View Full Version : Vacuum Advance
11-17-2003, 01:53 PM
I have a #1479 Pro-Billet distributor for my '77 F-100 2wd with a 302. I love this thing, but I get some hard part-throttle pinging even with premium (93 octane) fuel. I can't compensate for it any more by setting back my initial timing, because my fuel milage is hurting! I'm down to around 11 MPG, which really hurts considering I used to make 16-17 regularly. My engine generates 21-22" of vacuum at a lazy idle, so this is a very high vacuum engine that pushes a 4,000 lb. truck with 2.75 gears and a C4 auto trans. Please help me tame my vacuum advance! I would cap it, but then my milage would be seriously miserable!
Oh yeah, I'm running 2 silver heavy springs, and the biggest stop bushings I could fit on there with those springs (red, I think).
MSD fan for life
FRPP 5.0L-Edelbrock/World/Crane, featuring a 6A w/Blaster 2 and a Pro-Billet 1479
11-18-2003, 11:32 AM
Does anybody know how to adjust an MSD vacuum secondary cannister?
11-18-2003, 01:51 PM
Unfortunately there is not a way to adjust the vacuum advance. You can add a canister from Crane, Accell or Moroso to that distributor that will make it adjustable via an allen wrench. You will need to get one for a chevy points distributor. As far as the mechanical advance, have you tried any of the more aggressive curves? That curve is really the slowest curve available and if at all possible get more aggressive with it.
11-18-2003, 02:20 PM
Why would I try to go for a more aggresive mech. advance when I'm already pinging so hard that my spark plugs burn up every 3,000 miles? I don't know that I understand your reasoning there. I guess I'm going to have to cap the vacuum advance for now and bump up my base timing and see if I can recover some fuel economy.
BTW I'm running Autolite spark plugs 1 step cooler with a 0.035" gap.
11-18-2003, 03:13 PM
you should always adjust and tailor the mechanical curve first then add the vacuum. If you can get the total in at cruising speeds this will help over all efficiency. Then once the timing mechanically is sound add and adjust the vacuum so it helps just under cruising conditions. What do you mean you have to replace the plugs every 3000 miles? what is happening to them?
11-18-2003, 03:48 PM
I follow your idea about getting the mech advance strait first, then adding vacuum. I'll take that approach from now on.
What I mean by spark plugs burning up is that the ground strap turns white, and they get so hot that the motor doesn't shut off immediately after disengaging the ignition, it pops and bangs for a few seconds, then spits out some smoke and dies. I usually use NGK plugs that are 2 ranges cooler, but I had an availability problem at Napa that made me use the 1 step cooler Autolites because that's all they had in stock.
I genuinely appreciate your time and input. I will cap the vacuum advance, and play around with the mech advance springs until I get best efficiency, then I'll add in vacuum. I guess I should get this spark plug situation corrected first. Who would've thought that 9.5:1 compression and a factory truck hyd. roller cam would require a plug that much cooler?
11-18-2003, 04:08 PM
That is what we are here for, if you have any questions let us know. On the plugs, if that is happening go colder on the plug. You generally want as cold of plug as possible with out fouling. If you can get away with three steps go with it. the is no performance lost by going colder. If the plug is to hot it can actually cause pre-ignition problems and you are compensating for it with timing. I would try 3 steps maybe even 4 from where your at since you have already gone 2 and they where fine. The timing issue could be plug related also.
11-24-2003, 06:46 AM
Okay, I took off the vacuum advance, and changed to advance springs to the blue ones. No pinging, it definately solved the problem. I also got a set of Autolite #23 spark plugs. It's really funny that Autolite now offers a 2-year, unlimited milage warranty on their plugs. Of course that means free spark plugs for me, because I put about 25,000 miles a year on my hot rod in a hostile environment for spark plugs, so finding out that they'll replace them for free is fantastic news.
Anyways, pinging problem is gone, now on to the fuel milage issue. I'm only getting about 12 MPG now with no vacuum advance, and I clearly have less part-throttle power. I guess I'll keep tuning more aggressively with the advance springs and stop bushings until I run into pinging again to get some of my fuel milage back. Thanks for your help MSD
11-24-2003, 03:31 PM
Make sure you try a colder plug then what you have been running. Once you get everything dialed in, try hooking the vacuum up from there. If it does not take it, you can put an adjustable vacuum canister on the distributor so you can lower the total vacuum advance to suite the application.
11-26-2003, 11:01 AM
I have a 350 chevy with a pro billet dist and 6A ignition and I was in the same boat you are in now. The engine ran great with total advance at 36 degrees (18 deg bushing & 18 deg initial) but with the vacuum capped. To get better fuel mileage and quicker part throttle response I purchased the Crane adjustable vacuum advance. The great thing about that was it came with a vacuum limiting plate you can put on underneath the rear screw on the vacuum unit. I did have to modify the plate to fit inside the pro billet dist. but now the vacuum only advances about 10 degrees instead of 25 degrees and works much better than it did without a vacuum advance.
11-26-2003, 03:05 PM
That is correct, sometimes the 16 degrees in the distributor (vacuum) is to much for the application. The canisters that can be limiter work great in these applications.
11-06-2013, 12:08 AM
I don't understand why a distributor would come with an advance canister that can not be adjusted. Be that as it may, what exactly is the advance applied when the vacuum advance canister is hooked up? (0-?) My distributor is the 8477 on a 1977 Lincoln 460 ci engine.
11-11-2013, 09:11 AM
we never sold an adjustable cannister, but if your looking for one
try our friends at moroso ( part number 99601) .
ours , will do 10 degrees.
01-17-2014, 08:14 AM
Sorry for the delay. Thank you for answering my question and suggesting a part that may help!
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