changing q-jet secondary rods

[SS734]

With the Qjet it all depends on the size of Primary and Secondary bleeds.
Too small of a bleeds it will go dead Rich. Too big of Bleeds and you can't get a small enough tip.

[Dissident]

Originally Posted by Alan Nyhus

Dave, the tendency of QJets is to run rich as air flow increases (rpms going up). Secondary metering rod changes alone don't always fully address this. Also, the primary side can contribute close to 40% of the total fuel flow at W.O.T.

Hope this helps. -Al

Mr Nyhus and Others that might be interested in some facts from measurements,
1) Most 750 type QJs flow test to be from a very poor low of 164cfm to an average of 170-175cfm at 20.4"H2O test pressure.
2) Most 750 type QJs flow test at 660cfm to 675cfm on the secondaries at the same test pressure as above.
3) Doing some quick math to see how much that is in mass flow of air in lbs/hr at sea level, yields 798 lbs/hr on Primaries and 2622 lbs/hr on Secondaries.
4) Allowing for max power at somewhere near 12.5:1 A/F ratio would yield fuel use on primaries of 63.84lbs/hr (127.68Hp) and 209.76lbs fuel use on secondaries (419.52Hp) supplying a total of 547.2Hp

As you can see, that is 23.33% fuel on primary and 76.67% on the secondary. IF one sees that the primary is supplying the 40% as claimed in the quote, one can only assume the secondaries are not at WOT or something is categorically wrong with the carburetor or the fuel supply system tune.

Hope that helps.....No, I do not have a race car but have supplied assistance to make more than a few more than competitive.

Regards,
HB2
Dissident

[Jeff Stout]

I measured the diameter of butterflys on both sides and it worked out 20% primary 80% secondary. May not be exact science IDK

[Dissident]

Although that is a good observation, Mr. Stout,
The 800 type QJ have the same throttle plate and blades but flow considerably more on the primaries due to a larger venturi for those carburetors which produces an average of 207cfm to 209cfm. And the bias changes the percentage of fuel used, but that is yet another oddity of the QJ organized leaks. Makes for interesting work on the dyno long before heading to the track.

Regards to all that like this kind of stuff,
HB2
Dissident

[Stan Weiss]

Originally Posted by Dissident

Mr Nyhus and Others that might be interested in some facts from measurements,
1) Most 750 type QJs flow test to be from a very poor low of 164cfm to an average of 170-175cfm at 20.4"H2O test pressure.
2) Most 750 type QJs flow test at 660cfm to 675cfm on the secondaries at the same test pressure as above.
3) Doing some quick math to see how much that is in mass flow of air in lbs/hr at sea level, yields 798 lbs/hr on Primaries and 2622 lbs/hr on Secondaries.
4) Allowing for max power at somewhere near 12.5:1 A/F ratio would yield fuel use on primaries of 63.84lbs/hr (127.68Hp) and 209.76lbs fuel use on secondaries (419.52Hp) supplying a total of 547.2Hp

As you can see, that is 23.33% fuel on primary and 76.67% on the secondary. IF one sees that the primary is supplying the 40% as claimed in the quote, one can only assume the secondaries are not at WOT or something is categorically wrong with the carburetor or the fuel supply system tune.

Hope that helps.....No, I do not have a race car but have supplied assistance to make more than a few more than competitive.

Regards,
HB2
Dissident

Harold,
Just a question about the BSFC you used. Is that what a competitive engine will show or did you just choice it for ease of Math? Never seen a dyno sheet from a competitive S or SS engine.



Stan

[Dissident]

Stan,
Thanx for the question.

In the example I used here, I just used a BSFC of 0.5lb/hp-hr which is a pretty good target for planning. However, it is quite common for BSFCs to be somewhere around 0.45 to 0.47 when everything is complimentary. Stockers have to use OEM manifolds and SS stuff uses other than OEM so they can get a bit more fuel efficient as happens when CR go up as well. I did not get into an analysis of efficiency, simply wanted to point out the fallacy of following the wrong guidelines on how the goofy QJ works in practice. These things are more fun than a box of puppies when tweaking them on the dynamometer.
Regards,
HB2
Dissident

[Alan Nyhus]

Originally Posted by Jeff Stout

I measured the diameter of butterflys on both sides and it worked out 20% primary 80% secondary. May not be exact science IDK

The would be close if you were comparing primary and secondary sides that had the same fuel metering structure (like a typical Holley 4150/4160 style carb).

Measuring throttle bores or primary/secondary air flow and extrapolating that to fuel flow doesn't take into consideration how a QJet triple venturi primaries meters fuel as compared to the the non-booster secondary side that has a simple discharge tube. On the primary side, even the idle circuit contributes to fuel flow as the primaries open.

As quirky as they are, I love 'em. -Al

[monte385]

Anybody try different sizes of sec. air bleeds?

[Dissident]

Yes. Changing air bleeds in both primary and secondary is a sensible way to fine tune one of these organized leaks. Of course there are those that do not believe that is necessary and think that jets and rods are the only way to make adjustments. The things were designed around trying to pinch down on emissions and do so cheaply.
Regards to All that like this kind of stuff,
HB2
Dissident

[Alan Nyhus]

Originally Posted by monte385

Anybody try different sizes of sec. air bleeds?

Yes. The air bleeds/emulsion tubes are very important. -Al