Supra Forums banner

1 - 13 of 13 Posts

·
'95 TT
Joined
·
668 Posts
Discussion Starter #1
How much boost do you guys see in each gear with your BC cranked all the way up?

Second gear, for example, I can only seem to get about 15 psi out of it, yet at that same setting i can hit 20+ psi 4th gear.

Is this common?
 

·
Registered
Joined
·
1,288 Posts
I'm assuming your boost controller is set higher than 20?
 

·
'95 TT
Joined
·
668 Posts
Discussion Starter #4 (Edited)
flash_e_gt said:
I'm assuming your boost controller is set higher than 20?
Yeah, it's maxed out when I go to the track and run race gas.

I just noticed that in 4th on it can hit the higher psi but in first, second, and third it doesn't. I figured third would create enough load on the car to boost above 20 psi but that doesn't seem to be the case, or I've got a boost leak somewhere.
 

·
Registered
Joined
·
13,581 Posts
crossways said:
I bet you are seeing something like:
1st -13
2nd - 15
3rd - 17
4th - 20
That's absolutely normal.

Going up hill I see more boost in each gear, also normal.
 

·
Registered
Joined
·
401 Posts
Nick 95 6sp said:
Going up hill I see more boost in each gear, also normal.
A little birdy told me you can see 24+ psi in 4th in your car. :bigthumb:
 

·
Registered
Joined
·
814 Posts
obviously with the more load u put on the motor the more boost, seeing more boost in taller gears is normal, but with my auto i usually saw 18-19 pnds in 2nd gear on the stockers and 22 in third
 

·
Alpine Hardtopper
Joined
·
1,097 Posts
It has to do with the difference in accelation of the engine vs. turbocharger. If you plot your operating points on a compressor map, you can also read the RPM's of the turbocharger. These represent more of a "steady state" speed for a given flow and pressure ratio. Even at constant boost pressure, going from 3000 engine rpm to 5000 engine rpm could be a 30,000 rpm difference on the turbo shaft. The turbine isn't getting enough torque to make that transition in 1/2 second.

Now, with the car in 1st gear, the demands on airflow are changing rapidly, and the driving torque to accelerate the turbine can't keep up with those changes. Once you get in to 4th gear (assuming you don't have 3000 hp) the engine is a lot closer to steady state.

Going up a hill keeps the acceleration of the engine lower than if it were flat, so there's more time for the turbo to catch up.

What you're seeing is normal. If you see something like:
1st gear - 13 psi
2nd gear - 15 psi
3rd gear - 0 psi and lots of smoke.....
Then things aren't normal anymore.
 

·
DRiFTT
Joined
·
991 Posts
1st - 8.5-9.0psi
2nd - 8.5-9.0psi
3rd - 8.5-9.0psi
4th - 8.5-9.0psi

Thats how my readings used to look on the blitz dual sbc boost controller i had :( dunno why i sold it
 

·
Banned
Joined
·
2,999 Posts
Blizzz said:
How much boost do you guys see in each gear with your BC cranked all the way up?

Second gear, for example, I can only seem to get about 15 psi out of it, yet at that same setting i can hit 20+ psi 4th gear.

Is this common?
what boost controller are you running???
 

·
Registered
Joined
·
13,581 Posts
BlueGTS said:
A little birdy told me you can see 24+ psi in 4th in your car. :bigthumb:
I like birdies, they taste good ;)

And that has been known to happen under the right circumstances and with higher octane, as others will attest.
 
1 - 13 of 13 Posts
Top