A/F Ratio Tuning w/ Sensor - Team Camaro Tech
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post #1 of 13 (permalink) Old Apr 28th, 04, 06:42 PM Thread Starter
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Matt Jones
 
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I got my LM-1 wideband sensor in my truck and I'm going to do some tuning. Here's my vehicle info:
1986 Chev C10
Quadrajet (#49 rods, #72 Jets)
350ci
Stock cam

So I took the truck for a drive to check things out. At a steady 45MPH cruise, the meter says 18:1, so I'm way lean. Idle is 14.7-15.0, right where it should be. I really am not sure where I want the cruise ratio to be, since in one of David Vizard's books he says that a lean cruise can be as high as 16-17.5:1, and I thought 14.7 is what you wanted. Any thoughts on this?

Now I was wondering if there was a way I could get in the ballpark on what rod and jet combo I should use. My current combo has a flow orfice of .00219 sq. in. Since 18 (as in 18:1) is 17% more than 15:1, do I want the flow orfice to be 17% larger?

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post #2 of 13 (permalink) Old Apr 29th, 04, 02:14 AM
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I would believe your theory to be correct, but, I would install a vacuum gauge and conduct the test again. This will tell you which circuit is being used.

My theory is this, the engine doesn't suck the gas into itself. The engine creates the vacuum in the chamber between the intake valve and throttle plates. Atmospheric pressure is filling the void created. Remember, high pressure is always trying to equalize a low pressure.

Using a vacuum gauge to help. You could always get another pair of jets, lead them up, and use them for experimenting.

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post #3 of 13 (permalink) Old Apr 29th, 04, 02:41 AM
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Did the engine surg or ping when it was that lean? I have run my camaro as lean as 16:1 at cruise and didn't have any problems. I think that every engine is slightly different as to how lean you can run it before it becomes "unhappy" though.

I think it was Toyota that developed the first wide band sensor (or at least engine to use it) and they ran really lean at cruise (17:1 - 18:1).

How does it run a part throttle and WOT? That would help to determine the jet size as the rods are the same diameter on the end. Once you know that you can work backwards to find what rod would work best for cruising as at cruise the fat part of the rod is pulled into the jet to meter less fuel.

I built a spreadsheet to show the different changes that would occur due to rod and jet changes. Like I said above the jet determines WOT metering and you can figure the diameter as:

72 Jet = ((.072/2)^2)*3.14 = .004069
49 Rod = ((.049/2)^2)*3.14 = .001885 (at cruise)
49 Rod = ((.026/2)^2)*3.14 = .000531 (at WOT)

Metering area = .004069 - .001885 = .0021847 (at cruise)

Metering area = .004069 - .000531 = .0035388 (at WOT)

I'm not sure if there is a direct correlation (via %) from AFR to metering area, but you could at least try a smaller rod making changes in about 10% increments. So to change 10% richer from 49 rods/72 jets, you would want to make a metering area change of .0002 or so. That would be about 72 Jets/46 rods:

72 Jet = ((.072/2)^2)*3.14 = .004069
46 Rod = ((.046/2)^2)*3.14 = .001661 (at cruise)
46 Rod = ((.026/2)^2)*3.14 = .000531 (at WOT)

Metering area = .004069 - .001661 = .0024084 (at cruise)

Metering area = .004069 - .000531 = .0035388 (at WOT)


Anyway, sorry for the long post! I've just geeked out on QJet metering stuff in the past. I hope some of this helps!

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post #4 of 13 (permalink) Old Apr 29th, 04, 07:44 AM Thread Starter
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Thanks guys. I probably should hook up my vacuum gauge to see the engine's load.

You bring up an interesting point, Paulm. The engine didn't ping or surge at cruise which I found interesting. The main thing I'm worried about is excess heat.

So far WOT is not good. The secondaries (or something) needs adjustment. When I peg the throttle, it downshifts, stumbles, and the meter reads 9:1-10.5:1. So I'm not sure if it's sucking fuel w/o the air valve opening (if that's possible), or if its just dumping too much fuel in. I don't remember the secondary rod # or the hanger type, but it's the ones that are common for mild performance motors.

And I just remembered. When I did put a good load on the engine (say where the A/F ratio should be 13.5-14.0), it was hanging around 16-16.5. So it sounds like I should step up on the jets, correct?

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post #5 of 13 (permalink) Old Apr 29th, 04, 08:18 AM
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The best way (that I know of) to tune your primary jetting is to completely lock off the secondaries (that's a whole different task).

With the secondaries closed try making the WOT runs. That will show you where the primaries are at WOT. Adjust jetting as necessary....

After WOT on the primaries jets is squared away, tune for cruise by adjusting the metering rods.

When WOT and cruise are squared away on the primaries, then you can unlock the secondaries and begin tuning them.

WOT AFR (as I have heard) for a normally aspirated motor should be 12.5-13.0. I ran more like 12.5 with the QJet and now run closer to 13:1 with the TBI.

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post #6 of 13 (permalink) Old Apr 29th, 04, 11:10 AM Thread Starter
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That sounds good. It turns out I have the factory rods and jets for a 305, so I'll see what the 350's came with and go from there.

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post #7 of 13 (permalink) Old Apr 30th, 04, 05:58 AM
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Paul
 
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I think that I have a little box of later QJet rods. I never work on the later QJets, so you can have them if you want them. I don't even remember how I got them...

If you want them, let me know and I'll check what I have and I'll mail them to you...free of course. [img]graemlins/thumbsup.gif[/img]

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post #8 of 13 (permalink) Old Apr 30th, 04, 07:18 AM Thread Starter
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Hey, that would be great! I'll shoot you an email.

Boy, did I make a mistake with this whole thing. After thinking about how everything is lean, from idle to WOT, I just remembered... ...the air pump is hooked up. It was shooting all that fresh air into the exhaust. So it turns out, My idle is rich (13.5:1), cruise is slightly rich (14-14.5), and WOT is lean (14.5. Back to square one! At least I'll get it right this time.

Hopefully somebody can learn from my mistake... [img]graemlins/clonk.gif[/img]

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post #9 of 13 (permalink) Old May 1st, 04, 07:20 AM Thread Starter
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Ok, I set the idle to 14.7-15.0 and I got the cruise mixture where I want it to be (15.5:1). I also locked out the secondaries.

Now, when the secondaries not operating, if I give it alot of gas, the mixture actually leans out a bit, to about 16:1 instead what I think it should be (13:1). But shouldn't it at least richen up a little bit at all?

Maybe a jet change?

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post #10 of 13 (permalink) Old May 1st, 04, 08:04 AM
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Paul
 
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I would say that you need a jet change. That will also affect your rods for cruise.

If your cruise metering area is correct, you will have to maintain the same metering area after you change the jets.

Changing to 75 jets would be about a 9% change in WOT metering area, but would also increase your cruise metering area 13%. You would need like 53 rods to keep the same cruise metering area.

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post #11 of 13 (permalink) Old May 1st, 04, 08:16 AM Thread Starter
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Yeah, I've found that a 74 jet with a 50 rod will keep my cruise where I have it, and should give me the extra fuel I need. I also have the APT set out about 3 turns, so I may be able to keep the 49 rod and put the APT all the way down. Time to get some jets!

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post #12 of 13 (permalink) Old May 1st, 04, 09:06 AM
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I forgot about the APT thing! I've only read about what it does, never actually tried adjusting one.

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post #13 of 13 (permalink) Old May 1st, 04, 09:30 AM Thread Starter
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The APT is pretty nice. It doesn't make a big change in the ratio. About two turns out richens the ratio by about .25:1, so it works good to fine tune the cruise.

One of the nicest things about this meter is setting the idle mixture. I've got it smack dab at 14.7:1 with the air cleaner on, and what a rock solid, smooth idle. If you couldn't hear the exhaust, you'd never tell the engine was running.

Here's a question: What is the difference between the rods labeled, say, "50" and "50M"?

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