General boost question

[sirmudge]

When people say they boost 10 psi does that mean that it peaks at 10 psi and then tapers off as the revs get higher, or do they mean 10 psi at close to redline say 5500 or so?

 

[sirmudge]

Anyone?

 

[808MP5]

I would think it will hold at 10psi. If not spoke above it. If you are losing psi something must be wrong.

 

[BRIAN_MP5T]

Boost should hold and not fall off.


Horsepower curves from a dyno are diferent, they someimes will fall off at higher RPM even if the boost level is solid.

 

[sirmudge]

I was under the impression that the t25 was small enough that it has trouble keeping up with the demand created by higher rpms. Is this incorrect? Also i may have done a bad job of explaining my question. I believe what most people call it is a spike in boost that you get when you first go into WOT and then it tapers off to the regular boost pressure. So what im really asking is when someone says that they are boosting at 10 psi do they mean the spike or the normalized boost pressure that comes after it?

 

[tweety2300]

Not sure about the spiking but I know the t25 is efficient to 14-16 psi. The turbo is rarely the cause of loss of boost that's usually the waste gate or a boost or vaccume leak

 

[BRIAN_MP5T]

^^ This..

The T25 will not have any issues holding a steady pressure all the way to 16..

The problem with that turbo is that on its way to 16 it becomes less able to make boost effeciently, which is why people who run over 10 PSI daily usually opt for a T28

So if you are seeing a fall in pressure, that is an issue, not just a design flaw (Or all MSP would be doing it, not just yours)

 

[jdwk]

Our stock turbo setup definitely spikes before the waste gate opens fully. The fact is you have a pneumatic and mechanical reaction that takes time, especially since our stock setup sources the boost from the IM. As the turbo builds pressure that air must pressurize all the way to the IM and out through all vacuum tubes, then press against the WGA diaphragm and push the wastegate open. The turbo builds boost faster than the entire system can respond and stabilize and you get a modest boost spike about 2psi over the "set" psi.

But even the set psi is not stable. If you run a high sample rate data logger on your MAP, you'll see it jumps around quite a bit even after the wastegate is fully open. So it's really hard to give a very exact PSI that someone is running. That's why people will often use the "~" in front of their PSI claim.

The way to combat the spike is first tap your boost source as close to the compressor outlet as possible, and second use a weaker wastegate spring and a boost controller (either manual or electronic) to hide the boost from the WGA until you reached the desired PSI so the wastegate opens quickly. The second method also has the benefit of increasing low end response since the wastegate does not crack and waste exhaust engery until the desired pressure is reached.

Don't get caught up on PSI. It is much more important to have a proper tune and keep temperatures in check than to run a certain amount of boost.