What does the signal from MAP sensor look like?

[jjdalynh]

2002 softail.  Wondering if connecting map sensor output to LM-2 via analog inputs will give me any meaningful info when trying to tune ignition timing with DTT TC88.  Build is 95", static CR 9.3:1, late model heads, Andrews 37g cams, CV40 w/46 slow, screw 2.5-3 turns out, sportster needle and stock (195? I forget) main.  CVP emulsion tube.  Rinehart stepped duals with adjustable homemade baffles. 

Planning on mild street port and decking heads for 9.8:1 CR so want to make sure that I can tune it right on my own.   I already have the LM-2 and know my AFR is pretty close. 

[joe_lyons]

5 volt signal I believe

[Admiral Akbar]

As Joe says it is a 0 to 5V signal.. The output pretty much gives a real time manifold pressure while the motor is running.. Looking at the signal will show it moving all over the place.. A ignition system / ECU will read the signal at a very specific part of the stroke to reference the timing map.. Since your LM2 is not reading MAP referenced to engine position, the output is pretty much garbage.   You can filter the signal to get an idea of what the "average" MAP is but it won't be a direct correlation to the timing table. 

[speedzter]

I had the same idea with my LM2 and asked the same question previously, and also asked the guys at DTT.
Apparently the MAP is sampled at BDC on compression stroke, and as Max states, the signal would move around a lot,
but if you could filter the signal with a simple r-c network, you may be able to get some useful data.
I never bothered going any further, but I would still like to see the results.

[jjdalynh]

So basically, do the same thing the ignition already does and you can get the info you need..... 😄

[gordonr]

2002 softail.  Wondering if connecting map sensor output to LM-2 via analog inputs will give me any meaningful info when trying to tune ignition timing with DTT TC88.  Build is 95", static CR 9.3:1, late model heads, Andrews 37g cams, CV40 w/46 slow, screw 2.5-3 turns out, sportster needle and stock (195? I forget) main.  CVP emulsion tube.  Rinehart stepped duals with adjustable homemade baffles. 

Planning on mild street port and decking heads for 9.8:1 CR so want to make sure that I can tune it right on my own.   I already have the LM-2 and know my AFR is pretty close.

I would spend a little time reading your plugs for that info. You will get more information on what's going on in the combustion chamber that way.