how does the MAP readout work?

[j12piprius]

I've been using % engine load, and tried MAP today for comparison. According to the Ultragauge manual, MAP goes up to 255 kPa (kilopascals). Based on that, I figured 90% engine load would be about 230 kPa, but that isn't the case.

When % engine load got up to 90%, the kPa's were only approaching 100!
Interestingly, standard atmospheric pressure on Earth is defined as 101.325 kPa.

Does MAP have a direct relation to fuel use, i.e. up to 100 is efficient, and above 100 is excessive? Therefore 101 to 255 MAP is relative to 93 to 100% engine load?

If so, I think that's quite neat.

[ksa8907]

MAP is manifold absolute pressure. It is a pressure reading. The computer uses this data in part to determine fuel delivery and ignition timing. Think of it as a digital vacuum gauge.

Without forced induction, you will never get 255kpa.

[j12piprius]

Thanks for the explanation. I like it being like a digital vacuum gauge.

What might a maximum reading be, and how does it relate to percent engine load?

[Occasionally6]

Absent forced induction, the maximum, achieved at wide open throttle (WOT), will be close to the local atmospheric pressure i.e. ~100kPa. Perhaps a little less at high engine power outputs (high MAP & rpm) and air flow rates, where there are some more noticeable losses in the intake ducting and air filtration.

MAP doesn't directly relate to fuel consumption, although in general higher MAP = more fuel consumed. The other main variable is engine speed. If you put those together, high MAP + high engine speed = lots of fuel consumed and vice versa.

Most engines have still have the additional fuel added in with high MAP, especially with engine speed (and therefore power output) towards the higher end of the range. (To control - reduce - the engine and exhaust temperatures.) That will kill engine efficiency with really high MAP. You've probably seen that with open loop and high MAP.

Otherwise the higher the MAP, the lower the difference in pressure between that in the cylinder and on the underside of the piston during the intake stroke. That difference in pressure manifests as work that has to come from the fuel and is therefore unavailable to move the car. MAP corresponds pretty closely to the in-cylinder pressure (which is lower than MAP).

The lowest MAP you should see will be ~30kPa and that with a closed throttle. You may see it a bit lower during an in-gear, closed-throttle deceleration, depending on what the ECU does with the idle control during deceleration.

I expect the load percentage will be calculated based on the current MAP minus the minimum MAP divided by the range of MAP possible i.e. maximum MAP minus the minimum MAP. Like this:

("current MAP" - ~30)/(~100 - ~30)

[j12piprius]

[QUOTE=Occasionally6;444774]MAP doesn't directly relate to fuel consumption, although in general higher MAP = more fuel consumed. The other main variable is engine speed. If you put those together, high MAP + high engine speed = lots of fuel consumed and vice versa.

I expect the load percentage will be calculated based on the current MAP minus the minimum MAP divided by the range of MAP possible i.e. maximum MAP minus the minimum MAP.[quote]

Thanks much for that detailed explanation. MAP doesn't directly relate to fuel consumption, but the load is derived from the MAP. Here are some comparisons based on your formula (x100). The MAP got down to 24 today when coasting in gear. I'm considering the sweet spot to be 90 to 93 MAP ( 87 to 91 Load). Anything higher is likely to be open loop and excessive.

Load = 100x (current MAP - 24) / (100 - 24) ]

MAP - - - Load
100 - - - - 100
95 - - - - - 93.4
90 - - - - - 86.8
85 - - - - - 80.3
24 - - - - - - 0