about turbocharging and forced induction
As echoed by previous posters, turbo's add what my dad deems unecessary complexity to a car. The main reasons are outlined above, but I feel it necessary to add more.
Added Plumbing: Most decent turbochargers (read: IHI, Garrett, KKK, and the like) use not only oil to cool the bearings, but use coolant from the car's engine cooling system to cool the driven side of the turbo.
note: Turbo's have 2 sides: Drive, and Driven. The drive side is the side that bolts onto the exhaust manifold and houses the turbine. The driven side houses the air input compressor. The two are connected via the turbine shaft and bearings.
- Added wear: As with all forced induction systems, the positive pressure created at the intake manifold increases the compression ratio in the cylinder. While providing more power per stroke, it also wears the moving parts of the engine faster. These include valves, rings, cylinder wall, and then some.
- Extra heat under the hood: Generally, the hotter it gets in your engine bay, the worse the coolant and oil function. Which is why most cars that come stock with a turbo, recommend using a multi-grade oil with a minimum weight of 10-40. Else, the heat will cause the oil to vaporize, and it won't lubricate or cool well at all, at which point, numerous bad things will happen almost all at once . This includes, but is not limited to: engine seizure (due to pistons fused to the cylinder wall), melted bearing seals in the turbo, and a melted head.
note: Personally, I use Mobil1 10-50 synthetic. It's about $5.50/qt in California, but well worth it.
- Added weight: Turbo's function best when the air entering the turbo is hot, as it allows for greater kinetic energy of the exhaust pulse to drive the turbine to it's full RPM faster. Alas, this is best achieved by keeping the exhaust manifold hot. Thus, cast-iron exhaust manifolds are the most heat-efficient, compared to aluminum or steel headers. They're also heavier, and thus increased weight. This can sometimes offset the extra weight of a supercharger, in that super's can have ligher headers without them melting under high-boost conditions.
Notes about turbo'd cars:
note: I consider this last note to be the minimum level of modification you should apply to your turbocharged vehicle. It will cost you aprx. $1000, but the longevity and horsepower gain will more than make up for the initial outlay.
Be wary of purchasing a turbocharged car that does not have an intercooler. they do exist, and the Ford Thunderbird Turbocoupe was commonly sold in 1983 to 1987 without an intercooler. Such cars can't be boosted for very long without incurring much engine damage.
- Just as properly sizing the turbo to match the engine exhaust output is key to getting the most horsepower, sizing the intercooler to the output of the turbo is equally important. A 3-core liquid to air intercooler designed for a T66, used on a car with a small T03 will lead to a pressure drop across the intercooler that will cause poor performance, since not enough boost is reaching the intake manifold.
- Don't EVER let a dealership work on your turbocharged vehicle, unless it's something like Ford's SVT or Toyota's TRD. And that likelyhood is pretty slim. I will provide a short parable below.
- Change your oil every 3000 miles, and use a good synthetic oil, despite what your owners manual says. Remember, car manuals' maintenance spec's are optimized to bring people into the dealership at 100,000 miles for their signature service, NOT to make them last.
- NEVER use engine oil additives! EVER! Most of them have a high detergent concentration. Notice the oil spec on most turbo's doesn't include the HD (ie: High Detergent) marking, and you wouldn't use an out of spec oil, would you? Come to think of it, avoid any fluid additives, except for water wetter, an expensive substance that has better heat absorption and dissipation characteristics than the typical water/antifreeze mixture, and is turbo-friendly.
- Turbo lag is caused by the turbo having to spool up after venting out all it's compressed air when the throttle plate closes (ie: when you take your foot off the gas pedal to shift, etc). This is done to keep the back pressure from destroying your compressor blades, and the blow off valve makes a very nice sound in the process. However, the fine folks at Garrett have developed an electrically spooled turbo that will keep the blades rotating even during a blow off, so that the boost will build back up much faster when the throttle plate opens and the blow off valve closes.
- Lastly, any car with forced induction is going to be more finicky than a naturally aspirated (read: no turbo/super) car. This means you need to check your fluids regularly, maintain proper tire pressure, keep your engine bay clean, replace oxygen sensors slightly more often (due to higher heat at the manifold), use better quality, after market ignition parts (spark plugs, wires, cap and rotor), replace the stock boost gague (if your car comes with one, they tend to be wildly inaccurate), invest in a turbo timer and decent boost controller (APEXi and Greddy come to mind), and a high-flow air filter. I prefer K&N, but there are now a number of good manufacturers like HKS.
I own a turbocharged, 1990 Ford Probe GT. This car has been one hell of a learning experience. I comitted the 3 cardinal sins of used car buying, all in the same car! 1) I bought a turbocharged car used. 2) I bought it from a mechanic. 3) It was a salvaged vehice. It should be fairly obvious why these are classified as sins. If not, I have a car I'd like to sell you. :)
Anyhow, the car provided decent performance stock (rated at 145HP @ 4800 RPM, 190Ft/Lbs of Torque @ 3500 RPM, according to my Chilton's manual), and after the addition of the above mentioned components, I felt a marked increase in low end torque and horsepower. (I did not have access to a dyno to confirm this, but going from a 15.5s 1/4mi to a 14.2s 1/4mi, I'd say that's good enough.) Also, I started using synthetic oil. All was indeed fine for about a year after I bought the car in 1997.
Around late 1998, I noticed an increase in problems with blown coolant hoses. I was replacing hoses on a monthly basis, during both summer and winter months. (As anyone who's ever had to shop for parts knows, the odd-ball parts on odd-ball model cars gets harder to find as years go by. When you do find them, they become expensive. Case in point: $17 for a small 3" long coolant bypass hose!) After replacing almost all the hoses, a strage, white, acrid smelling smoke was billowing out of my tailpipe.
An independent mechanic inspected the car, and determined that the bearing seals in the turbo were shot. He readily admitted to avoiding turbos, and suggested I see the dealer. I did, and they replaced it to the tune of $1500 for a used turbo, plus labor.
Not sooner than i drove off the lot, did the engine start smoking again. I took it back that second, and they said I should just let the oil burn out of the catalytic converter. (If you're not shrieking at the horrible mistake of that advice, read on.) So I kept driving it. At the time, I was working out of town, and commuting every day. And no sooner than 2 days after I got back to work, I noticed the smoke again.
I took it back, they had to replace the turbo for the second time. This time, they cleaned out all the oil from the rest of the parts (like the intercooler, intake plenum, etc.), but suspiciously, not the catalytic converter.
After a while, I couldn't figure out why my car kept overheating. I took it to an exhaust shop recommended by a good friend. Upon inspection, the pressure at the top of the cat was 3psi under no load acelleration. This is abnormally high, it should be anywhere from 0-1psi, and no higher. And under load, this kind of back pressure was causing my turbo to glow cherry-red under even moderate to light driving conditions.
I replaced the cat, muffler, and down pipe, and the overheating slowed significantly. But it wasn't gone. As it turns out, the fuel mixture was far too lean, causing abnormally high exhaust gas temperature, and the water pump had started leaking. Combine that with a faulty oil pump, and you can see where this goes.
Moral of the story: Buying a used, salvaged, turbocharged car is a very bad idea. Taking such a beast to a dealership, is an even worse idea. Most of the low-grade tech's that work on cars at dealerships wouldn't know a compressor from an engine block if one fell on their foot.