Stumbled upon this site, found a few things that _may_ be of interest to a few.

http://www.superchargersonline.com/t...asp?RecordID=6
http://www.superchargersonline.com/content.asp?id=31
http://www.superchargersonline.com/content.asp?ID=76
http://www.superchargersonline.com/content.asp?ID=85
http://www.superchargersonline.com/content.asp?ID=80

http://en.wikipedia.org/wiki/Axial-flow_compressor

Andrew

 

That's a good find! Fills ina few gaps on the variance of the options.
Top Post

 

yeah its a little biased towards specific types of blowers but good none the less.

Andrew

 

I liked how they pointed out the centrifugal supercharger produces the lowest heat of all designs thanks to its more recent development. Good choice for RotorMaster to use one of those....

On another note, it did help me understand the Lyshom compressor in my 800SP a bit.

Good article

 

If you have to have one ...my pick by far would be the centrifugal... Like the article said the others were designed for trucks with buckets of torque.....RX8's have bugger all torque..( 211Nm ).

Rotormaster is on the right track

Smile heaps

Dave

 

Good one. A good read....

 

Yes, thanks heaps for the post. I don't know if I'm a supercharger expert, but I certainly understand them a hell of a lot better now.

And as it is one of the early things I would like to do, this has helped me understand them. It has probably been posted before, but does anyone know of anyone in Perth, or if not Perth, Australia that installs these things for RX8's?

My brother said Sprintex is good and being Perth based (I believe), that would be handy. But I'm not sure if they have done anything with RX8's. I will check it out anyway, and thanks again for the post.

 

Yes but its where they produce it. my understanding is that the centrifugals produce it at around 3-4k rpm, which is pointless for the 8 as thats where the torque starts to kick in anyway.

from what I know, I would find a rotary shop that has experience with the particuar engine manegement system you (or the SC kit) are going with, and has a good reputation. The fitting of the SC wouldnt be the biggest worry for me, its the configuration and tunning afterwards.

As an aside, I was talking to a guy here in Malaysia who has done a few motex/turbo installs on RX7's here who was talking about a configuration where a large turbo (I think a T-80 was mentioned) was rigged to the crank shaft instead of using exhaust gas. Is this even feasible?

Andrew

 

Andrew,

A large "turbo" driven by the crank is essentially a centrifugal supercharger. (Even a small one for that matter). It just doesn't have the turbine that is driven by the exhaust gasses. But the compressor part is effectively the same principle and design. It is just driven by a huge step up ratio to get the speed up. I sort of imagine like one of those old air-raid sirens you wound by hand. Lots of effort at slow speed to get the siren spinning fast enough

I really enjoyed reading the articles as well. It is interesting for me how reading those articles validated my passion for a positive displacement compressor.

Why did I choose the twin screw? Well it is the best of the positive displacements, and according to that article twin-screws have high efficiency - approaching that of a centrifugal compressor.

The article also mentions how high RPM's are required to get boost with a centifugal. The graph that has been posted for Rotormaster supercharger project does show that. The particular combo shows it makes 6 PSI boost at top end RPM. For every RPM lower than that, the boost drops, until it was only 1 PSI at 2 or 3K RPM. (Hard to tell on that graph, is it was in km/h. 1 PSI at 70 km/h up to 6PSI at 180 km/h, whatever gear it was in.)

So where does our RX-8 feel like it really needs a punch? To me, it lacks bottom end to start with. So what use is 1 or 2 PSI going to be there?

My current combo of the twin-screw currently makes 7PSI boost. No matter what RPM. So what I get to the bottom of the hill and I open up the throttle, I get 7 PSI boost on tap, right away. If I am high in the RPM, I get 7 PSI boost as well.

I could easily change this to by 12 or 14 PSI boost. It is all about choosing the right supercharger size and the gearing combo.

Centrifugals are renowned for being able to make high boost. Screws can make high boost as well. My mates boat shed has a twin-screw compressor for the air tools; she is running at 70 PSI or something, at some phenomenal flow rate as well.

The advantage with the centrifugals is that they are smaller for a given mass flow / boost. Yep - smaller, and easier to package. If I wanted to go the easy route and get something small and easy to jam in there, I might have gone for a centrifugal.

But my desire was to get low end grunt. For that, I needed boost-on-tap, and that is something only a positive displacement supercharger can deliver. (Or a larger capacity engine !!). And out of the positive displacements, the twin screw is king. Screws are even used as OEM - even Mazda have used them, athough the one in the Millenia was only a pretty small displacement.

It is good to have options. I just need a good sales pitch Boost on tap is a good start :D

Cheers,
Hymee.

 

not sure how you came up with this conclusion.. The articles only state that they are good for trucks because they boost comes in at low RPMS.. Trucks don't usually have very high revving engines.. there for no point putting a centrifugal in a truck.
Not sure about you, but it's not often that I drive in the 7000+ rpm... so I'd be reluctant to get with the RotorMaster Kit.. I really want more power from 2000rpm... That's where the twin screws come in handy... full boost on demand..

 

Amen to that!

Cheers,
Hymee.

 

Interesting, thanks for the explanation Hymee.

I doubt many owners would disagree that it is down low where we need the torque. I suppose people who want to race their cars might want the centrifugal S/C because the car will be spending more time in a higher rev range but for day to day driving it is some low down grunt that would come in real handy.

Look forward to comparing the two systems at the Nats.

 

Cheers Hymee, always a pleasure reading your comments. I have to say that I agree also that certifugal SC would be almost pointless for street purposes. My thoughts on the turbo thing were also that it was basically just a centrifugal SC.

So what was Sco's comment about? I totally missed the point.

Andrew

 

I think he was just being careful not to hijack the other thread. He has driven my car.

 

Ohh OK so Pettite is using the same SC as you?

Andrew

 

Sort of. There one looks about 1/2 the size of the one we are using Maybe not half, but I do know it is smaller, and I'm pretty sure I know what displacement/model it is. Autorotors have quite a range to choose from.

Yes, Pettit are using an Autorotor. I only found that out after I had begun my project, and had already chosen that brand. I'd even already selected the bigger size for this project, without any knowledge of their project.

And I am already experimenting with larger ones.

Cheers,
Hymee.

 

You have WAYYYYY to much money if you can just "Experiment" with SC's :D

Andrew

 

I tell you what - it's great fun reading all of these 'subtle' rejoinders on the merits of various SC and TC solutions.

I can't wait till thet Nats when you are all in the same room :D


I think both approaches have their merits, depending on the character of the car you want to end up with. I bought the RX-8 knowing it was a low-torque, high-revving screamer.

If I upped solely the low end grunt I'd have a V8 not a rotary (NOT saying this is what the twin screw is doing, all the way across the rev range sounds like a winner to me!).

Perception also has a lot to do with any sort of aftermarket mod. Having owned/driven SC and turbo cars before I'd have to say there is something to be said for the 'feel' of a centrifugal/turbo power curve.

Maybe I have been bitten by the turbo bug. Sc vs TC is a religious war, there'll never be a decisive outcome...

Unless....a 3 way drag at the Nationals....with independant drivers giving points on 'driving enjoyment'....

 

Who says it's his money?

 

let me count, a Hymee grill, shifter, sCANalyser.... :D

I strongly understand and taking in what Hymee said, but as Revolver has mention. If I want to track my car and it is a weekend baby traveller - having a big punch at city driving is less desirable - I don't want to shake my baby :p

 

I have to agree that low down torque is what this car lacks. If the twin screw can provide this as well as high end power....then I think we have a winner.

I have seen the the current form and it looks like it is well put together and I hope the finished product lives up to all the expectations

skc

 

Hey, im sorry but I am not an expert on superchargers, but I have been wondering if a supercharger can use a BOV? When you switch gears when having a SC on your car does it make that PSSST sound?

Just wondering...sorry.

 

No, thank goodness

As soon as the throttle is closed, the bypass valve opens and the pressure differential between the blowers output and the intake is balanced, plus the fact that the blower rotors slow down with the engine's RPM. Not like with a turbo where the compressor is still spinning fast making excess PSI's.

Cheers,
Hymee.

BTW - I am specifically referring to the positive displacement style of S/C, such as my twin-screw setup.

 

After jumping into Hymee's FI car on the weekend for a short spin, I must say that it has lots of grunt all the way through the rev band. (Spent most of Monday in a neck brace :D )

As he is still testing the different setups it could only get better.

I was a little disappointed though, as I had to drive mine home.

cheers,
rx88er

 

Now ain't that the rub.

As curious as I am, I reckon I'll wait until I can afford it and am ready to do it before taking a FI car for a spin.

Nothing worse than rooting a supermodel and then going home to the wife. :D :D :D