Imagine standing next to your own car while it is full throttle at maximum revs. It is truly a profound experience, as the sound, vibration, and actual gut feeling is amazing. I could not decide if I wanted to feel it or run and hide. Was it the sound of torture or power?
This is the third time I have done this, once with the NSX and now twice with the Seven. It really makes you think about the extreme stress an engine is under at full tilt, and make you reconsider running it to max rpm any other time in your car's life.
Measured power at the rear wheels was 100.3 horsepower. Charlie Rockwell, tuner extraordinare, started at 92, fiddled extensively, and managed to reach the magic number. He suggests a conversion factor to get engine horsepower,
engine hp = wheel hp x 1.28
giving me 128.4 hp. He even jokingly suggests the British add another 8 hp, at least this is true in his experience, giving me 136 BHP. This is on an engine with only 144 miles on it. Charlie beleives it will gain another 3-5 hp as it wears in. Mike Stimson, the guy that built the engine, was there as he was curious to see the results of his labor.
His log sheet looks something like this:
Stimson 1600 X flow 2 x 40 DCOE
rpm hp CO O2 AF (air:fuel)
3000 43 9.7
4000 73.1 6.9 12.1
5000 86.1 7.2 11.8
5500 90.0 7.5 11.5
6000 92.2 6.3 12.5
timing 26 degrees total
changed to 30 degrees
5500 91.6
changed to 35 degrees
5500 90.8
changed to 32 degrees
5500 92.3
choke 30 mm
changed to 33 mm choke and 130 main jets
4000 73 4.45
5000 90.3 4.2
5500 92.3 4.4
6000 98.0 4.4
6500 92.0 na
changed to 150 main and 185 air jets
power the same, running rich at low end, lean at high.
(He probably made three more jetting changes here searching for a better throttle response. There is a slight stumble at initial throttle off idle.)
adjust balance, idle mixture
remove exhaust sniffer
6000 100.3
Some of the discussion was relatively low compression on the motor, about 9.3:1, but with a good squish as a result of less than .040 clearance between the head and the top of the piston. (The crossflow has a flat head and a dish in the top of the piston.)
Total advance was done not using a knock sensor but based on how much power the engine created. Mike was used to more like 36 degrees and has seen as much as 40 in some race prepared engines.
Charlie is now out of the tuning business. He is looking for a job with an automotive firm. He has submitted his resume to Honda to help with their CART effort. I told him he should work with them on their reentry into F1. I would love to brag that my engine was tuned by a F1 mechanic...
This weekend is a club autocross. Trial by fire
0 comments:
Post a Comment