Driving Without HEI Vacuum Advance!

[savatreatabvr]

The HEI vacuum advance on my mid 70's 454 went out so I plugged the manifold vacuum, advanced the timing and I'm driving it until I find the correct vacuum advance. It still doesn't drive right but it drives better. My question is, will it hurt the engine driving it this way?

[wilkin250r]

Yes it can. However, you can drive a certain way to limit your risk and damage, but it's still an ever-present risk.

Background engine theory: Without getting too technical, your throttle is controlling the density of air getting into the engine. It can be thought of and explained as the AMOUNT of air getting into your engine. At idle and low throttle positions, less air means less fuel, and your carb controls all of this.

Really dense air/fuel mixture will burn fast, air that is less dense with less fuel burns slower. So when you're at idle and low throttle positions with very little air and fuel, you need lots of advance to give it enough time to burn, that's why you have the vacuum advance. You're firing the spark earlier to give it enough time to start the combustion process and build pressure.

However, if you start it TOO early, you run the risk of detonation, and it will destroy your motor rather quickly.

[wilkin250r]

Now here is how it affects YOU and your particular situation. If you stab the throttle wide open, you're going to get LOTS of air and LOTS of fuel, which will create a really dense mixture inside your engine, and that really dense mixture will burn really fast. If you couple that with lots of timing advance, you run a serious risk of detonation, which can destroy your motor in a matter of minutes.

Of course, I highly advise you to figure out the problem and fix it. If you continue to drive it like this, my advice would be to stay off the throttle as much as humanly possible, even if it takes you several minutes to get up to 60mph, and even if you have to downshift and crawl up every hill at 25mph. A heavy foot and max timing advance is a recipe for disaster. I imagine you can overcome the grief of people honking and flipping you off much faster than you can overcome the headache of a blown engine.

[savatreatabvr]

Thank you for the advice! I don't redline the 454 much so hopefully it will last until I find the correct vacuum advance which I'm having problems doing. My vacuum advance is about a 1/4" longer than the ones Autozone, O'Rielly's, Napa and Pepboys have and I can't find the HEI distributor casting numbers online anywhere so I'm kinda at a loss! I guess the junkyard is next to find the right one then go back and physically compare the units!

Originally Posted by wilkin250r

Now here is how it affects YOU and your particular situation. If you stab the throttle wide open, you're going to get LOTS of air and LOTS of fuel, which will create a really dense mixture inside your engine, and that really dense mixture will burn really fast. If you couple that with lots of timing advance, you run a serious risk of detonation, which can destroy your motor in a matter of minutes.

Of course, I highly advise you to figure out the problem and fix it. If you continue to drive it like this, my advice would be to stay off the throttle as much as humanly possible, even if it takes you several minutes to get up to 60mph, and even if you have to downshift and crawl up every hill at 25mph. A heavy foot and max timing advance is a recipe for disaster. I imagine you can overcome the grief of people honking and flipping you off much faster than you can overcome the headache of a blown engine.

[jhngardner367]

I just replaced the one in my '85c10.Itcaused shift problems/converter lock not engaging,spark knock/bad mileage.
Since replacement it's back to my sweet truck!

[Pyrotechnic]

I would set the timing back to stock specs. Running without the vacuum advance will just give you bad gas mileage. Running with the whole distributor over advanced is not a good idea.

[wilkin250r]

Originally Posted by savatreatabvr

Thank you for the advice! I don't redline the 454 much so hopefully it will last until I find the correct vacuum advance

And oddly enough, RPMS aren't the problem. In fact, your distributor will actually advance your timing at higher RPMs anyways, so you're not in danger at higher RPMs. The danger would be heavy throttle at LOW rpms, like mashing the gas off a stoplight, or accelerating from a stopsign to get into traffic.

If everything was working correctly, your engine would have the least amount of timing advance at wide-open throttle around 1000 rpm.

[1LowToy]

Go ahead and put the new one in. The difference is in the stroke of the advance unit. The new one appears to have a little longer stroke to it which will give you just a little more vacuum advance than your old one, which is more low rpm power.. If you end up with pinging back your timing up by 1 or 2 degrees. Set timeing at factory specs after you install it.

Note if you have problems at the smpg station just keep in mind you did this. I doubt that it will show up on emissions check.

[savatreatabvr]

The problem is that the new one won't fit and I'm having problems finding the correct one!

Originally Posted by 1LowToy

Go ahead and put the new one in. The difference is in the stroke of the advance unit. The new one appears to have a little longer stroke to it which will give you just a little more vacuum advance than your old one, which is more low rpm power.. If you end up with pinging back your timing up by 1 or 2 degrees. Set timeing at factory specs after you install it.

Note if you have problems at the smpg station just keep in mind you did this. I doubt that it will show up on emissions check.

[Desert1957]

Chevy ignition timing sometimes is a little confusing.

You are going to here terms like...

Base timing
Initial timing
Total timing
Initial advance
Centrifugal advance
Vacuum advance

Here is a GREAT article about HEI Distributors, How and Why things need to work properly

http://www.chevyhiperformance.com/ho...438/index.html

Desert

[savatreatabvr]

Lol, I have a basic understanding of Chevrolet timing, my problem is deciphering the casting numbers on my HEI distributor so I can get the correct vacuum advance I need.

Originally Posted by Desert1957

Chevy ignition timing sometimes is a little confusing.

You are going to here terms like...

Base timing
Initial timing
Total timing
Initial advance
Centrifugal advance
Vacuum advance

Here is a GREAT article about HEI Distributors, How and Why things need to work properly

http://www.chevyhiperformance.com/ho...438/index.html

Desert

[54Caddy]

Easy fix man go on ebay and just slap in one of the hei untis from there. I have one on my truck there not too bad im sure there not the best but for the bang for the buck it will work.