I'm curious how you're routing your vacuum line to need 8ft of vacuum. Are you wrapping it around the whole car?
I use 3, maybe 4 feet tops.

The NA8's have the ECU mounted behind the passenger seat, there is almost 4 feet of line needed to get from the firewall to the ECU once inside the car.

Even still, after you're at the firewall, it's like 4-5" to the back of the mani. So 4' tops

I thought the standard vacuum line to use for the MS was the one on top, near the throttle body, not the one at the back of the manifold? That's the one I'm using, I've probably got 2 feet of vacuum line inside the engine bay so that it's routed cleanly and has enough slack.

FWIW, the 4 bar MAP sensor sold by DIYautotune is a chip on a board, it's designed to go inside the ECU case, not in the engine bay. You may be thinking of the GM 3 bar MAP sensor which is a weather-sealed standalone module. I have the 3 bar GM sensor (I'm running 230 kpa, so the 250 kpa built-in sensor doesn't have enough headroom), but I have it in the glovebox with the MS3. Putting it in the engine bay would be marginally superior, but not enough that I'm inclined to go to the effort of moving it.

So the short version of the answer is that the MAP sensor is in the ECU box because mounting it externally is not enough of a win to justify the increased cost, effort, or complexity.

--Ian

Its the one that is connected to the FPR (in NAs). the one near the throttle body is supposedly noisy.

yeah, they have a tendency to fail if they get moisture in the line and then freeze.

Ok, stupid question incoming, but I have yet to come across an answer to it and I been wondering about it since I learned about speed density control.

Why does a speed debsity system care what the outside pressure is? 70kpa in the intake manifold is 70kpa. I can't wrap my head around why a second sensor is needed to compensate for altitude. Although I know that it is fairly often needed. Am I just thinking about this too simply and missing something?

because the load has also changed between those two point, and thus the fueling requirements have changes as well. Your whole fuel map would shift if you tuned at a high altitude, and then again at a low one. The baro corrections are actually correcting for the changes in VE (exhaust pressure, throttle position, crankcase pressure, humidity, etc).

Ok, so the baro is correcting for all the changes cause by things other than the pressure in the intake manifold. That was whete my hang up was. My thing was "the air is x dense in the intake manifold, meaning the engine can take in x amount of oxygen each revolution needing y fuel".

But since the exhaust pressure and other things change as well, the VE of the engine changes so it isn't taking in x amount of oxygen for x density, it is taking in some other amount (z maybe) and needs the fuel to change. Yes?

yeah, cause density is known like you said and it can calculate for that, but other variables have changed that impact VE and you're correcting for that.

And those other variables are directly related to absolute pressure outside the car, so rather than measuring and correcting for those, it is easier to just measure the pressure.

So I assume thay the baro correction curve needed will vary car by car? Or maybe engine by engine?

It will vary with the exhaust setup, particularly if there's a turbo on the car.

--Ian

From the bottom of the plenum near the FPR, routed up along the fuel rail and then back through the firewall, along the passenger frame rail to the ECU behind the passenger seat. I'm using an EUDM flat top manifold, there aren't a lot of options unfortunately.


And this is the type of MAP sensor I was talking about: https://www.summitracing.com/parts/f...2KAaAqcd8P8HAQ