"Fo-par" vacuum advance on Flathead

Alright guys, here's the sccop....

Just got my Chry-co dizzy back from the machine shop after being modded to fit into the '54 255 Canadian merc flathead, (thanks Rat Bastard for the inspiration!!) and am trying to decide if the vacuum advance will be going back onto it....

I will be running an Offy super dual with twin Holley 94's, so a obviously a ported vacuum source is not available, but, could I drill and tap into the manifold to utilize manifold vacuum aka will this setup even work with full advance while it is idling? (I think the dodge setup adds 14 degrees on the vacuum).

The dizzy will have 22 degrees of advance all in at about 1000 RPM.

What initial timing should I be shooting for with just the mech advance? What if I use this "manifold vacuum" for the additional 14 as well?

On another note, what power valve should I throw into the carbs to start with... Am thinking 4.5" will be a good SWAG....

Let the opinions fly.....

 

Don't know much about your timing but I run 2.5 power valves in both carbs and she runs great! Distibutor is a stocker w/ Pertronix and new cap, rotor. Runs great up to about 6,000 rpm and it starts to run outta steam.

 

On the advance, I think experiment on the road. It will likely want somewhat less than the Chrysler, a lot less if the original use had EGR. Make up some sort of simple stop like a screw through the slot or something and cruise up a hill at part throttle. A car with too much vac advance will ping until you either increase or decrease throttle from where you want to be. No or too little vac advance decreases your part throttle power...so play with it until you find a setting that is as close as possible to onset of ping under a variety of circumstances. You could calibrate the stop you make and study original advance specs from the manual, but I think trial and error will give you the best setup for your individual ride. Since most driving is at part throttle, this setup should be very worthwhile once sorted out. Start out with a ride with the fully stock advance can, then block its movement until you find the happy point.
Then--experiment some more, trying respose with full-time vac versus ported vac. Ford used full time vac on the '32-48 Mallory design, ported vac on the '49-53's. Figure out what yours likes!
Without any sort of vac advance, you lose power in most of your driving. The missing power is translated directly into exhaust heat--and you know where that goes in a flathead!

 

hi mate,

for what its worth i use my mopar dizzy without any vac advance, mechanical only and im very happy with it. having said that who knows if there would be a difference in performance with vac advance. all i know is it runs like an ape compared to the stocker.

danny

 

I'm no expert but it seems to me that if you tie the dizzy to manifold vacuum the advance will be in full at idle and reduce as engine speed increases and vacuum goes down. Seems backwards to what you want or go to the ported vaccum route.

 

This is not at all like the stock Loadamatic--this is a distributor with centrifugal advance to provide a reasonable full throttle RPM related curve. The vac can on this provides the additional advance needed for full power at part throttle. Some engines like their vac to be full time, others prefer ported vac. This vac advance has nothing to do with RPM, only with load differences at whatever RPM. 3,000RPM accelerating up a hill is different from 3,000 RPM cruising on a level road.

 

I would think the kind of vacuum you use would be the kind that the original Mopar unit used. There is nothing magical about Ford's ported vacuum. It just supplied the kind of vacuum that the distributor mechanism was designed to use. For the Mopar unit to work right, I would think you would give it the kind of vacuum it was designed for. Maybe not, but my fresh in from work and not having thought about it mind just leads me to think this.

 

Man ... do a search on the tech archives.... "got time" ....you'll NEVER need to know anything more .......It's ALL there ...post was by C9 i believe....EVERYBODY should read it ..... One HELL of a tutorial ....

jersey Skip

 

Thanks for the input guys....

Am thinking will get her tuned in without the can hooked up, and go from there...

B