: Distributor Advance Tuning Techniques for Fuel Economy and Performance!

06-06-13, 02:04 AM
Hi Guys,

I decided to start a separate thread here about a lot of information I've researched for another forum member on "Boosting Fuel Economy of a 472." That thread got kind of side tracked but I hope to revive the subject and possibly start another thread dedicated to improving the Cadillac Big Blocks fuel economy through many different ideas I had including these techniques below on tuning the distributor advance for economy and power.

So I'm just going to post up some great info I found on tuning the advance of the distributor for the original thread on improving economy, but there are many other improvements to both types of distributors that I will try to figure out how and where to post. I think it would be a good idea to consolidate much of what I saved into a dedicated Distributor Tuning Thread......

First, I'd like to thank a forum member "BARRY425" on the HAMB and HOTRODDER'S Forums for his posts that I found on the subject and answering my e-mails regarding his unique distributor vacuum advance tuning techniques which I thought would be valuable info. for others on this forum. When I asked Barry where he got this info. he told me it was from his 45 years of racing and engine building experience, so I think this guy knows what he's talking about because I've never seen anything like his tuning techniques. His tuning instructions are for using the CRANE Adjustable Vacuum Advance....... Cranes tuning instructions were not as thorough and as well written as Barry425's.

Here is the link to the better manufactured Point/HEI Adjustable Vacuum Advance that Barry recommends which includes a "limiter plate", (unlike others), part# 99600-1 :

Crane's Instructions:

"Barry425" Vacuum Advance Tuning Instructions:.....see full instructions posted below:

As Seen in post #12 here:

Same info. post# 5 here:

Barry's Vacuum Advance Tuning Instructions:

Get an HEI recurve kit from CRANE (NAPA Auto Parts has them) and follow the instructions:

1. Disconnect the vacuum advance from the distributor and plug the line.

2. Power time the engine at 3,000 rpm's. That means, attach your timing light with the engine off, then set the idle screw on the carb so the engine is running at 3,000 (a little higher is okay, but not lower). Adjust the distributor for the highest rpm, then back it off by about 2-4 degrees (using the light). You can back the timing off at idle if it is easier, since turning the distributor 2 degrees at idle will have the same 2 degree effect at 3,000 rpm's. That way, you don't have to put your head into a screaming engine compartment to adjust the distributor, and the timing marks will probably be easier to read at idle.

3. Set the idle screw back to a normal idle if you haven't already in the previous step.

4. Make a full throttle run with a hot engine and listen for spark knock or "ping". If it pings, back it off another 2 degrees. Keep doing this until there is no pinging. This will be your maximum MECHANICAL advance. WRITE IT DOWN! (advance at 3,000 rpm's).

5. Now with the idle set normally, adjust the distributor for the highest rpm with no ping, especially when accelerating right off idle. Usually, advance for highest rpm and back off 2 degrees, just like before. This will be your INITIAL setting. Make sure when you are doing this, that the timing marks are steady. If they are jumping around at idle, then you need stronger springs. Rule of thumb, choose springs to BEGIN your MECHANICAL curve approximately 500 rpm's above your idle speed. This will give you a steady idle speed. Recheck this setting AFTER every time you select new weights.

6. With a kit, select a set of weights that will give you the total MECHANICAL advance that you found in step #4. In other words, let's say that you found your INITIAL advance to be 14 degrees, and your total MECHANICAL advance to be 34 degrees. That means that you need to put weights in the distributor that will allow 10 degrees distributor/20 degrees crankshaft advance (remember, you have been measuring CRANKSHAFT timing with your timing light, but the distributor turns only half as fast as the crankshaft).

7. Make a full throttle pass to check your work so far. WRITE IT DOWN!

8. Reconnect the vacuum advance (it has been disconnected until now, remember?)

9. At part throttle CRUISE, listen for pinging. It won't be very loud, so you might have to find a place with a hedge or some other barrier on the left to reflect the sound back to you with your drivers side window open.

10. If it pings, the Crane kit comes with a vacuum limiter (a small flat serrated disk about the size of a nickle that mounts inside the distributor on the vacuum advance unit). Adjust it to reduce the total vacuum advance until it stops pinging. Don't get this confused with the allen wrench adjustment inside the vacuum tube port. The allen wrench only adjusts the RATE of vacuum advance. In other words, how fast the vacuum advance mechanism will move when exposed tio vacuum. Let it happen fast (all the way Clock Wise if I remember correctly), unless it causes ping when ACCELERATING (different than part throttle CRUISE) at part throttle. In that case, back it off 2 turns at a time until there is no ping at part throttle acceleration.

11. The intial advance gives you low rpm torque and drivability, the total mechanical gives you maximum horsepower, and the vacuum gives you the best mileage possible from your particular engine combination.

12. If you have a Holley carb, make sure that you set the accelerator pump on the front float bowl so that it has no slop, but no preload either. Do this AFTER you get your idle screw set where you want it for the last time.

Good luck! Barry425"

Barry also said: Don't get too excited about weights until you've selected your cam and compression ratio. You've got to start at the beginning.
Also, you can't get giant horsepower AND economy at the same time. Choose what you want to do and then ask Comp Cams for their recommendations for cam, compression ratio, and timing. That is a good starting point. You can always change gears later.

In addition, I'd like to thank John who is the owner of the "CADDY500 Forum". for his time and answering many questions of what he knew about the MTS#3 Economy Camshaft when Al Betker developed that cam and who was the former owner of MTS (Maximum Torque Inc). There is a lot of discussion about the old MTS from years ago on that forum. John is also on a few other Cadillac Forums and goes by "~JM~" so maybe some have seen his posts. Thanks for the great info. John ~JM~ ! His CADDY500 forum is another great resource dedicated to the Big Block Caddy Motors........

Here's a good link that John ~JM~ provided about Dustin's Timing and 472 Fuel Economy with the MTS#3 Camshaft in post#12:

Here are the instructions that John "~JM~" of the CADDY500 Forum provided titled: "Dustin's Timing": that he saved from the old CB7 (Cowboyseven Cadillac Forum) now known as "Cadillac Power" Forum....I think...:

"Dustins Car

I had a 75 CDV, stock weight, MT3 cam, K&N Filter w/Erson drop base filter case, 795cfm Edelbrock built Q-Jet for 1 ton Chevy truck, Accel built dristibutor, Fully ported Edelbrock Alum. intake, Flowmaster exaust, FlowKooler Water pump, and the below mentioned timing curve. Best mpg was 20 at 75mph, most of the time it was 18 @80-85mph. Around town, depending on the throttle angle 14-16mpg.

Advance curve recommendations

If you want to use premium unleaded gas use the combo that I have been using for years.
With the stock 8.5:1 compression use 10-12 degrees initial, one gold spring and one silver spring and turn the adjustable advance counter-clockwise 10 turns or until part throttle pinging is eliminated. That puts it at about 35 degrees total at very low RPM like 1800 rpm. I have tried using more vacuum advance but it detonates like crazy with the spring package described. If you use the stiffer springs in conjunction with more vacuum advance the car is real lazy at WOT. It takes more RPM to get the centrifugal advance to open up and delays throttle response big time. Using less initial timing can work but the car will not want to run well in colder weather. It leans out bad enough to backfire through the carb when trying to accelerate during cold engine operation.
I have tried every combination of weights, springs, vacuum and initial advance and have found the combination I described to be the best compromise between fuel economy (18-20 MPG at 2500RPM or 75MPH) and power (lots of bottom end grunt) at part throttle and full throttle. If you try and use lower octane fuel it will work well for part throttle only. At WOT it will ping lightly with 89 octane, never have run 87 octane in my car so don't know about that. I used the weights that came with the Crane kit due to their lighter weight, and besides the stock weights are usually toast by the time you get to them. They wear heavily around the spring posts. It was the Crane kit, but I think I used the Accel vacuum advance."

I hope this info. helps.........



06-06-13, 01:09 PM
Take it from a long-time Olds 455 builder that the part of your post by Barry - the Crane setup - is right on the money. Very close to the same process we use to set up rebuilt ACCEL/Delco points-type distributors for marine cruising engine service..... and we use the Pertronix Ignitors instead of points.

06-06-13, 01:40 PM
Good to hear "Sub", I'm glad someone with years of experience confirmed these procedures were valid tech. info. I found.:thumbsup: