HEI Limiting Mechanical Advance to 10? - Chevelle Tech
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post #1 of 99 (permalink) Old Aug 21st, 19, 9:25 AM Thread Starter
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Jeff
 
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HEI Limiting Mechanical Advance to 10?

Is there an easy way of limiting the machanical advance in the HEI distributor? I'd like it to max out at 10. I will be running 22 to 26 initial..

I am running 20 initial now with the stock timing curve, no issues, starts smoothly with slight pinging at full throttle so I won't run it up there until I limit the mechanical timing..

This is on a stock, low compression, 1978 454, 781 heads, Quadrajet, 160 thermostat, aluminum radiator, no AC or heater, turbo 400, 2.87 gears.
100% street driven, I like to use 87 octane fuel but I use 89 when its on sale.. It does slightly run better on 89 which is weird but there in no pinging.

Thank you..
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post #2 of 99 (permalink) Old Aug 21st, 19, 9:58 AM
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Re: HEI Limiting Mechanical Advance to 10?

Are you running vacuum advance ? You really should not have to limit the distributor with a basically stock engine.

Bill Koustenis
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Waldorf Md


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post #3 of 99 (permalink) Old Aug 21st, 19, 10:01 AM Thread Starter
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Re: HEI Limiting Mechanical Advance to 10?

No vacuum advance.

Like most low compression engines, this one likes a lot of initial. Maybe its not stock. The guy I bought it from didn't have too much info on it.. The roads in Western Pa are very demanding on a engine, it seems like you are always going up a hill around here. That's why I like having a lot of initial. It sounds better without the vacuum advance to me too..
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post #4 of 99 (permalink) Old Aug 21st, 19, 10:12 AM
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Re: HEI Limiting Mechanical Advance to 10?

Quote:
Originally Posted by hydro462 View Post
No vacuum advance.

That is the problem.


Quote:
Originally Posted by hydro462 View Post

This engine likes a lot of initial for some reason.



With the vacuum advance you would have a lot more initial and it would run even better but when you put a load on it at WOT the vacuum would back it off and you would not get the pinging.



With the vacuum advance you could set the initial at 10-12 then the vacuum would add another 15 or so for a total of 25 plus whatever mechanical is in the distributor. . But under load the vacuum would go away and you would be back to the initial 10 plus the mechanical.

Find a vacuum advance distributor and you will be a lot happier. Or hook it up if it is already there.
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post #5 of 99 (permalink) Old Aug 21st, 19, 3:49 PM
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Re: HEI Limiting Mechanical Advance to 10?

But if you put a little extra load on it with part throttle and vacuum stays above 10" during this time and you have vacuum advance you may have too much advance and it will ping.
My high compression small blocks with 200+ cranking pressure and a 396" bb with 210+ cranking pressure worked way better without vacuum advance.

Driving styles vary so it may work for some and not for others.
If you get the vacuum to drop out at lower RPM that tiny amount of advance will have you down on power and you just mash the throttle farther to make up for the lack of power MPG then suffers.

I just use the stock weights and center pivot and usually only have another 10-14 total depending on which weights.
Or it could be dependent on the slot in the distributor where the pins stop against.

I have welded up the slot and fine tuned it by filing it longer if needed with a round file I use to sharpen my chainsaw chain.

In the end the welded stop distributor worked best for me.

That required taking it out and apart many times. No biggie as I was driving 70,000+ miles a year and the wife was doing the same in another car so a lot of experimenting was happening with and without vacuum advance.

Winner was No vacuum advance when it came to MPG believe it or not just my results and that is a lot of miles of driving and 6 different vehicles.

But many people benefit from vacuum advance.
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post #6 of 99 (permalink) Old Aug 21st, 19, 5:01 PM Thread Starter
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Re: HEI Limiting Mechanical Advance to 10?

Quote:
Originally Posted by jeff swisher View Post
But if you put a little extra load on it with part throttle and vacuum stays above 10" during this time and you have vacuum advance you may have too much advance and it will ping.
My high compression small blocks with 200+ cranking pressure and a 396" bb with 210+ cranking pressure worked way better without vacuum advance.

Driving styles vary so it may work for some and not for others.
If you get the vacuum to drop out at lower RPM that tiny amount of advance will have you down on power and you just mash the throttle farther to make up for the lack of power MPG then suffers.

I just use the stock weights and center pivot and usually only have another 10-14 total depending on which weights.
Or it could be dependent on the slot in the distributor where the pins stop against.

I have welded up the slot and fine tuned it by filing it longer if needed with a round file I use to sharpen my chainsaw chain.

In the end the welded stop distributor worked best for me.

That required taking it out and apart many times. No biggie as I was driving 70,000+ miles a year and the wife was doing the same in another car so a lot of experimenting was happening with and without vacuum advance.

Winner was No vacuum advance when it came to MPG believe it or not just my results and that is a lot of miles of driving and 6 different vehicles.

But many people benefit from vacuum advance.
In my area with my driving style I never liked the vacuum advance. I like a solid throttle feel and not having to chase the sweet spot. As far as idling in heavy city traffic goes, hands down no vacuum advance is best! I notice no difference is gas mileage.

I played around with it today, I tried one slightly softer spring with a 16, 18 then 20 initial, 20 was best but 40 total timing is too much at full throttle, I don't even need to check it.
My goal with this low compression 454 is 24 initial with the mechanical bringing in 12 with the stock, slow heavy springs.. I have no issue with starting even with the stock starter! This thing must be real low in compression which makes me very happy because the performance is fantastic!! It's a 1978 with 781 heads..

I used to weld and file the single points distributors the same way. I actually prefer the single points distributor as a trigger for either the MSD6AL or the old Mark Ten CD unit. I have a Mark Ten and I'm looking for a nice iron single points distributor for it. I'll use and a Mallory Volt Master coil. This is more for nostalgia then anything but its still an excellent ignition...

Last edited by hydro462; Aug 21st, 19 at 5:19 PM.
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post #7 of 99 (permalink) Old Aug 23rd, 19, 12:23 AM
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Re: HEI Limiting Mechanical Advance to 10?


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post #8 of 99 (permalink) Old Aug 31st, 19, 10:48 AM Thread Starter
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Re: HEI Limiting Mechanical Advance to 10?

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Originally Posted by LeoP View Post
This is a guy who pays money to be on a forum that's free...
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post #9 of 99 (permalink) Old Aug 31st, 19, 12:32 PM
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Re: HEI Limiting Mechanical Advance to 10?

ALL GM high-performance cars came with vacuum advance.

If you are running a huge amount of initial advance, you are compensating for a poorly designed setup.

Hard starting and high temperatures at idle will result by not using vacuum.

http://www.camaros.org/pdf/timing101.pdf
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post #10 of 99 (permalink) Old Aug 31st, 19, 11:07 PM
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Re: HEI Limiting Mechanical Advance to 10?

Quote:
Originally Posted by nashville beth View Post
ALL GM high-performance cars came with vacuum advance.

If you are running a huge amount of initial advance, you are compensating for a poorly designed setup.

Hard starting and high temperatures at idle will result by not using vacuum.

http://www.camaros.org/pdf/timing101.pdf

Well said Beth, I used much different language and it got deleted!!

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post #11 of 99 (permalink) Old Aug 31st, 19, 11:08 PM
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Re: HEI Limiting Mechanical Advance to 10?

Quote:
Originally Posted by hydro462 View Post
This is a guy who pays money to be on a forum that's free...



This is a guy that comes to a forum for advice and says he isn't gonna change anything, Brilliant!!
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post #12 of 99 (permalink) Old Sep 3rd, 19, 4:46 AM
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Re: HEI Limiting Mechanical Advance to 10?

That's enough guys.

Don't make me come over there. Especially, don't make me use my mod/admin voice. It's the kiss of death!!!!
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post #13 of 99 (permalink) Old Sep 3rd, 19, 7:26 AM
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Re: HEI Limiting Mechanical Advance to 10?

Yeah, I dont get it, but.... you can limit it by plugging the slot with a screw or something. This assuming a bushing wont do it.

I've been advised not to run vac adv by my engine builder, as it may put the engine at risk. The 427 did like it much either, at 205 cranking psi. I've already limited the Crane can to maybe 4-5 degrees is all. We'll see, I hope Jeff is right and it's fine. Never done it before, , run w/o vac adv that is.
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post #14 of 99 (permalink) Old Sep 4th, 19, 3:57 PM
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Re: HEI Limiting Mechanical Advance to 10?

Quote:
Originally Posted by nashville beth View Post
ALL GM high-performance cars came with vacuum advance.

If you are running a huge amount of initial advance, you are compensating for a poorly designed setup.

Hard starting and high temperatures at idle will result by not using vacuum.

http://www.camaros.org/pdf/timing101.pdf
You are correct.

The flame speed/combustion burn rate varies with cylinder pressure. Every difference in throttle position and manifold vacuum changes the pressure in the combustion chamber. Without the vacuum advance to modify the spark timing point to try to track these changes in cylinder pressure, an engine will run at incorrect timing in 99 (or even 100) percent of the speed/load situations it encounters.
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post #15 of 99 (permalink) Old Sep 4th, 19, 4:40 PM
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Re: HEI Limiting Mechanical Advance to 10?

I'm not going to judge one way or another, but I was always taught that in all but the most radical street engines, if it runs better without vacuum advance, you were just crutching some other issue. Take it for what you paid for it.
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