changing q-jet secondary rods

[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.

[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!

[Stan Weiss]

John,
The jet is fixed at 0.135" or 0.014313881527918496 sq inches of flow area.


A 0.057" rod has an area of 0.0025517586328783095 sq inches


A 0.066' rod has an area of 0.0034211943997592849 sq inches


► 0.014313881527918496 - 0.0025517586328783095 = 0.011762122895040186 Total Flow Area



► 0.014313881527918496 - 0.0034211943997592849 = 0.010892687128159211 Total Flow Area


The metering rod is much smaller that the jet and so a large change in metering rod area has much less effect on total flow area.


► 0.010892687128159211 / 0.011762122895040186 = 0.92608173076923073 change.


I tried not to post to much math the first time as in other forum the math scares some people.


Stan


PS - Let me add that if they could change jets and keep rod the same size like you can on the Carters. Same weather as picture on previous page.

[GTX JOHN]

Got it Stan =
Thanks!

We run Carters TQs where Metering rods and in the
much smaller size Primary Jets.

[Mike Pearson]

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

I have been racing my car for many years so I know what it likes in certain air conditions and I set the carb according to that. I run the .066 rod when it is hot and humid. In Atlanta it was almost 90 degrees and the humidity was near 80% most of the weekend DA about 3500 ft. The first couple of passes I still had the cold weather set up in the carb. That was the .057 rods. I had that in from the Baby Gators just before the shut down. The DA there was below 1000 ft and the humidity was about 30%. I don't know anyone that runs rods richer than a .057 unless you are trying to air leak the carb. Personally I have never tried that. I have 2 good Q jets that were built by well known builders. They both respond the same to similar changes. My car always runs best on the lean side.

[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.

[James Perrone]

A question to the q jet experts.
Any of you own a FAST. Q jet car.?
All this formula On what secondary rods is kind of useless to tune a car
Trial and error is how you find what a car likes.
Computers in the car are overrated still gotta make runs
What you feel in the car and on the time slip is gospel

[Stan Weiss]

James,
Very True.

But when you roll into the track and the weather is nowhere near what you have run before it gives you a pretty good starting point.

In the end for me it is the time slip and reading the spark plugs (yes, low tech still works ).

Stan