Technical Vacuum vs. mechanical advance distributors

On the fence on which to get. For a SBC crate. Going to be driving in traffic, highway, etc. Mechanical is more aesthetically pleasing, but what are the pros and cons ? Thanks

 

The vac advance is an addition to the mechanical advance.
The mechanical advance works and doesn't care where the throttle is, what load is on the engine or anything but rpm. More for flat out performance and consistency.

The vac advance does all of that plus !!!
Allows the engine to run cooler by advancing the timing.
Allows Get better mileage By compensating in the tune.
Reads the load on the engine and adjusts timing.

Once you mash the throttle, the vac advance becomes inoperative and you have all of the mechanical advance attributes.

 

Thanks


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Anything driven on the street, you want vacuum advance. Take the time to learn how it works, because there's so much BS to wade through online. Very misunderstood.

 

Always had vacuum in the past with no issues, so why change ? Was just wondering what advantage mechanical would give you. Thanks


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All distributors have mechanical advance, the only ones that don't have been altered with some sort of lock. These are racing distributors.

 

Use manifold vacuum source too, not carb ported.

 

Vac-o-matic's only used vacuum.....

 

Or use ported vacuum, if it works better that way.

Vacuum advance is there mainly to increase gas mileage. If that's not a concern, don't bother with it.

 

You will appreciate vacuum advance sitting at the stop light/sign.

 

Thanks guys, I googled it before posting and there's a lot of talk about the engine running hotter and spark plugs failing sooner with mechanical. Any truth to that ?


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you can set up the advance curve in the distributor to work well with or without vacuum advance. If you set up the mechanical advance so it has a lot of initial advance, and not so much mechanical advance, then it will work pretty much like having vacuum advance, connected to manifold vacuum, at idle conditions.

I think many guys don't think too much about that, they start with a distributor that is set up with with a lot of mechanical advance, so it has to start out with very little initial timing, and they make up for it by adding vacuum advance at idle.

 

Thanks squirrel, food for thought


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For a vehicle that spends any real time being street driven, you're probably better off with a combination of both mechanical and vacuum advance. As already mentioned above, the mechanical advance responds strictly to engine RPM while the vacuum advance makes its adjustments based on engine load.

To get an idea of how and when the vacuum advance works, spend a couple of weeks driving a car with a vacuum gauge installed. Make sure the gauge is connected to a manifold vacuum source and not ported vacuum. Watch how the vacuum changes under different driving conditions.

In the end you may choose to connect the vacuum advance to either vacuum source.

 

Another train of thought would say to run vacuum advance because cruising down the highway the engine is in a lean mixture/ high vacuum mode and the extra advance helps burn the leaner mixture more efficiently. Tip into the throttle and the mixture is now richer but at the same time manifold vacuum and consequently advance drops off giving the best of both worlds.
If I set my initial at 20* and total at 34* for example I do get the benefit of the high initial but not the additional 16* that a vacuum canister could provide cruising down the road.
Mechanical only responds to RPM, but vacuum advance responds to the load on the engine and that will be it's greatest benefit.

 

Yup, the amount of advance that an engine "likes" under light throttle, mid rpm conditions, is surprising.

 

Exactly. The mechanical is designed to be "all in" by a certain RPM, but with highway gearing it won't be anywhere near that cruising at a steady speed. That's where vacuum advance comes in, it's based on engine load and adds in a lot of extra advance. Somewhere around 10° to 15°, this means there might be almost 50° of timing cruising down the highway on level ground lightly loaded. (The engine, not the driver.)

 

I have always used a vacuum gauge for setting the timing and I use the vacuum from the manifold.

For me this is a very good way of setting both timing and the carb.
Just my opinion.
Jimbo

 

Same with me, also works for valve trimming.
Thanks, I learned a lot from all these replies


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Not true.

Ford Load-O-Matic, when control by a SCV equipped carburetor, are all vacuum.

The have no mechanical advance function.

 

Depends! Straight Eight uses port.

Ben

 

Oh sure, you live in Arizona.

 

The original post's question was directly targeting sbc crate engine. The statement wasn't a "one size fits all"

 

He said all.

 

I should have said "my" statement wasn't a one size fits all. Very well other engine setups would fair better with different setups. Just trying to stay along the lines of the sbc in question.

 

This all started when I ordered a vacuum distributor from summit and they sent me a mechanical instead, so I was considering keeping it. Thanks again fellas


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Generally speaking, vacuum advance (in conjunction with mechanical advance) will add 4-6 MPG at cruising speed as well as reducing engine temps at idle/low engine speeds. One reason it usually isn't seen on race motors (besides the fact that it doesn't operate at WOT) is at high RPMs, the movable points plate can introduce spark scatter/timing errors.

 

You must be driving something I'm not, to be able to get 4-6 mpg out of vacuum advance

I did document getting one more mpg out of my blown 454 that I had in my 55, after adding vacuum. Over many tens of thousands of miles, that was probably the most cost effective modification I ever made

 

Maybe you know this already Dino, the trick is to setup the initial and mechanical distributor timing first, while the vac advance is disconn. & plugged. May take a few test flights and beer runs etc. Get that part dialed in to the gnats ass, then when it's running correctly re-connect and tune the vacuum can. I see a lot of folks start backing off on the initial timing when it's the vacuum advance pulling in too much lead on the cruise end of things. Wrong way to go about it. Or, they just disconnect it entirely. There is no downside to running vacuum advance if setup right.

 

It's all relative... LOL. I had a built FE 'back in the day' that went from 8 mpg to 12 after swapping out the dual-point distributor w/no vacuum for a single-point with it....