Hot-Rodded Engine---Vacuum Modulator Problems???

[Mooosman]

Ok guys, I'm in the process of building a hot-rod 300-6 for my 72 F-100. It's got a pretty good size cam (248/254 @ .050) in it, and I suspect that when I get it running, it will only have about 8-10" of vacuum at idle.

I may be converting it from a 3spd manual to a C4 auto when I swap engines also, and I read on a few websites that the tranny needs at least 15" of vacuum at idle to function properly. Am I screwed when it comes to converting to an auto?

Are there any tricks to getting the trans to function normally?

Thanks

Nick

[Randy Beck]

May have to use a vacuum reserve canister. Summit & Jegs sell them. The canister stores vacuum for vacuum powered

[Jake11]

It will work ok. Get a TransGo shift kit. Got pretty good tips about
setting up the modulator. Not hard to do at all. Should work good with
your 300.

[averagef250]

I've never dealt with modulator problems on fords, but chevies are notorious for big problems with low vacuum when running a TH350 or 400. I've seen several radical small block chev street/strip motors with wiped thrust bearings and scored cranks because the transmission line pressure was through the roof at an idle and pushed the TC into the crank. For chevies at least, the fix I've seen is to run a full manual valve body which eliminates the modulator input. A friend tried a vacuum can as a bandaid for this problem on a 475 horse 383/TH350 combo and didn't have good luck with it. He ended up wiring in a vacuum pump to a switch on the throttle to kick on and keep the vacuum signal to the modulator high at an idle. It works well and allows him to just leave the car in drive at the strip as he likes it.

[Jake11]

I've run 'GM' autos on low vac motors. Even got them to shift ok.
I'm a tranny builder.

The Fords are a lot easier to work with. The modulators are adjustable
and the pin can be changed in length.

Now, if you throw the pressure regulation system in the trash like all the
so called experts do, well stuff will break. Open your eyes. Learn how it
was made/designed/works. Pretty simple hydraulics. When you run 200
line its gonna be a problem.

[Mooosman]

I'm still up in the air about converting it to auto. I've never dealt with the vacuum modulated stuff. I'm a Chrysler guy originally, and all of that auto trans stuff is just based on throttle position. There is no vacuum reference; you have linkage between the carb and the tranny, and it takes that into account to adjust shifting.

Nick

[Clarko]

Go with an AOD. No vacuum modulator to speak of, only a throttle-valve cable that you hook up to the carb in place of a kickdown.

[FoMoCo]

I had a Comp 268 in my 300 and did well as far as vac and streetability.b I can't recall the specs other than it had no torque.

Nick