c00lduke

does anyone know why i would not be getting any boost whiling revving in neutral? but get a 12-10-8 boost pattern. Car is completly stock.

~Luke

Redrotor

you can't get much boost in netural because it doesn't
have load.
however you should see around 4psi at 4000 rpm
when you push gas hard in netural

mightyslash

You should see 4psi at WOT.

MakoDHardie

4psi should be about right. Boost is pressure. The airflow through the engine shouldn't encounter the resistance to back the pressure and raise the reading on your boost gauge while the car is in neutral. It just spins right through. The only reason boost builds high in the first place is because the pressure the engine fires with has to be greater than the force of the load on the engine in order to accelerate.

Axe

OK, so the boost gauge is reflecting the pressure produced by the turbos AGAINST the engine. If in neutral then it "spins right through," but does this mean that if there was a little more "load" on the engine then boost would build quicker? And if so, then would back pressure help not only in low end torque, but also reduce turbo lag?

And if that is true, then would a variable aperature on the exhaust side reduce turbo lag? (Kind of like the aperature on jets)

MakoDHardie

The boost gauge gets its readings from somewhere along the turbo system. You could install a boost sensor anywhere between the turbos and the intake manifold. If your air is passing right through, then there was nothing to slow it down when it hit the engine. The turbos spin somewhat directly proportional to engine rpm, but if for instance you pluged half of your intercooler pipe, or intake manifold, it would require greater ehaust pressure to turn the charging turbines once the pressure build up all the way through the air path.

With load on the engine, it does mean that boost can become easily accumulated at the intake manifold, pushing its way into the engine. It is not always that easily done, however. With wastegates and blow-off valves, excess boost is spilled to ensure that the turbos are not pushed when they don't need to.

As far as the exhaust backpressure and low end torque/turbo lag. You're right. Depending on what kind of turbo set up you have, you will preserve your low-end torque buy buying an exhaust system that has medium sized piping. Something with big piping like the APEX'i GT spec exhast (3.74"max) is made to optomize your high end. If you take this same exhaust and install the silencer, you'll preserve low end torque. If you're not sure how this works, its because the amount of exhaust pressure from the engine required to pass through the cat-back is greater than that of a large pipe with very low restriction. That means you'll have a greater density gaseous mix spinning your charging turbines. That relates to the intake turbine being able to encounter more air before it begins to lag. Don't rely on this totally doing away with turbo lag because I doubt you will be able to even feel the difference. And turbo lag is directly proportional to engine speed. If your turbo kit specifies that it spins until it hits 3000 rpm before it begins to produce boost, then that should be pretty accurate. You can improve the margins by your ignition system and computer programming. If you're trying to find a good exhaust, I'd recommend a more streetable system unless you really like to get your car into the high-end. If you're not sure what that is, check out the exhaust systems from M2 and Racing Beat. They're pretty good street systems.

If you're running a single turbo and can't stand the turbo lag, there's always the 100 shot of nitrous from NOS. That's a system designed to keep you safe out of the start before your big turbo kicks in.

Let me know if this answers all your questions.