turbos are definitely more efficient. Most energy from the exhaust gas is required to accellerate the turbine from rest (or very low speed) into its operating speed range. when the turbo is at max boost, the wastegate opens and most of the exhaust gas is free to take the path of least resistance out through the wastegate into the exhaust. Only enough gas to keep the turbine spinning has to pass the turbine.

Positive displacement blowers on the other hand are simply mechanical pumps which output a fixed volume of air for each revolution. The energy required to turn them rises proportionally with their speed. They require the maximum effort to operate when they are at their maximum speed or boost level.

ah, thats a goo point

although we are mainly talking about backyard turbo setups, which arn't as complicated as having a wastegate, surely yours doesnt does it?

all turbos need a wastegate, otherwise boost will continue to rise uncontrollably. almost all OEM turbos have an internal wastegate built into the turbine housing. You'll see a little flap valve in there. The setup I bought has a large external wastegate.

1200rallycar if you look at your turbos you will see on each of them a can that has a boost signal sent to it in the form of a rubber hose. This is the actuator, if you follow the arm off of this can it leads to the wastgate, its the little flap inside the turbine housing.

But hey I bet you were talking about Dattodude's backyard setup which has a big ####in turbo which has no wastgate and requires an external one to be fitted

ah yes true i got confused, i was picturing an external wastegate, and had forgotten bout the turbos having them, as i didnt fully understand what they did, now i do, cheers fellas

Hey Guys,
Now I know that many of you have had your say on the efficiency of turbos verses the roots blower9 I believe the correct term is adiabatic efficiency ), but there is another supercharging option that is worth mentioning/ exploring and that is the (excuse the spelling here please ) Lysham screw supercharger- presently made by Flemings Thermodynamics and marketed as the 'Sprintex' blower.
To the best of my knowledge these are actually MORE adiabatically efficient than a turbo- one drawback though, price!
Perhaps Dodgeman may be able to enligthen us further on these....
Dave

actually you dont really have to have a wastegate if it's 1984, you're a poor student and have a 72 corolla wagon with a homemade turbo setup on it.
Mate of mine was all of the above, and just used to juggle the backpressue in the exhaust to keep things under control... I think balls of chickenwire stuffed in the pipe was the prime means of doing this.....

and yes, it was painted matt black, with the obligatory hotwires.....

I'd be interested to hear how the Lysholm screw type supercharger is more efficient. I thought is was just another positive displacement design. I can see that it wouldn't generate the pulses that a conventional rootes type blower does, but I can't imagine it being more efficient than a turbocharger.

Harry,
Somewhere in my piles of papers/mags I have got an article in them. If I can dig it out I'll fax it to you.
I believe that they work on a rotary srew principle but I can't really help you more.
Dodgeman can you help here?....
Dave

Quote:

RX7 turbo built in our backyard and featuring large external wastegate, to show you how uncomplicated they actually are.

Another crude method of boost control that has been used is a valve in the inlet that opens and and vents the excess boost to atmosphere....not the most efficient thing ever!!