I’m hoping this has a factual answer, and doesn’t belong in IMHO.
I have a crush on the Subaru Impreza WRX. However, it occurs to me that forcefeeding the engine like that can’t be great for the gas mileage.
Could you put in a switch that would turn the turbo on for times when you want it? Around L.A., there would be a great many times when it would just not be needed. I’m sure the car does fairly well without the turbo boost, so my little fantasy is to be able to hit a button that turns on the turbo for when I really need to jump.
While typing this, it occurs to me that perhaps the biggest problem is that these days, computers are deciding how much gas to inject, and my idea would probably then need to incorporate a new program that would also turn the injection computer on and off in its “turbo spooling mode.”
It isn’t really necessary. Turbos pretty much do that on their own anyway.
The point to getting good gas milage with an engine is to make things so that at cruising speed as much of the generated hp is used to move the car insead of just turning over the engine as possible. A small engine uses less horsepower/fuel to sustain a specific RPM with no load than a larger engine (in general).
The point of a small turbo engine is having the small engine economy with big engine breathing on demand. All things being equal (and they never are exactly), a small turbo charged engine should get only slightly less fuel economy than a normal engine of the same size and quite alot better then a larger engine of the same power.
At low throttle conditions the turbo should be doing very little to impact performance or fuel economy.
What scotth said. Couple that with the fact that the WRX doesn’t get any turbo until around 3200 rpm (they say 3500, but in my experience, it’s a little under that), and conservative driving should cut a lot of the forced induction down, anyway. If I’m not mistaken, EPA stats for the WRX are like 21/27, which isn’t bad for a car that goes 0-60 in under six seconds.
As to your comment about the car going just fine without the turbo, I’m skeptical. I have a Forester with the Subaru 2.5L engine, and it’s not exactly a rocket. Since the Rex has a 2.0L engine, I suspect it might be quite the dog sans turbo. I do know from driving one that under the boost threshold it’s sluggish.
Of course, I didn’t actually answer your question: Yes, you can do what you want to do. All you need is some aftermarket engine management. One of the better ones is made by Link: http://link-electro.co.nz/
You can plug the Link module into your factory ECU harness and then use a serial connection to a laptop in the car to manage fuel delivery, boost, etc. I think the Link units cost about $1500.
I just want to reiterate what ScottH said, and how it relates to your question.
It’s hard to over emphasize that turbochargers do what you want already, in fact,** that is the whole point of turbocharging.** (on cars at least)
Also, those boost controlers and ECU upgrades that Necros mentioned are generally used to make the car produce more boost, not less.
Also, I’m sure you know this, but if you screw with the manufactures settings, you not only void your warranty, but you are likely to make the engine run lean, which will kill it very quickly.
The WRX has a 2.0 liter motor, which gives excellent gas mileage (for an all-time 4wd car). When you step on it, the turbo spools up and you now have the power of a 4 liter motor (at 15 psi). So you get the best of both worlds - good mileage when you want it, and power when you want it. Unfortunately, not both at the same time.
If you go ahead and jump on a turbocharged car, go spend the $200 on a turbo timer…
when a turbocharger is run hard, it runs very very hot. If you turn off the car after running it hard, the blades inside the turbo housing will cool without lubrication…needles to say, after a while, this is not good. The blades will warp. A turbo timer allows you to set an amount of time (say, 30secs), and after you turn off the car, it will continue to run the engine and oil through the turbocharger for the amount of time you select, allowing it to cool while lubricated.
Alternatively, you could just let it idle down yourself every time you stopped after driving it hard…but most people won’t do this after the first month of ownership.
Under light loads, the turbo is not really added or subracting anything.
So much more is to be gained by proper gear selection and general driving habits.
And, WTF! Can’t anyone on these boards buy and enjoy anything!!! It’s frustrating to watch a disproportionate number of posters disqualify the fun in so many things. Know what’s fun? A WRX. Know what isn’t? Any number of SDMBers owning/driving one.
As an aside and a suggested possible mod for the WRX turbo: If it is oil cooled then stick with that, if it is water cooled you might want to migrate to an oil cooled unit.
I had terrible luck with my Volvo until I went to an oil cooling unit. The only caution is that you need to be careful about how hot that thing can get, topping off your engine oil can become an exciting event when your turbo housing is hot and you drip a bit of oil on it.
To put some real numbers behind what scotth said, I had a Chevy Lumina Z-34 which was a 3.4 liter non-turbo v6. It was rated at 210 hp, and only got around 18 mpg. My next car was a Fiat Coupe with a 2.0 liter 5 cylinder turbo. It was rated at 220 hp and got around 25 mpg. So the smaller turbo was more powerful AND more economical. My current car is a Smart with a 600 cc 3 cylinder turbo engine. Rated at 61 hp and should get in excess of 50mpg.
KFERR !! You have a SMART car ?? That’s cool! While in Europe a few weeks back (Mitre pub also!), I was really admiring those and even went to a dealership there in London to ask about them. Tight little package there!! I should have known that you would have one!
Also, Turbo timers allow the engine to run for a while to let the exhaust gasses and oil cool the turbo, so the oil doesn’t cook inside the center section and coke up the inside. This helps the bearings last much longer. I doubt (IMHO) that the turbine blades will warp from the heat-soaking.
That’s all true, but (there’s always a but) I wouldn’t go challenging any well-tuned (lotsa valves, etc.) 4.0 at any speed above that 0-60 spec.
What you give up in a supercharged small engine is torque, especially over a wider rpm curve. The larger engine is at full torque by the time your blower kicks in, and holds it pretty close to redline. As the guy said, “There’s no substitute for cubes”. Of course the 4.0 is chugging the gas just driving around town, which, as mentioned, is the big advantage of your WRX.
So sure, stick with what you have. It’s a good machine. And kick it in the butt once in a while and enjoy.
BTW; One could add an aftermarket turbo to that 4.0. Hmmm…
Peace,
mangeorge
Do you guys run synthetic in your engines? Almost everyone on DIY-GT (who obviously run their turbos very very hard) use synthetic oil, usually Mobil-1 or Amsoil 0w30 or 5w30.
To keep an eye on what the turbo’s up to, I suggest buying a boost gauge. They’re cheap (you can get one and its mounting for under $50) and if you want to keep it from producing much boost on in-town driving to save gas, you can watch it and see what driving produces the most boost (or, more likely, least vacuum - the turbo isn’t always spinning fast enough to make boost) and avoid those behaviors.
