Opel Astra J GTC 1.6T 2012 - Total Build


Cape Town
Opel Astra J GTC
Hi Everyone

Great to be here.

I have some issues I was hoping you can all assist
I have done the following Mods to my car:
- Intercooler 600x300x76
- Boost pipes 63mm
- Mamba GTX2554 internal gate Turbo (wanted good mid power and good torque for fun daily) running 12psi spring
- 80mm maf housing with stock maf sensor
- GFB Gforce 3 electronic boost controller
- 57mm downpipe and 63mm exhaust
- bosch 613cc injectors #0280158123
- 3.0bar map sensor
- Progressive meth injection kit
Engine: 79mm forged pistons 129.7x19mm conrods Full ARP studs on head and rotating assembly. Race bearings No headwork done, rev limiter at 6700.

All torque limits maxed out in ecu since we control boost externally so no throttle closing.

But i cannot seem to get high power, i am getting 171kw and 314nm at 1.7bar spike holding 1.45bar at redline.

What could be holding us back so much?

We using HP Tuners to tune, If we turn boost up we not really gaining more power. Not getting detonation or knock retard at higher boost. Just not getting more power

Let me know you are willing to help i would appreciate it alot!


Staff member
Deal, Kent UK
A3 1.4 TFSI 150 COD
Have you altered the stock engine map in the ECU? If not this will be the limiting factor. Your map must take into account the tune, and be able to match fuel with air, the stock ECU maps are restricted to safe limits so pushing in more air is pointless as it will be trimmed back to match the "safe levels" the car is set to us.

If you have mapped it, either a remap or aftermarket ECU then look at the following suggestions.

Two guesses here, which is quite hard not seeing the car.

The stock MAF sensor could be failing to register the increase in air although you mention a 3.0 bar map sensor later in your parts list so I'm confused here unelss the car is designed to use both, in which case is uses the MAP sensor if it doesn't like the MAF readings, but could conceivably average out the two!

The fuelling may not be able to supply enough fuel, you've uprated the injectors, have you uprated the fuel pump?

You'd really need to get the car on a dyno and see where its peaking. Fuel and Air need to be matched, if one is not measured correctly or there is just not enough fuel delivered you could have the problem here.

Another issue could be boost leak, how are you measuring the 1.45bar - with a boost gauge?


Pro Tuner
Staff member
2002 Clio 172
Hi there, it's great to meet you. I'm sure you'll love the site, it's a really friendly and helpful bunch of guys on here.


Torque King
JB Starion
Sorry for the late reply :)

IMO Waynne is on the money with his answers |B

1 Down pipe is too small And are the bends mandrel bent? Fit the largest diameter downpipe possible for the best turbo response and power output as they DO NOT like any back pressure.
2 Need it remapped on a dyno by someone experienced and knows what they are doing.
3 Can the fuel pump supply enough fuel @ WOT?
4 What are the injectors flowing @ WOT and @ the redline? they need to be @ 85% max or lower.
5 Get rid of the MAF and use MAP sensor.