View Full Version : Smog equipment on the 68's

03/29/2005, 09:48 PM
Just like to get some opinions about keeping the smog stuff on my GT/CS. On one hand it's nice and original and I really want to keep this car stock, but on the other I'm hearing that it's robbing horsepower. So 2 questions that I'm looking for here and then I'd also like some of your opinions on the matter.

1) Does anyone know what the stock 302 4v Top Loader put out for horsepower?
2) Does anyone know what I would gain if removed?
3) Sorry I lied, one more question.. Does anyone here not love Paul N. - geeze, i've been around for just a few short weeks and this guy is my hero!! Keep up the good stuff Paul.

thanks in advance...

03/29/2005, 10:30 PM
if you decide to take the smog stuff off please let me know. I would give it a good home if it fits on my 289.

03/30/2005, 01:21 AM
If I recall the 302-4V was 230 HP. Might of been 225 but I think it was 230. Back then they measured the engine, not at the rear wheels so it would have been the same whichever tranny you had. I have no idea how much the smog stuff takes away but I would base the decision of whether to remove it or not on what you plan on doing with the car. If you're going to show it, leave it on, because you just don't see it any more. If you take it off, I'd save it, but if you feel the need to get rid of it, there is probably 2 dozen guys here, myself included, that would love to have it. Lastly, yes, Paul's the man! We're not worthy!

03/30/2005, 09:20 AM
"If" you removed your smog setup, you would get improved mpg and more horsepower. The reason is the car would NOT have to work as hard because of the smog pump.
I suppose you could just take off the belt and put it back on when it came time to show.

As posted by another member, I do recall reading somewhere, only the manual trans in Ca.had smog as the auto trans had another way to get rid of the emissions. I don't recall how though.

03/30/2005, 08:15 PM
You know--I need to chase this down for the new book. Smog gear was on automatics and 3&4 speeds, too. I once saw a Canadian '68 Mustang Convertible with no smog on it's 390 at all!

I have heard that on the 390's, the smog pipes in the heads makes the engine run hot, since the exhaust valves are kinda small. It's not so much the smog pump, etc., but how the engine timing is retarded, which literally knocks it in the head!!

Here in Calif., anything built in 1974 and earlier is not required for a smog check. I've had mine smogged something like 10 times since 1974, and it "barely" passed a few times (ever see a Ford blue air cleaner with snorkle and heat riser--on top of a Cobra Intake? It's not pretty). If you lean out a Holley, and use splitfire plugs it will lower the NoX levels. But that is all history now.

I think some of you might like to know how ot restore the tubes, etc., and what is what. Other states might need to know that stuff--but I keep seeing this "subjective" attitude from various smog stations and referees.

Paul N.

03/31/2005, 08:51 AM
In 1968, 289/302 engines used two types of emission control systems, called Thermactor and Imco. The Thermactor system was used on manual transmission cars, and the Imco was used on automatics.

The Thermactor system burnt gases in the exhaust ports of the cylinder heads. All V-8 small block cylinder heads were cast indentically, but the Thermactor system heads had air injectors that were drilled into each exhaust port (early style) or were drilled from the front of the head and then drilled again in the exhaust port (later style). The earlier Thermactor had tubular manifolds with a backflow baffle while the later style had the backflow valve bolted directly to the head. A smog pump, driven by a V-belt, forced fresh air into the heads on the Thermactor system. A great diagram of the Thermactor system is found on page 8-63, Figure 15 of the 1968 Mustang Shop manual.

The Imco system is much less complicated. There is no smog pump or direct air-injection system used with the Imco system. The heads have a raised "bump" in the exhaust port where the air injection holes of the Thermactor system would be drilled, but the Imco heads are not drilled. A PVC valve is used, the carburetor is specially set and the distributor was set to allow for the least amount of emissions.

If I was going to take off the smog system on a small block, I would try to find "J" Code heads for higher compression, or find a set of 1965 heads to get rid of the 1968 "bump" that restrict the flow of exhaust gases. Just removing the system won't do much to increase power or improve gas mileage.

Hope this helps

PB gtcs
03/31/2005, 04:28 PM
Or, remove the bump (port) the heads. We did that on our engine, see gallery. That will let her breath better.


03/31/2005, 07:32 PM
Thanks, Russ. That is really great information.