As the title suggests, I need to know what kind of boost I should be expecting to see from a GT28R.

I currently have one fitted on the car, with a 2.5" exhaust system inc decat. The inducion is standard until next week. After taking advice on fueling etc on here, I decided to take it to Leighton a couple of months back for some mapping. This produced results of 260bhp @ 1.1 bar via a mbc.
Although this is a decent enough figure, i was expecting a little more. I dont have the graphs to hand right now, but I shall update as and when I can.

My issue is that I have an insane amount of lag. So much so that i'm hitting 1.1bar at about 4200 maybe just a tad later. This may not seem too bad to some people on here, but the fact that my boost trails off to 0.7bar at the redline kinda makes it a little undriveable. You have to wring its neck everytime you want some power, and then when you get it, it kinda tails off.

Apart from a boost leak, what else would cause my boost to tail off?

Can someone on here with a simalar set up let me know (or knowelegeable people) what kind of boost I should be seeing at 4k, and what I should be seeing at the redline? The turbo seems to be acting more like a very lazy standard turbo, but not holding boost high up the rev range as i assume it could.

Cheers in advance.

My old one got full boost of 1.4 bar in the mid 3000s and didnt drop below 1 bar at 7000rpm. Sounds like you might have a boost leak or a restriction, maybe in the intercooler.

What other mods do you have?

I found that a manual boost controller was pretty rubbish at controlling the GT28R so I bought an AVCR which was amazing. I dont know exactly how much of it was down to that though as I changed other things at the same time.

Do you know what size exhaust turbine housing the turbo has? 0.64 or 0.86?

Either way, 4.200rpm is much too late for full spoolup

Another thing it could be is incorrect adjustment on the wastegate. If the actuator arm is too long then the wastegate wont be tightly shut when it's spooling up.

I am seeing full boost of 1.4bar at 3200rpm and it holds 1.2bar at redline on my GT28R. However i have a 3" DP and de-cat which may help

The only mods I have at the moment were listed in my original post. I plan to get a fmic and run a bit more boost, but I think it's best to get this lag issue sorted/figured out 1st.

The turbo was fitted by Barbz a couple of years ago by the previous owner. I don't know what size exhaust housing it has, but on the paperwork I have, it states that its a Garrett dual ball bearing turbo GT28R. Barbz part number was MD378 if that helps?

I have checked most of the boost pipes and replaced the fiat clips with jubilees but it still seems very laggy. There are no signs of any leaks on the standard intercooler either. I have the standard downpipe on there that ovbiously restrics flow, but I didn't think it'd make this much difference?

How do I check to see the the actuator is adjusted correctly, and how do i rectify this if this turns out to be the problem?

Cheers for your help guys.

Hi Rob, this must have changed since I mapped it as looking at the logs it was spooling much quicker?

I would be 90% sure of a boost leak, the symptoms are classic i.e. very slow to boost before eventually reaching full boost. Turbos are amazingly powerful, even with a big leak they can still pressurise.

If you can get the car back down to me I can run a full check on the car to diagnose the problem.

Hi Again Leighton,

I'm not entirely convinced that it's changed a great deal (in terms of lag anyway) since I had the car mapped. Looking at the graphs I have...

2000rpm 2.6psi
2500rpm 3.7psi
3000rpm 5.1psi
3500rpm 8.1psi
4000rpm 13.3psi
4500rpm 17.4psi
5000rpm 16.5psi
5500rpm 14.5psi
6000rpm 13.3psi
6500rpm 11.9psi
7000rpm 11.4psi

I can't be this accurate on how it's reading now, but it definatley seems to tail off a lot quicker than it did. Even still, surley 11.4psi at the redline is a bit low for this turbo? Or have I been mis informed?

Cheers.