Vacuum advance or mechanical advance

65fastbackresto · Dec 8, 2007

I have a built 302, and my engine builder talked me into buying a mechanical (or centrifigal) advance distributer, due to low vacuum by big cam. This car runs GREAT at wide open throttle or close to it, but its sluggish and doogie just trying to cruise around. I must have some crazy highway gear ratio too cause I can flog this car and run at least 120 mph in 2nd gear without taching over 5500 rpm. I assume had I have went a vacuum advance this car would probably run smoother but I`m geussing....

The dist is an MSD unit that you can change the springs in, and I`m currently running one heavy spring and one light spring, if any of yall running MSD`s know of a better combo on the spring set up let me know please.

Anyone with some experience on this please help me out.

fasttback · Dec 8, 2007

Fine tuning the timing curves on distributors are specific to the engine they are in, including vehicle weight, gearing, cam specs (rpm range) etc.

Most of our typical Mustang performance applications like to see full advance by 3000 rpm or something close to that. Track or dyno testing is the best way to test that.

If your gearing is a bad as you say it is, your "big cam" may be operating below its optimum range most of the time and acting sluggish until you reach the cam/engine optimum rpm range. My recommendation - fix your gearing to match that cam/motor then start fine tuning the dizzy.

65fastbackresto · Dec 8, 2007

Gears are my next project.

I dont know what I have now, but 60 mph is like 2200 rpms in drive.

66forfun · Dec 8, 2007

the msd book shows the different graphs, there is an advance stop bushing to play with too. the main diff from what i understand is mech gives better tuneability and worse mileage. the 2 silver heavy (out of box) give the slowest advance curve, then there is the advance stop bushing to play with too. red (smallest) =28deg, silver=25, blue=21, black (largest)=18deg
keep an ear open for valve clatter when tuning the dizzy.

step up the gearing, big cams and 2.sumpthing gears dont play well together. that is probably your #1 problem for the dogging. stall torque converter/gearing/cam/intake/exhaust should compliment each other.

fasttback · Dec 8, 2007

65fastbackresto said:
I dont know what I have now, but 60 mph is like 2200 rpms in drive.

So 60 mph @2200 with the C4 in 1:1...right? Do you know your tire height or size on the rear (gearing can be calculated)? If you have a stock Mustang 8" rear end it could be 2.79 to 3.00 (most likely). If you're lucky there will still be a rear end tag on one of the carrier nuts. Also, do you remember the optimum range of your cam? With this info we can get you in the ballpark with gearing.

brianj5600 · Dec 8, 2007

See what bushing is in there. Low initial timing will definatly make on sluggish. I think the blue or the black will work best with 33-35 total timing. Vacuum advance is a plus for any street motor. Vizard even said he likes it on a lot of circle track motors.

65fastbackresto · Dec 8, 2007

Tires are P215/50/R17`s

Its a C-4 tranny, and I didnt get a cam card but he said it was a high rpm cam, it dont even seem to pull good until its about 3000 rpm`s, then it goes freaking insane on me. I told the engine builder I wanted this motor to scare me a little, and I got exactly what I asked for, LOL. It has a 2400 rpm stall convertor, which is perfect in this application in my opinion. 2 1/2 inch exhaust thru 18 inch glasspacks out the rear.

Is an 8 inch rear, and I think the tire height is really close to the same as stock would be.

I have to get more traction with lower gears or I`m gonna hurt me or the car....or both.

On the timing issue, I have 14 degree base, and 34 total in at 3000 rpm with the factory stop in. At least thats what I was aiming for, I need to recheck everything again since I`ve got about 100 miles on it now. It dont clatter but it seems to heat up very quickly when i gas on it, like maybe the plugs are too hot or something. But I got medium heat plugs, so I dont think thats it.

fasttback · Dec 8, 2007

215x50x17, 2200 @ 60 mph = approximately 2.78:1 rear end ratio (at least good for the freeway). Play with your timing curves, but you will feel more with the proper rear end gearing. A 3.50:1 is a good start, but your cam/engine and driving habits will help you decide.

65fastbackresto · Dec 8, 2007

What would be best way to get gears?

Just replace the whole rear end or change the guts? I know its an 8 inch, but I dont know anything else about it, if its limited slip or any of that, so far I`ve only smoked the tires once and the cab of the car was so full of smoke I couldn`t tell if it was both tires or not.

fasttback · Dec 8, 2007

65fastbackresto said:
Just replace the whole rear end or change the guts? I know its an 8 inch, but I dont know anything else about it, if its limited slip or any of that, so far I`ve only smoked the tires once and the cab of the car was so full of smoke I couldn`t tell if it was both tires or not.

When you did your burn out go back an look if your car left one or two marks. One mark = conventional and two marks = limited slip or some other type (e.g. locker). Another way is to lift the rear of the car so the tires are off the ground, car in neutral and parking brake off - rotate tires. If they spin in the same direction you have a limited slip differential (or a locker type) or they spin in opposite directions - conventional differential.

You can change the ring and pinion of your 8", or buy a remanufactured 8" center section ready to run and switch them out, or upgrade the whole rear end with a 9" that fits a 65-66 Mustang. It depends on your funds and what you will end up doing with the car now and in the future (e.g street now - drag racing later???). There is some skill needed in setting up a rear end properly. If you don't have the tools and some basic knowledge on rear ends, leave it up to a mechanic.

runawayabc123 · Dec 9, 2007

fasttback said:
When you did your burn out go back an look if your car left one or two marks. One mark = conventional and two marks = limited slip or some other type (e.g. locker).

Are you sure this is true? I have left two marks before with a conventional diff before, but my application is a 4sp manual.

zookeeper · Dec 9, 2007

As have I. If you're car is equipped with traction bars, it'll probably leave two marks, but that still doesn't make it a limited slip. Jack the car up and spin one tire, if the other tiree rotates the oposite direction it's an open rear, if they both spin the same direction, it's a limited slip of some kind. Or a "Lincoln Locker" LOL!!

fasttback · Dec 9, 2007

runawayabc123 said:
Are you sure this is true? I have left two marks before with a conventional diff before, but my application is a 4sp manual.

Interesting. The three different convential rear ends I had in my three Mustangs never left two marks (manual and automatics). All the power goes to the passengers side only. Although, I never used traction bars on those rear ends.

runawayabc123 · Dec 10, 2007

zookeeper said:
As have I. If you're car is equipped with traction bars, it'll probably leave two marks, but that still doesn't make it a limited slip.

I know the wheels go opposite ways so it is not a limited slip. But, you hit the nail on the head, I have traction bars.

mrmustangman357 · Dec 10, 2007

with that motor combo you need to have prolly 3.73 gears to take advantage of that power. And swap to a 9" rear axle!!!

mrmustangman357 · Dec 10, 2007

what vacuum does your motor draw at idle? vacuum advance helps for a street car. at WOT, the vac advance goes to zero so it functions like a mechanical advance distributor anyway.

65fastbackresto · Dec 10, 2007

Ok

Tires spin both ways, so its an open diff.

Dont know how to check the vacuum (dont have guages or anything anyway)

Dont have traction bars, but I have 4 1/2 leaf rear springs and subframe connectors, guessing thats prob close to the same effect.

I dont mind letting a mechanic do it, but I`d prefer to buy something I could just install without having to gut the rear end, its a simple bolt on just changing the rear I`m geusing.

freddiecougar · Dec 10, 2007

Your best bet, and probably the cheapest solution (if in fact it is the solution) is to take your distributor to a shop that can check the curve. Those curve numbers in the instructions are estimations. THere are many variables that affect how the advance comes in. Did your engine builder give you any advice on what initial advance to put in the car? Usually recurved or aftermarket distributors take 10-14 initial. Your problem may be that simple, and would save you some time and money, so I would look there first.

QUOTE=65fastbackresto;7248072]I have a built 302, and my engine builder talked me into buying a mechanical (or centrifigal) advance distributer, due to low vacuum by big cam. This car runs GREAT at wide open throttle or close to it, but its sluggish and doogie just trying to cruise around. I must have some crazy highway gear ratio too cause I can flog this car and run at least 120 mph in 2nd gear without taching over 5500 rpm. I assume had I have went a vacuum advance this car would probably run smoother but I`m geussing....

The dist is an MSD unit that you can change the springs in, and I`m currently running one heavy spring and one light spring, if any of yall running MSD`s know of a better combo on the spring set up let me know please.

Anyone with some experience on this please help me out.[/QUOTE]

66forfun · Dec 10, 2007

buy a vacum gauge for manifold pressure, there is a port on the carb for it. it is one of the most basic diagnostic/tuning tools for the toolbox. you are peeing in the wind without one. Got the performance motor now learn how to get it & keep it running good.

65fastbackresto · Dec 10, 2007

YEa its cool trying to figure this stuff out.

I`m really enjoying it actually.

Could the timing being a little off make the motor heat VERY quickly when I flog it?

Vacuum advance or mechanical advance

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