Tuesday, Jan 09, 2007 at 09:14
Hi Kirsten,
There will be some difference between before and after the intercooler, especially at high rpm. How much will vary for every make and model. The intercooler represents a restriction to air flow and the pressure drop across a restriction increases with the square of flow rate. That means, in approximate terms, the pressure drop across an intercooler could be 4 times as much at 4000 rpm than at 2000 rpm. But it might still be quite small, depending on the design of the IC.
A properly-sized IC would probably drop less than 10% of the turbo outlet pressure, i.e. at least 9 psi at the manifold at high rpm if the turbo outlet is 10 psi. If you have a convenient manifold tapping point, why not try both sensing points? On the same day and conditions, swap the boost gauge tube between the two points (& block the unused one, of course) and see what difference you see - we'd all be interested to know.
John,
I don't know about 'tainted'? You are still measuring a pressure post-IC and it is the pressure that is actually feeding your cylinders.
But you are right about what this pressure actually means. If you've read my "intercoolers" article, you'll see how much effect the charge air temperature can have on the MASS of air available to the cylinders.
In practice, a boost gauge doesn't tell you very much of use, a lot of the time. Boost pressure varies rapidly between zero and maximum often while driving and rarely remains steady for very long. A boost gauge generally does two things - it tells you whether your maximum boost level is being controlled at full load and how quickly full boost is being developed.
Charge air temperature would be interesting to see, especially if you were comparing different intercoolers, for example. But on a particular vehicle, seeing the charge air temp isn't going to be of a great deal of use. Unlike EGT, you can't do anything about it - unless you're planning on using a water spray or similar.
FollowupID:
474433