MikeT wrote:XantiaMan583 wrote:Remember i'm talking about a 2 litre Activa which has a different turbocharger to the 1.9TD. It's a T25 the exact trim i do not know but the compressor wheel is very small.
At an estimate i reckon its knocking out about 170-175bhp at 12 psi, up from the standard 9psi/150bhp.
Sure, that's why I asked what turbo you had as even different 1.9TD's have different turbo's apparently.
Garrett T25 Turbo:
Garrett's T25 Turbos are compact, quick and powerful - very different from the slow starting Turbos of the 1970's and 80's. New technology has increased the efficiency of these Turbos, and their smaller size allows them to start up much faster. They deliver 5-15 psi to your intake. Get up to 230HP for engines 1.5 to 2.0
Perhaps you're right that the exhaust is the limiting factor rather than the turbo tailing off.
There are about 200+ variants of a T25, easily! You can get a turbo based on a T25 thats enough to flow to 300bhp. Infact any turbo can be called a hybrid as its always modified to suit its application.
In the case of the engine fitted to the Activa, it has to suit the characteristics of the engine size, compression ratio, the way the head flows, how many valves it has and how high it revs to.
Its for this reason that a diesel turbocharger is not a good idea for a petrol. It has to produce boost much earlier and this is done by smaller turbine and compressor wheels, the downside to this being the top end is reduced.
No problem for a diesel as most dont produce power over 4000rpm whereas a petrol will be making peak power typically between 5-6000rpm.
Diesels also run lower EGT's so the turbocharger can run at a higher boost, a good example being the XUD lump that runs about 14psi and can run more boost, to about 17-18psi before it starts to heat up the inlet temperatures.
There's alot to learn and know about turbochargers but alot of it goes over my head.
I do know from looking at an Activa turbocharger on the bench that the size of compresser wheel is ideal for low boosting, it does this well, as you can see 8psi just after 2200rpm, but the turbine housing is very small (as to be expected from a T25), the turbine wheel must also be fairly evenly matched.
A simple improvement would be to increase the size of the compresser wheel, which would mean running similar sort of boost but making more power because the wheel is shifting more volume of air and the inlet temperatures are also reduced. There would be very increase in turbo lag because the turbine wheel remains the same size.
Next step would be a different turbo with a better flowing turbine side an even bigger compressor housing and wheel. You could get big power from this engine because its head has massive valves, a bit of porting and matching on the inlet and exhaust sides of the head, bigger throttle body, more effective intercooling and inlet plus exhaust mods, your well into 200+bhp!
Watching you giving the bodywork TLC was like watching poetry in moton, always very enjoyable watching a craftsman at work..
I'm getting old.. For those that are not aware, Gareth's TD is the same colour as my Activa and is a dead ringer for it as it is an accurate Activa look-alike 
Turbocharged cars really do not like hot air and the Activa is no exception, it does become a bit flat despite the higher boost. 
But they have been very enjoyable miles, as the ECU has nicely adapted to my driving style and strong pull from 1500rpm onwards, very much like a modern diesel but without the fuel economy! Its doing around the 30mpg mark which is pretty good as it does get the occasional blast, higher boost is too addictive 
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