Using Vacuum Advance With FiTech Controlled Timing
Was just wondering if anyone had tried this, and if so, what were the results? It's what I'm currently running and I want to match those numbers while using ECU timing to aid in idle control. For those that are ready to fire back that 50* is way too high, the attached is for your reading pleasure (not mine, can be found all over the internet). Really looking for inputs from someone that may have tried this, or someone that's willing to give it a try if you're already using an MSD distributor with vacuum advance that is blocked off because you're using the FiTech to control the timing. Thank you...
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Interesting. Need to do some reading.
Hi all, I'm now using vacuum advance with my FiTech ECU timing control and it works great. I'll explain what I did below, but first to explain why it worked. There is an article out there written by Jeff Smith that states both mechanical and vacuum advance work by advancing the trigger. That's not always correct as my vacuum advance (and I believe all SBC's work the same) works by retarding the magnetic pickup (so the trigger gets to it sooner). Because Jeff is huge in car mags and on the web, I'm sure many folks accepted the trigger statement as fact and so figured locking the mechanical advance would also lock the vacuum advance, but that's just not the way Chevy distributors work. And just for the record, I'm a huge fan of Jeff as his articles are easy to follow and have great information. So anyway, I spent today playing with it and here's what I did:
1. Locked out the mechanical advance on my 2-wire MDS distributor with vacuum advance
2. Set my base timing to 10* by setting the marker on the crank timing tape to 10* and installing the dizzy with the rotor pointing to the #1 post
3. To account for phasing, I rotated the adjustable rotor 10* clockwise (SBC)
4. Put the cap on and rewired as required for ECU timing
5. For this part I used the ProCAL software on a stand-along computer - remember to save the changes back to the CAL file several times along the way
a. Set the base degrees and the idle advance at 10* (or as close as the sw let me)
b. In the BREAKPOINTS, I set my "Small Ign MAP kPA" to three values that I see at my 5500 ft alt. Engine is NA so no need for boosted value so useful table can be 3x3. Also set usable values in "Small Ign RPM" which for me is 1100, 2300 and 5800.
c. Selected "SPARK MAP" and the 3D table and on the first line set 20,34,36, second line, 20,34,36 and third line, 20,36,36 and I was finished
6. Went to the car and uploaded the CAL to the ECU. Because the software is different than the handheld, I then downloaded the CAL from the ECU to a different file than I had just uploaded. I disconnected the handheld and loaded the file I had just downloaded back into the ProCAL software (the computer was not hooked up using the k-driver) and checked to make sure the values had not changed; they had not.
7. Went back to the car and attached the handheld and a timing light, started the engine and ran through the normal FiTech procedures. I made a small adjustment to dizzy so timing on the crank matched the timing on the handheld at idle. I then ran it up to 4k and made the VR drift adjustment (mine was 6.5*). So at this point the engine timing is being managed by the ECU, the settings are NOT what you would use if only using the ECU to control timing, but it's functioning as expected.
8. I now hooked up my vacuum advance which I know gives 16* max and the timing light shows right at 26* at idle and just over 50* when I open it up. The handheld does NOT recognize the added timing; it continues to show the values the ECU is controlling.
So why did I choose to set it up like this? I essentially am replicating what I had when the ECU did not control the timing. The values I selected are to approximate what a spring/weight based mechanical curve would be...since my RPM breakpoints are 1100, 2300 and 5800, the ECU controlled portion is mostly in by 2300 (at my WOT kPA it is all in at 2300) and for all MAP values it is all in by 5800. The vacuum advance now makes all the little adjustments that are driven by load. Yes, if you have an OEM sized spark map (>30x30) you can get the same without the vacuum advance, but the FiTech has at best a 3x3 table, not the best to replicate all the combinations of mechanical and advance curves.
With my initial set at 10* I needed 26* degrees additional to get to 36* max for the ECU "mechanical"...so 13 dizzy degrees.
The vacuum advance has 16* crank so 8* in the dizzy
The total spread is from 10* to 52* (the 36* plus 16*) is 42* crank and 21* dizzy.
By selecting to advance the rotor 10* (remember, this is just the adjustable rotor that's advanced), the spark fires right about center on the post at 4000 RPM and WOT which for my engine is peak torque.
Drove it around and it feels great, just like the old setup but now I know the "mechanical" timing is spot on and the idle is smoother because the ECU can make timing corrections at idle which is better than trying to control with just fuel.
Bottom line, if you want to use your vacuum advance along with ECU controlled timing, you can. Just remember to setup the ECU spark map to reflect the mechanical curve you want, and the vacuum do what it always has! I think I captured everything above, but please let me know if you have questions and I'll try to answer.
Good luck to others that try this!
Rick Lenz
Edited this 10/30/19 to show now running 10* base/initial as the 14* was too hard on my starter. Engine turns over easier at 10*.
Another advantage I forgot to mention above is the ability to make the timing adjustments easily using the handheld instead of using weights, springs and bushings which can be a pain. Many of us "older" gearheads have grown up seeing the timing advance curve charts and tables that came with our aftermarket timing kits or distributors. I've attached the page that came with my distributor. By permitting the ECU to essentially just control the RPM related timing, we can match those advance curves...probably much more accurately than when using actual mechanical advance. I can also continue to think about engine timing the same way I always have as my initial advance is 14* and my total timing is 36*, I don't have to worry about MAP as the vacuum canister takes care of that.
Along those lines, I'll be playing with the settings more today. The FiTech instructions do not provide much/any definition for many of the settings that we can change. I spent some time looking over all the inputs and tables in the FiTec software and I now believe the "Small Ign RPM" values are just related to the spark table and won't impact other tuning factors. I'll be adjusting my breakpoints with the lowest RPM representing when the curve should start, the midpoint will be the RPM halfway through the curve I'm trying to replicate and the high RPM input will be when the "mechanical" timing is all in. Using the advance curve charts, I now have a visual aid to help me select the values the ECU is fed...and that helps this old guy easily blend the old with the new!
I've just been addressing how to replicate the mechanical curve, the vacuum advance still needs to be selected to work with your engine and the great paper (attached) written by the GM engineer addresses that.
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I have 1 tip for you when you set you initial 14 degrees; before you put the timing tape on and line everything up you MUST find true TDC before you do that or you actual timing numbers will be off ; remove cyl 1 plug use a telescoping dial indicator bring number 1 up on power stroke and find where the piston is at a true 0 degrees of travel set your timing tape to line up at 0 degrees on the pointer now set your dizzy at your 22 degrees because if the dizz is locked it cant have any degrees of timing in it I would almost be a 100 percent to think your piston in number 1 is off by 8 degrees and once it is set you can manually adjust it or use a timing control but once its locked and set it is what it is ... just thinking on it
I built the engine myself and ensured TDC was correct with the heads off; done when degreeing the cam. My system is working exactly as I describe it above; the ECU is essentially supplying the mechanical advance portion of the curve and the vacuum canister adjusts for engine load. The only thing that gets locked in the distributor is the reluctor wheel/rotor to the spinning shaft. The bottom plate that hold the magnetic pickup is not locked and rotates as vacuum is added/removed, thus adding/subtracting ignition timing. If you have a dizzy with a vacuum advance, take the cap off and add some vacuum and see how the pickup moves in a counterclockwise motion (SBC) to add advance...it gets to the trigger fingers faster. Bottom line, my engine is set up correctly and also runs great using the vacuum advance.
a locked dist that advances.. interesting okay great
I know, I should post a video. Maybe when Austin has that part of the site up and running I will. And maybe not all SBC distributors with vacuum advance work this way, but the MSD 8361 does...and I'm very glad it does because the FiTech spark map is pathetic to say the least.
Thanks for taking the time to post all this.
I've been doing a lot of reading on vacuum advance, and being able to keep it going with the fitech is something I definitely want to try.
Btw... If you wanted to provide a video as you mentioned, you could just upload it to YouTube and then post the link to it here.
Good luck if you give it a try. Please check to see if the plate your mag pickup is attached to rotates opposite to the way the rotor spins when you apply suction; otherwise this method will not work. Also, I was wrong above when I stated the small ign RPM breakpoints in the ProCal software only impacted the Spark Table, they are same RPM values used for the AFR table. Sure wish that was not the case. One other suggestion, if you're going to try this, I would set the system up without the vacuum advance, just block off the vacuum port, and once you've had some experience with the FiTech timing control, then add the vacuum advance and adjust the FiTech spark table to provide the "mechanical" advance. Please ask away if you have any questions.
One other overall edit...I would stay away from the ProCal software and just change the settings using the handheld. Seems the more I try to mix using both, the more I end up reloading a known good .cal because the system really acts out like a 3-year old on a candy binge.