Scottish Elises

Hey guys,

I know cock all about turbos, I'm hoping some of you lot do...

Practical Performance car mag have just run an article on a Peugeot 205 GTi where the rear silencer/muffer had been replaced with a small ish turbo... Everything else was stock.

Allegedly with the standard small bore exhaust and even with a long inlet pipe there is very little lag (Probably due to the stock CR so it's hidden) and its running 1 bar boost... the pipework returns under the car so isn't intercooled and because of the distance to the turbo the exhaust valves don't suffer with excessive heat. (The exhaust gasses have cooled somewhat by the time they get there I suppose)

Now I have no idea about turbo's but there is a large gap under the 924's fuel tank(!) that would easily swallow a turbo, there is even a nice big exhaust hanger on the gearbox to bolt it to. It will need an oil scavenge pump and some pipework clearly.

Aside from the lag (that would appear to be inherent in this design but who knows) Theory says 1 bar is safe on most engines - would a standard block at 9.3 CR take a bar of boost?

And should I turbo the Porker...?

If your VE is good then it will suffer from detonation when on song. If you assume the thing doesn't come on boost until about 4,000 RPM and your na VE is 60% or so at that RPM and beyond then you have a reasonable chance.

If I were to do this I would try to return the engine to as close to 100% na VE across the whole rev range - perhaps allow it to go to 110% so as to raise the CR to 10.5:1 max. That will give you peak torque across a wide rev range and thus make more peak power than it currently does without stressing the engine. If you really want to make your 2.0L like a 3.0L you'll need to drop the CR and then you start to create a compromised engine (only works when the turbo is on song).

So to work out the boost allowed at a given RPM, look at your current VE, then scale it up by boost to get to 100%. So if for a given load cell, VE was 66% you would allow yourself 1.5bar inlet manifold pressure (equals 0.5 bar boost) to get to 100% or 1.7bar inlet manifold pressure to get to just over 110%. Any more than that and your effective CR will exceed 10.5:1 and the thing will start to detonate.

I don't know if the MS has turbo compensation inherently, or whether you have to factor it into the base VE table. Assuming you're running with a MAP sensor of some sort I'm guessing it can compensate for the +ve pressure produced by the turbo system by increasing fuel and retarding ignition.

Placing the turbo at the far end of the exhaust system will mean it's spinning in colder gas - less cooling issues and less boost but more lag. But seeing as how you want only light pressure increase, it should be fine. Nevertheless it will still cause a restriction in the gas flow and thus a local hot spot - I wouldn't put it under my fuel tank - could easily reach >300C - if you must put it near the tank put a reflective heat shield between it and tank, and don't leave the car idling when you're not actually rolling along!!!

Cheers,
Robin

That makes sense, cheers Robin.

So basically you are cramming as much air in as the NA would need at max chat compensating for poor flow characteristics with boost.

Yes the fuel tank is a problem, it will need some serious shielding to protect it, especially in traffic, maybe a radiator style cowling, air temp sensor and fan? Maybe an overridable wastegate to bypass it altogether in traffic, might mess up the inlet tract though. Inlet tract of 2.5m might mess it up anyway!

MS can cope with anything up to 2.5bar absolute (1.5 bar boost) as standard and uses MAP vs RPM for the VE and spark tables so the fuel side will be straight forward to fix.

cheers
Rich

Just read the article and it basically sounds like a constant pressure turbo instead of the impulse system currently used on most cars. You will get HUGE lag when you first accelerate until the pressure builds up in your exhaust pipe but once it is up to pressure providing you are giving it some then you are going to maintain that pressure between gear changes..

So if you don't go crazy with the boost i can actually see it working quite well.