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| A customer recently asked me about the level of boost he’s been seeing on his Techco supercharged mustang. Concerned about the numbers he was seeing, he was wondering if there’s a more accurate way to read it. He’s using an aftermarket boost gauge Tee’d into the the vac/boost line that connects to the fuel pressure sensor. Here are some notes about reading boost… |
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- Generally, with the base 7psi Mustang kit, you’ll see 6psi for most of the RPM range and near redline it’ll creep up to 7.
- The published boost levels were recorded at sea level in Anaheim, CA. If I remember correctly, you’ll lose about 0.5 psi for every 1000 ft elevation.
- All lot of the time, aftermarket automotive boost gauges are not calibrated accurately. Get a GOOD pressure gauge from SnapOn, Craftsman, etc. One that goes up to 20psi or 50psi or something (you can’t acurately read low pressure levels on the ones that go up to 150-200psi). Hook both gauges up to a single air line with a regulator and make sure both gauges are reading the same. Depending on the boost gauge/sensor, some you have to calibrate.
- Make sure your SC belt is fairly tight
- Every once in a while, there’s a problem that people run into with the by-pass valve operation. When the by-pass butterfly shaft slips in the linkage, the boost levels will substantially drop. Here’s a link to a post about checking the bypass valve for proper operation
- As for the boost reference location, it shouldn’t make TOO much of a difference where you hook it up. I think the line that goes to the fuel pressure sensor is hooked up to a single runner. This usually causes some fast pressure fluctuation caused by the intake valves opening and closing. For PCM use, filters are applied to the voltage which smooth it out. For a mechanical pressure gauge, you MAY see smoother values hooking up to the bypass vac/boost line because it’s derived from the intercooler housing which is common for all runners. Additionally, you may see SLIGHTLY higher boost values from the bypass valve’s pressure reference port because it is hot air. There’s a slight pressure drop after the air cools from the intercoolers. Below is a diagram I made. I think the bypass pressure reference port is actually where we measured the pressure back at Techco but don’t quote me on that because I’m not 100%.
