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toon1 |
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Senior Member ![]() ![]() ![]() Group: Members Posts: 1,849 Joined: 29-October 05 From: tracy,ca Member No.: 5,022 ![]() |
I've been working with Megasquirt with timing and Fueling. The car has been responding well to the changes.
I had to take a trip to the foothills today and decided to take the 914 and really test the changes. Most of the driving I had been doing had been around town so I wasn't sure how the car would respond to a long freeway trip. while cruising up hwy 5 the cht's were 340.....I pulled up Tuner studios and leaned out the area around cruise a bit. Suprisingly the CHT's dropped about 5 * and were stable. I kept the setting there and was wondering how this would be when I started climbing hills, again I was suprised how good the head temps were. nothing over 370(which was for a very short time) on the long climbs. I don't have my 02 sensor connected yet so I can't say what the AFR is. At the end of the trip, I drove 176miles and it took 4.5 gallons to refill....39.1mpg......NICE!!! |
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jd74914 |
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Its alive ![]() ![]() ![]() ![]() Group: Members Posts: 4,782 Joined: 16-February 04 From: CT Member No.: 1,659 Region Association: North East States ![]() |
Tuning the entire load range at 13:1 would result in pretty poor overall gas mileage for a street car which spends 80% of its time at low loads. Race motors are a different animal because you're usually at WOT.
My experience has been that generally it's pretty hard to knock at partial load. Different animal, but I've run bike motors at up to 18:1 AFRs (part load) and not had knocking problems. Running WOT is a different story, though generally when you're that lean I don't think you have terribly extreme problems either. I believe this is because the mixture just doesn't burn easily enough to pre-ignite. Going super lean is a really effective means of torque control and is safely used on tons of production cars (especially direct-injected) rather than just pulling timing since it's more effective and less wasteful of fuel. In any case, the real place where you have problem is WOT with AFR's around stoich and a hot engine because the mixture so readily burns. The plot below has some pressure traces of normal combustion to knock. At WOT you really need lower AFR's if you are tuning timing for maximum brake torque. I believe AFR~ 13.5:1 is generally best for max power and 16-16.5:1 is generally best for max brake specific fuel consumption; both internal geometry dependent. When running really lean pulling timing isn't a bad way of preserving motors. With conservative timing I wouldn't be surprised if you could run loaded at 15+:1. (IMG:style_emoticons/default/smile.gif) ![]() |
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