Don Terrill’s Speed-Talk

RCJ wrote: ↑Thu Nov 02, 2017 12:45 pm Advancing the timing will show leaner on a flow meter also.

Is that so with both carbs and EFI ..?

I've only worked on carbs (dirt late models)

Warp? If anybody's got info on the effect of timing change on indicated A/F~~ carb vs. EFI, it should be you.

Here's what I've noticed/experimented on GM late model EFi closed loop. Let's say I have the idle timing at 18 degrees at idle in neutral and the fuel trims are hovering 0 meaning computer isn't adding or taking fuel away to keep stoich/lambda1. When I decrease idle timing to 13 the computer has to add fuel to keep it at stoich/lambda1. If I increase the timing to 23 the computer starts taking fuel away to keep the engine at stoich/lambda1.

Although I have not done this with a 5 gas it still tells me it's about combustion efficiency and not Nox as higher timing should increase Nox in your previous explanation making it leaner.

That depends on whether it's a MAF based system or an SD based system.
MAF is far better in just about every way in that it meters the air instead of guessing it based on manifold vacuum.

Changing timing changes manifold vac.

Joe, the GM controller I am speaking of(12200411 2002 vette/camaro/firebird) uses MAF and MAP. I can program in real time and can use either or both blended like intended and when I experiment I try as much as I can, to learn. Being able to program on the fly instead of having to flash for each change allows me to really see what is happening rather quickly.

Bottom line, yes I understand how I could easily make this test unreliable. So, for the lay person, it is good you brought that up as it could definately invalidate a test.

Another thing that messes it up is the target idle speed.

Most computers will change both timing and AFR automatically to try to lock in the desired idle speed.
Changing the timing deliberately upsets it.


That means that a proper test is done with a fast idle, above the normal set idle speed.

In-Tech wrote: ↑Thu Nov 02, 2017 3:41 pm Here's what I've noticed/experimented on GM late model EFi closed loop. Let's say I have the idle timing at 18 degrees at idle in neutral and the fuel trims are hovering 0 meaning computer isn't adding or taking fuel away to keep stoich/lambda1. When I decrease idle timing to 13 the computer has to add fuel to keep it at stoich/lambda1. If I increase the timing to 23 the computer starts taking fuel away to keep the engine at stoich/lambda1.

Although I have not done this with a 5 gas it still tells me it's about combustion efficiency and not Nox as higher timing should increase Nox in your previous explanation making it leaner.

Yes, a 5 gas would tell the tale. Our dear departed friend Shrinker (RIP) was a proponent of corroborating the WB with his 5 gas, and pointed out the influence of the other gasses on the indicated A/F, not as a criticism but for illumination, so we might all better understand.

Like Warp said, the sensors are accurate and the most useful tool for the intended purpose, especially when combined with the user whose experience provides insight to the little idiosyncrasies that affect the sensed values. The aberrations of indicated A/F from actual are small and merely artifacts of physics and chemistry which, when understood, should not be a distraction. On the other hand, a WBO2 won't make a tuner out of someone who isn't already.


My point about timing changing NOx is the O2 sensor sees NOx as if it is 50% as much O2. In other words, 50 units of NOx is measured as if it were 25 units of O2. To paraphrase the SAE paper (88059) describing this, "The O2 that came in at the beginning of combustion is still there in the end, just in a different molecule, so the effect is nearly a wash, so NOx can be disregarded." However, when A/F is relative far from 1λ I think the NOx cannot be entirely disregarded.

The effect of A/F on production of NOx is fairly well understood on the lean side of 1 λ, but concentration of NOx is influenced by other factors than A/F. Advance and retard timing with the same A/F changes NOx. Another example is detonation occurring in a rich A/F, where there should be little or no free O2 to combine with N to make NOx, makes substantial NOx and causes WBO2 readings leaner than actual A/F, sometimes much leaner if the detonation is severe.

When you're on the rich side of lambda 1 , you're sensing the combustibles (CO)in the pipe, not the oxygen concentration.
The pump cell is working in the opposite direction.

Having the WB before or after the catalytic converter gives different readings.
Same as having a NB before and after the cat to tell if it's working or not.

That's due to the cat converting the NOX into N2 and O2 and at the same time converting the CO to CO2.
It's the CO % that it's actually sensing, that reduces in concentration so you always get a leaner reading after the cat when it's rich of lambda 1.

So sticking a sensor up the tailpipe when you've got a cat.......it's inaccurate.

When in closed loop it's constantly switching from just over to just under lambda 1 . A sensor after the cat will just smooth out the peaks and troughs, the average should be about the same.

RCJ wrote: ↑Thu Nov 02, 2017 12:45 pm Advancing the timing will show leaner on a flow meter also.

I find that puzzling. Does the airflow go up, or the fuel flow down?

I never payed attention to whether it was from air flow or fuel.After any change the airflow will change some and the airflow can go down and hp pick up.I never tune soley off of afr ,I pull a plug and watch hp and torque.

I'm talking small amounts.When the plug strap is showing it would take 2 degrees more timing and the afr is showing it could be .4 leaner at high rpm.Adding the 2 degrees will make both better.6400 to 7800 rpm range

If you pulled a spark plug wire off, what would that do to the AFR?

John Wallace wrote: ↑Fri Nov 03, 2017 7:39 am If you pulled a spark plug wire off, what would that do to the AFR?

Exactly. And if the wideband tells you the engine is lean should you put bigger jets in the carb or should you re-attach the spark plug wire??