Fun71

The specs for factory 1972 350 engines show a substantial gain, but I am not 100% certain the different engines used the exact same camshaft.


Engine Carb Exhaust HP @ RPM FT-LBS @ RPM
- ------ ----- ------- ----------- ------------
350 2-bbl single 160 @ 4000 275 @ 2400
350 4-bbl single 180 @ 4000 275 @ 2800

350 2-bbl dual 175 @ 4000 295 @ 2600
350 4-bbl dual 200 @ 4400 300 @ 3200

MDchanic

Camshaft Part Numbers, 1972 F-85 350 Engine (a/p the 1972 Parts Manual):

2bbl: 400084
Auto Trans: 400084
4bbl Std Trans: 402486

- Eric

Fun71

Thanks for that info. I knew the MT engines used the .440 lift / 274 advertised duration cam but was unsure if the 2bbl and 4bbl auto cams were the same or not.

grampy

with a stock engine going to 4 bbl from a 2 bbl the power gain is small
on a car that runs in the 17 teens one or two tenths ,you would not be able to feel power diffrents

Fun71

I can not agree with your position.
A 14% increase in horsepower is not "small".

I have run my car with the secondaries wired shut and it was a very obvious reduction in power.

oldcutlass

The Butt Dyno is a finely tuned instrument.

grampy

here is something to think about 2 big 2G two bl flows about 450 to 500 cfm . a 750 quadrajet has a veary small primarys and huge secondays.The % of flow from primary to secondays is about 30% front to 70% rear so on the primary side of a quadrajet it flows 225 CFM ( which Is a guess) so by blocking off secondays you would have a big diffrence in power. that is about half of what a 2bl flows.even if you had 750 holley it would be 375 front to back and 600 holley is 300 front to back.It not about how many barrals it CMF. a 350 with 100% eff @ 5000 rpm only needs 500CFM

Fun71

Your numbers are close to what I discovered but I found that you have to account for different rating systems.

http://www.rasoenterprises.com/index...18-carburetion
Two barrel carbs are rated at 3.0 inches of mercury (@ 3.0" Hg) pressure drop while four barrel carbs are rated at 1.5 inches.


The info on this site shows ~30% less CFM at the 3.0" rating compared to the 1.5" rating. The chart shows a 227 CFM 2bbl carb @ 3.0" correlates to 161 CFM @ 1.5", so a 450 CFM 2bbl would be ~320 CFM when using the same ratings as a 750 CFM QJet. Conversely, the QJet would be rated 1056 CFM using the same rating system as the 2bbl carb.

The info I found shows the 2bbl came in a variety of sizes, with the largest being 435 CFM (corrected to 308 CFM using the same scale as the QJet):

1 1/4" flange, 1 7/16" throttle bore, 1 3/32" venturi - 278 cfm
1 1/2" flange, 1 11/16" throttle bore, 1 3/16" venturi - 352 cfm
1 1/2" flange, 1 11/16" throttle bore, 1 1/4" venturi - 381 cfm
1 1/2" flange, 1 11/16" throttle bore, 1 5/16" venturi - 423 cfm
1 1/2" flange, 1 11/16" throttle bore, 1 3/8" venturi - 435 cfm


While researching QJet primary CFM this came up:



So the 750 CFM QJet primaries would compare to the smaller bore diameter 2bbl above: 1 1/4" flange, 1 7/16" throttle bore, 1 3/32" venturi - 278 cfm

Comparing the QJet primaries to the largest 2 BBl carb
3.0" Hg:
QJet primary = 278 CFM
largest 2 BBL = 435 CFM

1.5" Hg:
QJet primary = 197 CFM
largest 2 BBL = 308 CFM

So the QJet primaries flow ~65% of the largest 2 BBL carb. I am curious to know what the venturi diameter is on a '72 Olds 350 2 BBL carb. The info above shows there are 5 sizes, so is it the same size as the QJet primaries or is it larger? The info I have read over the years was that its the same size as the QJet but that may not be accurate. Does anyone know for sure? If so, post up. I've been wondering about this for years.

Paladin31

I have a 7042156 that I might be able to get at tomorrow to game some measurements for you if you think it's what you need info from?

MDchanic

Whoa whoa whoa whoa whoa. Hold on there a second. That makes no sense.

You say that carburetor CFM ratings are measured at a specific level of vacuum, measured in inches of mercury.
All well and good.

But then you say that flow numbers are greater with a lower vacuum (.5") and less with a higher vacuum (3").

This does not compute. Flow volumes increase as vacuum increases.

Could you please provide a source for your information, and could somebody else please corroborate?
Following the laws of physics, if the different vacuum levels that you say are used to establish the flow rates of the different carbs, the 2bbl should flow more than the 4bbl.

- Eric

Shaunbo13

I plan on moving directly to the rear. Can u give me any pointers on the rear changing the gears? Anything I need to know? can i do it in my garage on a jack?

Fun71

Isn't that what I posted? If not, help me see where I went astray. It was almost 2am when I posted so I may not have typed what I thought I was typing.

3.0" Hg:
QJet primary = 278 CFM
largest 2 BBL = 435 CFM

1.5" Hg:
QJet primary = 197 CFM
largest 2 BBL = 308 CFM


It's in the link above. Look at the bottom of the table for the QJet numbers for 1.5" and 3.0" Hg.

Fun71

Changing gears is a fairly involved and precise operation that requires a fairly high skill level and experience, so it's best to have a pro do this. The other option is to buy a complete rear with the gear ratio you want and swap it for your original rear. That can be done fairly easily with basic tools and a floor jack.