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Jiu Jitsu Grappler
 
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Join Date: Jul 2006
Location: Nor Cal
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Air Fuel ratios vs weight?

Is there a correlation between the air/fuel ratio and the weight of the vechile? The reason I am asking is that I just put my 76 on a major diet and dropped about 200 lbs from the car. The car feels like a put a bigger motor in it! Prior to the work, I had the car dialed, running perfectly. Now its running way rich and I've had to completely readjust the AF ratio & pressure. It only happened under load, thats why I was wondering if the reduced mass had anything to do with it? I cant see that it would, but thats the only thing different. In other words, if you had a 3.6 that was in a 3100lb 911 and the engine was dialed, would anything need to be readjusted if you put the same motor in a 2200 car? (engine not having to work as hard under load may require different AF ratios?) Thats my logic, but I really don't know.
Old 02-26-2008, 07:26 PM
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Crotchety Old Bastard
 
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Join Date: Jan 2000
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Interesting question.
I cannot imagine that the ratios would change due to the extra load of only 200 pounds. That's the size of a passenger.
If you take that theory to its exteme you will see a change. For example, I had a V8 S10 back in the day and ran a 650cfm carb. When I decided to start using the truck to pull my car to the racetrack I found it needed a 750cfm carb for more balls. The A/F ratio took a nosedive under that much load.
How much of a change did you see?
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Old 02-26-2008, 07:45 PM
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Jiu Jitsu Grappler
 
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It was noticeably on the rich side. Bucking and sputtering under load at high rpms. I just got back from a test drive and I think I got it. I backed down the fuel pressure and its my old car again. (only much quicker) Maybe the regulator got bumped when the car was painted. On a side note, its amazing how much faster your vehicle is when you shed a few hundred pounds off these little cars. It breaks the tires loose in 2nd gear now!
Old 02-26-2008, 08:43 PM
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Acceleration Junkie
 
Join Date: Aug 2007
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Rule of the thumb is -0.1 sec/1/4-mile/100#.
It adds up quick.
Old 02-26-2008, 08:51 PM
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Wo ist die Rennstrecke?
 
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When I am street tuning, I find this issue interesting. For example, I can hold the throttle in a position that holds a specific cell in the fuel table that gives me a specific AFR. However, when I climb a hill, again holding the same fuel table cell, the AFR will change. The car obviously did not change weight, but there is a different load on the motor. This doesn't really answer your question, but it happens with me too.
Old 02-28-2008, 06:15 PM
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