changing q-jet secondary rods
 

how many sizes down should one go to see a difference (better or worse) on the secondary rods of a q-jet? This is on a small block stocker engine.

Re: changing q-jet secondary rods
 

What do you have in it now and who built the carb?

Re: changing q-jet secondary rods
 

I'd say 5-6 thou, depending on what's available..
If you're still foot braking, you might see a change with the hanger too.
Try to stay in the median alphabet range
It your carb won't run that way, there's always the seat of the pants method..Almost lost in this day and age.

Re: changing q-jet secondary rods
 

Mark,please define median alphabet range

thanks john

Re: changing q-jet secondary rods
 

Hangers are lettered A-V I think. Each letter down the alphabet will pull the sec. rod out of the jet .005 later.
Starting with say a K, L M will give you room to fine tune in either direction.

Re: changing q-jet secondary rods
 

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.

Re: changing q-jet secondary rods
 

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.

Re: changing q-jet secondary rods
 

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.:D
Otherwise just put your mask on and change the channel to reruns.:rolleyes:
HB2:)
Dissident

Re: changing q-jet secondary rods
 

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

Re: changing q-jet secondary rods
 

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

Re: changing q-jet secondary rods
 

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.

Re: changing q-jet secondary rods
 

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

Re: changing q-jet secondary rods
 

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!

Re: changing q-jet secondary rods
 

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.

Re: changing q-jet secondary rods
 

Got it Stan =
Thanks!

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

Re: changing q-jet secondary rods
 

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.

Re: changing q-jet secondary rods
 

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

Re: changing q-jet secondary rods
 

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

Re: changing q-jet secondary rods
 

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

Re: changing q-jet secondary rods
 

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 :D ).

Stan

Re: changing q-jet secondary rods
 

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.

Re: changing q-jet secondary rods
 

Mr Nyhus and Others that might be interested in some facts from measurements,:D
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:cool:

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.:rolleyes:

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

Regards,
HB2:)
Dissident

Re: changing q-jet secondary rods
 

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

Re: changing q-jet secondary rods
 

Although that is a good observation, Mr. Stout,:D
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.:cool:

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

Re: changing q-jet secondary rods
 

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

Re: changing q-jet secondary rods
 

Stan,
Thanx for the question.:D

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.:cool:
Regards,
HB2:)
Dissident

Re: changing q-jet secondary rods
 

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

Re: changing q-jet secondary rods
 

Anybody try different sizes of sec. air bleeds?

Re: changing q-jet secondary rods
 

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. :rolleyes:
Regards to All that like this kind of stuff,
HB2:)
Dissident

Re: changing q-jet secondary rods
 

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