Inliners International Forums General Discussion Hi Performance Q-jet 2bbl

Hello everyone, this is my first (of many )post. Let me say that before I Joined I read all 445 pages of engine posts and all of the hi perf. posts. My 8 yr old grandson and myself have a 78 nova 2dr. The car has a 1bbl intergrated head and 60k miles.The trans is a 350 and the axle "should" be a 2.73. We have gotten a 2bbl 1980 truck engine so the he can learn to build a motor.We are doing a mild budget build. The goal is to get close to the 250 cid version of a 327/300. High compression,cam close to the 929 sbc cam ect. We are going to keep the 2bbl intergrated head. The first Question that I have is has anyone run a rochester Dualjet before? I know that I would be giving up some cfm compaired to stock but I think it would be a lot easer to tune than the Varajet carb. Any questions or comments would be appreated Thanks in advance Jay.

I am surprised! No one has tried this before? I have been inspired to try this by DeuceCoupe's carb shootout test. In the test the 4bbl carbs were both faster and got better gas milage than the 1bbl carbs tested. The dualjet flows about 203 cfm at 1.5hg pressure drop. this compares with the 2g small base carb at 195cfm at1.5hg. Maybe someone who has run a small base 2g carb could chime in. I also hope to contribute info to the DeuceCoupe Gonkulator. Thank you Jay

Great project w your grandson!
I built my 1st carb at 5yrs old, and at 8yrs old one that ran on the car (a Model A Zenith).

Glad the carb shootouts were helpful.
I dont have any experience with the "half Qjet" Dualjet carb.
I saw an early one the other week - a full Qjet carb but with the big secondaries cast blocked off. I guess later they cut it down to just a 2bbl body.

I expect it would be good on gas, though on the top end its only a little bigger than the biggest Rochester 1bbl "B" jug.

If you have a good Varajet progressive carb it might be the MPG winner, cruising on that one tiny barrel.

Thanks for plans to help the Gonkulator, first step is of course to weigh the car, full tank & spare ie "curb" weight. Next if you can, I'd just get it running good as is and get some strip times, and MPG readings, as a baseline, then rebuild it.

Probably the trick with that integrated head is to keep the cool so they dont crack. Dont they also have dual exhaust outlets? That should help with your plan.

Maybe the Dualjet is such a new carb that nobody on here runs it (for most of us, "new" means anything newer than about 1972...)

Thanks for the reply. The current engine in the car is a 60k mile 1bbl(monojet)intergrated head. I have confirmed that the rear axle is 2.73 current tires are 25.4 in. diameter. the only mods to the car is an open element air cleaner,converter delete. The exhaust is 2 1/4 all the way back to a single 22in glasspack.I have marked off a 1/4 mile near the house and can get close with a stopwatch. I am off work friday and will try to get the car weighed. The carb question is for the "new" engine that we will build with the 2bbl head. I agree that the stock carb is so lean that the head cracks. The 1/2 QJet is wide enough to be adapted easly. So will the holly and autolite 2bbl. Thanks again. Jay

So is this the Dualjet off your 292 motor, and are you putting it on the 250?

The other motor is a 250 C10 truck engine. It comes with a Varajet 2 staged 2bbl carb which I have. (Its shot) The problem with the Varajet is tuning. There are almost no parts to be had. I am getting an adapter plate made to put on a Rochester Dualjet 210 2bbl carb. This is the front half of a Q-jet. The throttle bore spacing is pretty close. I will lose total cfm by doing this but I can tune it easly and will get both barrels at once when floored. The Varajet is so lean IMHO that it contributed to the cracking of cylinder heads. As far as I know no one has tried this so I get to be the first. I did adapt an Autolite 2100 to the 2bbl Varajet intake on my 79 GMC that I owned. The truck ran good but gas milage was down.(Ialso did other things at the same time,cam ect). Jay 6155

Ah, OK. I knew you had the 2bbl 292 engine and your Nova with 1bbl 250, I wasn't sure where you got the carb from. I had a '76 Camaro with a 2GC 2bbl on a 305 back in the '80s. I'm not sure how much different the Dualjet is. I would be tempted to do a dual jet upgrade as my first step, if it works for you. Then swap it over with a new head, 292 exhaust manifold and a 2 barrel manifold for the next stage. Are you mounting it perpendicular to the engine so the linkage lines up the cable, or parallel and changing the cable to work with the sideways linkage?

So the truck integrated head is cracked, I was wondering why you weren't swapping the 1bbl head for 2bbl head.

I am sorry if I am hard to follow sometimes. Please let me start over. I own a 78 Nova with a 1bbl intergrated head that runs fine for stock. The engine is a 250. I bought another engine, also a 250 that has a 2bbl intergrated head. Both heads are PRONE to cracking. The 1bbl head is ok. I haven't pulled off the 2bbl head yet.(Waiting on my grandson to "help"me) We are going to rebuild the 2bbl engine so he learns how. On the 2bbl engine is where the dualjet goes. It will be mounted like a 2bbl carb would be on a V8. The 2GC is a completly different carb that works fine and was made in a total of 5 different CFM ratings. If you are not going to change manifolds the 2GC is narrower and would work better with a 1bbl to 2bbl adapter. Sadly I have never owned a 292. Jay 6155

My bad, I forget that the 250 was available in the trucks too. That was my thought if you are going to go with a duajet and an integrated head, why not swap out a truck head which has the dualjet mount perpendicular to the engine allowing for the stock position of the cable. I'm also wondering if with the HD 292 trucks, they had the little half exhaust manifold with a 2.5" outlet too. My integrated exhaust has the same exit point as the older HD truck 292 manifold with three studs, but I presume it is 2". If that's the case, then the truck integrated head with dual jet and a 292 2.5" integrated exhaust manifold sounds like a decent daily driver build.

So is this going to be your grandson's first car in 8 years?

If you ask him my grandson thinks he ownes it now! I have the exact same engine in the car that you have now. My pipe coming out of the manifold is 2 1/4 inches. The stock exhaust stays 21/4 inches until the tailpipe which is 2 in. No 292 engines to my knowledge ever had an intergrated head either 1 or 2bbl.I worked at a Chevy dealer in 1978/79 when the 2bbl intergrated head first came out.(I was 3 years old at the time LOL) They were only used on 250s. 292s then still had seperate manifolds and a 1bbl carb. For some interesting reading go to the GM heartige center, then archive, then vehicle information kits. Download the 1979 chevy truck file and at the end is a good discription of the 2bbl truck engine. How did you determime that the 292 exhaust came out at the same place as what we have now? Jay 6155

Ah, the integrated head was a passenger car and SD truck part.

My integrated exhaust manifold on my 250cid has the outlet right under the #4 & #5 exhaust ports and a three stud clamp, just like the 292 2.5" exhaust manifold. Since my exhaust manifold has a 2.25" outlet, my shop should be able to run a 2.5" single exhaust to the integrated head setup, and then when I swap out the top end later it should at least be close enough to just bolt onto the 292 2.5" exhaust manifold. That way I can get my exhaust set up now, change out the top end later when I have more time and money, and avoid having to pay for two different exhaust systems. Routing the exhaust past the manual linkage will be a challenge, as I'd like to avoid the stock route of a 45 degree turn under the oil pan and then angled back under the passenger side.

I honestly don't know how many bolts the exhaust flange on the nova has. Like yourself I don't care for the under the oil pan piping either. In those days the cat converters were all in about the same place on the right side of the car on any GM car that I remember. Thats the reason for the whole under the oil pan thing with the exhaust pipe. I am lucky that I have an auto trans and don't have as much linkage to worry about on the left side. Jay 6155

I'm guessing your's is identical to mine, so three studs instead of two.

Yeah, I think I can squeeze the exhaust under the linkage but above the bottom of the crossmember and frame, so it is no danger of scraping.

I just had a bit of an a-ha moment. I was under the impression that from 1975 on, the integrated cylinder head (.......Half man-ifold, half cylinder head.....) was the only casting made, thinking 250 passenger car wise. Which would relegate me to searching for a 1974 or earlier 230/250/292 head. But since the 292 kept rolling out with a separate cylinder head, 2bbl intake, and exhaust 2" or 2.5" exhaust manifold until the last inline 6 rolled off the assembly line. I should refine my phrases be searching for a later model 292 cylinder head. I can probably find one of these that the exhaust surface hardening treatment is still good on and avoid the extra expense of hardened exhaust seat inserts (at least until I decided I want to upgrade to 1.84"/1.6" valves and intake lumps).

Here is what I can tell you for sure based on reading specs and my own eyes back then (78-80) and now. All 1977 car and light truck 250 engines were painted chevy orange and had an intergrated head with a 1bbl carb. Early 78 engines were also orange I don't know exactly when the color was changed to blue but I do know why. GM was caught putting 350 chevys in Olds delta 88s. This was done because at the time the Olds Cutlass was the #1 selling car and there wern't enough Olds 350s to meet demand. GM's responce was to paint all engines blue and put out the word that you may get any engine in your car. Back to topic. In 1979 all car 250s had a seperate intake and exhaust manifolds like 1974 and back. They were also rated a 5 more HP .I have seen this with my own eyes. 1979-84 light truck engines had the 2bbl intergrated head with a factory split manifold. The 1 and 2bbl exhaust manifolds do not interchange. Look at pictures of the 2 different gaskets will quickly explain why. I think it would be really cool if someone would make an "adapter gasket" to make the exhaust swap possible. Are you listening CNC Dude? Jay 6155

Well I know the '77 Camaros were GM blue, so they must have changed colors in passenger cars first, then later did it for the truck engines. After my first '76 Camaro got T-boned by a Ford Escort driver putting on lipstick, I shopped for a another one so I could use parts from the first one on it. I did look at a '77 L6 and remember it being blue. There might have been a phase in of some orange that year, but everyone I've seen was blue. '75 and '76 were the years for an orange integrated head. Yeah, I knew they started standardizing engines between the GM brands, Firebirds started getting Chevy V8s as the Pontiac V8 was phased out. In 1982 the Camaro and Firebird engines went heat friendly black, I don't know if the truck engines did too that year.

1979 was the last year of the L6 in the Camaro, in '80 they went to the small-block based 229cid V6. It sounds like they discontinued the mono-jet integrated head for that one last year. Weird for them to use it from '75 to '78 then switch back, but maybe there were enough warranty problems that they just went back to the old parts for one year knowing it was being phased out.

So I should include "1979 chevy 250 cylinder head" in my search criteria, along with "1975 to 1990 292 cylinder head". I was just searching "74 and earlier 250 and not having much luck. I searched, but this is the only pic I could find of the '79 passenger car 250:

http://rides.lowridermagazine.com/ride/1012999/vallejobox/1979/chevrolet/impala/photos/5.html

It makes sense the truck 2bbl 250 integrated head had a different exhaust manifold from the passenger 1bbl 250 integrated head. I'm assuming that both exhausts were 2 .25" outlets? I'm going to need to make a chart with all the running changes to keep it all straight in my brain.

On the 79-84 truck 2bbl intergrated head the dual exhaust manifold used 2 inch pipes for about 18 inches than dumped into a single 2 1/2 pipe then a cat then a 2 1/2 exhaust pipe, single muffler (oddly enough the same muffler used in 1960s full size V8 chevys) and a 2 inch tailpipe. On pickup trucks the 2 short pipes are different lengths to make the distance to the "Y" The same. I have one of these pipes and can post a picture if you want. It WILL NOT clear the sub frame in a nova/camaro. Hangs down way too low and pipe would hit the ground. When used in a van the "Y" pipe is different. Looks like it MIGHT clear in a car.NAPA's web site has pictures of both. On the color of engines I think we are both right My 77 Z28 camaro was blue yet I have seen unpainted 77 novas with orange. Jay 6155

Wow, the integrated 250s were really different between cars and trucks. I had seen some double outlet exhaust manifolds and didn't know what to make of those, I guess they were off of 250 trucks. Ah, found one:

http://www.amazon.com/CHEVROLET-fullsize-MANIFOLD-Integral-327-1006/dp/images/B000E4OJR2


Yeah, I suspect the change from Chevy orange to GM blue took place at different times for each of the engine factories. Camaros came from Van Nuys along with some Novas, but the 4th gen Novas coming out of the Tarrytown, NY and Willow Run, MI plants may have been getting their engines from plants still using up their orange paint. I prefer the Chevy orange, as the GM blue just looks like a Ford or Chrysler.

I've been searching for pics online of the 292 intake manifold, or the pre-1975 truck 250 intake, but I haven't found any yet. I can find pics of the post-'74 250 integrated truck head. I'm assuming the carb orientation was the same on both?

On the stock 2bbl intergrated head with a Varajet 2 carb,the linkage and carb throttle blades open "sideways" compared to a V8. When I put on the Dueljet 210 the throttle blades will be like a "normal" V8. Jay 6155

Wait, are you saying the Varajet blades open the same direction as the Monojet does on the 1 bbl integrated head? I guess that makes sense, as the truck 250 integrated head has the carb mount perpendicular to the engine, unlike the Offy intake which mounts the 4bbl carb facing the cylinders with the linkage working sideways like the Monjet. I haven't see the 2bbl 292 intake manifold, but I'm guessing the orientation is the same with the carb mount perpendicular. The manifold that CNC-Dude is designing has the 2bbl mounted parallel to the cylinders.

Believe it or not I had to look at the 1bbl on the car to see what you mean. Yes the 2bbl opens the same way as the 1bbl. The throttle shafts on both carbs are front to back. My Dueljet 210 will have the throttle shaft turned 90 degrees so that the ends of the shaft are side to side pointing to each fender. Jay 6155

So the dual jet doesn't need the extra manifold mounted linkage to change the direction. The throttle cable just connects directly to the carb. My 2GC on my 305 '76 Camaro was the same way, throttle cable directly to carb. One day I was driving along and the gas peddle slumped to the floor and I had no power. Pulled over and the throttle cable pin had broken or popped off and the cable just popped right off.

Exactly. Linkage won't have to turn 90 degrees. Might have to fab up a bracket for the TH350 kickdown. No big deal. Jay 6155

Yeah, I don't have to worry about that with my 3spd manual. However the exhaust pipes would be easier to route with an automatic.

I was searching for information on Rochester 2G and Monojet carbs and my Google search lead me back to this thread and I reread it again. Still scratching my head on the whole orange to blue paint transition in '77 and '78 (I'm sure the '77 L6 Camaro I looked at had blue paint). However I learned a few things on the integrated head 1bbl to Varajet that I missed the first time I read through. But that left me scratching my head on the '79 passenger car 250cid exhaust. The exhaust parts do not list a different header pipe for the '79 250 Camaro, so did it use the 2.25" 292cid SD exhaust manifold? I'm also assuming the 292 always had a different exhaust manifold in 2.25" SD and 2.5" HD, is that correct? I'm also wondering if the 292 250cfm HD Monojet was always paired with the 2.5" HD exhaust manifold, while the 210cfm SD Monojet was matched with the 292 2.25" SD manifold?

I know where there is a 79 nova without the intergrated head. I can measure the exhaust pipe next time I am at that junkyard. As for the 292 stuff I don't know any answers. Jay 6155

You should know at first glance on that '79 based on where the exhaust exits, below the carb or underneath the #3 and #4 exhaust ports. I'm betting that it will be below the #3 and #4 ports, so it is the same manifold as the 292 with 2.25" outlet to keep the same exhaust. A 2" exit is too small for the 2.25" catalytic converter, so I'm betting they kept the same header pipe from the '75 to '78 L6 passenger car. Pretty doubtful that they would make a whole new casting just for one year.