Boost on an GT 35/40R

[prosty]

Hi!

I'm using a GTBB 35/40, with a K26 flange, bought if from ATP turbo.

I'm using machined 7a cams, the power is awesome, the car is like a mad cow with ecstacy .

But I'm not happy with how "slow" the boost is spooling, I'm seeing 2 bar @ 5500 RPM, is this normal? I was led to believe that this turbo would spool around 4000-4500 RPM. Or might this be because of the cams?

The turbo is .70 A/R compressor side and .82 A/R turbine side (#10 exhaust according to ATP) , I have an option to go with .63 A/R , that would set me back on the top end I suppose, but I think that would be worth it just to get earlier spool.

Had the car driven on a track this saturday and the spool was "killing me" , but when I started to mash the tranny (putting it in first gear in a few slower spots) then I ran a few seconds faster.. But The GTBB turbos is supposed to be quick. A friend of mine is using a HX40 with a 1.8L golf engine in his GT quattro, he's seeing full boost at around 4000. I mean WTF????

I'm rechecking the cams now to see if I've done anything wrong, but considering the power the car is producing I'm sceptic that I've set the timing incorrectly. Maybe adjustable cam gears is the way to go now?

What kind of RPMs are you other 35/40 guys seeing boost at, if you're seeing it earlier, PLEASE tell me what kind of camshafts you are using

[Ross]

Prosty, 11.45 is blistering for an Avant! I'm jealous (at GTi my avant didn't even make it to the 1/4mile )

I've been watching your project thread, what kinda power do you think you're pushing? The AR's do seem rather huge (i guess necesary for the uber-high power), my GT3071R was 0.63hot/0.50comp and I was seeing peak boost by ~4k (at 2.0bar), but she made 1.0bar at 3k and 1.8bar at 3.3-3.5k.

I know your turbo is bigger, but still it seems like a significant difference to break into the 500s! What does Tom/Doug's hit 2.0bar+ at??

[Petya]

Do you use still original wastegate and N75 valve and motronic?

Because it look like you have something boost control problem.

[prosty]

Stock wastegate. 1.4 Bar spring (no boost when I remove the spring so the WG seems to be sufficient) , I use a hydralic pressure regulator, it regulates on both sides of the WG membrane. I've also tried using a manual boost controller (dawes device type) but then it won't hold high boost on the higher RPM. It drops slowly back towards 1.4 bar. When I use the pressure regulator I can go 2.3 bar until 8000 rpm without any hassle.

Still motronic ECU but I can promise you the guy who tuned this car is probably one of the best motronic tuners in the world..

I would think the car produces closer to 600 hp, maybe more with 102 octane race fuel.. The "know-it-alls" on the swedish forums say 500-550 , but with AC , and full interior and some light stereo equipment in the car... I would say 550+ anyway just to be safe.

My tuner said that the pressure regulator makes the boost come slower, but it regulates the boost better, and it has pressure on the upper side of the membrane, which gives faster boost between gear shifts. This is better for the transmission. But not so good for the performance. Maybe an electrical boost controller is the way to go here?


203 kph top speed on the strip, full interior, 30+ degrees celsius outside that day. Lot's of power that's for sure.

[Jamo]

Speak to Audiman he has the same turbo as yourself so does Mark Halligan. Ask them what their boost profile is like.

[audiman]

Same turbo, but not sure on housings?
Max boost at 3.8k-4k (2.5bar)
0.5bar at 3k

It comes in very quickly, and holds 2.5-2.6 until redline. I use std motronic and N75. Wastegate size sounds ok. My boost without N75 is about 1.8 bar IIRC.

Will see what housings i have in a bit

[audiman]

Cant see markings on hotside, but coldside is .70 AR

[Jamo]

Same turbo, but not sure on housings?
Max boost at 3.8k-4k (2.5bar)
0.5bar at 3k

It comes in very quickly, and holds 2.5-2.6 until redline. I use std motronic and N75. Wastegate size sounds ok. My boost without N75 is about 1.8 bar IIRC.

Will see what housings i have in a bit

That's quite impressive, I thought it was higher up the rpm range on your car.

Be interesting to see what hotside you have.

[prosty]

It must be my hotside that is too big. I have #10 exhaust.

But that is nothing to worry about now since my engine said POFF earlier today. Taking the head out tomorrow

[porkkis]

What kind of AFR figures you see when the car is building boost or a bit before?

[jogi730]

Hmmmmmm.... crikey I would so gladly settle for those sort of figures (actually what I was getting pre big IC install)
Can I ask if the setup suffers at all from surge (and maybe also, someone could explain exactly what surge is?)
Many thanks
Cheers
Chris

[Faheemo]

Same turbo, but not sure on housings?
Max boost at 3.8k-4k (2.5bar)
0.5bar at 3k

It comes in very quickly, and holds 2.5-2.6 until redline. I use std motronic and N75. Wastegate size sounds ok. My boost without N75 is about 1.8 bar IIRC.

Will see what housings i have in a bit

2.5 bar boost on Redline ?
Wher can i find details on your car ?

[audiman]

Surge is the situation when the compressor "spits out" more air than the engine can swallow, which causes a backup of air at the intake and it actually creates reverse-flowing pressure waves that can be very damaging to the turbo. You want to avoid surge at all costs.

I found that Chris, HTH

[S2central.net]

Good explanation Tom...

My gut feel on prosty's problems are that the hotside is maybe too big (A/R too high)... Well its great obviously for ballsout drag race, but lag and spool are going to be a problem on such a setup that is biased to top-end performance. Maybe the camshafts are a factor also.

Did I read this right for 2.3bar at 8000rpm - that must be quite some engine with a lighter/stronger bottom end and solid lifters up top. Wow !

Paul

[Ross]

Good explanation Tom...

Did I read this right for 2.3bar at 8000rpm - that must be quite some engine with a lighter/stronger bottom end and solid lifters up top. Wow !

Paul

...or a reason for the engine to suddenly go 'ppff'...

cant remember from prosty's thread what he did head-wise (other than exotic cams), but i remember the bottom-end built to a pretty solid spec.

having a car sat with a dud engine at present, i'm rather intrigued to see what could lie under my own bonnet here