Newby question - Olds Quadrajet fuel economy

[Guest]

Best mileage that I ever got with a V-8 and 4bbl was with my 1973 Cutlass (believe it or not) Have had several Quadrajets apart and have noticed that the Oldsmobiles typically have a MUCH leaner primary set-up than a Chevy or a Pontiac even with the same size engine. (Typically the Olds jets/metering rods will be .069"/.050" rather than the Chevrolet/Pontiac combination of .072"/.042" ) How can the Oldsmobiles be calibrated so much leaner on the primary side? Do the secondaries come in sooner to compensate? <BR>Anyone else wonder about this or do I have too much time on my hands?<P>Thanks!<P>Mike

[Guest]

Are you talking about jet size or metering rod diameter?

[Guest]

I was talking about the relationship between the metering rods and the jets. For example the Olds Quadrajets typically run a .069"jet with a .050" primary rod. For a Chevy you will typically find a .071" jet for example with a .042" metering rod. As you can see from the above examples the resulting metering area in the Olds Quadrajet is quite a bit less.<P>Mike<p>[ 12-05-2001: Message edited by: burgandycutlass ]

[Guest]

Without intentionally bagging on the chevy small block, I will say that in my opinion the chevy isn't as efficient at part throttle or off idle as an olds engine. This is probably the reason for the difference. The 455 being at the top due to its stroke and low end ability. The chevy requires alittle more on the primarys and as a result was one of the first to require the use of air pumps and such to kill the exhaust. I don't know about California, but out here Oldsmobiles didn't see air pumps until late in the game. None of this racket I'm making has any fact other than observation and opinion.

[Guest 70 Electra]

Since the metering rods are tapered, the actual jet "area" depends not just on jet/rod size, but what POSITION the rod is in. <P>The rod position is dictated by the power piston length, the power piston spring, and the engine vacuum signal that is controlling the power piston.<P>Don't know if this is the answer, but it's a thought!

[86 2dr.ltd]

Though I dont have the tecnical knowledge to answer this question I was suprised and pleased to find that my 75 & 76 Deltas w/350 are affordable on gas.Not the 12,13-14 mpg I expected. I should start keeping track and come up with average.<P>I remember reading somewhere that Oldsmobile claimed 17 or 18 mpg for these cars.

[NTX5467]

Back in those early "smog" days of the '70s, there was a manual out with detailed instructions on how to "retune" engines to regain the earlier power and economy. When this routine was performed on several '72 Cutlass 350 Q-Jet motors, they regularly and consistently did 20+ mpg on the highway and ran beautifully.<P>There are many other fuel system calibration points in the Q-Jet (or any other carburetor for that matter) than just jet/rod size interactions and considerations. All of those little air bleeds in the circuits are calibration points also. It's how all of these things interact that result in the economy and driveability we experience.<P>Then, look at manifold design considerations too. Plus cylinder head flow and combustion dynamics consisderations. Then, bore, stroke, and connecting rod length come into play with respect to the manifold/cylinde head flow issues.<P>Other than aerodynamics issues, from my observations the Olds engines of the early '70s were much better in fuel economy than their other GM cousins. Everything seemed to work together.<P>As underpowered as the 307 might have been in the wagons, I heard many reports of 25+mpg on the highway in the full size and mid size sedans they were used in when the Chevy 305 did good to get 20mpg--way back in the 1970s and 1980s.<P>As I recall reading, there were some particular air flow rates when the Q-Jet was super-efficient at metering fuel with it's triple-booster primary venturis. Meaning, it was much better at atomizing the fuel, which put more emphasis on manifold design to make things work better together. Perhaps Olds could or did spend more on refinement than the other GM divisions could at that time?<P>Jets, metering rods, power piston spring calibration, and such are only part of the story.<P>Enjoy!<BR>NTX5467