NoTec

iTrader: (4)

Is this compressor surge something that worries you? Is it a boost spike or something else? I never experienced it so if you could share some comments about the surging, that would be great. And how does your car feel after the mod in terms of drivability and responsiveness.

trinydex

iTrader: (12)

surge is what happens when your compressor wheel moves so fast that the fins stall and cease to move air. this can happen at different frequencies (fin speeds) of rotation. so typically only happen in a narrow range of rpms in certain load conditions. they of course can be shifted around with another type of load at a different rpm. i would say it happens at a certain air flow but that would be alleviating some of the blame from the compressor wheel.

so basically what happens is the fins are spinning moving air... then when they stall they continue to spin but don't move air so they start experiencing a huge drag force and the whole damn assembly slows down, but when it slows down you get the air flow to catch up again and everything runs as normal. now the same thing can happen while you're flowing through rpms ti's just ti's brief and then you kinda hopefully catch the flow and keep going.

the way it feels... should be a buck, a momentary loss of air in the intake side of everything, it's as if yer boost went to almost 0 for a moment adn then caught up again so it gives you that lil kick in the pants. if it's happenin' at 6psi then it won't be too violent.

there is the other surge that you get when your bov is too tite and you don't get a full purge on throttle lift off or partial lift off. and the symptoms are the same however the reason is different. you don't get a full purge and your air in teh intake tries to go backwards.

btw... surge is bad. cuz as i stated above especially when the air tries to go backwards everything inside the turbo tries to go backwards too, that's really bad... that means that exhaust goes backwards back into the engine, tho the valves may be closed from the cylinders it came out of... the cylinders that are now open might receive it. also... turbos don't like running backwards.

GPTourer

Compressor surge is when the turbo spools up very quickly, but the corresponding amount of boost isn't built. It causes the wheels to overspeed, but the air itself has "stalled" and puts stress on the turbo.

NoTec

iTrader: (4)

that cleared up a lot. thanks. So i'm assuming if Al thinks this turbo is a beast, it must be a worthy upgrade. We need dyno sheets!!!

WrX Kila

iTrader: (18)

Hmmmm

NocturnalEVO

iTrader: (39)

is there any way to fix this 'surge' problem?

TwStDeVo

iTrader: (62)

Al,

just got a couple of questions:

1) how does the 20g compare to the gt3071?

2) how would an evo w/ 20g do against a sti w/ 20g?

3) can we run 25psi on pump and alky?

4) best drag/street performance on pump and alky: 20g, gt3071, or gt3076?

Nez136

iTrader: (10)

some companies make there turbos w/ anti surg housings (i think) but it depends on the turbo design

trinydex

iTrader: (12)

well... one is to do antisurge porting on your compressor cover... inside the intake inlet you can put a ring (or if your turbo already has enough area there you can just drill out that area) and you basically put a series of holes in this ring, this creates a backwards pressure differential and keeps the air from getting too much speed and going faster than your fin blades.

the other way is a lotta trial and error with turbine and compressor wheel combos which don't make surge beacause they're "tuned (read selected) in such a way that the surge harmonics are minimized (this is what garrett does so you don't have to)

this is also why there's no such thing as a perfect turbo. if you have a turbo that is perfect in all but one way, chances are that one way is gonna bring you surge. which is why that turbo wasn't made and the one you have was made. hence this lil handicap on the 20g.

you see the reason why the wr doesn't have this is because they increased their airflo through the compressor without modifying hte compressor wheel's geometries (other than taking a fin out and centering everything). changing a compressor wheel's flow characteristics is costly testing cuz you never know when surge will come up and bite you. in a sense... the wr only modifies the HOUSING's flow by having one less blade the housing can move more air and the comp wheel actually ends up moving less... think of it as the difference between making a hole in something and getting a bigger fan. i realise there may be modifications to the wr's wheel but it's not really a bigger wheel in teh sense that the 20g is a bigger wheel.

kimletrim

iTrader: (18)

Compressor surge is definitely a bad thing. Remember the Starion/Conquest?

Erik@MIL.SPEC

iTrader: (94)

Here's a question. Would stroking an engine (thereby allowing more air into it) help with compressor surge? Or will surge be there no matter what?

Nez136

iTrader: (10)

if your going to stroke the engine your better off going with a bigger turbo anyways.

trinydex

iTrader: (12)

that would most likely make it worse... but maybe not... maybe it'd just be the same. you gotta know it's something totally dependant on the compressor wheel itself... that's why i said earlier i wouldn't wanna take any blame off teh compressor wheel. at a certain speed it flows a certain amount, at a certain speed it will accellerate air to a certain speed. when it gets too good at hte accellerating then it starts to run into problems with the physical properties of air and hence the stall. when it stalls you get the surge.

4G63>OOOO

iTrader: (1)

Triny,
Nice explanation, but the way I understand it is a little different.

Think of things not in terms of pounds of boost, but mass air. You have this turbo compressing this mass of air and trying to stuff it into 2000cc's of engine volume. At a certain rpm range, the motor just can't take in this amount of air and it gets backed up, so to speak. If your diverter valve isn't soft enough, it will get "backed up" all the way back to the compressor wheel and either slow it down or stall it. At higher rpms, the engine's volumetric efficiency is such that it can digest this volume of air compared to its capability at lower rpm's.

Think of it in terms of fluid dynamics and it makes a lot more sense.

Pruven

1 - I like the 20 G the best as it feels stock like and makes big power

2 - Sti with 20 G makes 390 tq and 425 whp on pump gas - (at least one I tuned did) Seems that the evo will hold more power after 6,000 than the sti will but the sti will kill it in low end tq. An evo with a 2.3 L would be a even match

3 - Yes - its good for 28 - 30 psi

4 I dont like the 3071 or 3076 at all

I would either do a TME or 20 G or go with the GT35R

AL on Dan's computer by mistake