Exhaust Back Pressure Tester – DIY with Boost Gauge
I’d put the stock exhaust back on a VAG 1.8T motor running 300bhp. The stock exhaust has 55mm diameter with some silencers, the performance exhaust was 70mm and basically straight through. I wanted to know if the stock exhaust was a restriction so I DIY’d a temporary exhaust back pressure tester, this is how I did it.
Exhaust Back Pressure Tester
First things first we need to get into the exhaust. The car I had was using an EGT sensor so I used the bung for that.
If you don’t have an EGT bung weld one on or use the lambda sensor hole.
Fittings
My original idea was to use a fitting with a barb and thread. The thread the same size as the EGT bung and for the barb end I’d force a pipe of aluminium or steel tube over it.
This didn’t work for various reasons. In the end I still ended up using a thread/barb fitting but the metal tube went inside the bard fitting. To stop the tube from being blow out I kept the pipe and the barb together using a silicone hose.
Yes silicone hose is only rated to around 170C but my thinking was this. The silicone is not going to come into direct contact with the exhaust gas. And this pipe I was using was aluminium, the pipe would transfer away the heat before it got too high, at least for a brief use.
I used around 50 cm of pipe, I though a good length to dissipate heat and at the end of the pipe I ran a silicone hose in the boost gauge already in the car.
Did It Work?
All in all it worked a treat. I drove the car for around five minutes with this setup which was plenty to get the readings I needed. When I came to remove my DIY exhaust back pressure tester from the car, all the parts seemed fine and the perhaps most importantly the aluminium seemed to be doing it’s job transferring heat away from the hose. At the exhaust the aluminium pipe was very warm to the touch but after 10cms it was much colder.
Summary
For short duration use, I see no issue using a setup like this. At the end of the aluminium pipe closest to the boost gauge it was basically ambient temperature so I don’t think it would do any damage to the sensor.
DIY Exhaust Back Pressure Sensor – Results
As for the results of the test.
Until around 3000 rpm will relative high boost 10 PSI approx, back pressure was non existent. But after this point back pressure spiked up. By around 4000 RPM back pressure was up to 0.8 BAR. Engine redlines at 6500 RPM so for sure, the restriction in the exhaust was far too much. This confirmed the but dyno, the car was hitting a wall at around 4000RPM. Sure it would still pull but nothing like as well as it had.
For the purposes of this test, my DIY exhaust back pressure sensor worked a treat. If you need something permanently mounted then the silicone at the exhaust side probably needs to be changed for another solution and perhaps the length of metal pipe needs to be increased. But honestly, I don’t see any circumstance when someone would have to constantly monitor exhaust back pressure if the exhaust system is staying the same.
