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MAP Signal Line Length

Old 07-04-2011, 03:09 AM
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Default MAP Signal Line Length

I have been experiencing leanness (12:1-13:1) which has been difficult to tune out.

It occurs when my MBC spikes. The spike lasts about a quarter second going from 14psi up to 25psi before settling down to around 21psi. Yes, boost builds that quickly.

The injector duty cycle indicates that I still have plenty of fuel left and despite richening up that row I am still seeing leaner AFRs than I should.

My map signal line is pretty long- about 15 feet or so. It is routed through a grommet on the driver's side of the firewall.

I suspect that the long signal line length results in MAP latency, just enough so that my ECU's MAP sensor is a step behind fast transitions like the boost spike.

I have experienced similar vacuum/boost "lag" before. When I sourced my MBC signal from my coldside IC pipe, my turbo spooled about 400RPM later.

I think that I will try running a short line through the passenger's firewall. Maybe one of the holes from the now-removed A/C. Another "fix" would be to drill out my MBC restrictor.

I searched the web about this and it's funny, the only thread that came up was a four year old thread from this site: https://www.miataturbo.net/megasquirt-18/length-map-line-11530/
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Old 07-04-2011, 12:39 PM
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accel fuel enrichment not correct?
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Old 07-06-2011, 01:54 PM
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You can further shorten it by moving the ECU to the passenger side under the dash, or inside the passenger airbag compartment. Requires moving the harness, but it's not too insane. You could just move the whole harness attachment points up there too.
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Old 07-06-2011, 02:57 PM
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Some MS'ers (I think the developers?) tested MAP latency and showed that it is barely measurable through something like 100ft of vac line. Unless you're running a very large (VERY large) vac line it won't make a difference.

Have you tried installing a small lawn mower fuel filter in the line? That has been known to smooth out jittery MAP signals and it's what I have on mine.

I'd also check your accel enrichments. Maybe switch them to MAPdot if they're not already. (I'm assuming you run MS, obviously).
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Old 07-06-2011, 03:04 PM
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I wouldn't suspect a difference between 10' and 100' feet of map line.
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