Vacuum advance for street BBC

[Bishop540]

Good grief. I thought I would call a few well known companies today to get advice on converting to vacuum advance for my street 540 BBC. One wanted me to throw my new stuff away and buy his. The other talked way too much (instead of being short with his time like most good performance companies) and he wanted me to back my timing to 34 (his head-dyno is greater than my own dyno results, i guess). So I decided to ask the people who have the experience in street cruising.

My 540 had its best dyno power with 40 degrees of timing. Don't ask me why because i don't know. And yes my timing pointer and balancer are correct. It is actually locked at 40 right now and has destroyed several auto parts starters -until i got a good aftermarket one. I was wondering if adding vacuum advance would help cruising MPG, say 8-10 more degrees. I know its a hotrod and mpg's shouldn't matter but if its there and it cruises better, than I'm all for it.

I don't mind backing off my timing to 25 or so and putting a quick advance back in it. It definitely idles and cruises better at 40 degrees but it may idle and cruise good with manifold vacuum advance too. I have a Mallory unilite with a Hyfire 6AL box. It cruises the freeway great at 2500-ish rpm, besides a mild transition stumble from my 1050 when accelerating mid throttle. But me and AED are trying to tweak that.

Is it worth changing my setup for vacuum advance?

[gnicholson]

what are the engine specs specificly the compression and cam? what trans and final drive ratio?

[RevTheory]

Is your manifold vacuum stable when idling at 40*? How much vacuum does it pull? Another question is will it idle in gear at say 28*?

[Bishop540]

what are the engine specs specificly the compression and cam? what trans and final drive ratio?

10:1, 200-4R transmission (.67 OD),
540 BBC
Lunati 40110515

Is your manifold vacuum stable when idling at 40*? How much vacuum does it pull? Another question is will it idle in gear at say 28*?

Vacuum is fairly steady in gear with the A/C on. Not much. 7-9" if I recall. I have hydroboost for brakes. At 28 I may have had to increase the idle screw. I turned the idle down when I went to 40, because I could at that setting.

[In-Tech]

Hiya Bishop,
As the engine gets larger, the combustion space for a desired compression ratio gets larger as well and of course BBC open chambers are notoriously lazy and require timing to burn that big area efficiently. I am not surprised to hear of it liking 40 total. Kicking back against the starter can break the block along with other things, make sure you have that starter braced. It's always a good idea to get a start retard setup or use a "hop out" curve in the distributor to aid in starting. I would not be afraid to try at least 10 degrees of vacuum advance, I bet it would like 14.
Good luck.

[Bishop540]

Hiya Bishop,
As the engine gets larger, the combustion space for a desired compression ratio gets larger as well and of course BBC open chambers are notoriously lazy and require timing to burn that big area efficiently. I am not surprised to hear of it liking 40 total. Kicking back against the starter can break the block along with other things, make sure you have that starter braced. It's always a good idea to get a start retard setup or use a "hop out" curve in the distributor to aid in starting. I would not be afraid to try at least 10 degrees of vacuum advance, I bet it would like 14.
Good luck.

Thank you Carl. My thoughts exactly. The guy I talked to today was surprised I hadn't blown my engine at 40. He must be a Ford guy.

[GARY C]

My 540 had its best dyno power with 40 degrees of timing.

What about best tq timing?

If you have any pulls with lower timing see if you can pin point the best tq and that will give you an idea of what you need for a timing curve so you can maximize both tq and hp.

I like vac advance for street but I run it with full manifold vac so it's really a vac retard not advance.

On my mallory distributor It has a tab to limit how much it travels so I could do say 30 and let it advance mech to 40.

[prairiehotrodder]

I run my 540 at about 38 or 39 total locked. It never kicks back. I have an old used stock 96-98 chevy 350 vortec starter on it for the last 5 years.

I used to have a MSD start retard on a 383 SBC with 11. CR. It broke starters, growled, hard to crank, hard to start. Got rid of the MSD start retard and left the timing locked and it worked way better. I wouldn't put one of those things on my enemies car.

As far as the vacuum advance, i don't use it but if you can make it work for you, good.
Brian

[Bishop540]

If you have any pulls with lower timing see if you can pin point the best tq and that will give you an idea of what you need for a timing curve so you can maximize both tq and hp.

What about best tq timing?

Good Point. I'm looking at my dyno pulls and at 40 its the best torque at 4500rpms, HP at 6100rpms.
At 38 that same torque was at 5000rpm, HP was at 6200rpms (but 2 less HP).
At 36 6 less torque at 4900rpm, HP was at 6300 (but 8 less)

I wish I would have had pulls under 4500rpm. I requested it but for some reason he never did it. He said it had a bad stumble anyway (I got it fixed once the engine was in the car). That would have been an excellent way to see what torque and HP was best at all RPM, granted the timing was locked in each test (I suppose). But I do regret he didnt run some tests down lower in the rpm. It was my 1st time to dyno an engine, so you could say I was a rookie.

[Bishop540]

I like vac advance for street but I run it with full manifold vac so it's really a vac retard not advance.

Thats the reason I am thinking it would be safe to run on manifold vacuum for idle and cruising (part throttle on the highway), since, when you punch it the advance doesnt work until the engine stabilizes (not under a load). Correct?

[Bishop540]

I run my 540 at about 38 or 39 total locked. It never kicks back. I have an old used stock 96-98 chevy 350 vortec starter on it for the last 5 years.

Right?!? I had the same starter that worked fine for a while. Until the bodyshop had it and started it twice daily for 3 months (moving it in and out of the shop). By the time my car was ready I had to tow it home, and a new vortec warranty starter didnt work either. The last one made a loud pop and a tiny puff of smoke . The autoparts finally wise-up and gave me my money back and I got an good aftermarket starter. They fought it for a while since the starter was already 4 yrs old -warranties disappear while a poor guy slowly builds his car.

[GARY C]

I like vac advance for street but I run it with full manifold vac so it's really a vac retard not advance.

Thats the reason I am thinking it would be safe to run on manifold vacuum for idle and cruising (part throttle on the highway), since, when you punch it the advance doesnt work until the engine stabilizes (not under a load). Correct?

Thats what I do with HEI and full vac with an adjustable can, on my SBC it was around 20 at idle 40 at part throttle and then dropped to 36 at wide open throttle.

You would just have to play with what works best for yours, I usually try to set initial timing that provides the highest vac reading and no more, you know peak is 40 from the dyno, you can usually run more than peak at part throttle just listen closely for knock at part throttle and adjust as needed.

Actually you my not need a vac can if you can get soft enough springs for the mech advance.

[Bishop540]

I like vac advance for street but I run it with full manifold vac so it's really a vac retard not advance.

Thats the reason I am thinking it would be safe to run on manifold vacuum for idle and cruising (part throttle on the highway), since, when you punch it the advance doesnt work until the engine stabilizes (not under a load). Correct?

Thats what I do with HEI and full vac with an adjustable can, on my SBC it was around 20 at idle 40 at part throttle and then dropped to 36 at wide open throttle.

You would just have to play with what works best for yours, I usually try to set initial timing that provides the highest vac reading and no more, you know peak is 40 from the dyno, you can usually run more than peak at part throttle just listen closely for knock at part throttle and adjust as needed.

Great. Thank you for your experienced advise!

[In-Tech]

Hi Eric,
Yes, usually peak torque likes less timing and a gentle ramp up from there after peak torque. Some of that can happen due to the "box" and/or the magnetic trail if using a mag trigger caused by "lag" in the trail or the box not compensating for it. Not well known that some "boxes" compensate for it and some don't, whether crank trigger or not.

Your idea for better gas mileage and efficiency is not unfounded and should be addressed with fun in mind that it will take a bit of trial and error.

Again, good luck.

[Geoff2]

Use a Crane adj vac adv unit. Connect it to Manifold Vacuum [ MVA ]. PVA is useless & a waste of a vac unit.

To dial in the VA. Leave disconnected for now. Engine idling, in gear if auto, initial timing at about 12* [ not critical ] loosen dist & turn CCW slowly to advance timing. Keep going until you have the highest idle rpm [ vac will also be highest ].Toggle dist to be sure of highest rpm. Now check what the timing is. Do not be surprised if it is as high as 50*, which it can be with some combos!!!

Say yours is 45*. That is what it needs for best idle. So idle timing can be any combination of Init + MVA. The Crane unit has a max of about 30*. So for easy starting, you could use, say, 18* init + 27* from the Crane.

Allen Key [ AK ] adj. Turn fully CW before installing. This is the softest setting, ~5-6" of vacuum; check that timing is steady with VA hooked up. Turn the AK 2 turns CCW & re check timing. Keep doing this until timing drops or becomes unsteady. Then go back 2 turns CW. Done!

More on MVA benefits by GM enginer Lars Grimsrud. Scroll down to post #6.

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