changing q-jet secondary rods

[Mike Pearson]

Originally Posted by Dave1695

Had 57s on a J in it last night, switched to 55s on the same J, no difference. Don't have too many rod sizes, just put 44s on the J for next week. Have an H and I that could go in too.

My car is a super stock car. I never run anything smaller than a .057 rod. In the heat I usually run .066 rods. J or K hanger. That’s what I had in at Atlanta last weekend and it picked up from the .057 that I had in for the first Q pass.

[Dissident]

Easiest thing to do is calculate the square inches of area of the main secondary jet which is fixed (I use 0.135" diam = Area of 0.014313915 sq in) then subtract the area of the secondary metering rod tip so the difference is the functional area of the secondary jet with that metering rod. As an example, the guy that raced his SS in Atlanta said he used a 0.057" and a 0.066" tipped rod so that is a change of area of 30%.....wow. Normally one would want to change only about 5% or so at a time, but that is just a suggestion. Whatever works for you.
Best thing to do is to make a chart or table of whatever your metering rod tip selection is and do the arithmetic.
Regards to all that like this kind of stuff.
Otherwise just put your mask on and change the channel to reruns.
HB2
Dissident

[James Perrone]

You want to see a difference ?
Go from a 55. To a thin 34 Rod.
It’s basic tuning. Throw a bunch of fuel. See what happens
It’s probably too much but then try something in between
It’s seat of the pants Tuning it works for me.
Hangers. Same thing B hanger is most aggressive. Then D. E. F
You won’t know unless you try

[Stan Weiss]

Can someone post up what the weather is when they run a 0.057" rod and when they run a 0.066" rod? Looking to see how far off my math is.


Stan

[Dave1695]

Thanks for the feed back gentlemen, sounds like the change from 57 to 44 rods should show something, either good or bad. Plus changes by percentages will help in the future. The small change that I made (57 to 55) was probably too small to mean anything. Plus DA that night was at 1100 feet on last run, probably wanted way more fuel. Thanks again, appreciate the advice.

[Alan Nyhus]

Originally Posted by Dave1695

Thanks for the feed back gentlemen, sounds like the change from 57 to 44 rods should show something, either good or bad. Plus changes by percentages will help in the future. The small change that I made (57 to 55) was probably too small to mean anything. Plus DA that night was at 1100 feet on last run, probably wanted way more fuel. Thanks again, appreciate the advice.

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

[Daran Summerton]

With the small qjet float bowl, too avoid starvation try to run the leanest combo.

[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

[Stan Weiss]

Originally Posted by Dissident

Easiest thing to do is calculate the square inches of area of the main secondary jet which is fixed (I use 0.135" diam = Area of 0.014313915 sq in) then subtract the area of the secondary metering rod tip so the difference is the functional area of the secondary jet with that metering rod. As an example, the guy that raced his SS in Atlanta said he used a 0.057" and a 0.066" tipped rod so that is a change of area of 30%.....wow. Normally one would want to change only about 5% or so at a time, but that is just a suggestion. Whatever works for you.
Best thing to do is to make a chart or table of whatever your metering rod tip selection is and do the arithmetic.
Regards to all that like this kind of stuff.
Otherwise just put your mask on and change the channel to reruns.
HB2
Dissident

Yes a large change in metering rod area but a much small % change in total flow area.


► ((.057/2)^2*pi)/((.066/2)^2*pi) = 0.7458678
► ((.066/2)^2*pi)/((.057/2)^2*pi) = 1.34072022



Stan

[GTX JOHN]

Originally Posted by Stan Weiss

Yes a large change in metering rod area but a much small % change in total flow area.


► ((.057/2)^2*pi)/((.066/2)^2*pi) = 0.7458678
► ((.066/2)^2*pi)/((.057/2)^2*pi) = 1.34072022



Stan

You lost me on this one = If the metering rod is inside the jet and
displaces a segment of the jet area does it not proportionately
restrict the volume or Air Flow?

Sorry I am OLD and not particularly sharp anymore!