Kings of timing – what do you think of this plan?

[Cobra 29]

I have a FMS 351W Crate engine. I want total advance (initial plus mechanical) to be 34 to 36 degrees at 2800 RPMs. Some sources say that I should have between 14 to 16 degrees of initial timing to get a better burn at idle given my cam which specs at 236 degrees @ .050” intake and 246 degrees @ .050” exhaust.

With my MSD distributor, I have tried 14 degrees initial and 21 degrees in the distributor to get a total of 35, all in by 2800 RPM. The engine does like 14 initial versus 10 or so. It runs smoother and the exhaust fumes are much less, I guess this indicates a more complete burn at slow engine speeds which is good.

My problem is that my mini FMS starter does not like this much initial advance especially on hot summer days in Florida after the first run. It does a better job than the old heavy original type starter but some cranks are slow and I want to avoid this stress (mechanical and emotional!). I have good Optima battery in the trunk, good cables and grounds. For even more direct current, the positive battery cable connects directly to the starter. When the key is turned to start, the starter is tripped by a smaller 12v feed from the starter terminal on the starter solenoid.

Opportunity: I want to use my vacuum advance which came with my MSD small diameter pro-billet distributor for better gas mileage, crisper part throttle response and to help my occasional slow starting situation on hot days. Before you tell me, I know that I should not be concerned about the mileage but I do find the engine revs quicker with the vacuum advance. If I can get quicker response and gas mileage too, I’ll take both!

Here’s the plan. I have physically limited the MSD vacuum advance module to only permit 8 degrees of advance (by installing a bushing on the vertical part of module arm immediately under the mag pick-up plate inside the distributor). I plan to connect the vacuum advance module to manifold vacuum so it will provide 8 additional degrees of advance at idle and less toward WOT. I will then set the initial timing at 10 degrees and keep the 25 degree bushing in the distributor for a total of 35 degrees. Since I will not develop much vacuum when starting, the engine will have only 10 degrees of advance when cranking but get approximately 18 as soon as it starts (the engine pulls 15 inches of vacuum at idle, the module only takes 5 inches to provide all 8 degrees of movement). The 18 degrees at idle will provide the advance for a cleaner burn.

I also understand that during quick acceleration and WOT moments the vacuum module (connected to manifold vacuum) will provide none or less than 8 degrees of advance which is desirable under these conditions. At part throttle, when the manifold vacuum returns, it will provide advance up to 8 degrees for better gas mileage and crisp part-throttle response.

One concern I have is the source of vacuum. Previously, I have had the vacuum attached to ported vacuum (side of carb) and that did provide the crisp engine response when accelerating. I am not sure I will feel the same when the vacuum source is manifold. When connected to ported vacuum, I also believe I achieved too much advance in total at higher RPMs under partal throttle which was due advance from initial and centrifugal and then vacuum a the wrong time!).

Before I re-do all my timing again, what do the timing experts think?

BTW, the vacuum advance modules from Crane and Accel work slightly differently. The Crane adjustment only affects the amount of vacuum needed to pull the entire movement (or advance). You must physically limit the advance by blocking the module arm or blocking the pick-up plate like what is done in some GM distributors. This is not easy to do in a MSD distributor. The adjustment on the Accel module affects both the required vacuum to move the arm and how far the arm will travel. It does not need a physical limiting device to limit the arm (advance). I wish I knew this before I spent the time machining a bushing to limit the MSD module!!!

[Excaliber]

Generally speaking there is nothing special about a ported vacuum signal, OTHER THAN, vacuum is not present until the throttle plates are opened. No vacuum at idle rpm, full intake manifold vacuum above that. If your using a port that does not give you full manifold vacuum once the throttle plates are open it is somehow 'restricted' and certainly NOT a port I would choose to use. You could check this with TWO vacuum gauges but the readings may 'bounce' and the variance of the two gauges may make it difficult to reach a solid conclusion. It's an 'art' reading a vacuum gauge.

Some feel, I tend to agree, that the vacuum signal reacts faster with a direct intake source as there is inherently at least some initial restriction with a ported signal. There is nothing in the carb to ENHANCE the ported signal, to 'increase' it's strength. It will never be greater than the direct intake manifold vacuum signal.

Depending on what you want to do I can think of a couple of reasons why I would prefer direct intake vacuum over ported.
1. Faster response time (but unless your racing for that 100th of a second, a moot point).
2. To prevent 'run on' when you turn the key off.

With intake vacuum the timing is advanced immediatly upon startup. The idle rpm will go higher with it than without it. To compensate for this higher idle rpm you would have to CLOSE the throttle plate idle adjustment screw for a lower (correct speed) idle rpm. Because the throttle plates are 'more closed' than would normally be required when the engine is shut off it is more difficult for the engine to draw in fresh air\fuel through the 'more closed' throttle plates. This causes the engine to 'starve' and prevents 'engine run on' or 'dieseling' when the key is turned off.

As far as 'crisper throttle response' intake vs ported signal, that leaves me scratching my head. I think it would be extremely difficult to 'feel' the difference UNLESS there is something wrong with the ported signal, such as being 'restricted' TO much.

I to run a vacuum advance MSD dist. I've played around with both ported and intake vacuum and intake works better for my application. HOWEVER, I was disappointed to see in MY case it makes virtually no difference in gas mileage using it or not. Certainly vacuum advance CAN help many motors to get better mpg, but it just depends.

Currently I'm not using my vac advance at all. I DO have an engine 'run on' condition, which USED TO be handled nicely by the vac advance allowing for more closure of the throttle plates on engine shut down. So why did I disconnect it? My feeling is it's to much of a variable in the overall timing control. I like the idea of knowing exactly what my base timing is and my mechanical advance timing is. To compensate for 'engine run on' I turn off the key and let the clutch out at the same time with the car in gear to stop the motor dead instantly.

Why does my engine 'run on'? I have twin carbs on high riser heads with a fairly wild cam. This setup means I need an idle speed of a 1000-1100 rpm, just enough to cause run on sometimes. 800-900 rpm, no run on at all, but the engine will all but die at the low rpm anyway.

[Cobra 29]

The vacuum at manifold and at the carb (ported) provides a good reading either way. No restrictions. When increasing the throttle the ported source will begin to advance and provide advance from none to max via the vacuum module. Under the same situation manifold vacuum (and advance from the module) will move in the opposite way; start higher and then become zero or near zero at WOT. It was my impression the engine rev'ed easier (from slight throttle to near full throttle) with the advance "map" provided by the ported source.

I have had more time connected to ported vacuum so maybe when I switch to manifold it will provide the same crisp throttle response I think I am getting with the ported source.

At any rate the bigger question is what do timing experts think of 10 initial, 25 in the distributor and the vacuum limited to 8 and connected to direct manifold vacuum thus providing approximately 18 degrees at idle? And having decreasing advance from the vacuum module as the throttle is moved toward WOT?

[vettestr]

Cobra29,
You are making this way to hard !!!!! First MSD makes a start retard module to reduce timing but also try this. On an engine with enough initial timing to bump against the starter just flood it a little. I forgot this only works on carb'd engines but can set electronic injection programing to do the same but that is another issue.
When you jump in to start her up turn key on to run pumps and wait till they turn off (about 3 to 5 seconds as a rule) then pump gas pedal to the floor 2 times. This is just enough to wet plugs a little so starter can gain RPM before it tries to start. The momentum at higher RPM will allow her to fire up without fail, play with yours to find if maybe 3 pumps or just 1 pump will work but IT DOES work.
The old school high compression guys or roots blower guys all have to flood them a bit to start when hot. I had a blown Camero that I hooked a fuel pump to a single injection nozzle mounted in the hat. It had mechanical injection on top of a 6/71 blower with a horn button to turn pump on for a few seconds or until she fired up. When hot I would have to run pump before cranking engine over so it would not bump starter

[jeffko]

I would rather say you are trying to do something difficult and really should be performed on the dyno. Most of your assumptions are correct and what you are trying to do is a great theory and the best of both worlds. Unless you have a 2000 lb trailer to haul around or a very long steep hill (which would involve lots of trial/error and some guesswork) you will not effectively get what you want without a dyno. Exhaust sampling (and I don't mean air/fuel ratios) and power can be measured through the rpms with the car sitting still and the ability to adjust. You simply can't compete with a dyno. Anyway here are a few things to know:

Use ported vacuum because it is indeed special. Direct manifold vacuum will add timing to your initial timing at the damper at idle and nobody with any idea about what they were talking about would suggest that. The ported vacuum will give you the throttle position which indicates low to mid throttle load. This is a huge distinction because it advances the timing as you use more throttle.

Initial timing is best to start around 12-16 but your engine will have a preference which is most easily indicated by a vacuum gauge. This is also the time to set your carbs idles screws again watching the vacuum gauge.

You should have all your timing in by 3000 and no earlier with also using vacuum advance. I would not exceed 38 total.

By the way, you will feel a better off idle to half throttle response with vacuum advance since that is exactly where it operates. Mechanical advance is simply that and is dependent on engine speed only. So your theory is correct. Using ported vacuum your vacuum advance will react more quickly and fall off with the loss of vacuum at wot which is where the mechanical advance will have taken over. I have to tell you though, you will find it very hard to find a dyno guy that will want to tune this way and has the co, carbons and nox testing for the efficiency of your combustion by sampling the exhaust. Dyno, air/fuel and combustion sampling = power, economy and peak performace. But until you find this guy (looking myself) you will pretty much be chasing your tail. Good luck and keep us posted.

[Excaliber]

Jeffko says...

Direct manifold vacuum will add timing to your initial timing at the damper at idle and nobody with any idea about what they were talking about would suggest that.

Guess you missed at least ONE reason why using intake vacuum could be BETTER than ported vacuum. It's a way to address engine 'run on'. And yes, I know exactly what I'm talking about!

Jeffko says...

Using ported vacuum your vacuum advance will react more quickly and fall off with the loss of vacuum at wot which is where the mechanical advance will have taken over.

Not in MY opinion, and the 'theory' can be easily proved with a vacuum gauge and a timing light. As soon as the throttle plates OPEN the carb port there is no measurable difference between ported and intake vacuum, they are the same, originating from the same source. As ported vacuum falls off, so will intake vacuum at the same rate until the carb port closes. Intake vacuum will respond FASTER than carb ported vacuum because it doesn't have to 'wait' for the port to open. 8" or 20" of vacuum is vacuum, regardless of the 'source' (port or intake).

No offense to Jeffko, just a differing of opinions on a complex subject.

[jeffko]

Ex, whether direct manifold vacuum stops engine run on or not is debatable at best. I personally don't know since I have never had an engine so far out of tune that it needed such a tactic. The rest of your opinion on the matter is incorrect. The ported vacuum is metered by the position of the throttle plate and how much vacuum is available in the intake manifold and in my opinion you really don't understand the subject.

Cobra 29, here is a tech article that is helpful.


http://www.mustangmonthly.com/techar...on_tuning.html