Aaron,
A turbocharger compresses air as a function of the speed/volume of the exhaust gasses. Volume is static in this system because of the size of the exhaust pipes. Speed of the exhaust gases is dependent on cfm out of the engine into the diameter of the pipe. Turbochargers are designed to fit those size /speed parameters. Usually they are sized to mid-rpm flow. This means that there is excess pressure available at high rpms, requiring more fuel, more cfm of air etc. To keep from exceeding the amount of fuel in the fuel/air mix the air available must be limited. Since PV=Rho RT in a system with constant volume you can either cool the charge or limit the pressure. Both ideas are used. Cooling charge=intercooler. Pressure limitation is through a waste gate (wastes excess pressure) also called a blow off or pop off valve because it opens and blows off excessive pressure from the turbocharger before the mix becomes too lean and the excess heat melts your pistons. Hope this helps. Scott

 

The reason turbocharged aircraft engines use a wastegate is because intercoolers don't work very good at high altitude where the air is thin. Same thing will happen trying to drive your car to Leadville CO.
d.

 

What does a wastegate do?
I ask because it seems like it could be pretty cheap to put together a single turbo system if you can fabricate the pipes.

Also, how do you determine pipe size and turbo size?

 

Aaron...

What the wastegate does is limits the amount of exhaust gases reaching the turbine, once the desired boost pressure is reached, the waste gate opens so the turbo can maintain that specific pressure.

The blow off valve is a valve that opens when the throttle is closed so the excess pressure before the throttle body is bled off, and thus preventing it from going back to the turbo and shocking the system.

Answer your questions?

Let me know if you need some more help.

 

Too add...usually the waste gate is a part of the turbo itself. You can however buy manifolds and turbos with separate (external)wastegates. And the amount of pressure can be changed by changing when the wastegate opens.

As far as pipe size, it really depends on application. Most small displacement motors (like mine) can live w/ 2" charge piping, something like a small block would require a bit more IMO..but then again, I haven't ever really run a v8...just my four banger.

hth

Jamieson

 

Turbo's are free power. If you can control fuel and live with slight lag, you can make lots of power with almost any engine, more than you could expect from a normally aspirated setup. The wastegate works because of the multiplication factor...the turbo makes way more pressure than the engine can handle so you port it back into the exhaust. The BOV is there to reduce the excessive pressure when your throttle plate slams shut, trapping the pressure at the compressor side of the turbo. Without it, you could damage the compressor. I've never used the valves yet, but now with the new project, I might have to. Instead, I learned to "nurse" the throttle closed.
Turbo's are great.
-John Spina
www.gnspyder.com

 

Where does the wastegate exhaust to? The atmosphere or back into the exhaust?

 

Back into the exhaust, it just bypasses the Turbo. The blow off valve vents to atmosphere.

HTH, Rod

 

the wastegate typically vents back into the exhaust, some (usually custom) setups have it vent to the atmosphere..but that's not really street legal due to emissions regulations. If you pick up some old sport compact car mags (can you tell where my experience is?!?! hehe) You might run across a few examples.

Jamieson

 

Originally posted by John Spina:
The BOV is there to reduce the excessive pressure when your throttle plate slams shut, trapping the pressure at the compressor side of the turbo. Without it, you could damage the compressor.

I've read this for years but I own a Lotus Esprit with 72000 miles of 1 bar boost, no blow off valve, and a turbo that still looks/works great. Old wives tale?

 

Originally posted by MikeyRide:

Originally posted by John Spina:
[qb]The BOV is there ... Without it, you could damage the compressor.

I was always told the BOV was there to keep the rotor spooled up to help prevent lag when you nailed the throttle again.
My $.02
Should I add this one to the "guaranteed to start an argument" thread?
d.