How much boost on an STi?

[Philip Procter]

I have a new, US Spec '05 STi. The highest pressure I get on the
standard Subaru boost guage is 0.09MPa, which converts to about 14PSI.
Does this seem to be the correct amount of boost to you?

Philip

 

[BlueSTi]

I have a new, US Spec '05 STi. The highest pressure I get on the
standard Subaru boost guage is 0.09MPa, which converts to about 14PSI.
Does this seem to be the correct amount of boost to you?

Philip

That's right. It's right in the 14-15 PSI area. You can up that a
couple pounds but the turbo is pretty much running at it's full
potential. More boost just blows more hot air. There is not a whole
lot you can do to increase boost short of replacing the turbo. Then
you are looking at race gas and turbo lag. The boys at Subaru knew
what they were doing with the STi.

BlueSTi
"Scary-Fast"

 

[Patrick Fisher]

That's right. It's right in the 14-15 PSI area. You can up that a
couple pounds but the turbo is pretty much running at it's full
potential. More boost just blows more hot air. There is not a whole
lot you can do to increase boost short of replacing the turbo. Then
you are looking at race gas and turbo lag. The boys at Subaru knew
what they were doing with the STi.

Agree that 14-15psi is right for stock turbo and boost control. But the
stock turbo can make more boost, and race gas and turbo lag is just silly

I ran 20-21psi on my VF39 (of course, that tapers to about 16psi toward
redline as the turbo runs out of steam). There's plenty more torque (not
necessarily power) to be had with increased boost on the stock turbo.

And on my new turbo (APS SR50), I hit 22psi by about 3200RPM or so - quicker
spool than stock, and on pump gas of course.

There are huge gains to be had with flow and efficiency mods on a US STi,
while keeping a very drivable, pump-gas street car.

 

[FNO]

Agree that 14-15psi is right for stock turbo and boost control. But the
stock turbo can make more boost, and race gas and turbo lag is just silly

I ran 20-21psi on my VF39 (of course, that tapers to about 16psi toward
redline as the turbo runs out of steam). There's plenty more torque (not
necessarily power) to be had with increased boost on the stock turbo.

And on my new turbo (APS SR50), I hit 22psi by about 3200RPM or so - quicker
spool than stock, and on pump gas of course.

There are huge gains to be had with flow and efficiency mods on a US STi,
while keeping a very drivable, pump-gas street car.

I would find it very hard to believe you could get 20 PSI on pump gas with
the STi. The thing will knock on 91 octane on a warm day. You get better
results with 94. However, it will retard ignition like crazy if you
increased the boost much beyond what it is stock on pump gas. You may get
better performance because you can get faster spool-up with a bigger
turbocharger, but increasing the boost pressure would be dangerous because
of detonation.

 

[Patrick Fisher]

I would find it very hard to believe you could get 20 PSI on pump gas with
the STi. The thing will knock on 91 octane on a warm day. You get better
results with 94. However, it will retard ignition like crazy if you
increased the boost much beyond what it is stock on pump gas. You may get
better performance because you can get faster spool-up with a bigger
turbocharger, but increasing the boost pressure would be dangerous because
of detonation.

Where I am I can get 93 (used to get 94, but Sunoco recently stopped selling
it). 91 wouldn't be too much different though.

The STi doesn't need to ping, the problem was the stock ECU which tried to
advance timing too far. There was a service bulletin to re-flash the
original '04 STi ECUs with a less aggressive timing map, which alleviated
the pinging.

Regardless, I increased boost through a UTEC piggy-back engine computer,
which overrides the stock ECU's timing and fuel values. It was quite
capable of running as much boost as the stock turbo could throw at it.

Currently, I run 22-23psi from 3200 RPM to redline, and on race gas, I run
around 25-26psi and a lot more timing advance, with no detonation or
pinging. The engine handles it like a champ.

 

[gpatmac]

Yup, 21 psi here on 93. No problems.

Where I am I can get 93 (used to get 94, but Sunoco recently stopped selling
it). 91 wouldn't be too much different though.

The STi doesn't need to ping, the problem was the stock ECU which tried to
advance timing too far. There was a service bulletin to re-flash the
original '04 STi ECUs with a less aggressive timing map, which alleviated
the pinging.

Regardless, I increased boost through a UTEC piggy-back engine computer,
which overrides the stock ECU's timing and fuel values. It was quite
capable of running as much boost as the stock turbo could throw at it.

Currently, I run 22-23psi from 3200 RPM to redline, and on race gas, I run
around 25-26psi and a lot more timing advance, with no detonation or
pinging. The engine handles it like a champ.

 

[FNO]

Where I am I can get 93 (used to get 94, but Sunoco recently stopped selling
it). 91 wouldn't be too much different though.

The STi doesn't need to ping, the problem was the stock ECU which tried to
advance timing too far. There was a service bulletin to re-flash the
original '04 STi ECUs with a less aggressive timing map, which alleviated
the pinging.

Regardless, I increased boost through a UTEC piggy-back engine computer,
which overrides the stock ECU's timing and fuel values. It was quite
capable of running as much boost as the stock turbo could throw at it.

Currently, I run 22-23psi from 3200 RPM to redline, and on race gas, I run
around 25-26psi and a lot more timing advance, with no detonation or
pinging. The engine handles it like a champ.

I'm sure it would handle it just fine, but the ignition would have to be
significantly retarded, which reduces power as well since it will also
reduce the efficiency of the burn. So, increasing the boost but retarding
the ignition won't really increase power.

 

[Ken Gilbert]

I'm sure it would handle it just fine, but the ignition would have to be

significantly retarded, which reduces power as well since it will also
reduce the efficiency of the burn. So, increasing the boost but retarding
the ignition won't really increase power.

you're wrong. well, you're PARTLY right.

also consider that with increased cylinder charge pressure comes
faster combustion. you run less timing because of two reasons:

1) indeed it is easier to knock, and you don't want that, _especially_
at high boost pressures

and

2) you don't NEED as much timing to get peak cylinder pressure at 15
degrees ATDC which is the sweet spot for max torque.

you're right in that it is not NECESSARILY true that you will get more
power out of the configuration, and indeed you will certainly
eventually get to the point where more boost pressure requires so much
ignition retard that you do NOT get more power (at this point your
only friend is octane really), but nonetheless there are absolutely a
lot of gains to be had on a stock STi by *carefully* upping the boost.

jm2c
ken