brown7373

I had a question on the major difference in stock settings between Pontiac and Oldsmobile. 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 72 442 455. The 72 Pontiac 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.


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.

Seff

I'd suggest asking someone who works with Q-jets from all four divisions, like Cliff Ruggles. He has his own forum for questions.

I'd be very interested in hearing his answer, though!

White_Knuckles

The dark art of carb tuning is a science. There are additional variables such as float level, air temperature and altitude affecting this.

My thought is these factory carbs are setup to operate from sea level to Denver altitudes and somehow work! The first time buyer will never tweak it. So the two car makers may of had different ideas of how to spec what they were delivering across the US? The engines are very close where the jet/rod variations may yield nothing in the ol' butt-dyno? I think they just made a reasonable, do-all guess for the specs.

Carbs are best optimized to a specific location (altitude). A selection of jets and rods with considerable trial and error swapping may reveal the ideal setup for you is way off from either factory "do-all" spec.

Guess I don't know if your question is based on curiosity of the differences or you're planning surgery? I play with Edelbrocks 'cause Holley's are too fussy for me. But dialing any of them in is kind of fun if you're gonig down that road.

brown7373

I am not planning any surgery, but own both a Pontiac 455 and a Cutlass 350. Both use very similar Quadrajets, but the jet/rod combination between brands is very different, and I was just curious. Both brands run fine.


In Doug Roe's book, he talks about jet and rod changes and cautions that you shouldn't make changes, calculated from the rod/jet combination that exceeds 5% (or was it 10%). I also posted on Cliff's site, and the answer seems to be because the internal circuitry between the brands is different and also has a big effect.

droldsmorland

A simple answer is....Do you currently have a drive-ability problem? If so what is the symptom?
Before going at the carb, I (we all) suggest making sure all the simple stuff (ignition sys, timing events, vacuum leaks etc) is all checked and in known good order. Verify the float floats and its level and drop are in spec for your driving conditions and type of fuel you run. Clean air & fuel filter?
Next read the plugs and tail pipes. Determine which side of stoich your on, rich or lean. After determining which way you need to go then order an assortment of primary rods, secondary rods, jets, maybe a hanger or two and start playing. If you have the availability to put it on a chassis dyno that's the best way of dialing it in. But the trial n error method using the seat of your pants-n-eyeballs works with patience.
Be aware that the secondary vac door (butterfly) spring can weaken with age causing it to open to much/to early causing a rich bog when your going from PT (part throttle) to WOT(wide open). Thats a simple adjustment. So you see its important to know which circuit in the carbs causing the problem. Dont just throw parts at it or you WILL get frustrated.

Charlie Jones

Just a guess here, but I think it may have something to do with the 105 cubic inch difference in size of these engines.

The Quadrajet design was very flexible. It was used in everything from a 230 cu. in. Pontiac OHC six to a 500 cu. in. Cadillac , with variations in tuning and/or throttle size .

brown7373

Actually the jet/rod combos for the Olds 350 & 455 are almost identical, as are the Pontiac 400 & 455. But the difference in the Olds vs. Pontiac is huge. The jet/rod difference in the Olds is 19, but it is 29 in the Pontiac. ( 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 72 442 455. The 72 Pontiac 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.)


I state again, my Cutlass is running just fine. But since I also own a Pontiac with a 455, I noticed that the Pontiac has a much richer jet/rod combo, and was curious why so different for 2 similar GM products using a similar Quadrajet.

Seff

I think the solution to this puzzle lies in the different designs (duh), namely that the Olds engines are more torquey at certain RPMs and so forth. Look at the torque curve of each engine and you'll see a pattern, I think.

brown7373

Which rpms are Olds engines more torquey? My 1970 Pontiac 455 is rated at 500 lb. ft of torque at 2700 rpms. I don't have the specs for a 1970 Olds 455, but I don't think it's much different. Olds, Pontiac and Buick 455s were ALL considered Torque Monsters.

Charlie Jones

You got your answer right there.

jpc647

Can you post the link to the thread you started? It'd be interested in reading through out.

brown7373

This is the link I started. https://classicoldsmobile.com/forums...treet-use.html

White_Knuckles

... "a 1972 Cutlass 350 uses .069 jet and .050 rod,... the same for the 72 442 455"...

Okay that would be quite the spread of cubic inches across the 350-4XX range. Up to 100 c.i. difference! I would have bet money the carb spec would be richer for the big boys.

None of this makes sense and I'll contend the factory engineers back in the day were a bunch of lazy guessers. They used vacuum tube powered Dynos made out of wood that displayed random readings and graphs that only trained dolphins could decipher.

Engineering note: If the sucker runs well and can operate in various climates and altitudes, we're good. Ship it. Further testing is futile and the dolphin is mad.

jpc647

You posted the classicolds thread, not the cliff carb site one. :/


That's bizarre. But how could some stock 350's need to be more rich than a 455 same year? I mean, enought to make the car not run right with an exhaust.

brown7373

I think it's incorrect to think a 455 needs to be richer than a 350. A bigger engine typically burns more fuel, but not because it is running richer, but because it has bigger volume cylinders. The carb mixes the air and fuel. A bigger engine will draw in more fuel and air. It doesn't need to draw in a richer mixture, only more of the correct mixture.


http://cliffshighperformance.com/sim...p?topic=1944.0

jpc647

Very good point. The Q-jet only opens the secondaries as the engine breathes, I could see how the same carb could be used, but the exact same rods/jets? Even at idle, a 455 must use more fuel than a 350...

brown7373

I don't think the fuel goes through the jets at idle, does it? I think it goes through the idle circuit. But even on the primaries, a bigger engine will draw more fuel/air mixture because of bigger cylinders, but the optimum mixture (rich/lean) should be about the same, assuming a similar compression ratio.

GEARMAN69

The Olds are set awefully lean at part throttle from the factory and all I can say after doing high performance Quadrajets tuning , street and strip racing etc for about 25 years is that if you do any deviation from stock meaning cooler thermostats and or after market open air cleaners, headers , dual exhausts etc.. on that horrid lean 20 point gap on the Olds or any other engine set like that its going to be way lean and far from optimal. This setting is only good for hot air supply and economy nothing else. Any normal default performace minded Qjet build regardless of brand engine under it should start off with a 30 point gap between the jet and primary rods. Most application are best with 70-73 jets and 40-44 rods. I just saw something similar when I cracked open the 70 model 310 hp 350 carb I am trying to salvage it had a 70 jet but a 50 or 51 rod.. I normally didnt see combos like this till smog 1980 and later. Heck a 150 net HP 301 P in 1980 has a 70 jet and a 50 rod. But just a couple years earlier a 77 350 P would have 71 jets and a 44 rod and the smog 400 had a 72 jet and 45 rod so that was the leaned up smog version of Qjet in the late 70's. This may explain why so many guys are jumping over to Edelbrock 600's , maybe they have not put a richer primary rod in the carb to fix this lean part throttle? I have used one of my quadrajets on vehicle to vehicle from 455 Pontiac to 400 SBC to 400 Pontiac etc.. It needs very little if anything changed on the tune either. Has ran 11.0-121 in the 1/4 in one vehicle then 11, 12 or 13 in another depending on the setup but there very versatile. All I can suggest is if you have a 69 jet bump it to a 70 or 71 and drop the rods down to 40-42 and see what you think depending on what parts you have available consider a 72 jet and no larger than a 45 rod too, and thats assuming you have the cooler air available to the engine and less than a 195 degree thermostat thats only taking power away from you. Had another Qjet that was a strong 11-12 second street/strip runner on a 455 P and it got thrown on a solid cam big 462 BBC on the dyno with a adapter and was side by side with race 800 Holley and 850 doubleppumpers and made 560 hp. 160 thermostats and open element air cleaners are part of this tune as is lighter distributer springs with proper total timing. Always remember with a quadrajet build the default setting to start off with a rod 030" smaller than the jet.

brown7373

Gearman69, good info. In fact, the exact reason I started this thread was because I noticed the HUGE difference between the stock settings of Olds and Pontiac. I am much more familiar with Pontiacs, and have owned many through the years, but this is my first Oldsmobile. My 1972 350 is a street only car, that currently runs 69 jets and 50B rods. It starts and runs fine, but the question always comes up, if fine could be a lot better. With a low compression 350, I am going to try 70 or 71 jets and the 40-42 rods.

White_Knuckles

Interesting. I never considered the atomization of the mixture would be consistent with large cylinder demand. Picking up ponies was always a balance of flow and smashing more fuel/air in the party. I assumed with a big bore engine, they need more fuel to peak performance with that monster? That seems to not be true for factory builds. They want reasonable mileage to sell them. They leave the jetting and flow tricks to those who dare to unlock secret HP.