Corvette Forum : Corvette Forums banner

Everyone needs a vacuum advance

994 Views 4 Replies 3 Participants Last post by  Strider
I no longer worry about horsepower but do worry about a clean running engine and good fuel economy. Yes I know 540 blown cubic inches and gas mileage do not belong in the same sentence but I am looking for 18 mpg or so and believe I can eventually get it and I don’t mean just highway mileage. I mean in town with combined driving.
The first thing I did was read everything I could find about vacuum advance. Everything I read said it was a good thing and even dedicated race cars could benefit from in. ALL car idle at some time and it does nothing but improve idle quality. Yes it does increase emissions but I wanted the clean idling motor. One with a 270 @ .050 cam and still idle all day long without loading plugs.
VACUUM ADVANCE seemed to make a big difference in gas mileage, clean idle and taming the big cam.
I run 4 MSD boxes so I naturely bought a MSD billet distributor. But I was running a Mallory and my wiring harness only fit the Mallory so I took the MSD back and bought the Mallory, the billet Mallory with the vacuum advance.
I installed the distributor, set the initial timing at 24 degrees, I have a 20 degree electronic retart for starting. Had to also take the distributor apart and lock the timing curve so I got NO timing out of the distributor. I set the electronic timing curve at 12 degrees all in by about 2800. The nice thing is I can just turn a dial and get any advance curve I want.
I set mine at 12 so with 24 intial I have 38 degrees.
I then hooked up the vacuum line to manifold vacuum and went to 60 plus. Way way too much.
Out came the distributor and after a couple of hours of machining and fabrication I put the distributor back together.
I made a stock, a round aluminum stop with a allen 10-32 cap screw down the center, drilled the plate, taped the hole and with loctite installed the stop.
I then machined an aluminum block to bolt on the side of the distributor, drilled the body, taped it for 2 10/32 cap screws, applied expoxy to the back side and installed the block on the main body and filled in the existing wire hole since it was in a bad location and let the epoxy set up. I then installed the long 10/32 screw with locking nut to act against the stop

This is the completed distributor

I installed the distributor so the adjustment for the total vacuum advance is readily accessible so timing could be set while running and the vacuum pot is in a good spot.
I then marked # 1 plug where I wanted it to be and proceeded to install the plug wires.

I also run a voltage surge protector and keep it high up and cooler running. They are necessary for Mallory distributors.
My original Mallory billet distributor has no vacuum advance but it is a great spare parts distributor or replacement in the even of a failure. Takes me only minutes to change .

Click on this link and read this post. It is one on many I read and am convinced vacuum advance it needed
How did it run?? Fantanstic. Started right up, idled way too fast, turned the idle all the way down and still idles a bit too fast. Lugs down in 5th below 1500 without bucking, runs I believe cooler, sat below 180 with just one fan running in town.
I set total for all 3 timings at 52 degrees.
The other side effect is that my wide band LM1 oxygen sensor has become very stable at idle. It use to bounce around at idle, that would indicate a miss that I couldn’t find. Now it is steady indicating no more miss in a cylinder at idle.
I highly recommend everyone runs a vacuum advance for a cleaner running motor, again not emission wise but a nice idle, better gas mileage and a cooler running motor.
Read my link and see how with 38 degrees you are still running a retarted ignition as cruising speeds.
I am not looking for advice just telling you what I believe and what it felt like to me.
See less See more
1 - 1 of 5 Posts
1 - 1 of 5 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.