What is the Effect of Load and Gearing on A/FRs?

[Paulporsche]

Thanks to Mark 87Turbo I'm about to do some A/F tests on my car using his LM-1

What effect should gear selection and load have on my results?

In other words, can I expect to find a different ratio cruising @ say 2500 in 2nd gear vs 2500 in 3rd? If I go to WOT from 2nd or 3rd or 4th? Or say I accelerate to 6000 @ half throttle from idle vs rolling in first to 6000 @ half throttle?

What combination of gear and/or type of throttle would give me the best overall view of what my engine is doing?

BTW I will be doing these tests on the road, not a dyno.

[gumba]

I found a slight up hill in second gear works the best on the turbo, and 3rd gear on a n/a car. I use a freeway on ramp. The up hill provides resistance like a dyno. You would want to read from 3K to red line at full throttle to get an accurate a/f reading. You also really need the rpm controller so you can see what the a/f's are at different rpm's. I usually do 2-3 runs to compare the findings.

[T77911S]

interesting question. i would also be curious about same RPM's in neutral.

[Paulporsche]

Ty,

Thanks for mentioning that. I forgot to write that I'm planning to try various rpms while idling.

[berettafan]

great question! i know there is a difference but why and how much???

[Paulporsche]

I'll try some tests tomorrow and post results.

[Steve W]

There should be no difference what gear you are in, although in a higher gear you will be at a higher speed, where the cooler air and high velocity can have some effect, although slight on the AFRs. Best to do it in the highest gear you can reasonably get away with an not endanger yourself or get a ticket, as that will get you a better log. For example, 3rd gear, at 6700, you'd probably be doing over 100 mph.

[cgarr]

Are you friendly with any local airports? If you know what your doing they will usually yet you make a few runs down the runway. Dont use a grass strip

[Paulporsche]

Thanks, Steve.

Should I see a difference between no, or minimal, load (ie while car is stationary and revving) and while driving?

Now I've got to find a straight, unpoliced run of pavement. No, no grass, Craig!

[Steve W]

Of course, you should see different AFRs for no load, light load, heavy load, and full throttle. You want a fuel efficient mix at no and light loads, a power optimal ratio at full throttle and high loads, if you able to tune to that, which I don't see that simple with Bosch CIS.

[911pcars]

Your EFI system should maintain stoichiometric A/F ratios (around 14:1) at all vehicle speeds and throttle openings except for WOT and cold start.

Sherwood

[Paulporsche]

Sherwood,

Alas, no E for my FI; just a simple CIS, so, as Steve W says, tough to get it tuned to optimum in all situations.

Since noone was able to say definitively what to expect if, say for example, I did some WOT while stationary vs WOT from cruising in 3rd gear, I'll try to do my best to stage various typical conditions and post results.

[RoninLB]

Originally Posted by gumba

I found a slight up hill in second gear works the best on the turbo, and 3rd gear on a n/a car. I use a freeway on ramp. The up hill provides resistance like a dyno. .

I do same on a carbed engine. I find that a constant throttle in 4th going up hill causes the engine to produce more HP, a clearer A/F reading and gives me a clearer picture. I also find that acceleration extra fuel distorts readings.


then again like Sherwood said on a EFI it seems like a moot issue.

[T77911S]

hey paul,
i did a few tests last night with an LM-1. i set my idle mixture to 14.3, it did move around a little between 14 and 14.3. 2500RPM in neutral, 1st, 2nd, or 3rd all resulted in about 15.2 AFR. same results for 3500 RPM. around 4K it was starting to get richer at 14 AFR and continued to get richer, i think 5k was around 13.5. flooring it at 2500- 5500 produced 13.2 AFR.
in the end, 2k to about 4K was a consistent 15.2, + or - a few tenths, that may have been due to my foot .
then i did a quick test changing idle mixture to 13.3. steady mid range driving was about 14.5. about a consistent 1 point change across the RPM range.

before i changed my mixture to 14.3, it was set to around 13.4, that is about 3% according to a chart. the engine was still dipping down low then coming back up to idle when i let off the gas, this was after i lowered the idle down from just over 1k.
when i set the mixture to 14.3, it seemed like it was very lean. if i richened it from 14.3, the idle would increase a little and seemed like it ran smoother. this "edge" where it idled up was at about 13.7 AFR.

[Paulporsche]

TY,

Did you get any hesitation @ 2500 when you were @ 15.2? That's pretty lean.

[T77911S]

no problems in the low RPM's at all. 4200 to about 5k i get a miss. i thought it might be fuel related but i have run it rich and now lean and it is the same for both. sometimes it seems to be ok, mostly when cold. i also have a 914 rev limiter in there and i was thinking it might be causing the problem. i hit it last night, it came in at 5500. all ignition parts are new.

i dont know what is too lean or too rich. i guess a dyno would tell that for sure. the bosch cis book says it is suppose to go lean in the mid RPM range.

i left the mixture at 13.7, about 2.2%.

[Paulporsche]

OK. I was wondering if you were getting symptoms similar to Craig (mca) but it looks like you weren't.

Why did you leave it @ 2.2% Did it just feel right? Sounds like you still had a way to go before hunting.

[T77911S]

when i set it to 14.3, it idled the engine down just a little. 2.2 was where it idled back up an seemed to run a little smoother, than the 14.3.
why i left it at 2.2? i dont know, just felt good at the moment i guess.

it was craigs LM, i was hoping it would help him too. i wanted to do the check with the timing because i am wondering if the timing at 2500 is causing his problem.
with the more agressive cams would he need a different timing curve?

[Paulporsche]

I'm thinking he might. There is also a limit as to how far you can go w/ cams on CIS. Unfortunately I don't know what that limit is, although I suspect he's still OK.

My own diz seems to have been recurved 20 yrs ago to allow for mods. Since I still run single plug, however, I always go for 0TDC @ idle which gives me about 32 (I think) @ full advance.

[T77911S]

i was thinking about what you said about 15 being too lean. should i set i for 14.3 in the mid RPM range? that would mean low 13's at idle. 13.2 is 3.5 %. should it never go leaner than 14.3, unless you let off the gas?