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1970 |
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Ranchero Squire |
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429 Cobra Jet |
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1970 |
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Ranchero Squire |
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429 Cobra Jet |
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Zoom! |
I'd recommend not wasting your money on one of these things. If you want to skip
all the theory and operation, you can just skip down to
my conclusions.
The O2 sensor has to be HOT to work correctly, so the closer you can get it up the pipe the better. I welded mine about 10 inches down the pipe from the exhaust manifold. That was a close as I could get without completely dropping the H-pipe. I've heard some people drill and tape the exhaust manifold. Of course this means the sensor only reads one side. But I figure both sides will be close enough to each other so if I tune one side the other will be OK too.
The wiring should go straight to the battery with new leads. At least that's what the instructions said, so that's what I did.
The sensor isn't as good in theory as it is in practice, LOL. My problem is the sensor give me pretty erratic readings. I'm almost certain the A/F ratio itself isn't that erratic. I think it is the meter, but until I get a VOM on the sensor LEDs I can't be sure. A idle, cruise, and very light acceleration, my meter "blinks" on and off. My best guess is the "on" reading is the correct one and the "off" can just be ignored. (For those not familiar with the meter, it includes a series of 7 LEDs. Just one LED lit = A/F ratio 15, 2 LEDs lit = 14.5, 3 = 14, 4 = 14, 4 = 13.5, 5 = 13, 6 = 12.5, and 7 = 12. The A/F 15 single LED is red just to reinforce 15 is too lean. And the last two LEDs to light indicating 12.5 and 12 are yellow, to reinforce that these ratios are too rich.) What I get at cruise is 5 LEDs on for a couple of seconds and then the meter goes dark for one second or two seconds or 20 seconds, then the 5 LEDs all come back on together. I talked to Edelbrock and they said this was NOT expected behavior, but the unit is out of warranty. (It sat on my self for a year before I installed it. Duh!)
Anyway, my interpretation is the meter is saying my cruise A/F ratio is 13. I can move this around a little with jetting so the meter is really showing me something.
The stoichiometric air/fuel ratio is the chemically correct ratio, theoretically all of the oxygen and all of the fuel are consumed. The mixture is neither rich nor lean. However, due to the fact that combustion is never perfect in the real world, there will always be a small amount of oxygen left in the exhaust. This small amount that is left is what the oxygen sensor measures. The smaller the amount of oxygen that is left in the exhaust, the richer the A/F ratio is, and the higher the oxygen sensor voltage is.
The
A/F Ratio Meter is a voltmeter with a range of 0 to 1 Volt. The meter displays
the output voltage of the vehicles O2 sensor through the LED's.
The oxygen sensor is very accurate at indicating a stoichiometric A/F ratio. It is also very accurate at indicating an A/F ratio that is richer or leaner than stoichiometric. However it can not indicate what exactly the A/F ratio is in the rich and lean areas.
It
isn't easy to achieve ideal A/F rations with a carburetor across all RPM ranges
and throttle positions. That's why EFI is the predominate fuel metering
system used today.
Carburetors are still popular in racing where you really only need to turn WOT. And besides, when you are racing if you are just a little rich it doesn't really hurt, does it?
With stock out-of-the-box jetting on my Edelbrock 1411 carb, the meter said my cruise was running 12.5, light acceleration 12.5, medium acceleration 12.5 and WOT 12.5. (FYI, the Edelbrock carb has three distinct tuning settings, high vacuum primary jetting, low vacuum primary jetting, and the secondaries that only cut in at low vacuum.)
My carb is adjusted rich or lean in the stages shown here.
First I tried one stage lean for cruise and left the power primary alone and the meter moved to 13 for cruise and light acceleration. Moderate acceleration stayed at 12.5, no change as expected.
I wanted to try one stage lean power and two lean cruise, but I didn't have it in my toolbox so I tried two stage lean for both primary jettings and the meter moved 13-to-13.5 for cruise and light acceleration. Moderate acceleration moved a little but not all the way to 13, given the flickering LEDs I'll call it 12.75.
(FYI, I did call Edelbrock about the step-up springs and learned how to set these. Edelbrock says after tuning idle for max vacuum, divide that setting in half and use the nearest spring. At 14 hg/in of idle vacuum I changed the stock #5 spring for a #7.)
Next I wanted to try three stages lean both power and cruise, but, alas, I didn't have that either. But a little mix and match from the jets and rods I did have let me try some more extreme settings. I tried what was around 4 stages lean cruise and 3 stages lean power (these were off the Edelbrock tuning chart) but A/F ratio = 14 wall all I could get.
As far as I'm concerned, I can't believe the original jetting was that much too rich. One of the problems I'm facing is an O2 sensor is ideal for closed loop EFI but lousy for tuning a carb. A cheap O2 sensor is great for telling you the A/F ratio is below ideal or above ideal, but not good at all at telling you HOW FAR off you are. In closed loop EFI the computer iterates the A/F ratio very quickly up and down in smaller and smaller increments until it learns exactly what is correct. In theory, I could jet the carb leaner and leaner a stage at a time until the cruise A/F ratio goes lean and then jet it back one stage rich. (For max power you want the secondaries jetted substantially rich, e.g. 12.5.) Maybe if I trusted my meter 100% I'd try this. But since I went off-the-chart lean and still didn't hit 15 I don't want to try this. (I tried the other rule-off-thumb technique: I checked for medium acceleration ping. An extremely lean mixture might cause pinging. I didn't get a ping, but that doesn't prove much either way.)
So what did I do? I went back to two stages lean cruise and power settings. I left the secondary jetting stock. My meter says I'm cruising and light accelerating at more-or-less 13 and at 12.5 at WOT. I'd really like to be at 14.5 for cruise, 14 for light acceleration, and 12.5 for WOT but until I get the car on a dyno with a $1000 lambda O2 sensor I'm going to live with what I've got.
A final note, once you get all the A/F tuning done, you still need to play with accelerator pump settings to eliminate any hesitation.
After jetting leaner and leaner to satisfy my A/F ratio meter I was getting suspicious. Several people I talked to said their experience was the Edelbrock #1411 carb I had came lean from the factory. Confirming this, the same carb from Edelbrock but with a mechanical choke and targeted just for high performance is jetted way richer from the factory.
I made back to back runs at the drags to confirm a consistent baseline, then I swapped out my lean jets for the factory stock jets and my ET dropped 0.4 seconds and my trap speed increased 3 MPH. That is equivalent to picking up about 20 HP at the rear wheels. So while this A/F ratio meter was saying I still needed to go leaner, I was clearly way to lean already. I'm dumping this piece of junk!
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