Super vs Turbocharging the car
 

I'll most likely keep the engine N/A and do some simple - moderate modding to get the BRZ to 190-195 wHP. What I'm wondering is which would you want under the hood if you where looking to do forced induction?

Personally I'm thinking SC for this car. Normally I'm a big turbo guy (I have an evo), but it seems to me the small area under the hood combined with the already crappy low end torque, a centrifical SC would be perfect. Not only would it help low end, but there wouldn't be any need to cool the cSC b/c they are so small and effecient.

Centrifugal SC has bad low end torque assuming you're not doing something crazy like electrically driven, since at low engine speed the SC is turning slowly and not pumping much air.

Still its going to be better low end than a small turbo, easier to install and more reliable.

Nope, turbo is the easiest way to get low end torque because the turbine spins independently of the engine, this is how BMW, VW etc. get massive massive torque. Take a look at the HKS dyno, the torque improvement over stock goes up as your rpm goes up.

Perhaps your right, but it seems sub 3k in my car when boost is low, the engine really slumps. Given the way this car and engine has been designed from the start with driver responce in mind, the instant boost a SC provides makes more sense.

What car do you drive?
A turbo is a bit of a compromise. If you want low end, then you make the turbine housing "restrictive" since when the engine is turning slowly you don't have as much exhaust gas pushing through. That saps some extra power from the engine to give you more boost, and you get a great kick of torque. But then what happens at higher engine speed is that the turbine housing is too restrictive and sapping too much power, so the overall power gain is small. Again, think BMW/VW, their engines have massive torque at low engine speed, but then it dies off at higher speed.

A turbo set up for max power has a freer flowing turbine (in vague terms at least). At high rpm there is a lot of exhaust gas going through, and the turbine is able to extract a larger quantity of energy, and put it into the compressor. However at lower engine speeds the turbine can't build up enough pressure.

Variable vanes are an attempt to fix this, and it works pretty well. However you still have the problem of transient response and difficulty of controlling boost with your foot.

The best way to do it is if the compressor is driven separately to provide anywhere from 0 to the maximum boost the engine can take at any speed, and turbine optimized to collect only "waste energy" (this is likely a compromise too since the turbine size is also going to change how things work). Having the turbine fixed to the compressor but then connected to the engine or electric drive somehow should work mostly as well too. Rumor has it Subaru and BMW are working on this and we might see it soon. That will be a glorious day for car enthusiasts, completely linear throttle response, Roots blower like transient response, the efficiency of a turbo under boost, the fuel economy of a bypassed/declutched SC when cruising around town.

SC is missing the turbine component which is why it has slightly lower efficiency under boost, but it's easier to control something tied to the engine :)

i think its actually subaru and toyota

subaru and toyota have 3 projects in the works atm

1.brz/gt86 - finished
2.electric (EDIT)turbocharger - 2014-2015
3.unknown

it was brought up in the off-topic section sometime in January i think

EDIT: it was beginning of feb http://www.ft86club.com/forums/showthread.php?t=3611

I'd love to see a positive displacement supercharger designed to replace the intake manifold with a built-in air/water intercooler. It'll add weight :( but the boost to the torque curve across the entire rev band should make up for it.

This car needs immediate response, something that a turbo just can't supply without an anti-lag system and it's host of issues.

At least, that's my take on it.

+1 and I'll add that the more efficient (but expensive) twin screw option has me intrigued.

Meh, at low boost levels there's not much of a point in a Lysholm twin screw since a Roots blower will do the job pretty well.

Looking at Eaton TVS SC maps they tend to be most efficient at 1.8 ish pressure ratio or so, which may be a bit too high and need a reduction in compression ratio or other compromises. I get the feeling the higher pressure design limits peak efficiency probably due to increased friction as a result of more robust sealing or something like that.

If it's designed for lower pressure the Roots supercharger's efficiency problem is ameliorated, and seeing how the Eaton TVS compressors are >60% efficient, lower pressure optimization could put them at comparable efficiency to dynamic compressors.

My early understanding is that Lysholm twin screws can hit mid 70s efficiency which is superb. More efficiency (less heat) is going to be important for fi with the CR that the motor has. Also are they quieter than the roots?

Turbochargers can indeed have instant response, or close to it, and anti-lag systems exist for racing teams running big turbo's that have much different needs than street cars. It's already been said in this thread, but it bears repeating - turbo's can be designed for low rpm power. Put a low-boost small hotside turbo on a high compression motor, and you'll be making boost from 1500rpm. It's just that it is no magic wand for power, if you make low end, you sacrifice top end.

Superchargers are popular because they are easier to fit than turbochargers, and I expect for that reason we'll see more than a few of them for this car. They are not, however, more efficient, and on a 2.0-liter that may be an issue. There's a reason many manufacturers moved away from the superchargers to turbochargers over recent years. Another reason is that superchargers don't do as well as turbochargers in extended operation, such as at circuit events.

Regards,

Paul Hansen
www.avoturboworld.com
www.facebook.com/BRZSportsCarClub

Still transient response delay even with a small hot-side. It's not just about low rpm boost.

I imagine they're quieter than Roots under boost since the pressure difference between the air being pushed along the compressor and the manifold is smaller.

Like I mentioned, I don't know anything about the details of Roots blower design but seeing an efficiency peak at 1.8 pressure ratio when by concept an ideal Roots blower is less efficient at higher pressure ratio signifies that there is probably some sort of friction or related tradeoff going on. All of the Eaton TVS superchargers have efficiency peaks occurring at pressure ratios in the 1.6-1.8 range. Of course most people who supercharge their cars are looking for a sizeable power boost anyways so it makes sense that they'd design them like that but if they're capable of hitting 76% efficiency (albeit only on their largest one) at that sort of pressure ratio with a Roots design then they can do even better at lower pressure.

The reason I like Roots blowers is because if you can figure out a 2 speed drive you can do crazy things, such as increase cruising fuel economy :O If the Roots blower is set up to give you just a little bit of boost like on the Lotus cars, the throttle response can be very intuitive and linear too without any fancy bypass control mechanism.