Koma

iTrader: (3)

I think I may have just started a war...

THE_CEO

ha trust me this is nothing new for us here...

CEO

qtb33

Well I pick Turbo, but just for efficiency reasons.

If you are comparing an S/C to a Turbo you have several options to consider.

1. If you go S/C over a turbo then you need to think about this. Do you want a roots style or a centripetal style? This brings in the point, what type do you want?

2. A roots style is commonly referred to as a blower. This type is considered under driven and is creating max boost (@ WOT) at all RPM’s for all intents and purposes. This will yield a power band very similar to a NA version of the motor provided you keep a similar state of tune respective to the level of boost you create.

3. However, if you go with a centripetal type, you will keep increasing boost with RPM, or a linear boost increase. So depending on your redline and Volumetric Efficiencies (VE) at higher RPM’s you’ll generally see a “very peaky” power band.

4. Turbo, this can get very tricky and goes from blower type performance all the way to the centripetal style S/C. The great thing is you pick. If you want a fast spooling turbo with a lot of low end punch, you choose a smaller appropriate turbine to get the job done. If you want a high RPM monster, then you step up the A/R and trim to something that will flow the high mass rates and maintains efficiency. However this is only half the story, you need to also match a compressor. This is based on two things, pressure ratio and you overall mass flow rate, which is also based on your level of boost. So if you keep you stock redline and want 10psi. You pick accordingly, but if you want 10psi and 8500rpm redline, you need to find a turbo that will keep that 10psi pressure ratio but flow a much higher mass flow rate.

I’m sorry but the old argument of “turbo lag” is dead. (As an example an Audi S4 with stock KO3 turbos will fully spool by 1900rpms and that is also where peak torque is, and it still revs to 6400rpms and is making power) With today technology and excellent management systems, a turbo can spool as low as you want, and create any power band you desire. There are so many options for sizing a turbo, it’s not what do you want, it’s can you afford it and figure out which components to buy.

So pick your poison, I choose a turbo b/c it produces cooler air at the same boost than a S/C.

Koma

iTrader: (3)

Honestly I don't know too much about either. I know the basics.
Superchargers use belt driven power for a roots and centripital is basically a turbo using intake to spool rather than exhaust. I don't know too much about twin screw setups.
Turbos are spooled by exhaust.
It'd be great if we could coincide these two ideas and create monster of a forced induction car.
I don't know physics too much so I don't know if this is physically possible.
But if it was I'd lay down the money for it.

qtb33

It has been done but with limited success. The system is called “Twin Charging”. It is however common on 2-cycle diesels. It typically has a turbo that compresses air into the roots style S/C. However, I think one reason it isn’t commonly used is because of the enormous amount of energy that is required to compress already compressed air.

A car that it has been done on with some success was the NXS, I think. I don’t remember too much about it though. So I may even be wrong about which car it was.

FYI, all super chargers are belt driven not just roots type. The difference isn’t in the driving power but rather in the method of compression. A roots type uses lobes that interlock to compress the air while a centripetal is the same as a turbo in using a radial flowing compressor housing.

morris

iTrader: (2)

centrifugal not centripetal.

morris

iTrader: (2)

"So pick your poison, I choose a turbo b/c it produces cooler air at the same boost than a S/C.
"


THis also is not neccessarilly so, turbos are much hotter than Centrifugal superchargers, that is why you are always concerned about cooking the turbo and have to add devices that keep the car running after the car ignition is turned off (ie turbo timer) . Turbos have this problem becuase the hot exhaust gas heats up the turbo and heats the incoming air before it is even compressed. The compression then heats is even more. Since SC's are not using the exhaust gas they do not have this added heat.

qtb33

Yeah sorry my bad.

This is true. Although, if you give a turbocharged car two minutes of normal driving before you shut it off, coking isn't typically a problem. That is all assuming it isn't a water cooler turbo, in which case, you can rail the crap out of it and the turn the car off and it still generally isn't a problem. For general daily driving heat isn't an issue, if your abusive, then you give is the last few blocks onthe way home to cool down and your set.

As an example, my old turbo would drop from a peak temp of 1400* F to an operating temp of about 600*F in less than 2 min. At higher speeds even faster.

In regards to the heating of the compressor housing from the turbine, or this non-adiabatic process, is mostly eliminated by using a turbine blanket or heat wrap. Even taken that into consideration, the power lose from the absorbing heat is still less that the power requirements that is needed to drive the gear box of a centrifugal style S/C.

So yes your correct but there are steps that can be taken to offset the negative results.

Koma

iTrader: (3)

Oh god I'm torn between forced induction worlds!

morris

iTrader: (2)

So explain this for me this how does the blanket stop the transfer of heat from the exhaus to the turbo? I meain the impeller that is in the exhaust stream is metal and the side that sticks through into the fresh aire to do the compression is metal in fact it is the same shaft right? Also isnt both sides of the turbo metal and bolted together so wouldnt the heat transfer pretty easily?

morris

iTrader: (2)

Oh and dont get me wrong, I dont have anything against turbos, I'm just trying to expand my mind.

qtb33

I hear ya! Both S/C and turbos have their place, I think thats what this thread is meant to be about.

Every case can be so different its truely a personal choice and we all have different reasons.

qtb33

Yes you will get some heat transfer through the shaft. But keep things in perspective, the shaft is only a few millimeters thick and has cooler oil flowing over it acting as a heat sink, absorbing much of the heat, not all but a lot. I wish I could give some numbers but I would be talking out my butt.

Any heat via the center section is also cooled by the oil and/or water as well as the surrounding air. My old turbo actually had some cooling vanes similar to an air cooled engine. Also the area of contact isn’t all the large. Not perfect insulation but effective enough to limit things to resonable.

I do see your point, and it is relevant. Let’s not forget the gear box from a S/C also heats up with use, to metal surfaces constantly rubbing creates a significant heat source as well.

The biggest downfall to a turbo is that it creates a huge back pressure between the head and the turbine. I have never personally measured any numbers but in Garrett’s catalog they used a number in a sample calculation along the lines of 20 psig in the turbo manifold. I’m sure they used a number that is practical. I have talked to a mechanic who is also a personal friend and is now an instructor at a very reputable post secondary school. He made the comment about a NA engine probably wouldn’t even run with that much backpressure. Keep in mind this only occurs at WOT full boost at your peak VE.

morris

iTrader: (2)

Cool thanks for the explantion, I thought a good bit of heat went throught the housing. I guess the heat from the gears inteh SC could be an issue but I bet it is no whwere near the temps of that exhaust gas hitting the SC

qtb33

Yeah more than likely your right.