I'm wanting a turbo to play around with. The engine is a 3.5 litre inline 6 cylinder. I need a single turbo (for logistical reasons) and I'd like to make around 500bhp eventually (440bhp to start with). I don't want lots of midrange so I'm also looking for a reliable very low pressure actuator system say around 4psi. Do they go that low and still work properly? I want to limit the midrange boost you see but bleed the actuator pressure away at higher revs.
Also has anyone tried diverting exhaust away from the turbo at low revs .. ideally i'd prefer no boost at all until about 4000rpm. I want to essentially recreate the effect of a centrifugal supercharger to some extent .. no boost lower down but ramping up exponentially or thereabouts with revs. I don't want to go down the supercharger route though for a variety of reasons.
Any help appreciated.

Have you considered a supercharger? Not a positive displacement unit but a centrifugal one. They are the ones that look like a belt driven turbo. Sounds like one would meat your requirements (little to no boost low down) better than a turbo... Just a thought

Can you explain what you are trying to achieve? I had to re-read your post as I thought it was full of typos!

You want no boost and no low end/mid range until after 4k rpm?? Why??!! 99% of tuners/car manufacturers are striving to find more......

Fill me in I'm intrigued!

Surely the way forward is to fit a large turbo, (GT30 etc) then electronically control the boost pressure - just bleed it off (or the exhaust via the wastegate) when you don't want it.

hello spitfire ,

yes an actuator will work down to at least 4 psi . the old metro turbo has a 3.5 to 4 psi actuator ,and the saab gt1752 has a 5.5 actuator .

if you are trying to do this cheaply ,then i would find a turbo from a diesel ,around 300 bhp. this should flow enough for around 450 to 500 bhp at least ,prob a little more. if you find one from a large capacity engine , it will have a bigger a/r turbine housing ,and this should create much less bost at lowe rpm .obviating the need for electronics .you would also benefit from a lower exhaust back pressure ,this is very good for power and economy ...low rpm responsewise ..er..not so much.

if eyeballing turboes down at the scapyard ,look for something in the 55 to 60mm inlet at the blade tips ,and the biggest turbine housing .

this is all fairly vague non specific advice,but might shed some light .

regards
robert

Many people do use diesel turbos.

But a 300hp turbo is a 300hp turbo regardless of what it is installed in. It isnt suddenly going to supply 500hp worth of air unless it was incredibly undersized on the diesel engine to start with

And unless you're into large trucks, 500hp turbos arent that common second hand.

IMO just buy a new unit. Precision turbos offer a huge range and you'll get a new oil journal turbo for a few hundred quid and you'll know exactly what you have and what it's capable of.
I certainly wouldnt opt for an overly large turbine housing in some strange attempt to make it laggy....is there will be no getting around the fact it will be laggy after gearchanges etc.

Something like this would put you close to your 500bhp goals, and at $600 why on earth would you even consider a second hand unit ?
It'll still only be around £600 when sourced from a UK dealer.

http://www.precisionturbo.net/turbochargers/entry-...

Or with internal w/g

http://www.precisionturbo.net/turbochargers/entry-...

But they can build/supply pretty much anything you could ever want.

But you've taken a turbo from a (say) 1.7 litre engine and put it on a .5 litre high revving petrol so I would say a pretty good match (about 3 times as big), to put that theory to the one in question you would need a turbo from a 10ltr diesel.

I'm probably missing something here!

Also, I read that diesel turbos won't survive on a petrol engine due to the extra heat output. Internet bull****?

I'd check the TVR forums on here. Pretty sure someone was offering a turbo kit and poss even a supercharger kit for the speed 6 engines. No point re-inventing the wheel!

Also is the engine going to be strong enough for much boost?

Chris.

Hi all. many thanks for the interesting replies so far.
I don't want any (or very little) midrange boost because the sp6 engine I'm boosting isn't the strongest base to start from. I'm looking to limit the cylinder pressures to the same 310lb.ft it's making at peak normally aspirated, but holding that 310lb.ft all the way to the limiter .. that way I'm not asking the combustion volume to cope with any more stress than it's under already. It looks like some people have got what I'm trying to explain already so thankyou for that
A slow spooling, laggy low boost turbo is exactly what I'm after, but realise it's exactly what 99.999% of other people aren't after..

May as well not bother then. As every time you change gear it will still be horrible and laggy giving an almost useless power band and making it pointless turbocharging it in the first place.

There's just no point even doing it if you're going to do it so badly as it will just make it horrible to drive.

Why not fix the weakness and do it properly? What are the weaknesses?

A couple of other ideas spring to mind, run a turbo from three cylinders or if there is room underneath at the back of the car then mount it there fed by the exhaust - it's been done before with (apparently) great success.