Different Boost in different gears...

[yesidoeatpie]

Hay all,
I have a bit of an interesting boost thing going on with my car at the mo.

Check this..
Below is what boost my car holds in each gear with my foot to the floor (max)
NOTE: I do NOT have any kind of boost controller

1st Gear: 11psi
2nd Gear: 11psi
3rd Gear: 13psi
4th Gear: 13psi
5th Gear: 13 - 14psi

My boost guage doesnt jump around at all.
When i have my foot to the floor my boost doesnt creep in any gear it solidly holds the above. Apart from 5th gear - Sometimes it like 13psi and sometimes it likes 14psi. WEIRD!

Im not really complaining as it goes like a rocket at the mo :D But its just something ive never come across before.

Is 13psi too much for my standard turbo? (stadard gtx turbo)
Why is my car boosting different in different gears?

Thanks

[Tazman]

Lower gears = less time for turbo to boost more because of the motor kind of out running the turbo in a sense. So thats why you get more boost at higher gears. Thats about all the i can explain as i'm too stupid to really explain it technically! :alchy:

[PseudoRealityX]

Boost is a pressure differential, not an absolute pressure.

[yesidoeatpie]

Lower gears = less time for turbo to boost more because of the motor kind of out running the turbo in a sense. So thats why you get more boost at higher gears. Thats about all the i can explain as i'm too stupid to really explain it technically! :alchy:


Ok yep that sort of makes sense. I see where you coming from but....

In second gear I am at max boost by about 3000 - 4000rpm (11psi for 2nd) but why does it hold solid on 11psi till red line from there? Should it not hold the same boost in each gear?

1st gear is very fast if your looking at my rev guage and yeah 1st doesnt really have much of a chance to boost much more than 11psi BUT in 2nd gear there is heaps of time for it to hit 13psi if it wanted. :confused: :confused: weird ay!

[yesidoeatpie]

Boost is a pressure differential, not an absolute pressure.

I wish i knew what that ment:D

[WorldHazard]

I wish i knew what that ment:D

pressure differential... means it's not goin to be exact everytime...

[PseudoRealityX]

If the engine is accelerating quickly, it means that's desiring more air, and that "need" for air is rising quickly.

Since turbos have to "catch up" to the demand, they don't catch up fast enough in the shorter gears, because the demand for air rises faster than the turbo can provide it. Hence you run less boost.

The initial post was more to the idea that say you have a stock head with a roots blower on it running 8psi. If I now port the hell out of the head, and change nothing else, it will no longer run 8psi. Maybe 6.5 or 7 psi with the new head.

[yesidoeatpie]

ok. I can understand that.
So I dont need to worry about it boosting low in the first couple of gears?
I have no control over my boost at present as im just running a vac hose to my piping and wastegate soleniod valve.
If i want to drop my boost how should I do it? If i get a boost tap will it only let me increase my boost? Can i decrease?

Here is a pic of how my vac lines run from the actuator..

Since ive added the bigger front mount ive been boosting like above. I am loving the pull from extra boost but im worryd I may pop something.
What do you guys think? try and restict my boost a bit or just leave as is now...

[95ProLeila]

what kind of EM are you using?

[yesidoeatpie]

Em?

[95ProLeila]

Engine management

[yesidoeatpie]

Standard GTX ECU

[Lex]

You are fine at that boost. Once you open up exhaust and intake on the GTX it will overboost. Mine moves beyond 15 psi in 5th if I keep my foot on it. Your GTX ECU is equipped with a boost cut. If you run out of fuel maps, your car will cut out and you will know when that happens (kinda like hittin a wall). If this happens a lot, you will need to port the turbo wastegate to control boost a little. It sounds like you're not there yet (happens around 15 psi) so I wouldnt worry about it.

[yesidoeatpie]

cool thanks. Yeah i havnt had any cutting just yet. I get the random cut when i boost 12+psi at low revs..

Next question i have for you inregards to what you said Lex.. By adding custom extractors i will over boost more im guessing BUT i wont be installing custom extractors without an external wastegate so that should manage my boost a-o-k right? With something like a 38mm external wastegate with like a 12 or 13 psi spring in it, it wont boost over that right?

[Lex]

Firstly I don't know what custom extractors are ... :(

If you are installing an external gate, you need to weld the wastegate hole you have right now on there shut. On a VJ20 it's not worth it to have an external gate. Get a bigger turbo and an external gate.

And a nice unrestricted external wastegate will keep steady boost. But for the VJ20, save yourself a lot of trouble and port the internal gate and get a boost controller.

[Lex]

This should be in the forced induction section btw mr admin of this section please sir :)