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 Post subject: MAB?
 Post Posted: Wed May 02, 2012 6:05 pm 
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If I go from a 026 mab to a 025 about how much richer will it get??

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 Post subject: Re: MAB?
 Post Posted: Thu May 03, 2012 7:58 am 
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two numbers is 1 jet size. That is what I have always been told. Yes they make a difference.

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6.7772 @ 101.51 1/8th 10.747 @ 122.24 1/4

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 Post subject: Re: MAB?
 Post Posted: Thu May 03, 2012 8:40 am 
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WR169 wrote:
two numbers is 1 jet size.


But where in the fuel curve?

Normally we use the MABs to tweak the fuel curve, ie, it will have more of an effect up top than in the lower RPM range. What we see with the data loggers is that the AFR line is usually not "flat" and up top end it usually goes either rich or lean from where it is @ lower RPM. Tweaking the MABs usually shows an overall change but also more of a change toward the end of the run.

PPL I know with a fair amount of dyno experience say that a slight leaning of the AFRs up top will usually make the best power. In other words, Low and mid range RPM likes to be slightly richer than top end.

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 Post subject: Re: MAB?
 Post Posted: Thu May 03, 2012 8:41 am 
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Beretta wrote:
If I go from a 026 mab to a 025 about how much richer will it get??

Not enough to tell a difference.

I totaled the fuel flow in a given rpm range with different dyno pulls with air bleed and jet changes. 3 air bleed sizes (1050 carb) changed fuel flow about the same as 1 jet size. It would probably change a different amount on a smaller carb. I think the air bleeds were around .030 and jets around 92.

Rick


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 Post subject: Re: MAB?
 Post Posted: Thu May 03, 2012 8:55 am 
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And since we're on this topic, I usually go .003 steps when I change the MABs so I'm with Rick on the .001 not doing much. When I do make MAB changes I usually do a jet change at the same time to compensate, especially if I'm trying to lean the curve up top, ie, MABs +.003 and MJs up 2 numbers.

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 Post subject: Re: MAB?
 Post Posted: Fri May 04, 2012 2:07 pm 
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The best way I can think of explaining a MAB on paper is like a see-saw
The MAB will tilt the See-Saw one way or another but will not raise or lower the pivot point of the See_Saw.

The Main Jet will raise or lower the height of the See-Saw.


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 Post subject: Re: MAB?
 Post Posted: Sun May 06, 2012 2:37 pm 
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This may give you an Idea on MAB. Thoughts?

This was from the Semi and Final at Atlanta. All I did between runs was change the MAB from 31 to 28. There was about an hour between runs. Jake also got out of it on the last run right at the line once he got by the other car.

Semi Final
Temp 81 82
humid 45 47
B Press 29.38 29.35
Water Grain 73 78
DA 2265 2355
60' 1.5860 1.5670
330 4.5250 4.5000
1/8 Et 7.0180 6.9760
1/8 MPH 98.24 99.11
1000 ET 9.1870 9.1270
1/4 ET 11.0450 10.9690
1/4 MPH 120.68 121.49

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6.7772 @ 101.51 1/8th 10.747 @ 122.24 1/4

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