Turbocharged = Supercharged?

"Don Bruder" wrote

It's fake. He's actually got a straight-6 under there, with a phony blower pop-riveted to the hood, and a little electric motor that spins it when he wants to look "kewl". :-)

But turbos are not drawing any *power* except when they supply boost (open throttle). The speed that the turbo spins is proportional to engine *load* not engine *speed*. If you rev the engine up to 4000 RPM in neutral, the turbo won't spin up to full speed, won't build any boost, won't raise exhaust back pressure, and requires essentially zero power because net airflow through the engine remains low. Conversely, if you floor the car in 4th gear at only 2000 RPM, the turbo will spool up and begin producing boost because the volume of air flow is high.

Superchargers always draw a certain amount of parasitic power since they are mechanically driven and their speed is *solely* related to engine speed. That means that even when coasting downhill with the throttle closed, a supercharger drive mechanism is consuming a non-negligible amount of parasitic power.

In article , "MasterBlaster" > '69 Camaro (tags read "BLWN69", so I'm assuming it's a '69...) if only

Y'know, I'd maybe believe this explanation... *IF* the car *HAD* a hood...

Nah, definitely not a fake blower on this one.

But thanks for playing :)

um the pt turbo is on the exhaust side. built into the header

I've always wondered if anyone had attempted to use a solenoid activated supercharger, something very much like how an air conditioning compressor is kicked on.

The way this would work, is it would idle most of the time and be activated either by vacuum manifold pressure via a sensor in the intake manifold, or by a manual switch.

This way you could have the additional power when you want it, but it would idle without adding any load at all when deactivated.

Corky Scott

This would complicate the plumbing, as an alternate airpath to the engine is needed. One that is not too restrictive, but closes up when the boost comes on.

In theory it can be done, but is it worth it?

For one thing, superchargers draw many tens of horsepower (up to more than 100 HP in high-performance applications). A/C compressors draw 3-5 horsepower tops, so any clutch used with a blower would have to be massive.

Also, roots, whipple, and other positive displacement blowers will halt all airflow through the engine if they stop turning, so an automatic bypass mechanism would have to be incorporated. Centrifugal blowers can be stopped without obstructing airflow.

Turbo lag has more or less been reduced to the point of being hard to notice in most applications worth driving, from what I understand. Things that reduce it...

a) reduced weight of the turbine and better bearings means it spins up quicker due to having lower inertia and lower resistance. Build them out of high tech metals and ceramics so they can withstand the heat and forces/torques with less weight.

b) designing it so it is "spun up" more of the time, with a waste gate keeping you from over pressurizing the intake

c) utilizing more than one turbo and staging them so they operate on different rpm ranges.

All these can be found on on production cars today (although not necessarily all on the same car and/or on the cheapest cars)

Approximately 10/1/03 12:10, Bob Hetzel uttered for posterity:

Smaller turbos to spin up quicker.

Pressure relief on the downstream side of the turbo so it has less inertia.

As with most things, turbo design is a trade-off.

Small turbos spin up quickly, but they turn into potatoes in the exhaust pipe at higher engine speeds and preclude higher horsepower because of excessive backpressure.

Bigger turbos take longer to spool, but don't choke the exhaust side at higher engine speeds and thus are better for high horsepower/high rpm designs at the expense of driveability.

And its not *just* turbo size. Turbine and impeller designs play a part also. Less restrictive turbine designs can be made to spin up reasonably quickly without becoming obstructions at higher speeds, but there is always some degree of trade-off. Compare and contrast the different turbine-side design features (aside from the fact that the left one is used) of these two turbochargers:

You're forgetting the existance of the wastegate.

No, I'm not.

Once the wastegate opens, the engine will not build anymore horsepower because the turbo's boost is at its max and will go no higher. So a small turbo with or without a wastegate limits maximum horsepower... PERIOD.