Tuning homework
I had an interesting tuning scenario come up last night at the stadium drags.
I was helping out a fellow racer with a tuning problem he's having. I belong to a racing club and there were a few of us adding in our 2 cents here and there throughout the night. Let me give you some simple specs on his engine.
360 some odd cubic inch small block ford.
Single plane intake with a dominator adapter weld fitted to it. The plenum was worked to provide simple flow turns into the runner. Runner dividers unmolested and protruded into the central plenum area. The configuration provided for a built in 1 inch riser for the carb.
Dominator carb with modified venturis for better signal reaction from the small engine.
Car fitted with an O2 alert system. Provides engine cutoff if a pre-entered A/F ratio (lean) is encountered.
250 shot of nitrous on for the whole run. Under carb plate distribution.
Intermittent re-ocurring problem: Engine is cutoff by the O2 alert system during launch in the middle of a big wheelie.
Ouch! O2 alert system is set to cutoff if 13s to one A/F is seen.
The nitrous system is setup rediculously rich. 4 something to one. Can't remember the exact ratio.
On the motor only the A/F records as high elevens at the end of the run (1/8 mile).
The funny thing is the A/F at the start of the run will record lean and drop (richen) in a fairly linear line until the end of the run. I feel it is the O2 sensor warming and changing reading and not a carb tuning issue. He was only testing last night and was not doing any warmup burnouts. I asked him to do a burnout the next run, which he did, and the record showed a little richer at the start of the run with the same linear richening until the end of the run. Still recording the same A/F at the end of the run as previous runs showed.
A new O2 sensor was installed last night, because I felt the readings were lazy. Readings were more active with the new sensor. Finally started to see slight A/F stepping between gear changes. Nothing outrageous. We recorded some graphs to test the accelerator pump tune and no leaning was occuring. A smooth slow richening was recorded.
During one run we caught a lean spike right at the launch. It was at the edge of setting off the O2 alert system. Obviously we had captured the very thing that was his main problem.
I will give clues if no one comes up with the answer. Razor will probably get this one.