twincharging prototypes needing constructive critisism (SC14 + turbo)

[Nim]

You could spring load the valve to be half way open on a fairly spring. You might lose a little boost at times, but it'd make things come on much more smoothly.

[Rcubed]

only problem is the supercharger will hold the turbo valve closed completely while the turbo is not spooling enough to make pressure. seeing as a turbo is a non-positive displacement type fluid pump, blocking the outlet will mean the turbo will never make any pressure and so the valve wont open....ever.

Sorry, that's not true. a turbo is a rotordynamic pump. this means that it relies on the fluid dynamics to produce pressure and flow, and inherently also to dynamically "seal" the pressurising chamber.

the idea is that if you spin the turbo or any other such pumps [turbines, propellors etc] an inertia is imparted to the fluid. in the case of a turbo the fluid is flung outwards, like being on a merry-go-round. the fluid is directed by the casing to an outlet under pressure. the behaviour is such that the pressure builds as the square of the rotational velocity and the inverse of the velocity of flow. the energy put into the compressor comes out as potential energy in the form of pressure, kinetic energy in the form of flow and heat [wasted energy]. so if the flow is zero then all energy comes out as pressure and heat.

the idea described by NIM is similar to the one i saw in an RA60 about 15 years ago but had no BOV or intercooler. i understand the delivery was quite good throughout the rev range with a fairly seemless transition from SC to TC.

This will cause boost to build up behind that SC body, and the blow off valve would open, dumping the extra boost in front of the turbo, adding extra boost to the turbo.

a BOV operates by having a vacuum line - i think you mean have a wastegate arrangement after the SC.
It sounds nice, but having the supercharger run like this will just mean it will vent to the atmosphere wasting the SC effort as it is not a closed system. sure the turbo will suck it in but the turbo is already sucking more air than the SC can produce anyway.
your second idea is the one most talk about when the mention a bypass for the SC. Eventually a series mounted SC will bcome a restriction for the turbo so you have a bypass to allow the turbo more airflow.

the other thing is that the valve in front of the SC will never close and so the pressure difference on each side of the SC will be very high if the turbo was to make significant pressure. the problem with this is that greater the pressure difference either side of the SC, the greater the load the SC puts on the engine.

I think i understand what you said here? i think you mean that the higher the pressure difference is across the SC then there is more engine power drawn. again low pressure on the intake + energy from engine [pressure + velocity + heat] =>high pressure on the delivery side

[2JZR31]

Sorry, that's not true. a turbo is a rotordynamic pump. this means that it relies on the fluid dynamics to produce pressure and flow, and inherently also to dynamically "seal" the pressurising chamber.

If the SC makes 10psi that means the turbo will need to make more than 10psi at a ZERO flow rate before the valve opens. I think this will be past the surge limit on most turbos.

[RAd28]

i had the idea a while back similar to NIM's idea, where by the S/C and turbo work totally independent, except the supercharger is over driven so it's spinning 7000rpm to the engines 4000rpm (or something like that). and once the engine reaches 4000rpm (just prior to the turbo hitting max boost) then the supercharger shuts off via a clutch. this way, you could use a nasty sized ball bearing turbo for massive top end.
the turbo would spool up faster 'cause the engine is now pumping 10+ psi so the exhaust flow would be much higher.

my plans were to use this with an AFM on each inlet, basically just to destroy lag, but retain the stock turbos... but i've decided i'm just gonna re-design the stock setup.

[2JZR31]

The only way there is going to be seamless operation is if you switch the SC off when there is zero pressure drop across it, and if you keep the SC on all the time you have lost the best benefit that turbos have over superchargers, making the twin charge pointless.

If you have a turbo and a SC in parallel you have the worst of both worlds as far as top end power is concerned. Heaps of backpressure and heaps of lost power to drive the SC. It would make more power with just a SC.

[brett_celicacoupe]

interesting points you put foward(ta22-akjf-nnsrnal)

although i was thinking along the lines of a vacuum cleaner(in blow mode) whereby you can put your hand over the outlet and then start up the motor and no force is placed on your hand.

that said, a vacuum cleaner is not a turbo

[2JZR31]

a vacuum cleaner is not a turbo

But it is a centrifugal air pump just like a turbo. Check the compressor maps out for the turbo. See if it can make 10psi at a zero flowrate. I would be worried about surge.

[brett_celicacoupe]

i thought i was on the right track

[2JZR31]

Even it it does work it won't be very good IMO. See my explanations in my last few posts.

[Rcubed]

If the SC makes 10psi that means the turbo will need to make more than 10psi at a ZERO flow rate before the valve opens. I think this will be past the surge limit on most turbos.

mate,
of course ur right. I should have known this.
Still doesn't explain what i saw on the RA60 - which i admit i didn't drive. the guy was selling it... maybe this is why.

[Rcubed]

from another post i found this.
http://www.autozine.org/technical_sc...tm#Twincharger
http://www.vwvortex.com/artman/publi...ter_1496.shtml

[RAd28]

ha! nice, i'd like to see the boys from topgear test this one!

[2JZR31]

That VW uses basically the same technique as I was describing....

Very good link BTW

[mic*]

Brett,

Sorry to have disappeared. This seems to be coming along nicely. The last pic you posted and asked for criticism looks right on the money. But you will still need a BOV for the Turbo. I think nice and close to it - plumback of course.

IMHO, i think that the other pic using a JZ TB with two pressure switches WOULD still work; Going back to i think my first post in the thread, the biggest point of interest for me in the whole project is how the turbo & SC interact as Turbo starts blowing into the SC.

My reason is (and i hope i make sense) is this; Think of the SC as the engine for a moment, if the turbo can make boost against it, it means that it is fundamentally FLOWING more air than the charger can displace. The only alternative for air, other than to build ridiculous pressure at the SC inlet, is for it to travel the bypass. Although the air on the other side of the SC will theoretically being "double boosted" and be at an even higher pressure, my thoughts are any hickups with this will be negligable coz the unregulated FLOW of the turbo through the bypass will almost instantaneously balance then exceed the pressure difference, and start building your turbo boost.

Depending on turbo selection, I have honestly thought that the bypass may never need to shut completely at all...? Just a thought... It could be very bloody close to closed between say -5psi to +5psi. Under that, slightly more open, over that WO.

The SC in my opinion is really only here for good torque and response whilst under vac, since we can get boosted power with any decent turbo setup. Thats why im not the biggest fan of the clutch. It has its place, but generally i think the only advantage of SC over turbo is that it helps the engine breath better at lower manifold pressure and lower revs - ie response. If you clutch the charger off under these circumstances, why have it?

Clutch it off under high boost (if you can) to save a bit of heat and get an extra few kilowatts, coz lets face it when you drive WOT are you thinkin bout the squirt of extra fuel?

Nick, how many peaks on your SC belt? I go through belts like they're goin out of fashion too. My clutch was on the worn side, but it was slipping so much that i didnt even look into it further. Even without the spacing shim so it bolted hard up against the pulley (permanent drive) & the clutch! Oh and out of curiosity, where did you come across the working limitations / efficiency map's of the SC14?

[2JZR31]

I think the method I am describing is better. (VW thought so because that is pretty much what they did) Its easy to cool the air with one cooler. To make life easy you just run 2 air intakes. One for the turbo and one for the SC. This should be much easier than the alternative as it reduces piping. There should be plenty of room on the other side of the engine bay for the extra air filter. There is heaps of room there in my engine bay.

Therefore the only extra pipes over a normal turbo set up would be, the SC air intake, a pipe that goes from the SC to the turbo inlet. The BPTB inlet would just be T'ed between the SC and turbo and take the place of the normal turbo inlet. It would just be like normal but with the BPTB inline.

Take a look. At the modded VW pic. The only difference is the SC intake pipe, the SC outlet pipe and the BPTB. That = 2 extra pipes. These pipes can be small because they only have to flow enough fot the SC further making plumbing easier.