With hundreds of pages on the topic it appears that superchargers are capable of producing a lot of hot air. The axial flow system looks pretty nice, but so far it's just on paper. The Petit twin screw looks well thought out, but it's not quite ready - although should be very close.

Now comes the big question; Which one really works best?

Looking at the fan systems like the average turbo or even a Paxton "turbo on a pulley" they tend to have a limited "efficiency zone" for pumping. Barely wide enough for the powerband of a typical up&down motor no where near wide enough for the mile wide powerband of the Renesis.

Granted those are centrifigal systems where the Axial Flow is, well, axial. Fan driven just the same, and like a turbine found in your local 737-500, even a Beech 1900, they too have fairly narrow band.

On the other hand positive displacement types like twin screws have a much wider band.

So which one(s) will give decent boost at both 3000 RPM and 9000 RPM?

Any answers from the experts on this?

 

I'm no expert but want to discuss this too .

I'm chaffing at the bit to buy one at the moment & the only ones on the market are the Blitz (roots) & Rotormaster (centrifugal) . The roots is not a good option for high up the rev range , however the centrifugal does shine at higher rpm which I think better preserves the rotary characteristics .
I really want the AFSC because I think it would be the best compromise but don't think I can wait much longer .

I like the way the twin screw gives good low down power but I think it becomes more inefficient the higher you rev it .

 

IMO whatever produces the most power down low but still allows power increases to 9k is perfect. I am hoping that the AFSC provides this. I am anxious as hell for SOMETHING.

 

The other thing to consider (i think) is that one of the best things about the stock motor is that it can operate in the 6-9000 rpm band all day & beg for more . Any SC that does not like this sort of punishment (or is not efficient at these revs) is going to detract from what the rotary is all about .

guys who have the greddy turbo which runs out of puff around 7k may like to add how they feel about that .......

 

Answer: Dynamic range.

Question: What does a turbo and a rotary motor have in common that a supercharger and a rotary motor do not?

 

i was waiting for somone to say something like that +1 for MM

have to admit though, that i am holding out for AFSC myself still

 

yeah - I knew you would say something like that . I must admit I have got an anti turbo thing that relates back to a few yrs ago when they used to blow up left right & centre .

I also see a turbo being more difficult to tune & therefore a higher risk option for a high compression engine that came out as N/A .

 

Dynamic range?

Not quite. Turbos have a very narrow pump efficiency band - about 15% of the operating range. Granted pop-off valves can double that range, but barely enough to make a turbo acceptable on a piston motor. A decent gas piston motor with variable valve timing has about 25% of range. A diesel has about the same, mostly because it's low RPM. The Renesis has about 45%.

However the question was based on superchargers, not turbos. Turbos are good things, I really like the turbo on my CX7. Yes it pumps a bit down low and up high but never really as well as in the middle, probably why Mazda had to use 6 gears to keep it within the 15% band for full power.

 

thats somewhat my reasoning for avoiding turbo as well

 

I must have missed it. You don't like power and efficiency?

Especially at higher boost, there is no comparison.



Just need to replace the turbo

 

+1

 

no, no, of course i do. its the more complicated installation, tuning that stears me away from turbo. turbo's bloody history is always in the back of my mind, but really its just installation and tuning.

anyway, i would like to see this thread not get sidetracked. interested in hearing certain ppl's responses

 

having driven the mazda 6 MPS - I know what you mean . Its kinda like ye-ha!!! for a few seconds then - **** is that all ya got ?

 

Shifting at 7K would take away some of the fun for me personally.

 

Then don't do it. You can still go to redline. The power drops off about 1,000 rpm earlier than stock ONLY because people are pushing the turbo to 10 or 12 psi... far beyond the 5 - 6 psi greddy designed the kit for.

Getting a higher flow turbo would also fix this situation.



You guys are also comparing theoretical superchargers with a specific turbo greddy supplies, then pointing out the low flow rate as a problem with "turbos" in general. That isn't fair.

The fact remains, with this turbo setup, I can get the FULL desired PSI at 2,700 - 3,000 rpm up to redline. RPM does not matter. And I can vary the pressure with the push of a button.

With a supercharger that matches peak output with the peak engine rpm, you will not get your max CFM till redline.

So I will get my power earlier, and I will not lose it every time I shift.


None of this takes away from the fact that the SC sounds great, and it will still kick *** in the RX-8 - but please don't diss the greddy for under performing when people are pushing it beyond what it was designed for.

 

Fair comments - don't disagree with any of that .
Now we have debated Turbo vs SC - lets get back onto the debate about which SC is best .

 

i think it would be a good idea to start this when a sc is being sold with a ecu in the usa..

a complete package... i am sure pettit will win the first, but then who are you going to compare it to????

this is a usa comment...

beers

 

hopefully accessport gets rolling, by rolling i mean, FINISHED!! and then we can compare it to AFSC

i also see no problem with starting this discussion now. sure it will be theoretical, but as of now, we have all the data and information to fairly closely determine what each of the two SHOULD be able to do under good tune

 

Something I really like about the AFSC is that it does not require an intercooler with all that extra piping . This really makes it an easy factory like 'bolt on' . That and the fact that it should outperform a centrifugal down low .

 

^ +1 for that. i personally like the idea that boost increases with rpm as well. as nice as full on instant boost is, for a daily driver i prefer boost to start out low. but i dont like lag obviously. AFSC shouldnt lag, and should come on smooth

 

Turbos and SCs are equal in their tuning demands. The "N/A" characteristics need to be accounted for equally in either configuration.
Get past the idea the somehow either is "magic" in any respect as far as temps, pressures or flow.

I think the big issue is that no one seems to set realistic goals for their projects and expects one system or another to deliver dyno-queen type power with OEM-like, razor-sharp drivability (not to mention high fuel economy, low maintenance, easy installation and upgrade path, low cost and retention of factory warranty).
Even the relatively "modest" expectations I hear around this place are really too broad to be used to define an actual system.

I won't contribute any more to the "SC vs TC" debate - its been pounded to death and no one on the "wrong" side is listening, anyway.

 

I woke up this morning and realized that what I said doesn't make any sense. The greddy turbo should run out of steam at 7k rpm at 6 psi too.

 

turbo'd 8's are MUCH faster than S/C 8's.

The reason? You can actually buy turbo kits for the car. I shake my head in disbelief every time I think about those waiting for one of those S/C kits that still haven't made it to market yet.

So as far as which S/C is best, it really doesn't matter. Just choose one you like and imagine it in your car.

 

I hear it all the time. "A turbo is the worst thing you can do to this car." Why do people say that? Why does that not hold true for a supercharger as well?

 

Haa yes! What a deal nada for a complete imagined S/C....

here's mine w/under the curve, low and high end power....