SlayerRX8

You're right - of the ones on the market, the centrifugal would be fastest. But still, two twin-screw kits are coming soon and should be in the same peak power range with more low-mid power.

shaunv74

Just for my education what's the difference between a twin screw, Roots type, and scroll supercharger (I'm comfortable with the centrifugal concept). I understand what a Roots type does but not sure the difference between it and the other types.

rotarygod

Since that will take a while to explain, it's easiest just to go look them up on the internet. Here's a good article explaining them but other sites can give you diagrams.

http://www.superchevy.com/technical/...0510sc_theory/

strokercharged95gt

I understand the pluses of a roots blower at low RPM, but since 90% of the race in an RX8 would be between 6-9k where centrififugal are most efficient. It would make sense to me to use the blower that is most efficient in the high RPM range. Im not a spokesman for centrifugal blowers, im just playing devils advocate for everyone that automatically gives the positive displacement blowers as being superior when they do so only by word of mouth on message boards. From my observations, other than funny car and top fuel dragsters cetrifugals rule in drag racing for the most part.

For the record, I would rather have turbo than any supercharger and will probably be going to a MP t-70 setup for my mustang in the next few months.

*edit: NM

Taken from Wikipedia-

"Roots blowers tend to be 40-50% efficient at high boost levels. Centrifugal Superchargers are 70-85% efficient. The Lysholm style blowers are nearly as efficient as their Centrifugal counterparts."

Good discussions though,

John

P.S.

that jump in the comptech dyno is all VTECH

rotarygod

Actually a positive displacement blower is perfect for a rotary precisely because they are good at low rpm's. The rotary or any other small engine needs help here the most. Who wants a peaky engine that only makes power and boost on the top end when daily driving isn't done here? A roots makes far more sense on the street. A centrifugal makes none at all for this use.

Boost pressure is irrelevant. Flow is all that counts so it isn't a fair comparsion to say that unit X makes more power than unit Y at a certain pressure. There are so many other systems out there that can be based on the same comparison such as turbo sizes. It's not really anything useful. Too many people concentrate on boost numbers only when all they really should care about is hitting their power goal. It's true that a roots is less efficient and heats the air up more. If it makes 300 hp (or any other positive displacement blower for that matter) and a centrifugal makes 300 hp, who cares what boost pressure it's at? The roots would walk all over it all day every day and laugh at it in the process.

A centrifugal is the single worse choice for forced induction that you can use with a small high revving street engine. Yes you'll go faster than stock but with all the other foreced induction options out there, why sacrifice so much for only top end gains? That makes no sense whatsoever.

strokercharged95gt

Id rather have it over the electric supercharger

BTW are there even any positive displacement blowers available yet, i stopped paying attention once the manufactureres posts became over 2 years old.

BaronVonBigmeat

Correct me if I'm wrong, but the Hymee and Petit s/c kits do not have roots blowers. They have screwchargers, ie Lysholm compressors. Uhh...right?

So if that's true, then you've got nearly the same efficiency up top with a lot more low and midrange power. And besides, a lot of people aren't basing their choice of FI based on all-out 6-9k racing conditions. Having a broad powerband may be more useful in everyday street driving.

Having said that, I think we can all agree that the turbonator is an incredible piece of engineering and has unique features no other s/c does. Can you unhook your centrifugal and take it fishing, and then use it to suck out water that spills in the boat? Yeah I didn't think so.

rotarygod

^lol

shaunv74

Thanks! Much clearer now.

r0tor

iTrader: (1)

Correct.


If you then want to compare the twin screw vs the axial flow, Richards real advantage is his compressors are even more efficient and therefore do not need an intercooler - while Pettit is going to need to use a water/air intercooler which adds some cost/complexity. So powerlevels will probably be pretty close with equal amounts of boost.

On the flip, I have a feeling boost drunk people will try to mod the superchargers by swapping pulleys. Richards compressor needs to be sized correctly to reach the efficiency thats desired for no intercooler and changing the pulley for more boost may harm the efficiency too much (just speculation at this point). Pettit's will probably have more leave-way with the water/air intercooler unit to bump boost (again more speculation)...

Brettus

iTrader: (2)

Surely it depends on what you want from the engine . If we all had wanted a torque monster we would have V8s or turbo 4s . The renesis is an engine that needs to be reved hard to get the most from it . the Cent SC retains that feature & seeing as I don't tow a trailer any more the lack of low down torque is not much of an issue for me .
If I'm reading this forum right - none of the other superchargers coming out will be over 300hp
And 350hp CF SC will blow away a < 300hp roots, twin screw or AFSC .

If you want the fastest accelerating Supercharged 8 the CF will be the way to go

rx8_nooby

from what i can gather it sounds like a twin crew desing would be the best for rx8, if only someone would make one

rotarygod

So you basically would be happy with a car that makes less power below about 3000 rpm than stock? The supercharger is a pretty good drag on the engine. Go try to turn a centrifugal by hand sometime. It makes an air conditioner compressor feel like a ball bearing. Since the centrifugal doesn't make any appreciable boost until a little ways up the powerband, it's nothing more than a drag on the system. It has to top a psi or 2 just to get you back to where you were without it. Depending on how much boost you want that may be 3000 rpm or 5000 rpm (since we have a very high rev limit. It's much lower on a low redlining engine) but you are slower below that. Which part of that sounds like a good idea for a street car? Forget it. Leave those pieces of junk alone. They belong on big engines that have low end power that don't need anymore down there. Remember we're talking about street use here. We aren't talking about fantasy dyno internet bench numbers racing that means nothing in the real world. That would be like the mega worthless horsepower Supra dyno's numbers.

rotarygod

Pettit does.

shaunv74

HaHa!! Love it! This is the quote of the day! This is going in my signature...

Brettus

iTrader: (2)

Well - no i wouldn't be happy with that - if it were noticeable . The reviews I've read suggest it is not noticeable . This is a good discussion though & it has clarified a few things for me .
As far a drag vs boost at low rpm - which SC will be the best ?

I imagine a roots or twin screw will have more drag than a CF ?

shaunv74

I suspect the one in your avatar.

rotarygod

Anything that makes more power is a drag on the system. The difference is that the roots or twin screw actually make more power than than they take to spin earlier in the rpm range. You won't feel it as a loss down low because you have more power than stock. A centrifugal is a drag on the system until it starts to make power. A positive displacement starts making more power right off idle. When it takes power to turn it but it isn't producing enough air yet to make more power, it's a loss. That's why they suck. Ultimately on the top end they do make more power. Average power across the rpm range is kind on the street. Peaky power is king for bragging rights. I'll gladly lose a few peak horsepower in order to gain far more average. At low to mid rpms a centrifugal will lose to a positive displacement every single time. It's a peak power device.

I'm not inclined to trust many reviews. If they don't think it's noticable on the low end, their perceptions are a little off. It's always a loss. It's hard to tell unless you go back and forth between 2 different cars but even then the perception will be skewed since they can step on the gas and start getting more power as rpms go up. A centrifugal needs to stay on large engines with fairly low rev limits. That's the only thing they make sense on. For street supercharging a twin screw is the perfect setup.

Brettus

iTrader: (2)

OK - just trying to get my head around the differences so I did a sample chart that is my best guess as to what the hp curve is going to look like for the different SCs around or about to come out.
The centrifugal is the stage 1 from RM - which does not have fuel or ign. upgrades.
Anywhere close ?
Edit : updated twinscrew to try & match Petit dyno (pg 51 on their thread)

rotarygod

Theoretically although that may be a bit of an exaggeration in spots.

r0tor

iTrader: (1)

my theory is if i'm paying $5k for forced induction, i want to get hit in the chest by gobs of power no matter where i am in the rpm band -shrug-

gh0st

from what i gathered at 7stock. the petite will put out more HP then the axial flow.

ЯX-8

Brettus

iTrader: (2)

Blitz ????
are you teasing us armxn ?

rotarygod

You couldn't pay me to use that thing but that's just my humble opinion.