So I've had a couple people now advise me that manifold vacuum advance is the way to go. I did a little test a while back and it didn't seem to work for my setup but I think I might make another run at it. I know most of you guys are original types so I figure you're running ported adv like me but they say it was mostly emissions related even back in the day and not the best setup. Anybody else running off manifold vac for adv?

Manifold vac is the way to go since it provides advance during idle and partial throttle cruising but dials it back under full throttle. This is the way god intended it for drivability. Racing may be a different animal entirely

If you use manifold vac, do you still set the initial timing to 9-12degress btc? How does it dial it back under full throttle? Doesn't is just stop advancing?

Yes, when you open the throttle the vacuum advance goes away and you only have mechanical advance. The timing you really care about is the timing at full throttle which has all the mechanical advance and essentially no vacuum advance. If you optimize that, the timing for idle and partial throttle cruising will be fine with the vacuum advance working

Ok so I got out in the garage today for a few minutes. Checked my vacuum. My gauge says 7-8 inches/pounds(whatever) at idle. Checked the vac can on the car and it doesn't start to move until 8-9. So manifold vacuum isn't going to change anything for me with the current setup till I get up off idle anyway.

Vacuum is usually measured in inches of mercury, from standard air pressure down to absolute vacuum. Kind of like a backward barometer. Standard air pressure of 14.7psi is usually measured as zero on a pressure gauge. Any pressure above 0 is that number plus 14.7. Both can be measured and stated in a multitude of units but these are usually the units we use when dealing with our car's pressure and vacuum.

Vacuum in the carb and intake manifold is created by the evacuation of the downward movement of the piston, with the intake valve open, and the throttle plates closed. Once the throttle plate/plates are opened the carb and intake valve ore open to atmosphere and the vacuum is no longer there. The vacuum advance is operated by vacuum on one side of the advance can diaphragm and atmospheric pressure on the opposite side causing the can's arm to advance the timing in the distributor. In an HEI it rotates the pick up coil, thus advancing the timing. With the vacuum advance can hooked up to the intake manifold, with a vacuum present, the timing advances but when the throttle is opened the vacuum signal goes to zero and the spring in the advance can returns the advance to zero.

As Bob S says, the advance you are concerned with at full throttle is the total, initial and mechanical. The vacuum advance is not considered in total timing. The mechanical advance is dependent on rpm and doesn't vary with load. The timing is advanced so burning the fuel air charge results in maximum cylinder pressure when the piston is in the most effective position. The faster the engine rotates the earlier the fuel charge has to be ignited in order to have max cylinder pressure in the correct piston location. Mechanical advance starts off idle and is full in at somewhere about 3000 rpm. Optimum total timing varies with each vehicle depending on the camshaft, static and dynamic compression ratio, rear gearing, combustion chamber design, etc. Most Pontiac engines run well with 12-16 initial and 18-24 mechanical but a simple cam swap could change what the car needs. Timing affects power, drivability and cooling, too much may cause engine damaging detonation, to little could cause low power and overheating. We want to set the timing so the car will start easily, idle smoothly and give good throttle response, once you set your total timing then tune your vacuum advance but do not adjust the total to vary the vacuum. Set total by driving the car and adjusting the timing up a degree of two at a time until you get a ping with a good pull in second gear, then back off two degrees. Usually best power is had just below detonation. If, once you have your max total set, you have too much initial to start the engine easily you will have to adjust your mechanical advance system to get the total with less initial.

The fuel air mixture is leaner at idle and cruise than it is pulling torque at wide open throttle, it may also be defused with exhaust. My engine idles at an air fuel ratio of 14.5:1 and has a richer mixture of 12:1 when pulling with the pedal to the floor. The leaner mixture at idle takes longer to burn than the richer mixture. To keep the fuel charge providing max cylinder pressure at the appropriate time vacuum advance is used to fire the charge earlier. As the intake manifold has vacuum at idle and stepped vacuum source does not, the manifold source is usually used, although not always. Some of the smog era cars had the idle timing retarded quite a bit in order to burn the charge as it went out the exhaust port to aid with burning contaminants along with the air injection reaction system. The problem with a late burning charge is it transfers heat to the coolant around the exhaust port. Adding 10 to 15 degrees of vacuum advance at idle and cruise will improve fuel economy and aid in engine cooling.

Every engine is different of course and what works for one wont necessarily work with the next. For mostly stock V8 an initial setting of about 12 degrees, a stock mechanical advance and the vacuum can connected to manifold should give satisfactory results. Some engines with a larger camshaft may benefit from less mechanical and more initial. My engine has the highest power and torque at 28 degrees total but runs smoother and cooler with 18 degrees initial and 5 degrees vacuum advance. In order to obtain those settings I had to modify the mechanical and vacuum advance systems. I got the info while tuning on a dyno,, before that I just listened for detonation and adjusted the timing accordingly then connected the vacuum advance to manifold. I don't think I can notice the difference while driving. I did notice the cooler idle though, it heated up when I had a lower idle advance.

Good write up Al. Wish you were close by to help me through my distributor setup.

So I got a little time in on it today. Got the Crane adjustable vac can installed and hooked up to manifold vacuum. Still running the same initial ( 12 ) and mechanicaI(24) I had before. Now at idle my advance is about 34 with the vac can hooked up and runs up to about 54 advance at about 2200 RPMs. Have not taken it for a spin yet so I can't report on any detonation issues. I'll report back later.

Me too you're already cruising and it'll a month until the weather is good enough that I get out.
Welding and testing the distributor plate gets one quite quick at removal and installation.
Did you tune to carb for max vacuum and did you get it so it stays in the idle circuit?

Yes and yes. It does seem to idle a bit smoother( I'm actually not all that thrilled about that) so I guess that's good. The vacuum's still pretty low. I was lead to believe that the vacuum signal would increase with the advance but it didn't happen. No difference from before so no worries there. When I get a bit more free time to spend on it I'll hook up the wideband and dial in the A/F ratio. I'll give it the summer and see if I like it better. Or if I even notice a difference. If i don't find anything detrimental by running this way I'll leave it be. One can hope there will be some kind of enhancement by running these parameters. That would make me happy.

Based on your cam. That vacuum sounds reasonable. Also one of the reasons I don't care with LSA at 110.

Advertised Duration (Int/Exh): 276/284
Duration @ .050 (Int/Exh): 233/241
Gross Valve Lift (Int/Exh): .504/.527
LSA/ICL: 110/106

I ran an Ultadyn H10 cam that was similar. Ran good when it was on my 455 with Torquer II intake a Holly 3310 and a 3 tub headers. I put that motor in my FB with LBM, stock intake and Q-jet than it was terrible. Had nozzle drip and stunk up everything around it. Burned my eyes just thinking about it.

My conclusion was the Qjets just don't work to well on the idle circuit with that low of vacuum (Cliff Ruggles agreed) and I didn't wan't to alter the Qjet to make it work better. I tired trick after trick eventually I changed to the Stump puller roller cam on a 114. The car now sounds stock and has loads of vacuum, but it faster then the old cam.

I watched your youtube and the car sounds awesome.

You shouldn't need much of any vacuum advance if you have the A/F setup well. 6-10 deg should be about it. If you get a skip sound at idle your have too much. That is really the sign if you should use ported or manifold vacuum. I try and go for the most amount of initial I can and still stay at the 36 total for iron heads. I have my initial at 18 and 10 on the crane vacuum can. I did some mods to my small body hei to make this all work and a limiter plate that cam with the can. 18 initial gives me much more off idle torq. Some people however may have starting issues with that amount of initial.

Can't see the vacuum changing by ignition timing, unless the timing increased the rpm with the throttle plates still closed, but you would lower the rpm again with the idle screw.Valve timing yes.If you're getting your total all in by 2200 rpm and your initial is 12-14 you most likely have to put stronger springs in the mechanical advance. You should have your mechanical max out around 3000 rpm. Your old springs may be worn out or just too light.

Rohrt, is that an SD Performance Stump Puller? If so it's the same camshaft I have. I get 12-13 in. vacuum max
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Some do have starting problems with a high initial timing,. That can be cured by modifying the advance rate to get the max at the rpm desired with a lower initial, then increase the vacuum advance to get the higher timing at idle. Some have used a hold off switch to allow the crank to start rotating before the ignition is powered, a relay can be used as well
I don't have a problem with 10.5:1 but 12:1?

Yes its the SD cam. As I mentioned mine is on a 114 LSA instead of the 112. I get anywhere from 13-15 in depending where I set the idle. The idle is very smooth and I can lower it down to like 650. I usually keep around 750-800.

You can play with timing forever trying to figure out what works for you. I usually say to work backwards. I think this was already mentioned. Shoot for the most power at WOT. This is usually 36 deg with Iron heads and 31 deg with aluminum heads much depending on chamber shape and quench.Then you have to see what your left with on the initial. It might be way to low. 12 is usually the mark most shoot for. If you use a HEI you have to find a center plate and weights that will work for you. If you use points you can do what I did and put in a set screw in to limit the amount of mechanical advance.

I've always tried to keep it at 36 deg all in. Now that you mention it, it might be too much since I am running alum heads. Only way to really see is hooked up to a dyno I suppose. It's always coming in pretty early as well but I've never heard any detonation so I figured it was ok. Might try some stiffer springs. Bump it to around 2800 all in. Probably get some road time this Sunday. They have Indy car testing at Sonoma. Free for spectators. It's about a 45 minute cruise to get there. Sounds like a pretty good tryout.

test

I have the E-brock Performer aluminum heads on my 455. My engine builder includes break-in and tuning on a dyno before he releases his engines out the door.

We found that my 455 ran best at 36 degrees total timing.

Thanks Salmon. That's good enough for me. I know there's a lot more to getting it just right but a few ponies one way or the other is not tragic. Even if it's not dead nuts it's got to be pretty darn close.

WOW, technical stuff. Thanks for the write up Bluebird, even though 90% of its over my head , I'll be able to atleast get a start on vacuum and the system with that.

Thanks bill v

Having your total 36 degrees in by 2200 rpm could be a bit too much advance at that engine speed, OK for a faster rpm. Could be but I'm no expert.

Rohrt, So you had them grind a cam with the same lobe profile, lift, duration etc but widened the lobs out? Do you still have it installed at 108 degrees? That would have the exhaust peak earlier than the 112 degree Stump Puller. You'ed have less overlap and must give you peak power at a higher rpm. Or did you install it at 110 to keep the centerlines as equally spaced from TDC?

I was going to put a machine screw and nut through the plate to stop the advance but I was afraid of it falling off and damaging something. I welded the plate on the low side to keep the curve, allow me to peak at 2800 RPM and have the higher initial.

Yep. I have the cam card in front of me. The card shows a 109 intake center line. I didn't do the install and not really sure on the details. There are some notes to the side 150 + 70 = 220 / 2 = 110 Then they wrote crank..gear?? 4 deg retard Cam Key 11+ Advance.

Well it's the same Part number as mine and the same grind number except yours ends HR 114+5 and mine ends HR 112+4
Same lift and duration at .006 and .050. Your intake opens and closes one degree later than mine and the exhaust opens and closes three degrees before mine. That four degrees is 2 cam degrees giving you the 114 degrees lobe separation angle.My cam is advanced four degrees and your exhaust centerline of 119 degrees and the intake centerline of 109 gives you and advance of five degrees. I wonder if you have an adjustable crank gear or you have a five degree offset cam key to give you an additional four degrees of advance to give a cam advance of 10-11 degrees, and why? The 150 + 70 = 220/2=110 must be crank degrees converted to camshaft degrees.