I'm working on a Model A that was purchased as an un-finished project. It has a SBC with a Fenton 3X2 intake and a set of 97s. The tranny is a stock Th350 that has a vacuum line. And the car has a vacuum-advance distributor which I am converting to Pertronix. The intake has a single vacuum port located in the intake below the center carb; this port is epoxied into the intake and the epoxy is cracked and the tube is loose. Question: 1) Can both the Th350 and the distributor share a single vacuum source with a T-fitting to allow both to receive vacuum signal? If so, where to place port? 2) If not, where to place two vacuum ports? Discussion: The Stromberg site says that I should buy one of their vacuum-modified carburetors. If I did that, will the T-fitting plan work to give vacuum signal to both accessories? Thanks, Keith
On my avatar 39 with 3x2 97's on flathead, found insufficient vacuum on a T-fitting to properly operate a C4 auto trans.........req'd separate vacuum port for this..........other vacuum port was for PCV setup as mechanical advance in distributor.........plse note that full race cam may have been issue with insufficient vacuum on T-fitting initially, but didn't mess around, just made provisions for a 2nd vacuum port...........
Manifold vacuum for the tranny and ported vacuum for the distributor works best if you have a vacuum port above the throttle plate. Otherwise 2 separate sources. You can always put a vacuum storage can to help if you can't get a second source.
Manifold vac is high at idle, drops rapidly when you slam the throttle open and will be high when at cruising speed. Ported will be lower at idle, as it is taken from the carburetor venturi. With increased airflow through the carb it will rise. The smaller the carb typically you will see more ported vac compared to a big carb.
Ported vacuum by design is absent at idle RPM, to keep for a steady idle. That's it. At other speeds vacuum signals between the two are more or less identical, for all practical purposes. When load is low, like at highway cruise speeds, the vacuum is high and ignition advance will be as much as 50 degrees or so BTDC. Most stock engines should be connected to ported source. Modified engines are usually connected to a manifold source.
The transmission should be full time vacuum. Just drill and tap a hole in the back of the manifold, down low and out of site. Vacuum advance can be ported or full time vacuum, depends on how the engine responds when hooked up. Type of ignition has no effect on vacuum portion of distributor.
Ok, thanks fellas! So, I need a vacuum source above the venturies for the distributor (will look into the modified Stromberg 97 for that). And, I need to close the leaking port in the intake manifold and drill a new port that is in a convenient location for the tranny. Finally, if I ever install an old vacuum-operated windshield wiper, where should I source THAT vacuum. And, if I install a PCV system, where should I put that vacuum port? My initial plan is to just put breathers on the valve covers.... Sort to ask so many basic questions-I've been an LS guy.
You don't want vacuum wipers when there are so many good electric choices. I sure am glad you aren't putting one of them ugly ass LS motors in your hot rod. Great motors ,just ugly.
The electric wiper motor that i have found is so thick that opening of the windshield is restricted. Been thinking that an old vacuum wiper might be more "traditional."
Rain x treat that windshield and use a hand operated wiper. You will probably never need to wipe it anyways.
Thats probably where I'm headed on the wiper: Rain-X. On the vacuum subject: My plan is to remove the leaking epoxied vacuum port on the driver side of the maifold. Then, I'm gonna get carb spacers and drill and tap the middle one for a barbed port for manifold vacuum for the transmission-will probably use the middle carb for that. I will get one of Stromberg's ported carb bases to provide ported vacuum to the distributor. Thanks, fellas! Any suggestion on carb spacers-how thick is best?
You could use manifold vacuum for the advance like Chevy did; it will run cooler at idle and in stop & go traffic. Those cheap long shaft electric wiper motors are relatively easy to mount thru the windshield frame on an A and won't interfere with swinging it open.
Enough to absorb the heat from intake so it don't boil the fuel in the bowl with this crap gas we get now. I make mine from marine plywood as it absorbs heat better. My old 57 312 would boil the fuel like crazy when in traffic when it was hot, almost died in a fast food drive through line. Made a plywood spacer 1/2 inch thick and cured the problem even with a/c on.
While car is in body shop I ran over and pulled the intake off and my buddy got the cracked intake vacuum port it on the driver side and capped it. He then drilled and tapped for a manifold vacuum port on the passenger side for the T350 to pull signal.
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