Hi guys,
Finally making some progress on a cam and manifold upgrade to my 390. I'm looking for some opinions on carb jetting and timing.

This is for a 69 stang, 4 speed, 3.70 gears. Street performance use.
Engine is bored .040 fresh stock bottom end. Heads are 428 CJ (C80E-6090-N) Performer RPM manifold, cam is .562/.565 274/286 adv 230/236 @.050 110 LSA. Have headers, Holley 750DP and a Mallory dual point with mech and vac advance. I'm probably going to drop a Pertronix in the dist.

My questions are is this a decent carb for the app and what jet size should I start with and what to set the timing at- both initial and total.

Do you think this vac advance distributor will work with this cam? Or should I plan on getting something with mechanical advance only?

A new carb is not really in the budget right now so I would like to make this one work for now but I would definitely consider something else in the future.

Thanks!!

 

For a Mustang the DP is just fine and start with the jets already in the carb. Should be close. As to the dist., try it and see. You can always recurve it and not use the vacuum portion.

 

who's turn is it to make pop corn ?

 

Mine

Subscribed

 

I guess you 2 were wrong. Popcorn's gone, and no entertainment.

 

Thanks Bear.
Since the new cam is much larger than the old one I wanted to err on the conservative side and jet rich if anything.
Do you or anybody else have any idea of what initial and total timing advance should be? I was thinking around 10/36
Thanks again!!

 

BUMMER!!

 

Man, this is getting old. At first I thought one of our posters just didn't understand. But after seeing the engineer explain it, along with myself and others, I guess it is just beyond his ability to grasp. But we'll try yet again.


Sixtyniner, keep in mind I bear you no ill will. We ( another poster & I ) will in all likelyhood have a long discussion about vacuum advance. I'll try to adress your question rather than teach one of our own who, lets say, disagrees with me. If you are going to drive the car on the streeet ( ever )and you care about the differance between 7 or 8 miles per gallon and 12 or 14, then you should use the vacuum advance. Vacuum advance has a HUGE effect on fuel economy and driveability. You should use a single diaphram can, not a double, and you should hook it up to manifold vacuum. The double vacuum diaphram cans and ported advance hookups are strictly emission contol band-aids that won't help you and they WILL make things harder. Rather than getting a kit for the Mallory, how about a Duraspark from a '75 or '76 truck engine ? You'll have to change the vacuum can and adjust the curve, but if you are tuning anyway......why not ? DinosaurFan

 

Thanks for jumping in Dinosaurfan. I have to admit I'm not familiar with the Duraspark. But from what I think know the cap is much larger in diameter than the Mallory and that may cause a problem with my Monte Carlo bar in the Mustang. Also, and I know I may not fully understand, but if I have to buy the Duraspark, (new, used?) then modify it buy changing the vac can I think I might as well stick with the Mallory and add the Pertronix. I also have to say the Mallory looks kind of cool with the machined aluminum base.

 

With this cam he should run ported vacuum as it idle better than with the high timing the manifold vacuum wil cause.

 

To think I almost missed this one!!!
dinasaurfan--give me a minute or so, its a popin!!
I have to say that amazingly "ported" is suggested, which is good and acknowledges that vacuum advance is needed for street use. I would prefer full manifold, but I can live with whatever is used at this point! I can tune for either.

Mines finished poppin---proceed

 

Just so that those who don't know what the point is, there are two ways of hooking up vacuum advance.

One is with "ported" vacuum, which only applies vacuum to the vacuum advance when the throttle is partly opened and RPMs are low. (this is ignoring the fact that there IS some "ported" vacuum at high RPMs with the throttle wide open, but ignore that for now).

"Ported" vacuum applies MORE advance when you hit the gas to take off from a stop, or are cruising on the highway and push on the go-pedal to speed up or pass. The reason being to either help with fuel economy, or to get a little more torque/power without too much mechanical advance at any one point.

"Manifold" vacuum is high when you are NOT on the throttle, or are coasting, and is LOW when your foot is into the gas pedal. The more you push the gas pedal, the lower the vacuum.

"Manifold" vacuum is usually used for vacuum advance to pull the advance way up at idle. This is done because of a very big cam ruining low-end vacuum, the more advance you have at idle, the more vacuum, and the better it idles. This helps cover a lot of problems with big cams. Idle quality, power-valve opening, brake booster function, lots of things. ALSO - if you have a lot of compression and the engine is prone to pinging at light throttle, "manifold" vacuum will back off the timing as you lean into it so that the pinging goes away.

"Ported" vacuum was used in STOCK vehicles from the factory. Some say it was for emissions, which is partly true, less timing advance does help emissions. However, that is not the only reason.

"Manifold" vacuum is used to cover issues with high-performance cams, mixed with a carb that is too big, and/or high compression. It also helps the engine start more easily because the timing advance is not so high, causing the starter to be overworked. There are ways to deal with the issues, that involve cam timing, not going with TOO big of a cam, or keeping the compression within reasonable limits. Some say having to go to manifold vacuum points out you didn't build your engine correctly. Which is also true. You also need an adjustable vacuum advance, and to recurve the distributor to get the best results.

I think I have explained it as it stands, there is NO NEED for an argument for the pros or cons of either.

This does not cover dual setups where the vacuum advance has two ports on it. To some degree, this was a big band-aid from the factory, and has limited applications.

 

Umm, Art, just where are you getting your ideas ? Your explanations of how they work is sort of okay, and with out getting into a Pro/Con thing.............ported spark adavance is an emissions thing, pure and simple. It has no other function. If it dores, i'd love to know what it is. Yes, lots of vehicles came with ported advance. But lots of vehicles came with manifold advance as well. It depends on when it was manufactured, where it was sold, and which emission laws applied at the time. The idea that manifold vacuum is a 'crutch' to cover build mistakes is just plain wrong. Look at the schematics for most pre '68 49 state cars. DinosaurFan

 

Interesting statement about ported vacuum, but totally false. Ported vacuum being is NOT emissions related. Ported vacuum with Holley carbs appeared in 1957 with the then new 1850 series of carb in the 4150 configuration and there were no, none, NADA emissions requirements then of for at least 10 more years. So how the hell do you support your emissions statment.

 

Agree on ALL points.