Hi,

I have a scanguage, and monitor manifold pressure measured from OBD. Manifold pressure (MAP) = atmospheric pressure (roughly 14.5psi) + boost.

I am interested in what people are seeing. I have a N14 factory JCW and have been looking at MAP racing and driving around for a long time. I find this useful, and in the past have been able to diagnose minor boost problems by looking at the gauge. I was expecting to see the same thing on the GP, but it is very different.

On the N14 JCW in full throttle runs in 3rd and 4th gear the MAP hits the low 30's and occasionally peaks out at 35 (ie 21psi boost). When coming off throttle, or going down a hill, the MAP will dip below atmospheric (often as low as 6-8) ie negative boost or vacuum. This is what you would expect, as the boost is diverted and the engine is sucking air from the manifold when off throttle.

The GP is different. If I had not been monitoring this I would not have suspected anything, as the car drives normally, and feels like it has loads of power etc. However, on third gear full throttle runs the MAP sits around 29 and occasionally peaks at 30.5. Also, when off throttle it always sits close to atmospheric around 14.2. There may be a very transient dip to 13.9 for a fraction when lifting off throttle, but certainly no prolonged vacuum.

On the scanguage I can't look at the what the requested boost level from the ecu is, to see if the car is making the target boost. To do this I would need to datalog with my mechanics obd system, and haven't had the opportunity to do this yet.

I reiterate, the car feels fine. I don't think there is a problem.

This puzzles me. I can think of a few possibilities

• There is something wrong with my car, and it is not boosting properly. This seems unlikely as the car seems to make loads of power, and this does not explain the off throttle behaviour. I have not had the car on a dyno or on the racetrack yet, which is the best way to truly determine the power
• The GP is somehow set up differently. Maybe it does not run as high a peak boost, but holds it longer at a certain level? Maybe something clever is going on with the diverter valve / recirculation I don't understand?
• Maybe what I am seeing is actually different measurements, ie on the N14 JCW this may be a real manifold pressure, but on the GP the OBD is giving a pressure measurement somewhere else in the system, explaining the different pressure and lack of vacuum off throttle?

Does anyone else have a better technical explanation for what I am seeing?



Can anyone else with a scanguage or similar OBD device have a look at manifold pressure (or boost) and tell me what their GP is doing? Maybe this is not just the GP but all N18 JCW engines?


Robbo

 

I don't have an answer for you, but you know the Scan Gauge can automatically display just boost pressure right? It's a lot quicker and easier than having to add and subtract

The XGauge code for it is somewhere around here if you search, too lazy to find it right now lol

 

Yes I do have that code thanks