Hi Mr Jimster,
You can use one of our standard fuel/oil/boost pressure sensors:
http://www.race-technology.com/oil__fue ... _2_67.html
This is a very high quality sensor, and ideal for this application.
There are 3 things to be aware of:
1/ The exhaust is very hot(!) and the sensors maximum temperature is about 100C... so you need to mount the sensor remotely from the exhaust. When I did this some years ago, I used 10mm copper pipe and mounted the sensor on the car bulk head. Obviously the pipe needs to be long enough that the heat doesn't conduct back far enough to fry the sensor! I'm not aware of any sensors that are able to take full exhaust temperatures.
2/ The gasses in the exhaust "pulse" with each exhaust value opening, but you want to measure the average pressure - so I would recommend having an external filter. This is very simple to implement and will work better than just using the software filters in this application. If you want details of how to do this then let me know.
3/ Not a problem - just an observation. Since exhaust gas pressure (I think) is proportional to the mass flow squared.. and mass flow is proportional to RPM x engine load..... basically the pressure is very close to ambient until you are at max RPM and full throttle and then it goes up very quickly indeed!!
Hope that help?
Andy