Why don't 4-speed cars have thermacs? Do they have heat risers?

davepl · Poncho Master!

Why don't 4-speed cars have thermacs? Do they have heat risers?

427carl · A Poncho Legend!

rteal men don't need heat riser or chokes

4SPEED427 · A Poncho Legend!

davepl wrote:
One part I'm still looking for (69 2+2 427) is the proper air cleaner. From everything I've seen they do not have thermac valves and heat stoves (with the little heat pipe that comes up from the stove to the air cleaner). So I've got this open-element one on there for now, as pictured.
So, I have two questions:
1) Anyone know why four-speed cars don't have them? Cold stalls wouldn't be as big a problem in a stick car, but is that all it is... they just don't need them as much?
2) Anyone know if the four speed cars still have heat-riser valves to force hot exhaust back up to the intake manifold? Looking at my manifold, there's no way it could, but I'm not certain.

-- Edited by davepl on Sunday 23rd of October 2011 02:42:43 PM

Hmm, interesting observations/questions. I may tackle the parts book to see if it has any answers...

68beaumont · Guru

Pumped gas at a Supertest station from 1971 to 1973. I do remember one customer's car (69 Beaumont factory 350 300hp 4spd) which had no thermac but had A.I.R. system. Can't recall if it had heat riser.

davepl · Poncho Master!

427carl wrote:
rteal men don't need heat riser or chokes
..Real men have a clutch pedal, and you can't three-foot a cold start car!

davepl · Poncho Master!

Some years back I scanned and OCR'd the thousand page parts catalog, and remebering they call it a "silencer", I was able to search for and find the air cleaner assemly part numbers, I put them below.

It doesn't tell me what's what, of course, bt you can see that stick cars have different part numbers, and additionally, the 335 and 390hp cars are different as well. So just for 427s you have four different part numbers.

-- Edited by davepl on Monday 24th of October 2011 12:49:25 PM

davepl · Poncho Master!

While I'm at it, I just cut to the chase and looked for the heat stove. It's an "air heat stove" for the small blocks and a "carburetor air heater duct or shroud" for the big blocks. It looks like ONLY the TH400 cars have them. I think that answers my own question.

Below that you'll see the part numbers for the exhaust manifold, and the big blocks all use the same left side one. But big block cars have different right side manifolds for AT and ST. Which is why, I'd imagine, my manifold looks different than all the AT cars I see.

The ST manifold looks like a better design, and I'd argue the ST 427/390 makes more power than the AT 427/390, but I can't prove it

2qwik2c · Poncho Master!

I have seen a 1968 Impala SS427 all original low mile 427 4spd CDN car with the thermac valvle and heat stove. So some did come like this in 68.

66 ponchoboy · Veteran Member

Hi could you list the air cleaner for a 66 390hp 427 AT I need one and also need the exhaust manifolds ? if its not too much trouble.. I guess you shouldn't have admitted you have the catalog eh.

davepl · Poncho Master!

6484409 1966/ALL WITH 427 /390 HP/ ENG /SUP 6421773/

67Poncho · Uber Guru

66 ponchoboy wrote:
Hi could you list the air cleaner for a 66 390hp 427 AT I need one and also need the exhaust manifolds ? if its not too much trouble.. I guess you shouldn't have admitted you have the catalog eh.

Actually Steve, many of us on here have the catalogue! Ask any of us and we would be glad to help out!

CdnGMfan · Canadian Poncho Superstar!

For 1968 & 1969, V8 Chevrolet (& Chevrolet-powered Canadian V8 Pontiac) used the A.I.R (smog pump) with any manual tranmission V8s and / or any with open element air cleaners (eg, manual trans Beaumont SD396 396-325 plus ALL Beaumont SD396 396-350 with any trans).

The Thermac was used with all the rest of the V8s with automatics & closed-element air cleaners (307 through 396-325).

The engineers may have thought about certain dirty phases of operation that might arise specific to manual transmissions (closed throttle or perhaps lifting during a shift). I really don't have the answer.

In most cases the A.I.R. system was dropped for 1970 and replaced with the TCS (Transmission Controlled Spark) system. It delayed vacuum advance for a time period (after 30 seconds in high gear continuously) before vacuum advance would happen (there was still centrifical advance, but total timing was lessened). Certain "tough to tune for smog" engines like 11:1 compression, solid lifter cam big blocks still got the A.I.R. pumps in addition to TCS.

By 1973 they threw all they had at the engines in order to meet the toughening standards. A.I.R., TCS, Thermac, plus the new EGR valve. By 1975 the only realistic way to meet the tightening standards and still be driveable was to use a catalytic converter. Note that some Canadian fleet cars could be ordered without the catalytic converter by basically running the 1973-74 equipment with further (de) tuning. Finding unleaded gas was a temporary concern around 1974-75 so by deleting the cat & substituting alternate tuning they could legally sell them (take that Mr. MOE Roadside Inspector!

davepl · Poncho Master!

It sounds from your description that the thermac was smog-related, but I'd always assumed it was a cold-weather driveability thing. Maybe both? And indeed, "flipping the cap" as we did in the day, turning the car into an open-element, caused it to bog and stumble when cold. Which is also weird, because I had an automatic 1963 Imperial with a 413 that had no air heater system, and it was fine.

AIR injects air into the exhaust (presumably to help with unburned carbon), so that's an entirely different beast.

CdnGMfan · Canadian Poncho Superstar!

davepl wrote:
It sounds from your description that the thermac was smog-related, but I'd always assumed it was a cold-weather driveability thing. Maybe both? And indeed, "flipping the cap" as we did in the day, turning the car into an open-element, caused it to bog and stumble when cold. Which is also weird, because I had an automatic 1963 Imperial with a 413 that had no air heater system, and it was fine.

AIR injects air into the exhaust (presumably to help with unburned carbon), so that's an entirely different beast.

Dave, you're right. It is both driveability & emissions related. It is to do with preventing carb icing, promotes fuel atomization. It also helps with getting the choke off sooner. There is something else to do with more closely controls the intake air temp, thereby controlling conditions for certain exhaust emission components to form

Flipping the lid. I used to do that in my friend's Laurentian Safari with an orange Chevy 400 4-barrel. Those Quadrajets really whumped open, what, 1.45 primaries & 2.25 secondaries?

I love your 69 Pontiac Dave. Do you have a current like to some pictures online?