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Internal or External MAP?


alexcrosse
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Hi, 

I've got an R32 GTR that I've run on a Link G4+ for a few years now. 

Although I still use it, I've never really liked the idea of the internal map sensor and the convoluted route some vac pipe has travel through. 

Has anyone got an opinion on this? Perhaps advantages of putting an under-bonnet MAP in with a short vac pipe to source? Or data showing there is no advantage in terms of quality of reading?

Sorry it's a bit of an open ended question. Thanks,

Alex 

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Having had both setups myself, I wasted a fair few brain cycles on this one myself... Going to rant the whole lot out here for you.

When I had my plugin installed I made use of the onboard MAP although it bothered me a fair bit. Unlike most situations, I was sorely lacking holes through the firewall on my car which left me routing the vac line all the way around to the side of the engine bay, down the inside of the fender and in the passenger footwell. It was a good 3meter or so run of hose. Being relatively ignorant as far as maths/science goes, I asked a friend of mine who was very much that way inclined what kind of a delay that would place between the pressure change in my manifold and the ECU being able to read it. He said it would account for between 10 and 12ms of delay (based on speed of sound for air movement over the distance of the line). 

Depending on your background, thats going to sound like an eternity or non-existent to you. To me, it was quite a large number. This is the time it takes BEFORE THE ECU CAN REACT to the change. So I would then expect there to be another few ms before the ECU can implement a change. Depending on what kind of build you have, I feel you could possibly run into some issues here, but my theory is you would need to be running something thats got bonkers power figures and is capable of extreme changes in manifold pressure within a very short window. In that kind of an environment, one could theorise there is capacity for the engine to run lean due to the VE calculations "lagging behind" the actual intake pressure. I could also be drastically overthinking it.

I would expect that there are going to be people a lot more scientifically inclined or even just someone with more first-hand experience with the matter who could tell us more. Not long back, I changed from the plugin to a wire-in with an external MAP and a very short vac line run. What I can tell you from my own experience from having installed and tuned both is that there was no observable difference (to my albeit inexperienced perceptions) in my 300-400hp Evo. :)

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As MAP is a reactive measurement rather than a demand based measurement you actually want some damping, fast changes are made based off throttle position.

You can choose a long pipe where it may react slightly slower or a long run of wire which may induce interference the choice is yours.

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