Why do turbos have a wastegate?

[makaio]

I've heard of turbo powered cars not having a waste gate, I know Porsche's do. So what does it do?

[Doug Zielke]

Matt, I thought *all* turbo-charged motors have a waste gate to bleed off excessive boost pressure. Too much boost and it's....boom. Ja?

[Targa77]

Actually, they control boost by using a spring in a canister that is actuated by manifold vacuum. If your wastegate fails (I know), bye-bye pistons and/or head gasket.

[Steve W]

Turbos create boost by spooling the impeller within the compressor side. The compressor impeller is driven by the exhaust impeller on the exhaust turbine, which is driven by the hot exhaust gases exiting the exhaust manifolds. The more exhaust energy there is, the faster the impellers spin. When the desired boost level is reached, the wastegate bleeds off some the exhaust gasses going to the turbine section to keep it from over boosting, with the extra exhaust either bled to the atmosphere, or refed back to the exhaust before the muffler. As boost level drops, the wastegate recloses just the right amount to modulate the right amount of boost pressure. Most cars have turbos sized to the engine which if not modulated by a wastegate, would create too much boost to the motor and destroy it from detonation. The wastegate is spring loaded, and controlled by the intake boost pressure. Intake boost fed to the wastegate, starts opening the wastegate valve at the spring pressure it is set at. Some factory turbo cars, intercept this boost signal to the wastegate with an electronic selenoid valve, controlled by a computer, to dial in boost up an down based on knock sensors and programming. Aftermarket computers such as an HKS EVC can dial boost from your cockpit with a push of a switch or turn of a knob. Cars without wastegates are few, and those that don't have them don't need them because the turbo is really undersized for the motor and already maxed out, incapable of creating excessive boost. They also create boost very slowly, so you don't get that instantaneous surge that comes when you floor the throttle. This is not a optimal solution, since undersized turbos are inefficient and limit the car's power. The inefficiency is converted to heat - hot air - with inefficient turbo systems being as inefficient as 65%. On the other hand, sizing the turbo larger, and controlling it with a wastegate, efficiencies as high as 78 to 80% are achieved.

[Mike the mechanic]

Actually, the only cars that do not use a wastegate are turbo cars using a variable nozzle turbo. These cars are few, the 1999.5-up VW TDi engines and smoe early 90's Dodge Shadows. These turbos have the advantage of variable turbine a/r which helps create almost no turbo lag with great top end flow too.

Mike
86 Carrera t

[makaio]

I was talking to this air cooled VW gear head the other day, and he said one of his old drag type 1's, was tubo charged with no waste gate.

I was just curious.

[Wayne 962]

Here's a quote from my new book (on engine rebuilding):

Many people incorrectly think that a larger turbo charger will generate more boost, and horsepower. In reality, this is not necessarily true. Installation of a larger turbo charger must also accompany other important changes in the engine. The turbo charger generates compressed air, known as boost when the blower is spun up to operating speed by the exhaust. This compressed air is then fed back into the engine’s intake system. The entire system is regulated by a pressure relief valve called the wastegate. The wastegate acts to release excess pressure so that the engine doesn’t suffer from too much boost being applied. Installing a larger turbo charger without making adjustments to the wastegate will result in no increase in boost levels.

-Wayne

[beepbeep]

Wayne, you are a bit wrong actually...

First of all, there are two "valves" on most turbocharged engines.

One is called "wastegate" and is used to re-route exhaust gases around turbo to prevent turbo boosting too much or overreving...this as often spring based vacuum thingie. On newer cars, there is a solenoid valve in-between that is pulse-width-modulated by ECU to achieve right boost. Wastegates can be integrated with turbo (smaller Garrett-turbos, modern cars) or separate (older Porsches). They can also vent directly into athmosphere creating a helluva of noise @ full boost. (like mine :-)

Second one is caled BOV which is short for "Blow Off Valve" and is used to prevent excessive pressure build-up when turbo is spinning wildly and throttle is suddently closed (as in upshifts). Ir releases air between turbo-compressor (the "cold" half) and throttle-body. If BOV was missing, turbo would face sudden pressure-rise in pipes going to the throttle-body (and intercooler if available) and would likely start to surge, with potential compressor-blade failure as result. There are two types of BOV's, recirculating and open-air. Recirculating ones short-circuit turbos compressor-intake and outlet, recirculating the air...thus the name. Open-air variant just vents compressed air into the athmosphere...creating "pshyyy"-sound of questionable coolnes. Open-air dumps are not to be used on engines with hot-wire AMM as it usually confuses ECU who thinks dumped air is still in the loop and richens the mixture considerably.

There are some cars without wastegates:
- Old ones which use so called pop-up vavle which just vents air to athmosphere over certain pressure (BMW 2002 Turbo)
- New ones with VNT-turbos (SAAB 9-5 TiD etc.)

There are also rally cars which don't have BOV's but use funky anti-lag ECU's to prevent turbo-lag...reducing turbo lifelength considerably...

About bigger turbos:

Yes, bigger turbo *can* produce more power @ same boost, due to fact that it will be nearer it efficiency peak for given volume of air compared to small one. With other words, small Garret T3 blowing 1 bar into 3-liter engine @ 6000 RPM will be out of limits, surging and creating a lot of backpressure, where KKK K27 will be just on-spot, creating moderate backpressure and heating air nearer it's optimal PV=nRT limit...thus producing more power at same boost. But all that is another complicated story...