milage at light throttle doesn't even begin to make up for the ugliness you impose on your motor with the vacuum pod and hose.

[mylesdw]

Too much advance at ANY throttle setting is a disadvantage. That is why these things are adjustable. Production cars are set up with a wide enough margin that a fixed vacuum advance canister is OK. Performance cars running higher compression may indeed experience detonation at part throttle from too much advance using a standard canister, so here an adjustable one is an advantage.

scottj: I am intrigued as to how your set-up works. Do you mean you statically time the distributor to around 35 degrees and have no mechanical advance? I can't imagine how you would start an engine as that is surely way too much advance for cranking.

[mr0077]

If it pings on light throttle, guess what, you don't have it set up right. The motor needs more advance at light cruise than at full throttle at a given RPM to burn the mix efficiently. Opening the throttle even slightly causes the vacuum to drop, and a properly set up vac advance will take out ALL the vac advance when the throttle is opened...when you get away from station wagon cams and ignition curves and into aftermarket stuff, you may want or need an adjustable vacuum pot, both for rate that timing is added or deleted and amount of timing added in, to tailor it to your particular motor. They come with instructions to set up the pot to your particular needs.
I guess we'll have to agree to disagree...I agree with Myles on this one.

[Cobra 29]

I am running a 351W FMS crate engine and I believe it does have a quicker part trottle response and mileage with the vacuum advance. Just like Mr. Fixit points out the issue is limiting the vaccum advance module and thus the total degrees of advance. The initial and mechanical are easy to limit.

I have 12 degrees inital + 25 in the MSD mechanical all in by 2500 RPM but my vacuum advance module will apply 12 more degrees. While I do not hear any pinging (how???) this is about 49 total degrees at part trottle above 2500 RPM. I was able to limit about half the vacuum module's throw crudely by making a sleeve that limited the arm travel to the rotating distributor plate. My design is not easily adjusted!

Who makes a vacuum advance module where the THROW is adjustable? The Crame Cams unit I purchased limits the spring tension thus required vaccum amount to pull the full travel. Will the "adjustable" Accel unit truly limit the throw or just the amount of vacuum required like the CC unit? MSD should make such a unit given the expense many of us have in their total systems.

[mylesdw]

Cobra 29:
49 degrees sounds a lot but theory suggests it is OK-

If we know that 12 degrees initial is Ok and if we assume a tickover of 600 rpm then that tells us that it is OK to light the mixture 3.33 milliseconds before tdc under no load conditions.

At 49 degrees advance and 2500 rpm we are firing 3.26 milliseconds btdc, in other words slightly retarded in real terms.

In a pure overrun condition (foot off, no load) this would be fine. How quickly the vacuum advance is removed as you apply load is the more important factor.

[scottj]

Originally posted by mylesdw


scottj: I am intrigued as to how your set-up works. Do you mean you statically time the distributor to around 35 degrees and have no mechanical advance? I can't imagine how you would start an engine as that is surely way too much advance for cranking.

mylesdw- Yes it is is timed @35 deg Btdc, determined by dyno testing, and no mechanical. I have a start retard box from MSD, p/n 8984, that retards the timing 20 deg while cranking but I've never had to install it. It cranks fine with a 153 tooth flywheel and a Tilton high torque starter and has never popped back or run on.

I have similar engines, with 14:1 compression and an 8" starter ring, that will blow teeth off if it so much as hick-ups while cranking. I have to use the MSD retard box with that combination.

This is a standard circle track ignition system set-up. In that application we don't take a chance with ignition timing because 2 degrees too much will destroy the engine. By locking the distributor you eliminate any chance of a problem.

On my low compression street/track engine it just runs much better than a mechanical advance distributor AND I always know where the timing is.

Lastly, I do it this way because it is the only way my engine builder wants their engines set up.

Scott

[mylesdw]

scottj: I can see how that would work on a race engine that exists mostly in the upper rev range but it amazes me that this set up works for the street without getting detonation under load in the lower rev range. Are you using special fuel? Lots of cam overlap?

[scottj]

No special fuel, just 93 octane pump premium.

Cam is a special grind solid roller 264/272 @ .050 & .680 lift with 106 LS.

It is really a very reliable, very mild, 9.5:1 pump gas small block that happens to make 625hp & 600 ft/#.

[rob frink]

..........

[mylesdw]

Rob: nice interesting reply.

In answer to your "dunno":
At 600 rpm and 20 degrees btdc you are firing 5.5 milliseconds btdc. When you lift off at 5000 rpm and get the all in advance of 45 btdc it only equates to firing 1.5 milliseconds btdc so in real terms you are still significantly retarded compared to tickover.

Set up correctly vacuum advance should make no difference to the power output at any point but should allow it to run leaner and cleaner at cruise. I agree with your point about not driving these things for fuel economy but if the set up is slightly rich under WOT, to be on the safe side, then it is nice to have some way of leaning out somewhat under light load.

[Mr.Fixit]

mylesdw,

how exactly does the distributor's vacuum advance have any effect whatsoever on the fuel mixture? as per:

"Set up correctly vacuum advance should make no difference to the power output at any point but should allow it to run leaner and cleaner at cruise"