turbo boost vs. manifold boost, vac question and boost question


New Member
Mar 21, 2002
How much will the turbo boost vary from the wastegate line vs. the the intake manifold?

I asked my dad to install a boost gauge in my car and he installed it direct to the wastegate line (between the turbo and wastegate actuator). The boost pressure seems awfully high (like 15-20 when the factory boost gauge reads 10-12), and i dont get an ounce of vacume.

Do our cars make vacume? How much under acceleration? I have an autometer -30 through 30psi boost gauge. I only get a positive reading.

If this is installed properly my car is getting way too much boost.

I need a damn adjustable wastegate! I have one around here somewhere I just gotta find it.

Thanks for the space,

There is pressure drop between the compressor outlet and the upper manifold, most of it in the intercooler I suppose, but also some in the hoses, up pipe, TB, etc.

Most guys hook their aftermarket boost gauge to the harness on the upper manifold. It's good to bear in mind that the compressor is making more pressure (and heat) than reflected by the upper manifold pressure...
In order for your gauge to register vacuum, it must be installed on a vacuum line that taps in BEHIND the throttle plate. The air passing through the throttle is what reduces the air pressure to vacuum in the intake manifold. That's why it's called a "throttle" ;)

In addition, your boost gauge is going to read very high. When the air leaves the turbo, it must go through the intercooler, all of the piping, and past the throttle in order to get to the intake manifold. The air will lose several psi while doing this at WOT. So, the air pressure at the turbo outlet will be much higher than at the intake manifold.

Good luck,
sweet so my car might not be over boosting,

i just installed an adj fpr and a fpr gauge

set FP @ 40psi

car feels great

jutst wont boost it cuz its showing too much boost

no vac either

I've seen numbers like three psi for the drop on a stock intercooler, but I think those were for the stock turbo, with stock boost. As you pump up the boost, and flow, the drop will go up, too. The "big neck" intercooler should cut the pressure drop by a pound or so, good return for the money.
Not only that, but you can damage your boost gauge by leaving it there on the wastegate line. After a WOT run, when you lift off the gas pedal and deadhead the turbo the pressure shoots up. Unless you have a blowoff valve, the pressure will go a lot higher than what your gauge is good for - and could hurt it. I know, cause I've done it! All in the name of science, of course :) With the gauge reading from the other side of the throttle body you don't have to worry about that.