Quote:
Originally Posted by Ecky
Makes sense. I don't want to get the thread too off topic but the issues you can expect to see from changing how the engine breathes are thus:
Big cams (broadly speaking) move the torque curve up. You lose torque at low RPM, and gain it up high. There's no such thing as a cam profile that does both. I haven't looked into the exact functioning of the D17's VTEC system but my guess is that it's either 1) a low and a high lift lobe, and each lobe drives both valves depending on VTEC, or 2) a low and a high lobe, where the low lobe actuates one valve at low RPM and the high lobe the other, and a locking pin allows the high lobe to operate both valves after VTEC engagement. This arrangement is more likely.
More lift and duration degrades torque and combustion quality at lower RPM. This can be seen in extreme cases such as in classic large domestic V8s which have loping, rough idles from their high lift and duration cams. VTEC can minimize this, depending on how it operates, but you'd be trading mid-range for high end torque in a best case.
At low load the engine runs in closed loop. The O2 sensor can compensate for differences in airflow, but the ECU has to relearn the fuel trims on every startup, and you may find it runs rich for a very brief time when changing throttle, as it expects more air. There will likely be some changes to cylinder scavenging if the low cam has changed, which would have a minor effect on what ignition timing is needed.
At high load, most likely the ECU goes open-loop and relies on its fuel tables without O2 feedback. If you're seeing a 15% difference in torque, for example, you'll be running 15% more rich in the mid-range and 15% leaner up high, since the tables won't be corrected and that air would have been moved from one spot to another. If the base target AFR was 13.5:1, that could mean at (for example) 4000rpm it could be as rich as 11.7:1 AFR, and up around 7000 it might be as lean as 15.5:1. I doubt from a cam alone you'd get 15% more high-end, but the principle applies.
My 2 cents, and my word of caution about changing anything past the butterfly valve without adjustment of ECU maps.  |
You are totally correct, great information! This is a great cam, the idle quality is just as good as stock and I don't feel a loss in low end torque, if anything it feels a bit better. The mid range and top end improved noticeably though. The main difference I noticed is that the car now pulls strong all the way to redline, the power doesn't drop off as it approaches redline anymore. It now feels like it would keep pulling strong past redline if it wasn't for the rev limiter.
IIRC what Crower did is they kept the non VTEC cam lobes the same and they increased the lift and duration of the VTEC lobes so low end torque and idle quality are retained, but the engine breathes better at higher RPMs when VTEC comes on. Or something like that, it's been a while.
Great point about air fuel ratios. My Scanguage shows AFRs, but I don't know how it calculates it or if its reading can be trusted. For what its worth it shows low 12s at full throttle though, so that should be good. I told myself I had enough gauges when I added a dual temp gauge for my oil and trans temp and an oil pressure gauge, but you are making me want to get an AFR gauge too lol.