[MalteseFalcon]

The "distributor spark thermal advance vacuum switch" (that's what my '78 service manual calls it) that originally sat in the intake manifold cross-over on my '78 Trans Am was broken off when I got it and I just installed a temp sending unit in this location. The car seems to run ok withoutit, but I'm wondering if I'm missing some performance and should replace it (and find the necessary vacuum hoses that go with it). Any thoughts would be appreciated.

Thanks for a great forum.

Steve

[MalteseFalcon]

The "distributor spark thermal advance vacuum switch" (that's what my '78 service manual calls it) that originally sat in the intake manifold cross-over on my '78 Trans Am was broken off when I got it and I just installed a temp sending unit in this location. The car seems to run ok withoutit, but I'm wondering if I'm missing some performance and should replace it (and find the necessary vacuum hoses that go with it). Any thoughts would be appreciated.

Thanks for a great forum.

Steve

[prboylan]

The thermal vacuum advance switch (and similar devices) are part of the 70's era emissions systems. You can safely bypass or delete the switch unless you need it to pass visual inspections. For best efficiency and performance connect the distributor vacuum advance to the ported vacuum port on the carburetor. This will apply vaccum advance at idle and cruise, and almost no vaccum advance at full throttle.
Adjust your initial timing to obtain maximum manifold (unported) vacuum at ~1500-2000 rpm. Then road test. If it pings under load back the initial timing off bit by bit until the ping goes away.

[Will]

Um, Phil, the ported vacuum source on the carb will supply NO vacuum to the distributor at idle. A manifold vacuum port could be used if vacuum advance at idle is desired, or use a port on the intake manifold itself.

Neither ported nor manifold vacuum sources will supply significant vacuum at WOT.

FWIW, I prefer to use a ported vacuum source exactly because it does not induce vacuum advance at idle.

If your car is not subject to visual inspection, you can eliminate all of the factory emission controls and not lose any performance, provided you retune the engine afterwards, however several systems don't hurt performance at all and cleaner air isn't really a bad thing (evaporative emission controls, and possibly EGR, though the apparatus that delivers the exhaust gases to the engine tends to be intrusive in the intake manifold, reducing flow and plenum volume).


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'72 Formula 400 Lucerne Blue, Blue Deluxe interior - My first car!
'73 Firebird 400/4-speed Black on Black, mix & match.

[prboylan]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Will:
Um, Phil, the ported vacuum source on the carb will supply NO vacuum to the distributor at idle. <HR></BLOCKQUOTE>

Thanks Will...
Dangit I KNOW that!! Check the timestamp on my post above. It's my only excuse.

[MalteseFalcon]

Thanks Will and Phil. We don't have emissions inspections at all in Alberta, but I agree with Will about leaving in place the stuff that doesn't hurt. My car is missing some lines from the evap system, and I plan to replace them. Mind you, my car was missing the cat when I got it, and while I have thought about replacing it with a pair of hi-performance cats when I do the exhaust, I have to admit that would be a fairly low priority. I'm also considering replacing the manifold with a Performer or a 68-72 cast iron 4 bbl.

Steve