I’ve added 02 sensors to my 455’s with qjets. Yeah, it’s a little weird, but I like qjets. I’m an amateur, so I’ve never really developed the experience to really _know_ what’s right.

I use dual 02 sensors on my cars to help me fine tune the qjets. At the moment I’m using 49 rods / 74 jets on the primary side. Judging by the 02 sensors, I’m pretty close on the jetting/power piston/APT operation. Secondaries seem a lot more forgiving, but then again, I’m not racing ‘66 big cars…

My cars idle at lambda of .93- .96. I appreciate that these values are <0.00 (aka perfection combustion), but they both seem to idle better a bit rich. I’m trying to not to get hung up on perfect combustion in favor of good running. I can tweak the idle screws to get to .99 or 1.02, but I’m aiming at smooth idle for a great driving experience.

When we all stab the throttle from idle, qjet accelerator pumps enrich the mixture to balance the quick rise in incoming air. So lambda drops reflecting the enrichment in real time.

My question is how far should lambda drop to get ideal enrichment on the pump shot?

I’m seeing .80.-.85 on my cars when the accelerator pump kicks in. Is this too lean, too rich, or about right? Then the power piston spring action kicks in a lambda drops back to .97 or .99 which makes me happy. I can change the length of the pump plunger to vary the amount of extra gas on the shot, but I’m not sure what’s optimal.

I’m not seeing any performance problems, but I’m curious about what kinds of enrichment numbers EFI systems use or aim for on off idle acceleration enrichment. If my pump shots are too rich or too lean, I’d like to avoid random pump length changes in favor of a goal. Otherwise I could burn a lot of time chasing my tail with no gain. Again, I’m not unhappy, I’m just wondering what EFI people are aiming at in terms of lambda goals for off-idle acceleration.

Thanks in advance for any guidance you all can offer.

Chris

 

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With a 455 and a Holley 780 on my car I see Lambda drop to around .8 as well and the engine responds well. When going to WOT the lambda stays at about .85 though for max power. I did not see a reason to go richer than target on the step in.

On my truck with a 455 and EFI I have tuned it to be similar but since it is more tunable, I have calibrated it to try to maintain my target Lambda when tipping in to the throttle. So that when I do a slap WOT it goes slightly rich and land right on my target Lambda.

I don't think you'll ever be able to get a carb to do that since the total volume of fuel is based on the size of the accelerator pump and with EFI you can tune based on throttle and MAP rate of change as well as temperature and engine speed.

 

Give the engine what it wants, not what you think it needs.

Hot rodders did just fine before O2 sensors became common. Personally, I think tuning for a specific number is a mistake. You can tell a lot from the plugs and your own observations. I’ll explain.

Many years ago I got serious about setting up the Q-Jet on my car. I ordered a bunch of metering rods and power piston spring.

I used a long stretch of fairly smooth highway, on a typical summer day. I drove that same section of road countless times, I kept leaning out the APT until I got a lean miss (best description is the sensation of a fish nibbling on the bait, just a faint bucking feeling) unfortunately, I had to go one step leaner on the jet, I was out of adjustment. Once you find the lean miss, richen it up a bit. Once you have found the “happy place” play around with the power piston spring. Is it lazy when you crowd the throttle? Try a different spring.

I tuned the metering rods at the track, I tried a bunch until I zeroed in on the best mph.


Other than moving the pump rod into a different hole, or drilling the discharge nozzles, what tuning can you do to the accelerator pump calibration?

Play around with the accelerator pump settings.

Fast forward a few years, I got an Innovate LM-1 wide band from a friend and installed it. Imagine my surprise to find cruise AFR around15.6-15.8. The WOT AFR was around 12.8-12.9, about what you would expect. I guarantee if I had the AFR first, I would have chickened out long before I got the sweet spot.

Now that I have Holley EFI, the cruise AFR needed to be a little richer. It’s happy around 15.2ish. The car gets great mileage , runs VERY well, and has yet to melt a piston (the usual comment when I tell the story) and the plugs are clean.

 

Matt
I completely agree on not aiming at values, but rather tuning to how an engine runs best.

As an example, when I got the Innovate 02 sensors, I initially I tuned my idle lambdas to .98-.99 (aiming at 1.00), then found when I enriched them down to .94 or thereabout the engines idled better. I got to exactly the lesson you described.

I’m not trying to tune the accelerator pump shot to any particular value, but I have a few different pump lengths in my qjet parts collection I could try if any change is warranted by the views expressed here by people with more experience than me with EFI lambda values.

What I can’t really tell from driving around is whether I’m already on the money or if it’s too rich on the shot. Neither engine is stumbling or smoking, so I think I’m close on the shot values, but wanted to ask here before I go tearing the top off the qjet just to make things not better or maybe even worse with a different pump length.

In both cases my carbs are the stock pump length provided by SMI (Sean Murphy Induction) who I respect as a builder. With his experience and the engine specs I provided, I believe he chose what he thought best, but there’s nothing like testing in the field. I’m just trying to avoid random testing unless I have confidence I can do better.

The answer may be I just need to test, but I thought a question here might give me some guidance on what direction to go.

Cheers
Chris

 

It’s actually high 14’s. ALL Holley O2’s show leaner than they really are. I believe they do that on purpose. And you won’t melt pistons if your WOT air/fuel is correct. That would be a comment from Internet idiots. A lean cruise is inconsequential as well.
But you’re right, give it what it wants.

 

I think you're pretty close on that accelerator pump shot. Might be able to try the outer pump arm hole just to slow it down a bit. I am working my way through tuning my six pack mopar right now; I was seeing .7x / 10.6 AFR, it was actually starting to load up/stumble at that point. If you have a very brief period down in the 11.5:1 range it doesn't seem to be a problem from my experience.

 

It's not easy to tune the qjet accelerator pump, so if it responds well to your driving conditions then just let it be what it is. Where it really matters is starting moving from a stop. Just experiment with different throttle rates. Say the car reacts great if you ease into the throttle, stumbles if you moderately stab it, but runs great if you just floor it. You know there's a pump issue to work on.
Chasing down too lean vs too rich on the pump can be fun - both cases are a loss of power. In my experience a rich stumble recovers more quickly, while a lean stumble is like turning off the ignition and possibly a backfire. You can track that since you have the O2 - basically if it stays in power and makes a good transition back onto the main circuits - then it's right. Sure, a tiny bit less or more fuel might be "more perfect", but getting that kind of change with a qjet is not simple. And the big problem is the carb doesn't know all the conditions: it only knows throttle position and rate. So really you want it on the rich side because that will work better across all conditions than being on the lean side.

But, also, I've never had a reason to mess with the pump in a qjet in my car across .... jeeze.... 6 different engines and at least 12 different qjets?
Now, pump on a holley (admittedly I have far less experience with holleys)? good lord, endless messing with those.


The best use of the O2 with a carb is to watch how the carb is reacting with the different circuits. It helps to isolate what the primaries are doing vs the secondaries and tune them appropriately. Same with the power piston spring - you can see when the spring reacts and make changes as appropriate. It helps nailing down whether you should change a rod vs a jet and if the piston spring is way too stiff or soft. Converting to adjustable part throttle base plate is also highly encouraged.

For me, I did all the typical carb tuning first, then used an O2 to fine tune and tweak a few things. It can also give you clues to what's happening when things go wrong. But just like OBDII codes on a modern car it's only a clue.

 

Are you saying the handheld display or software is calibrated to show slightly learner than it really is? I have replaced the O2 sensor a couple times over the years, the replacements always read the same as the previous sensor.

The “melting piston” comment has come up a few times, along with “more than 34 degrees of timing will shatter pistons!” People run their mouths and repeat the same bullshit myths without a clue of the theory behind them.

I admire the small business owners who deal with the public on a daily basis. And especially the ones that do custom work. Having to deal with the “experts” west all the different hats required to run a small business, actually do quality work, and make a profit has got to be tough.

 

Hope this helps.

 

What Mark said about the scaling of the input to the electronics that measures the O2 sensor output is spot on. I have an older AEM sensor that I will not be able to get a display for since the curve output doesn't match anything out there as far as gauges go. I would have to build and code my own A/D - microcontroller - display to match the curve of the AEM gauge to read right. Or make an old school analog meter movement that I calibrate the meter face by hand based on the Volts vs. A/F chart.

Anyone want my old AEM wideband? It has a chart of Volts to A/F that came with it...

 

When it comes to acceleration enrichment tuning, I don't target a specific number. Usually the goal is that you can transition to your full load target AFR smoothly. If it stumbles and goes lean, you don't have enough enrichment, if it goes richer then comes back and stabilizes, you have too much.

Now what your full load target AFR is going to be is going to depend on your combination, power adder, ignition timing, temperatures, fuel type, etc... On a pump gas NA engine with a pretty safe timing curve, 0.82 is a pretty typical number. If you're really chasing as much horsepower as possible or have a really really mild engine it might be more like 0.85. If you're worried about playing it safe, are throwing a lot of timing to it or not enough octane, you might go more like 0.78. Below that you're usually talking boost or alt fuel situations.

Generally for most combinations if you're cruising around at 1 and it goes down to 0.8 nicely and stays there, you're in good shape.

I think a lot of the Innovate stuff lets you program the gauge to any calibration you wish.

 

Are you using lambda for regular gas or E10?
If you’re using regular gas, even an air/fuel of .85, or about 12.5:1 under load, is TYPICALLY still on the rich side.
Put the car on a chassis Dyno, or run it at the track and see what it really wants. The O2 is just a reference.

 

Gang,
Many thanks for your views, I believe my enrichment (for street cars with mild cams) is about on target. I’m not racing or looking for tenths in the quarter mile. I just wanted to be sure I wasn’t missing anything big.

Each one is a little different but they’re running fine. I just wanted to be sure I was near optimal. I don’t need to get to the third decimal to the right in precision.

The other data point that helps me as I drive them around is the economy - around town I’m seeing 10-11 mpg and about 14-15 mpg on the freeway. Without getting too far into the engine combo I’m getting those numbers with 455’s, .030 over, 800 cfm qjets, TH-400’s and 3.23 open rear ends.

Thanks again. It’s great to get your thoughts in one place for reference for others down the line and for me too.
Chris