Here's the situation: My friend/tuner has a motec lambda meter. Earlier tonight we tried to do some tuning with my hondata. Above 3000rpm the car runs great - about 12.3 at WoT all the way to redline. Under 3000rpm was another story. We couldn't get an accurate reading out of the lambda meter all night. The numbers were all over the place. On the other hand, my crappy autometer a/f meter was stable and reading rich even under 3000rpm during acceleration. It actually seemed more accurate that the lambda meter.

The thing is that we just attatched the lambda meter to the end of my exhaust pipe. I have a 3" thermal exhaust. My question is if it is possible that with a 3" exhaust and measuring at the tail pipe if there is not enough exhaust (or exhaust velocity) toallow the lambda meter to read properly? That is pretty much the only explaination that we could come up with because my friend has tuned 4 other cars this week with no problems.

Thanks in advance.
-A

 

Did he have an insertion "sting" that went into your tailpipe? A smaller tube, about an inch+ in diameter that would extend into your exhaust abut 18 inches to 24 inches, with a bung welded on for the wideband. The sting tube should extend about a 12 inches past the wideband. You want to insure that ambient air is not being aspirated into/onto/past the wideband. If it is aspirating that will knock the readings to heck.

Regards,
BigMoose

 

I think the stinger is about 8 inches or so. I'm not exactly sure how long it is but it is definitely shorter than a foot, I'd have to say. I'll have to bring that up with him next time I see him. Thanks

 

This may sound dumb, but did you turn closed loop off?

 

Yup. It was in open loop.

 

I worked on a car for 20 minutes trying to figure out why the a/f was fluctuating so much only to realize it was in closed loop mode.

 

How does one get the car out of closed loop mode? do you just disconnect the O2 sensor?

 

I might have overkill on my sting for minimization of ambient contamination. I never cut it down to see at what length it gets flakey. The smaller the diameter the less length needed after the sensor. So there is some give there.

I go deeper in than most to gather a more pristine sample. What I am afraid of, especially with the big exhausts like your 3 incher, is potential ambient incursion on pulse resonances because of low mean gas velocities. In english big pipes = slower gas velocity, add on organ pipe resonances etc, and you can suck air in the exhaust a bit.

Just some ideas. Thanks for listening. When you find the solution, please be sure to post it so we can all learn form you.

Regards,
BigMoose

 

We have a similar set up and it is definantly slightly off. I usually tune for 12.5-13 on there and it seems to like it and i have never had any problems with detonation or anything. Even when running 20 psi and a 120 shot 13:1 was still working fine.

 

You turn off closed loop mode by going into your programmable engine management software and turning it off. There's no way to turn it off on a stock ECU other than throughing a MIL and then your a/f won't be accurate anyway.

 

It is recommended that your "stinger" extends into your tailpipe a minimum of 6" for each inch of tailpipe diameter. IE for a 3" tailpipe, 6"*3=18". This prevents ambient air from reaching the sensor due to reversion at low RPM.

 

Yeah, I think we have come to the conclusion that we are getting some ambient air screwing up our reading. Next time we do it we may just try hooking it up where the stock o2 sensor sits. It will take much longer to setup but I don't think we really have too much of a choice. I'll report back once we do that.

Thanks again for everyone's help. This is why I love HT!

-Archie

 

Thanks, I look forward to the results!


Thanks also to B20C5Turbo, you confirmed my intuition with a rule of thumb.

Regards,
BigMoose

 

Alright, to avoid the whole inversion thing we decided to put a new bung into my exhaust (right after the cat). Tuning the car this way worked much better so I think ambient air reversion was my problem after all.

There was something odd though. I noticed that when I turned my car off teh lambda meter would still give a reading (usually around 18.x). Why does it do this?

Thanks for all your guy's help, BTW

 

There is still exhaust there.

-Dustin