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Originally Posted by mbtoloczko
Hi Chike,
When you did the dyno runs in your car, I would guess that the shop simply put a wide-band O2 sensor in the tailpipe of your car. If that's the case, the afr reading is not going to be terribly accurate because its taking a reading of the exhaust after it goes through catalytic converters. From what I've read, dyno systems add a generic correction factor to the afr reading to attempt to compensate for the exhaust gas composition being altered by the cats. Bottom line is that the afr readings from your dyno runs are only approximate.
Be careful about running too rich. Running too rich can also cause detonation. 11.6:1 is probably not the optimum afr for timing advance.
I have not been able to understand why, but fuel flow from a fuel injector is not linearly proportional to the fuel pressure. Its proportional to the square root of the fuel pressure. I've talked with RCEng about it several times. Its what they see when they measure the flow rate of injectors. So going from 36.3 psi to 43.5 psi will only raise the max fuel flow by 9.5%.
I'd like to throw the SVX fpr back in the car, but I don't think I'm going to have time for about a month.
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Mychailo,
You are right that the dyno shop only measured my afr from the tail pipe. As a result, I'd be very interested in learning whether the stock SVX ECU actually richens the afr to somewhere in the 12:1-13:1 range in open loop operation. That'd be pretty optimum for a factory fuel system!
I came across the following graph today showing the pressure created by burning gas at different afr's taken from 'The Sports Car Engine: Its Tuning and Modification' by Colin Campbell:
http://www.ackthud.net/shawnfogg/mixture.htm
It is noted on the above site that: "Maximum power is usually obtained running around a 12.3:1 AFR. Going richer then that will cost a little power but you loose less power then being leaner then 12.3:1"
Depending on your wideband results with the SVX fpr, I may switch back to one. I just don't understand why the afr was first richened to 12.5:1 in open loop and then went all the way to 11.4:1 above 5000rpms???? (I was expecting it to actually lean out slightly in the upper rpms)
My dyno plot at Altered Atmosphere on 12-15-04 showed the slightest richening in the afr's above 5000rpms, but nothing to the degree that you measured...
My thoghts:
If the max fuel flow was truly only increased by 9.5% by raising the fuel pressure to 43.5psi, then an afr of around 12.5:1 in open loop would be the expected result. The increase to 11.5:1 makes me think that something else may be at play...
- Is the vacuum line that goes to your Impreza fpr properly hooked up and in good condition? (any small cracks in the rubber???)
- You mentioned you have aftermarket spark plugs. Are they running a colder or hotter spark compared to the stock NGK's? I know certain types of plugs work better in richer or leaner afr's. Is it possible that your aftermarket plugs might not be the best suited for running in a rich afr mixture?
Just my $0.02...
-Chike