Dynojet with A/F tailpipe meter question

[deleted]

Hi All,
I was on a dynojet the other day and they used a tailpipe sniffer type A/F ratio meter to graph the results. I was wondering how accurate are these things especially since they are after the catalytic coverter?

Also, my car showed pretty lean results ~14.5 throughout most of the power band even though it just has an high flow cat and 2.25" exhaust and short ram intake on a OBD1 P72 GSR motor with idle fuel pressure at 42 psi ( within stock specs ) with hose connected running 94 octane and 17 degrees timing. However, it does have a small exhaust leak between the header and cat before the o2 sensor. Would this cause the A/F to read lean on the dyno's tailpipe meter? The dyno charts look fine and match up with other stock GSR motors; its just the A/F ratio looks too lean.

Also, does the OBD1 P72 have long term mixture compensation? I saw this information in my 98 Integra Helms but I don't have a 94/95 Integra helms to look it up. Also, I noticed that in my 92 Civic helms, there is no mention of long term mixture compensation.

Thanks.
-Cliff

[AndyE]

An exhaust leak shouldn't affect the O2 sensor reading...the composition of the exhaust gases will be the same, leak or no leak. Exhaust pressure on a running engine is greater than atmospheric pressure, so you shouldn't be sucking in any air through that leak, only blowing exhaust out, unless it's a really friggin' big leak.

The P72 (and nearly every other ECU in existence) does indeed have long-term fuel trim. This is why folks always say that you should reset your ECU after making modifications...what you're really doing is resetting the long-term fuel trim.

-Andy