I'd like to hear from 70-72s with 2bbl 350s who left them low-compression, but swapped for a 4bbl intake & carb (particularly Rochester).

How much difference do you notice in power, driveability, gas mileage, etc.? Are you happy with the swap?

 

The power difference is an absolute 20 horsepower gain. Confirmed by Oldsmobile themselves.

Bear with me, just got a late breakfast ... I will put together detailed specs and repost.

 

a good swap would be to the Edelbrock 2711 dual-plane intake and a '1975-76 model Oldsmobile Qjet 800 (I believe the model# is p/n 17056253) which has been performance modified and/or upgraded for current fuels use. It's a factory electric choke model that still preceded the computer-controlled carbs to follow and still fairly available in good unmodified core condition for not a lot of $$.

Also upgrading to a dual exhaust (minimum 2-1/4" but maybe up to 2.5" depending on other engine mods you may eventually make) would help.

Those were the initial mods I did with my original 350-2bbl when I first got my ragtop - I've since gone other directions but there was a gain you could feel with just those basic mods and a good tune up (i.e. fresh plugs, wires, cap, rotor and a decent engine timing).

 

Check the timing chain condition especially before increasing power output. I'd also suggest a compression test.

 

I upgraded a '71 Cutlass 350 from 2bbl to 4bbl as described above. I used an Edelbrock dual plane manifold with a rebuilt Rochester Qjet and 2 1/4 inch dual exhausts. I was very pleased with the results.

 

I did this swap to my father's 71 cutlass.

Headers with dual exhaust
Correct Q-jet with an edelbrock intake,
Recurved HEI distributor.

the car ran fantastic. It was a big difference..

 

Look for a 704X250 (X=0,1,2,3,4) Quadrajet or a later 800 cfm (170xxxxx) that is calibrated for a 350, they’ll have appropriate internal sizes and “in the ballpark” configuration. Hot air or electric choke, whichever is more appropriate in your mind. Stock cast iron or aluminum (A4) intake, or an Edelbrock Performer.

If headers are likely in your future plans you could also consider an Edelbrock Performer RPM, without headers there is probably no benefit above the others.

On the Edelbrock manifolds, make sure they have “spread bore” carb configuration if using Qjet.

Dual exhaust should be a strong consideration.

Ignition recurve, a few extra degrees of initial timing, ensure that tune up components are in good shape.

If car has long legged gearing (2.5x - 2.7x), depending on your usage, a lower rear gear ratio (3.08 - 3.4x), along with limited slip could be considered. This is likely the most noticeable single mod.

To go along with that rear gear, overdrive transmission…. (Then you can go 3.7x - 4.1x) gears!, or not.)

See how this works?!?

Oh ya, there’s lots of ways to whip that smushy suspension & steering into shape!

…

 

Thanks, everyone. I bought the car with a rotted-out exhaust, so duals were done first thing. Just wondering how much I was leaving on the table with the 2bbl, since it's a fairly easy upgrade.

#69CSHC, if it's really only 20 hp, it's hardly worth the time and money.

And #bccan, I had the front suspension rebuilt, used KYB shocks*, and boxed the rear arms and put a rear sway bar on. I find that it handles remarkably well now, even with these minor upgrades. The car came with a quick-ratio steering box, which I like and recommend, but I still find it a little indistinct on-center. That's my biggest complaint. Maybe I've just been driving modern cars too long. I'd love an overdrive transmission, but the TH350 in it works too good to rip out. If it dies, though...

*Yes, I've had mixed results with KYBs. But looking back, it's mostly struts that have been bad. I really haven't had trouble with their shocks, and I'm pretty happy with them in this instance.

 

All good advice, as said, the 76 to 80 800 cfm 350 or even 403 Qjet will work really well with a quality rebuild. Dual exhaust and a distributor recurve as said, also beneficial. Also replace the nylon cam gear and timing set, if original, while in there.

 

I think the 4 bbl is definitely worth it. Generally, Qjet in particular, can provide better economy and throttle response along with a noticeable performance gain. Just the WHOOOOOOMP is arguably worth it, even if the car won’t change your optical prescription when accelerating.

If you haven’t put a big front sway bar on your car, give it consideration. IMO it’s the biggest suspension & handling bang for the buck and will enhance what you’ve already done.

​​​​​​….

 

I DO love me some WHOOOOOOMP...

 

I have a Q-Jet for 1975 350. It is an 800 cfm. But it is a one year only and the later would probably be a better carb. Personally, I would probably go with a new Edelbrock. Most of the Q-Jets you find are really old and will require a total rebuild. But I will say, there is nothing better than a good Q-Jet.

 

I did the conversion with a stock intake and 7042250 quadrajet. I was able to pick up a few MPG's along with noticing better acceleration and power. I wasn't looking at it as a specific increase of a number but instead I was looking at the performance and I would say it was worth it.

 

Significant improvement comes from multiple moves made in tandem...

Sorry for the delay, my day ended up getting a little busier than I anticipated. In any case here goes.

Oldsmobile 350s all single exhaust specs.

Compression 10.25:1 4bbl vs 9.00:1 2bbl vs 8.5:1 2bbl vs 8.5:1 4bbl

1970 hp --------------- 230 vs 170 ----------------------------

1971 hp ----------------------------------------------- 160 vs 180

1972 hp ----------------------------------------------- 160 vs 180

Below are average miles per gallon per model. For automatic Supreme coupes. All of course are TH350s, provided as well are rear gear ratio per model. As well as curb weight.

1970 230 hp = 11.1 mpg with a 2.78 gear and 3620 lbs
1970 170 hp = 12.1 mpg with a 2.56 gear and 3620 lbs
1971 160 hp = 12.6 mpg with a 2.56 gear and 3587 lbs
1971 180 hp = 12.5 mpg with a 2.56 gear and 3587 lbs
1972 160 hp = 12.2 mpg with a 2.73 gear and 3541 lbs
1972 180 hp = 12.1 mpg with a 2.73 gear and 3541 lbs

Performance 0-60 and 1/4 mile
230 hp = _7.5 and 15.5 (1970)
170 hp = _9.0 and 16.6 (1970)
160 hp = 10.4 and 17.7 (1971)
180 hp = 10.0 and 17.4 (1971)
160 hp = 10.0 and 17.4 (1972)
180 hp = _9.6 and 17.1 (1972)

17.4 to 17.1 only represents an additional 7.5 hp to get accomplished.

For good measure I will include the top of the line 350 automatic offering for 1972.

C/S coupe 200 hp, 4bbl, duals, 2.73 rear, 0-60 = 8.6 and 1/4 mile = 16.4 while MPGs = 11.7
(Automobile Catalog)

That may not seem all that impressive but 17.4 down to 16.4 is an 8 car length victory in the 1/4 mile...

 

4bbl does more than duals, although the enhanced sound may have you thinking different. Duals = 15 hp, 4bbl = 20 hp, but combined = 40. In other words your current setup specifically will see a 25 hp gain. (All 350s that have 2bbls are detuned. The detuning tool is the 2bbl. They should all have been 4bbl.) As you can see in my previous post, economy wise it's a wash. The 2bbl 350 likely existed just to make the old fashioned customers happy, those who feel more was overkill. But in this case it wasn't. Malaise eras only positive was proving the 2bbl should have been discontinued a decade earlier.

Here is a formal document to confirm power production variances. Right of center are both 2bbl and 4bbl 350 specs.

 

Fantastic info, 69CSHC. Thank you. Thank you everyone.

My new question is for the several people who recommended a later model Quadrajet rather than the originally-correct one (in my case, 1970). I would have thought that the later years were well into the malaise era.

Why a 75-76 or a 76-80 Quadrajet over a '70 or '71-'72?

 

For me, it was the fact they were just about the latest and most modernized run of electric choke Qjet models you could get before they became computer controlled, thus the chance to get better, less worn and/or cannibalized cores that in and of themselves, were a well-engineered foundation and easily modified for greater performance/current fuels use.

When doing the 4bbl conversion on my motor back in '16, I scored a really nice unmolested 17056253 that I ultimately had built to suit the motor specs of my build at the time - used it for about 6+ years after that without a flaw.

Still have it stored away actually - but I in the last year went to a different motor build and now running a Quick Fuel 780

Joe P provided this outline awhile back you might find useful

17056250 - 49 state emissions, non-A/C, early model year
17056253 - 49 state emissions with A/C, early model year
17056550 - CA emissions, non-A/C
17056553 - CA emissions with A/C
17056258 - 49 state emissions, non-A/C, late model year
17056259 - 49 state emissions with A/C, late model year

 

20 hp might not sound like much, but it's over a 10% increase. You'll notice that

 

Finding the right 2 barrel can make a big difference also, shown below are Rochester 2bbl differences. Specialty race carbs have tweeked carbs with ratings up to 600cfm. I've found the larger venturi models on 400cid Chevys.
https://biggcarbs.com/shop

1 1/4" flange , 1 7/16 Bore x 1 3/32 venturi = 278 CFM.
1 1/2 Flange carbs all have 1 11/16 bores :
1 3/16 venturi = 352 CFM.1 1/4 venturi = 381 CFM.1 5/16 venturi = 423 CFM.1 3/8 venturi = 435 CFM.
All ratings @ 3.0 in HG.

 

#shiftbyear Fascinating! I'll have to go out and measure my carb. Anyone offhand know the part numbers or applications for the higher cfm 2bbls? Will Chevy units bolt up and work on Olds as stock?

 

A public service message…

https://classicoldsmobile.com/forums...rajets-184900/

…

 

Never laugh at a 2 barrel.
I forget where I read that . . .

 

.......especially if you have THREE of them.