vac advance or not for FE Cobra
 

I know that vac advance doesn't add performance, but it certainly add fuel mileage.

An FE MSD distributor without vac advance is about $100 cheaper than one with. Would you ever make up the difference in gas mileage on a Cobra?

If a 447 gets about 15mpg with vac advance, how much would it get with?

John

OK, forgive me in advance. I just don't see why guys are still buying MSD. Is it not old tech. with the big giant red box. My cell phone does a 100 times more things. A friend just bought a new, not rebuilt ford dist., with adjustable vacuum advance. He then put in a Pertronix Ignitor III conversion. It has adjustable rev-limiter and multi-spark, all in this little module in the dist. Mileage is performance.

Probably not an issue for your ERA but vac advance also will result in a cooler running engine - sometimes significantly. Provided it's not on a ported vacuum outlet it will also smooth the idle out a little with the additional timing. But, if you already have your centrifugal advance limited and are running 15 - 16 degrees of initial advance, it won't make much difference.

On the downside, too much additional vacuum advance at cruise can create an overall over-advanced situation with the plug firing too far in advance of TDC and create what feels like a constant surging or light missing situation. An adjustible vacuum advance canister can be used to dial it back some.

Yes, it will improve gas mileage but I'm not sure it will be very measureable with these light cars and big motors - I would guess less than a mile per gallon but someone will probably come along and prove me wrong. I'm a believer in vacuum advance but I'm running a 427 dual point w/o vacuum advance in my ERA and it seems to be doing fine.

At WOT it does not matter. At cruise it does matter. How much time will you spend cruising? How much time will you spend with it mashed to the floor?

Let's look at it another way. Suppose you had a perfectly tuned engine that obviously has vacuum advance (it is perfectly tuned so it has to have vacuum advance). What would you say if I asked you to retard the base timing 20 deg, and drive it on a cruise? If you did that, how much power would you expect to loose at WOT?

Now understand that when you retard the timing you loose power. So at a cruise you have to burn more fuel by opening the throttle more. Or you could back off on the throttle, turn the timing up where it belongs, and make the same power, while burning less fuel.

It's just my opinion, but it is foolish to not run Vacuum advance on a street driven car. Now if you have an engine that is so radical it shouldn't be on the street, or if you have a race only situation, then it is foolish to waste your time tuning vacuum advance.

That is just my opinion, and I think you asked for it.

I posted this a couple of years ago. Still relevant. Rick

Here is one of the best definitive articles on vacuum advance. I converted my 66 428FE from a non vacuum MSD back to a Ford dizzy with a Pertronix ignition and vacuum advance. Unless you are running your car at WOT, you should run vacuum as this article so perfectly explains. You will improve cooling at idle, drivability and plug fouling. Take your information from a power train engineer and not from a backyard hobbyist. Rick

http://www.hotrodders.com/forum/dist...1-a-59033.html

Faron Rhoads is suppose to be the Dist. man according to everybody on the FE forum. This is what I got from him, recurved and all. Looks great I think.
http://img.photobucket.com/albums/v6...pslngwqr4z.jpg[/url]

http://img.photobucket.com/albums/v6...pspfzd4mfh.jpg[/url]

http://img.photobucket.com/albums/v6...ps6j7tnrb7.jpg[/url]

http://img.photobucket.com/albums/v6...psqgmmceqk.jpg[/url]

This post has had some good discussion. Most seem to be in line with my thoughts. I wonder why the big engine builders seem to push MSD type distributors without vac advance.

Is it cost or another parameter they don't want to mess with.

For my next FE build I am thinking about a Pertronix distributor with ignitor III. It gives you multiple spark discharge, built in rev limiter, vac advance and no need for a separate control box.

John

I've never worked on FEs before my current 427 but from 4 to 12 cylinders even race engines I've always found vaccum to be an advantage.

Some says there's no difference between vaccum and not, particularly on track cars. I challenge this especially on powerful engines - when driver maintains engine mid-range during a curve before the throttle gets hit WOT for the straight. The well timed ignition system - so vaccumed in our case will have more linear response ensuring a progressive power curve thus better times :)
Once we tested same track same driver with and without vaccum.
Driver thought his times were better without, it was actually the opposite.
No vaccum compensated advance will increase the feeling of kick in the bottom thus make the driver believe it's more powerful. Don't be fooled ;)

Btw i'm replacing the current pertronix with a stock recurved Ford distri with vaccum. Will share results if anything tangible can be measured...

I use an MSD programmable 6530 with a MAP sensor to give me the adjustability of a vacuum advance and even more flexibility. I like vacuum. Better light load response and fuel mileage plus a cooler engine.

Lippy,

Was the MSD unit easy to program? For a street driven Cobra, does it offer any additional flexibility over a distributor with vac advance?

Can you give us an idea of how much advance you are adding using the MAP sensor?

It sounds like a pretty good product, but I have no experience with anything like it.

John

John,

I think it's easy but I'm pretty good with computers. You just need a PC and cable.

It does have some advantages over a standard vacuum dizzy. It is infinitely programmable and you don't need to rely on springs and a mech advance, it is more stable at high rpm, and you get multiple sparks.

PM me if you want more details.

http://www.clubcobra.com/forums/igni...ap-sensor.html

I have been learning on tuning EFI. Most people focus on the fuel, which is important, but the timing map is where most of the power is at. When you look at the timing maps from Ford, they are more sophisticated than duplicating the old mechanical and vacuum advance. A lot has been learned over the last 50 years.

I think Lippy is giving you great advice. I would expect significant power and fuel mileage could be delivered with the unit his is talking about. Think of it this way. You would be gaining the better half of an EFI system.

You may have to change your Logon name if you're going to learn EFI and computers. :LOL:

Great info.

Olddog - can you expand on your comments related to the current Ford timing maps? I haven't seen them.

I worked at GMC Truck in the early 90's when they started using crank triggers. It was a dramatic change in performance and mileage over the distributors. I assume the automakers have made many more leaps since then.

-I am planning the next build and it will have the MSD 6530. It isn't much more to get the added flexibility.

John

Same thing I did on my last Cobra. Locked the dist @ 35 degrees which is where it made the most power on the dyno, then used the MSD box to retard when necessary. Had it mounted right under the passenger side dash with the serial port facing down for easy access.

Just proves you CAN teach an old dog new tricks.

Old dogs do learn faster than pups sometimes. ;)

I am wondering about the vac advance question. Vac advance is very important to your engine. It is not a nice to have. Do your research. First step could be to read the instruction manual of your distributor...
Then you have to measure how much vac your engine produce at idle and with full open mech advance (~3000rpm). Sum of mech and vac advance must fit your engine specs. Otherwise you must adjust mech and/or vac advance. On newer distributors this will be realised with e.g. microswitches.
I fully agree with olddog :)!

I just graphed out the timing tables from a tune called A9L2 I got from a dyno tuner. He basically modified the A9L for a 89-93 Mustang GT manual trany. He eliminated the WOT timing and sea level tables, and made one table for everything. It makes the older EEC4 A9L act more like to modern Ford EFI.

At light loads 10% - 30% cylinder fill (high manifold vacuum), the timing comes in very fast at low rpm. By 2500 rpm the timing is coming in at a much slower rate. It is fully in at 44 deg from 4000 rpm up at a 10% load. The graph looks much like a square root function, for the mathematically astute.

At a 90% load (Wide Open throttle) the timing comes in very slowly from 750 - 1250 rpm, then starts to increase more rapidly until about 2000 rpm. From 2000 up the rate of increase in timing is starting to decrease. At 3000 rpm the timing is at 26 and it slowly goes from 26 to 28 by 4000 rpm. I would describe this graph as looking somewhat like a stretched letter "S".

Now this is for a bone stock 5.0 engine, and they do not take a lot of timing. Ford is only putting about 28 in at WOT high rpm.

But the biggest point is that the timing maps are curves. Mechanical advance is a Linear straight line. I believe the Vacuum advance was Linear too.

This map was done for boosted engines as well. At 200% load, which would be 16-18 psi boost, it is real interesting. He pulls all the timing out (o) and then steps it up a bunch at about 1750 rpm. Obviously he is concerned about destroying the engine at low rpm.

It would be interesting to see what Ford is doing with their ecoboost engines.

A thought occurred to me.

I am talking about a Mass Flow system, which measures engine load by measuring the volume of air entering the engine and dividing by the volume of air that the engine is displacing (what the engine would suck in at 100% efficiency).

Manifold Pressure or MAP, is another way to measure engine load. However when cams get too far on the wild side, vacuum can be very low at idle. This gives you less resolution to try to measure the engine load, so accuracy suffers. If you get to the point that vacuum is very low at idle and then increases as you open the throttle and rev the engine, before dropping off again, it makes MAP much more difficult to use. At this point you are not going to get ideal results.

Personally I do not think cams of that magnitude really belong on the street. Actually unless there are some racing body rules preventing you from doing the right things, putting on better heads, intake, and exhaust will make more power without such a radical cam IMO, but I digressed.

Using a MAP sensor to map out your timing is a great idea, but it would give less than stellar results if the cam is too wild. However a vacuum advance distributor could do no better on the same engine. I would bet the programmable distributor would outdo the mechanic distributor with vacuum advance on all engines.