View Full Version : Anybody run a Thermoquad at hi altitude?

08-16-2014, 02:00 PM
Am looking at running my car at Bandimere next month and curious if there's anything required to make the TQ's secondaries work as well up there as it does at 1000' alt. I'm presuming an air door adjustment will be all that's required, if anything, to make it operate correctly, but admit I do not have the vaccum pot attached to control door opening and haven't had one in the 12 years I've run the carb both on the original 360 and now on the 440. Alternative is the carb I've been running for about half of this year, a 3310 Holley with its adjustable secondary opening springs.

9.5" Dynamic converter, 3.55 gears (I could put 4.30s back in and just might do that) and 28x9 slicks, car turns a 1.7 60' in bad air, 1.65 when all is perfect.

I've got a wideband A/F meter to tune the mixture, just looking for any potential issues with the secondary door operation.

08-17-2014, 08:05 AM
I don't think you'll have to do anything to the air door. I am confused about the vacuum pot you mentioned. Thermoquads don't usually use a vacuum pot to operate the secondary air door. There is a vacuum operated choke pull-off, which does have a connection to the air door. But that doesn't really enter into normal operation once the engine has warmed up. The air door is opened by the vacuum in the secondary venturis and delayed by a spring to eliminate bogging at wot. With the lower O2 at altitude, the air door will be just a little slower to open, but that should compensate for the leaner mixture. So I bet you won't need to adjust anything. If you find you do need to adjust anything, it would be either the air door delay spring or the the metal piece that sticks up from the metering rod cover that acts as a stop to the air door. Or you might want to consider going to richer jets and/or metering rods. That requires pulling off the top of the carb. So you would have to replace the well seals and carefully torque the top.

08-25-2014, 12:42 AM
C-rod, I've read numerous times about that pull off controlling air door operation, but like you am confused about it being true as I've not used them on my TQs. I'm certain I've read it from Ehrenberg in HPM/MA, but here's a typical warning from Mopar Muscle's Steve Dulcich, cut 'n pasted from their online article at http://www.moparmusclemagazine.com/techarticles/engine/mopp_1304_rebuilding_a_carter_thermoquad_carbureto r/#ixzz3BN2B6lzF :

Always test the dashpot (choke pull off) diaphragm. This dashpot also controls the secondary opening via a controlled bleed-down orifice in the vacuum fitting, and is critical to the operation of the secondary.

Like you, I've not found it to be critical so far. I'm guessing when we quickly flop open the secondary throttle plates a dashpot would keep the door from opening instantly and bogging if the spring setting is not optimum for every condition. I could see such a function as a worst-case scenario protection, but fortunately whether staging at 1800rpm or 3000rpm my car hasn't bogged at launch. (Knocking on wood here.)

The A/F meter showed me how my Mighty Demon was leaning out while cruising to the track and I found how to adjust the air bleeds to cure that, which was good until 4 of the helicoils stripped out of the main body to turn that carb into a paperweight. Holley/Demon will not sell a main body separately, so it's my TQ or the 3310.

I've got a good range of TQ jets and needles, have found a combination that has the primaries in the mid-to-low 14s at most all cruising speeds and loads, .098 jets with 1996 or 2086 rods. Secondary side I've got 110's, 125's, 137's and 143's, have tried all of the latter 3 and the TQ has 137's now. Picked up some even leaner secondary jets from a rebuilt IHC TQ, will take all of that to the track along with the tuning kit for the 3310.

08-25-2014, 08:24 AM
A marvelous thing about the tq is it is very forgiving. So it can be off a little and not be noticeable. Just means you can get away with stuff that isn't possible with other carbs.
But, at the track, you want everything perfect. I don't think altitude will have any effect on the air door. At wot, vacuum is low anyway. The business of delay with the dashpot vacuum is more of a nicety than anything else. As long as it's not bogging, the air door is working ok. There is a little adjustment to position the air door at full or nearly full opening which changes the venturi speed a little.
I think you have just about everything you need as far as metering rods and jets are concerned.
Another adjustment is the metering rod piston height. Since the air is thin at altitude, that adjustment might be the most critical. And that takes all of a second to do. Important to do just a little at a time.

08-25-2014, 04:57 PM
I keep a very thin screwdriver under the passenger side floormat, I think the starting point is 1.5 turns from bottomed out, but believe mine was at 4 turns out at one point. You can only go so high because the metering rod hanger is limited in its upward travel, but within that range is a good variety for cruise mixture. I found a fine line with two sets of needles so if I wind up driving the car out I can make adjustments if needed on the way.

Here's the only article I've seen of a TQ run and adjusted on a dyno.


They showed some interesting results when messing with the air door setting. In the past whenever someone would dyno a TQ in an article they'd give up on it as being too rich and swap on a Holley for the actual tuning. Rare to find this detail.

A portable dyno showed up at our club's annual show, I caught the 'Dere running with the TQ.


08-25-2014, 05:53 PM
That article is what I was talking about as far as air door adjustment is concerned.

09-18-2014, 10:38 PM
Well, it worked fine. I drove the 550 miles to Denver on the primaries (I assume) and tuned as I went with the hanger for the metering rods. Easy to get up to 15:1 A/F for the highway, which in Denver went to 14.2:1 or so. 110 jets in the secondaries gave 12.7:1 A/F past the 1000' line at Bandimere, and the car still ran 12.60 using the 3.23 gears I drove out with. Never had to go to the 4.30 gearset I brought along in a bucket as the car cut lights of .010 and .016 in time trials (when I did my part). Sportsman class has a dial limit of 12.75 at Bandimere and when the air finally warmed up on Sunday and the car slowed it backed right into that 12.75. Had my opponent not cut a .014 light to my way slow .069 I might have made the third round! :crying:

First time trial on Friday, I got a very slight stumble maybe a car length off the line. A quick tightening of the secondary air door eliminated that for the duration. Now that I'm back at the home track, I want to try the 3.23's in the 1/8th with our air. 3.55's get right at 7.50 here on good days, and at Bandimere the 3.23's showed 8.05 in the 1/8th. Back to 137's or 143s in the secondaries for the weekend.

09-21-2014, 08:05 AM
Sounds like it was a little rich at altitude with the thin air. But 12.6 and 12.75 is incredible! I am amazed that you had the stumble at all, but glad that a mild air door adjust took care of it. Did you adjust it at the tension spring or at the choke pull-off?

09-21-2014, 04:38 PM
Just tightened the tension spring, I do not have a choke pull-off on the TQ.

I checked the last two time trials I made at our track on 31 August with the 3.55's and Holley 3310. They called the races because of rain/track conditions and the weather was pretty good for late August. Car showed 60' times of 1.698 and 1.683 which I'd call typical for the conditions, 1/8th mile 7.492 and 7.520 at 91mph. With the 3.23's and the TQ yesterday the best 60' for 7 runs (2 time trails and 5 racing laps in 2 classes) was 1.70 before I started deep staging to get a better reaction time. 1/8th mile times were 7.65-7.68 at 89mph. I'd say the air was at least as good as on 31 August, maybe better, and with shift points at 5400rpm each time I'll call the 3.23's just a bit too tall! Good cruising gears, though. 3.55's will go back in for next racing season, or I might try to get some 3.91's going. The 4.30's are definitely overkill for the combo, but I imagine the transmission sees less strain with them.