KJL

A performance engine builder told me that if I am getting all 36 degrees of mechanical advance in by 2300-2800 rpm I should not use the vacuum advance. He said it is used for economy. Because the total mechanical advance of my distributor is 15 degrees, in order to achieve 36 total, my initial advance must be 21 degrees. My engine runs fine with this much intial advance and ported vacuum advance. (which gives me 52 degrees at 2300 rpm). I tried it without vacuum advance and it seemed to have more grunt of the line. Any thoughts?

68shark

I think you'll have overheating issues in prolonged idle situations. Proper vac advance is important and not just for economy. Heck...if that was it's only use, you would be hard pressed to find a car on the forum with one!! I wouldn't disconnect it, in fact I'd make sure you get one that gives you the right amount of advance to match your engine specs. The base unit (say for a 300hp base engine in the late 60's) might not give enough advance for an engine upgraded with higher perf. cam etc. Just my .02. :cheers:

Fevre

Theoretically you should have no diff with the vac adv hooked up when you hammer it off the line, your vac should drop to 0 giving no vac adv. You might have a restriction some where that is allowing some vac but at WOT you should not have any vac adv. Reason racers don't use vac adv is that it gives no performance increase and is one more thing to fail. You will get better economy and run cooler at higher rpm's with it hooked up.

red76vette

I don't use my vacuum advance and I have no problems with performance.

jn79vette

my engine builder told me the same thing.i left it off.ive been driving it daily for over a year.

Larry B.

I have a BB so correct timing is important. I cannot use any vacuum advance . I Have 28 total with 6 initial.. I have no problems with heating even with the A/C on. I run 93 Amoco and still get a slight pinging but only in very hot weather. It's a compromise...

Fevre

You guys are missing the point, there should be no vac signal at WOT so you should have the same timing with or without the vac adv at WOT. You could spend $25 on an adj vac adv can and reduce the vac adv to stop any pinging and gain mpg. More MPH means less time standing at the pump and more time :steering: . Too each his own I guess.

yellow 72

:iagree:

Larry B.

I tried virtually every combination (ported/manifold) with an adjustable unit. I had a problem with ANY application of throttle except WOT unless I ran 10 degrees retarded. Vacuum advance just didn't work for me. Need 100 octane.... that'll do it

Obdoc

Fevre is absolutely correct. Listen to him. :iagree:

Tim H

I disagree.
If you have the advance hooked up to the right carb port then when you nail the gas that port "creates" vacuum to pull open the advance lock and the weights fly apart to advance the timing. That port has to be above the butterflys.
I don't use the vacuum on anything.
I unplug it and use lighter springs to open the advance around 25/2800 rpms.
Some people on the other hand hook up to vacuum at idle then advance there timing and thats like running a lawnmower with the govoner wired open.

KJL

I believe stock engines were usually set up with stiff springs only allowing the mechanical advance to come in above 4000 rpm so the vacuum advance was used to provide some advance in the lower RPM range. This was most likely with fuel economy in mind. When you swap in a set of soft springs and bring in the advance at 2500, I think this changes the logic.

My engine also specs 8 degrees of initail advance. Timing to 36 degrees at approx 2500 rpm gives me an intial advance of 21 degrees. So driving at 2800 rpm assuming full vacuum advance has kicked in, I would have a total of 52 degrees.

Now, if an engine timed to 8 degrees initial with stock springs plus 16 degrees (full vacuum) and maybe half mechanical (8 degrees) you would be a 32 degrees total advance at 2800 rpm.

So, if you remove the vacuum advance off case 1, this would bring the advance down to 36 degrees total at 2800 rpm.

I guess the question is, do you need 52 degrees of advance at part throttle for better performance? I don't care about economy.... that is what my daily driver is for. :yesnod:

Fevre

Def if you have high comp and iron heads you will have trouble running it without using a gas that is similar in make up to what the eng was orig designed to run on. But on an eng designed to run on today's lower oct gas, running the vac adv should not be an issue. GM put vac adv on for a reason not just to create problems or lower performance. No need to run it on a strip only car but for street machines there is advantages to running it. :cheers:

Stewart's74

Lars tells us to use the vac advance. I'm taking his word as the gospel. I would bet he is one of the premere vette tuners in the country.

Stew

pws69

Me too. All hail his Larsness.....

theandies

:iagree:
Read this: https://www.corvetteforum.com/techti...=115&TopicID=3

yellow 72

Huh? :eek: :confused: You need to go back and study the principles of ignition advance...Engine vacuum has nothing to do with the mechanical advance weights.....

Fevre

Not to mention the fact that when you open the throttle vac readings actual drop. Hook a vac gauge up and try it, it should drop to 0 when you are at idle and drop the hammer. If you are getting a reading then you have a restriction somewhere in your set up.

Rockn-Roll

I bow to LARS on this. But in doing so while I'm looking down I read my engineering books just to make sure it's correct. I think I explained this before, but here it goes again:

Combustion does not occur instantaneously, it takes time for it to completely burn. Ignition is the beginning of the burn, and TDC is the optimal end when combustion would be most desired to be completed. However, it can't be exactly TDC since some of the combustion would have already taken place and will work against the momentum of the piston cycle. So, combustion will end slightly after TDC, but hopefully you get the point that ignition must begin at BTDC and end at ATDC due to the time required for combustion to completely burn the air/fuel mixture.

As RPM's increase then ignition needs to be more advanced since the air/fuel mixture still requires the same amount of time to burn while the time allowed for it to burn is being reduced by the rate at which the piston is moving. Also, when more air/fuel is packed into the cylinders then ignition needs to be more advanced because there is more air/fuel mixture in the combustion chamber and it will take longer for all of it to burn.

So, the mechanical advance provides the increase in ignition timing to compensate for RPMs while vacuum advance provides the increase for additional air/fuel. Everyone seems to agree that mechanical advance is required, but not everyone believes they need vacuum advance. I hope I have illustrated that vacuum advance is required especially at WOT! Unfortunately carburated vacuum signals are not very efficient which accounts for some of the more creative vacuum hose routings with many switches and sensors so that the distributor gets the best signals.

Ideally ignition timing should be handled by a computer which can detect engine RPM as well as density of air/fuel in the chambers. Modern engines have computers which have air density or air volume sensors and are patched into the distributor. These computers control the ignition timing as I indicated by increasing advance with RPMs as well as when the throttle is opened. This is the major reason that modern engines with half the cubic inches can develop more horsepower than our carburated engines, along with better metering of the air/fuel mixture.

OK, I'm getting up off my knees now unless LARS comes in here and I have to bow again.

yellow 72

Thank You........