Primary Jet and Rod Diffferences Pontiac vs. Oldsmobile

[brown7373]

After reading an article in (the last?) HPP about 8 Dyno-proven Tuning Tips and a comment by Dan Jensen about jets and rods, I had a question on the major difference in stock settings between Pontiac and Oldsmobile. I realize the article was more geared to drag set-up, but the question for stock or street driven applies.  Comparing same size engines equipped with Quadrajets, the jet/rod combinations are very different. Pretty much all the jets used from 70 to 72 across the board on all V8 engines were .069 to .072. But the primary rods on Olds were considerably larger, making a much leaner part throttle mixture. All rod tips are the same at .026, so wide open throttle is unaffected, but for instance, a 1972 Cutlass 350 uses .069 jet and .050 rod, a 19 difference and that is the same for the 72 Olds 442 455. The 72 Pontiac on the other hand uses .072 jets and .043 rod for 455, a 29 point difference. The numbers for 1970 higher compression engines are similar with the Pontiac engines much richer part throttle than the Oldsmobile engines.


 Anyone know why the huge difference? Is it the adjustment on the part throttle screw or the internal passages and orifices? I learned years ago that when you increased jet size, you usually needed to also increase rod size or the carb would be much richer for normal part throttle and lose it's responsiveness. Without the discussion turning into which brand is better, it seems to me that the mixture between the 2 brands should be similar. Why the big difference?
 
 
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[Frank400]

The metering is a combination of the jet, metering rod AND air bleed size.  Two carbs can have the same size jets and rods but the one with the smaller air bleeds will be richer (for the same jet/rod combo).  Larger air bleeds lean the mixture, smaller ones richen it. 

   

[brown7373]

So the Olds must have larger air bleeds than the Pontiac so the leaner jet/rod in combination with the larger air bleeds would then result in a similar mixture compared to the Pontiac.  I have always used carbs correct for my brand, but have read lots of examples of guys having real difficulty getting the carb from another make dialed in by only duplicating the jet/rod combo.

Years ago, I saw advertised in one of the car magazines a huge 3 binder set of Factory Rochester Manuals.  They have a wealth of information on all Rochester carbs and all applications for all brands, including Ford, Checker and Marine applications.  I never saw any specs on air bleeds or the differences between the different major components (air horn, float bowl or base plate).  Maybe they are in there somewhere. 

I didn't pose this question because I am having a problem.  I just noticed the differences and was curious.  But it surely is a factor when someone uses a carb from another application.

[Cliff Ruggles]

It's the opposite.  Old's carburetors at least thru 1974 used smaller MAB's than Pontiac carburetors, so typically use smaller jets.

There are other differences in them, specifically upper idle airbleed size and location, would get a bit lengthy if I went thru all the details. 

For the end user what really needs to happen is that you calibrated the carburetor for exactly what you are doing, based on the carburetor number and how it was set up by the factory.  Every q-jet was application specific, so you will find minor differences in almost all of them when it comes to airbleeds, idle tubes, jets, metering rods, etc......Cliff