AT/MT Quadrajet. What is the difference between the two?

cdnpont · Canadian Poncho Superstar!

AT/MT Quadrajet. What is the difference between the two?

4SPEED427 · A Poncho Legend!

http://www.gmpartswiki.com/getpage?pageid=64722

http://www.gmpartswiki.com/getpage?pageid=64723

Here's the two pages with the 2 carb breakdown side by side. At a glance the only thing I find is the float bowl part number, base plate (linkage difference maybe?) and the metering rods.

dcneric · Poncho Master!

The auto carb would have extra linkage bracket on the primary throttle for the kick down. As Carl noted. Jetting maybe different but if the engine is remotely modded, ie different cam, you may want to fatten up the jets a bit anyways.

HK1837 · Addicted!

Only main difference will be the distributor vacuum port. On an auto it is full vacuum, on a manual it is ported. Generally once you get into cars with carbon/charcoal canisters there is no longer a difference, they are all the same as you need ported vacuum to operate the canister purge. Generally these have a full vacuum port (often at the back) that has a plug in it on manual cars, and the dizzy and canister share the front port via a T piece.

cdnpont · Canadian Poncho Superstar!

Thanks guys.

HK1837 · Addicted!

Just to expand on the factory full vs ported vacuum on auto vs manual. The auto uses full vacuum on vacuum advance so that the distributor advances, increasing the idle rpm to overcome the torque converter. This is why on an auto you disconnect the vacuum advance to set your idle in PARK. A manual gets ported vacuum as there is no vacuum on it at idle, otherwise the engine would race at idle due to the vacuum advance adding idle advance.

This is on older cars. Newer stuff once pollution equipment started to get added used other methods like solenoid operated throttle nudgers.

cdnpont · Canadian Poncho Superstar!

This ported vs non ported between and auto/manual is interesting.

I've been playing with my timing, and yes, like in most all vintage cars I've known, the rpm goes up quite a bit when hooked up.

Would it be on any benefit in my case to worry about this in a manual car?

HK1837 · Addicted!

The revs rise as its an auto carb. The best solution is to get a manual one and get it rebuilt, and keep your original auto one aside. They are pretty cheap to rebuild if you find someone that knows them well.

Your 7027202 (DB) is an earlier 1967 model year auto non-A.I.R carb for 327/240hp, 327/275hp or 350/295hp. Just look for a 7027203 DZ (early) or 7027213 DZ (late). The A.I.R versions in 1967 are 703 prefix, but in 1968 A.I.R was across the board in the USA so they go back to 702 numbers. You'll still find the 1967 carb numbers in use in 1968 though outside the USA, like in Australia and most likely Canada. The 7027213 DZ can be found in the traditional 1967 Rochester style with the round casting on the throttle side where the tag goes, or in the 1968 Rochester style. You'll probably also find them in Carter Quadrajets but will use the 1967 Rochester casting with the circle, with the ID numbers stamped upside down compared to a Rochester. The carbs we got here on out GTS327 HK Monaro released in July 1968 were 7027213 DZ almost always with a 0048 date code on the throttle plate (4th January 1968), these were all 1968 Rochester bodies.

Here is what you'll need, but you'll find them cheaper used at swap meets:

www.ebay.com.au/itm/1967-CAMARO-CHEVYII-CHEVELLE-275-295HP-7027202-ROCHESTER-Q-JET-DATED-L6-RESTD/283937571460

I also can't remember where the vacuum porting difference is achieved, that is if it is done in the main body or in the throttle base. If it is the throttle base you may be able to rebuild yours with a manual throttle base. I'll have a look at a few carbs I have here pulled apart and see if it is obvious.

cdnpont · Canadian Poncho Superstar!

Thanks for the great info Byron.

unruhjonny · Guru

I believe an abbreviated/simple explanation might be: metering

Auto versus manual often entailed different camshafts, which made different requirements for fuelling.

I dabble with Pontiac Quadrajets;
I normally see that the automatic non ram air castings will have a rear vacuum break, while the manual applications will not.

HK1837 · Addicted!

As far as I am aware the auto and manual Quadrajet carbs (at least for a SBC) were identical tune, they even become the same carb number once charcoal canisters are fitted. The only difference is the vacuum advance port source, and of course the kickdown for the auto trans if a Powerglide or TH350. Some have an idle compensation setup on the back of the carb, not totally sure how/why/when these were used.

On a SBC, manual and auto used the same camshaft too, for the same power level engine. From the later 60's period there was basically only one hydraulic camshaft. There was a high output hydraulic as used in L79 and L46 engines but I don't think these came in auto, only manual so only the one carb for these anyway plus I think the L46 Corvettes had an early electronic distributor with no vacuum advance port anyway.

The rear port on the Quadrajets we got here (once charcoal canisters were fitted) is for full vacuum for the distributor, and they also have the front port which is used for canister purge. On the manual cars the canister and distributor share the front port via a T-piece, and the rear port has a bung over it.

unruhjonny · Guru

My Rochester books still show manual and automatic application numbers for Chevy's last time I checked, but I don't have that many;
My newest one is for model year 1975.

I ask this because I am curious;
When would you surmise that Chevrolet went to a "one size fits all" approach on carbs?

Typically with Quadrajets at least:
If the last number is even, it's for an automatic application.
If the number is uneven, it's for a manual transmission application.

HK1837 · Addicted!

That odd and even only applied pre charcoal canister as far as I am aware, as the difference was the vacuum advance source for the port on the front right of the carb. There may be some instances through when TH350 or TH700 were used that have a difference on the throttle attachment for the cable kickdown hence the need for a different number for the auto carb. I have seen a few with a bolt on attachment though. In Australia we got the following:

7027213 DZ (1968 GTS327).
7028212 DH (1968 Impala and Parisienne with 250hp 327).
7029281 ZA (1969-1971 HT-HG 308 manual).
7029282/29282 ZB (1969-1971 HT-HG 308 manual).
7029283/29283 ZC (1969-1971 HT-HG 350 manual (L48 engine)).
7029284/29284 ZD (1969-1971 HT-HG 350 auto (LM1 engine)).
7041281 ZE (1971-1973 HQ 308 manual).
7041282 ZF (1971-1973 HQ 308 auto).
7041283 ZH (1971-1973 HQ 350 manual (L48)).
7041284 ZG (1971-1973 HQ 350 auto (L48)).
7042202 DJ (1973-1974 HQ 350 auto).
7043283 (1973-1974 HQ, HJ (and LH Torana) 308 manual).
7043284 (1973-1974 HQ, HJ (and LH Torana) 308 auto).

These were the period where the auto and manual Quadrajets were different, and the only difference is the vacuum source on that front port. We got unique numbers to North America after the GTS327 carb as the USA went to smog in 1968, we didn't get that until the mid 1970's. Plus the 308 was a different engine to anything else in the USA at the time so got a different tune (4" bore 308ci 4BBL engine) you can see the return to a US smog spec carb on the 1973 HQ L48.
From January 1st 1975 we got charcoal canisters fitted and from then onwards there was only one spec carb for auto or manual. Auto on these were either a TH400 or a TH180 (Trimatic) which both have electric kickdown. Examples:

7045281 (1975-1976 HJ (and LH-LX Torana) 308 manual or auto).
17056280 (1976 HX (and LX Torana) 308 manual or auto).

unruhjonny · Guru

I like to say that there are exceptions to every rule, because it's so true.

The charcoal canisters came here starting in 1970 for California built (or destined??) cars;
I am unaware of any California built cars being shipped into Canada outside of one occasionally getting to the lower mainland.

Since I know Pontiac's the example of these would be is:
7040263 = Pontiac 400/4bbl 'L78' s/t - except CA
7040264 = Pontiac 400/4bbl 'L78' a/t - except CA
7040270 = Pontiac 400 'ram air' 'L74' & 'L67' a/t - except CA
7040273 = Pontiac 400 'ram air' 'L74' & 'L67' s/t - except CA

7040563 = Pontiac 400/4bbl 'L78' s/t - California
7040564 = Pontiac 400/4bbl 'L78' a/t - California
7040570 = Pontiac 400 'ram air' 'L74' & 'L67' a/t - California
7040573 = Pontiac 400 'ram air' 'L74' & 'L67' s/t - California

There after all Pontiac V8 cars (that I am aware of) had charcoal canisters.

The last Pontiac V8 application carbs still had a/t & s/t application numbers:

17056261 = 1976 455 s/t (Firebird WS4 only)
17056262 = 1976 455 a/t (full sized only)
17056263 = 1976 400/455 s/t (purported to be a 400-s/t application, but too many original owner 455 cars are reported to have this carburetor to call it chance)

17057263 = 1977 400 s/t
17057264 = 1977 400 a/t
17057266 = 1977 400 a/t (W72)

17058263 = 1978 400 s/t
17058266 = 1978 400 s/t (W72)
17058278 = 1978 400 s/t

17059263 = 1979 400 s/t
17059271 = 1979 301-4bbl s/t
17059272 = 1979 301-4bbl a/t

I believe with a reasonable degree of certainty I have the above application numbers to be correct.
I believe that the above numbers serve to illustrate how there were still divisional differences in light of you citing Chevrolet small block applications using the same carb regaurdless of a/t or s/t.

HK1837 · Addicted!

As i said it may well be the auto cable kickdown that caused the difference post canister. What boxes were in those auto Pontiacs listed? If it is a TH400 something else is going on, but TH350 would explain a difference.

unruhjonny · Guru

I believe that the only post-1975 Pontiac's (err, USA-Pontiac's) that received the TH400 were the full sized cars with 455's.

All my cited examples (except for 717056262) were Firebird application numbers.

Pontiac revised the throttle arm in 1975 so that they all* had provisions for the TH350 kickdown.
(* = some s/t applications had the bracket cut short.)

Pre-1975 Pontiac V8's with 4bbls all had TH400's - with one exception, The 1974 GTO.
For the 1974 GTO there was an add-on bracket that was connected to the throttle arm, and all 1974 Pontiac Quadrajets had a sublet modification which permitted the use of this bracket.
I have a 1974 GTO/auto carb complete with said bracket.