Racer X-8

Two days later, Hymee discovered that his new intake runner adaptors also adapted very well indeed to the distillation of rum!!! Since then, every time he tries to get some work done on this project, he winds up stinkin' schnockered before he gets anywhere with it.

Hymee

iTrader: (1)

Not for us. We chose to use the best in the world so we wouldn't have such issues.

Hopefully get to try some more things mentioned earlier real soon now. Heck, sco is even getting me to commit to a ready to ship date.

Cheers,
Hymee.

PS - I was reading an article about a 13B with a twin screw Opcon Autorotor putting out 343kW (460HP) at the wheels with 1.65 bar (24 PSI) boost. Its in a drag car (Datsun 1000) running 8.76 ET @ 149.75 MPH. A bigger blower than we are using, but scale it back a bit, and it gives me some confidence with the appropriate R&D we can get near what we felt we could initially achieve.

It is just getting the time to work on it between making a living and juggling everthing else.

Richard Paul

iTrader: (5)

See that, now he's buying it in jugs no less.

His tastes are simple, he only likes the best.

Hymee

iTrader: (1)

FFS - isn't it bedtime in your neck of the woods?

Rotarian_SC

Bedtime, it's only like 1:30 in California. It's like 4:30 here.

Richard Paul

iTrader: (5)

It's Terri, she's keeping me up again!!
She's always figiting with something, maybe you can talk to her?

Hymee

iTrader: (1)

Why not go read about someing silly I did youinsomniacoldbastard. https://www.rx8club.com/showthread.p...97#post1229797

Cheers,
Hymee.

wileecoyote

Guys I have a question about positive displacement s/c. in general I understand how the diff. types of s/c work in general and their boost behavior. What I don't get is how a positive displacement can provide full boost (let's say 10 psi,) at 2k rpms, and still supply it at 9k? With a centri s/c boost builds as rpms build because it's run off a pulley on the motor, faster the motor goes the more boost. Positive diplacement is run off a pulley too, so if you're getting 10psi at 2k, I would think that by spinning the s/c faster as rpms rise you would be getting more than the 10 psi at max revs? It's not like the s/c has a constant velocity trans to keep it spinning at a certain rate regardless of engine speed. I never really thought about it before but on my drive in this morning I was thinking about it and had no idea how it worked!

Thanks for your help, I really like this forum, some smart people here. As a matter of fact these superchargers have changed my opinion on whether to get an Rx8, I'm seriously considering it now that the low\midrange torque deficiency has been solved!

rkostolni

A positive displacement SC pumps a set amount of air with each revolution. You match the SC pumping capability to the engines needs. At 1krpms the engine needs some amount of CFM say 100CFM. At 9krpms the engine needs roughly 9X that amount or 900cfm. That is not exactly right because I haven't accounted for the change in SC or engine VE with rpm, but it gives you the idea. So the SC will be spinning faster at higher rpms and the pumping amount is synchronized to the engines requirements.

crimson-rain

I'm kind of dumb on this subject. But I am learning a lot from everyone here (heck, this forum "helped" me buy my 8. DAMN YOU GUYS!!!).

From what you just said, if the the SC is spinning faster at higher rpms and the pumping amount is synchonized to the engine, wouldn't that cause a surge or backup of air?

rkostolni

The engine requires more air at higher rpms. The formula for the amount of CFM an engine requires is

CFM = RPM X CID/(1728*2)*VE
- But you can treat VE as 1 for illustrative purposes. CID is the engine size in cubic inches.

The engine is just an air pump. A positive displacement SC is just an air pump. The SC just needs to output more air than the engine requires. This additional air is called boost.

With a Roots SC, since the compression is performed outside of the SC, it can cause the air to backup into the SC. That is one of the reasons a Roots SC is so inefficient. With a twin screw, the compression is performed inside of the SC so the air is forced out and into the engine.

crimson-rain

Good analogy.

Since the twin screw compresses inside the SC doesn't that heat up SC dramatically?

SoFL_RX8

Why is this such a hard application for the renesis? My good friend has a whipple TS on his 98 E36 M3. Another guy I know has one on his 99 SVT Cobra. Im not an idiot, I know a rotary is a different monster than an inline six or a V8, but what major setbacks are you having? Is it tuning or just physical application?

wileecoyote

Ok, that makes complete sense, thanks for the explanation. I'm not sure what I was thinking, now that you said it I can't beleive I didn't realize that earlier.

The twin screws do have a demonic sound, my turbo eclipse is rather fast, but a plainish looking Mustang GT dissuaded me from racing after he gunned it from a roll and I realized it wasn't so stock. That sound is crazy, like running a manual car in reverse at about 60 mph! It goes well with the rotary exhaust note, I'm getting really excited about this project, might even cancel my plans on a new M3. Similar performance, better looks for a lot less!

rotarygod

Just remember that a positive displacement supercharger pumps a fixed amount of air per revolution into the engine. Theoretically of course. that changes a bit with rpm as three is some leakage at low rpm's. This means that if the blower is turning twice as fast as the engine at 3000 rpm, it is still turning twice as fast as the engine at 8000 rpm. It's the same amount of air into the engine. If you have enough air at 3000 rpm to get 8 psi of boost, since the relationship is a fixed amount, you should have 8 psi at 8000 rpm also. It always maintains the same ratio of air to the engine unless efficiency gets in the way.

A centrifugal is too small to be able to provide a constant amount of air. You run into other issues with a centrifugal such as surge which is when you try to get more air out of it than it can provide. A centrifugal is geared in a way that it rises at a set ratio with the rpm. Typically boost rises with the square (or similar curve) of the rpm rise. This means that it isn't even linear and most of your boost will come on the extreme top end.

A roots blower is just that, a blower. It does no compression. Think of the air in the intake manifold as a punching bag and the rotors of the roots are the boxer. It just beats the air into the manifold to hold it in there. This is alot of work and causes the air to heat up much worse. A small amount of this hot air gets carried back around back to the intake side due to leakage past the rotors. It's a big fight to keep power up and heat low. These are the least efficient types of superchargers.

The twin screw are also positive displacement as they always rotate at a fixed ratipo to the engine but their rotor design is different in that it compresses the air internally before letting it into the manifold. The roots was a boxer beating the air into the manifold but the twin screw is more like a vice or a press. It squeezes tha air down into a smaller package and then happily sneds it on it's way to the engine. It makes more air fit into a smaller space. The roots is trying to bully it into ths space. Image 2 women from the Renaissance era trying to squeeze into a small dress. One uses a girdle. The other just tries to squeeze her more than ample figure into a less than ample dress. Which one takes more effort? Strange analogy but accurate.

Compression makes heat but internal compression makes less heat than beating it into submission. That's where the efficency gain is. A centrifugal also has an internal compression ratio.

I don't think there's an issue with using a supercharger on a rotary. I think the big reason we don't see them is because so much of the rotary community is just against the idea as a whole. People just do what they know and turbos and rotaries are a very well known thing.

Spin9k

Thanks RG, I love a good analogy... you make understanding the dif types of SCing plain and simple.

crimson-rain

thanks RG. What part of Houston are you in? I went to school at Prairie View, right outside of Hempstead. Right down 290.

KrisD1

Maybe a dumb question, but for calculations, which size do you use for the engine ,1,3l or 2.6l?

rkostolni

2.6l

Hymee

iTrader: (1)

The 13B consumes 1.3L of air per revolution. So yes, using a traditional 4-stroke "equation" you would treat it as a 2.6L engine.

Hymee

iTrader: (1)

I just look at it like this, for simplicity...

Engine displaces 1.3L of air per revolution.

Assume the blower displaces 1.3L of air per revolution.

Assume the blower is driven at 2 x the engine speed.

That means each revolution of the engine, it is "taking" 1.3L of air, and the blower is pumping 2.6L of air. That is a pressure ratio of 2.0, which in simplistic terms is 2.0bar (absolute) or 1.0 bar of boost = 14.7 PSI boost. (The engine took one atmosphere naturally, and the compressor pumped in another atmosphere's worth. 1atm = ~1bar = ~14.7PSI).

The mass flow of the Autorotor compressors if very linear with RPM. The graphs I have been supplied with from the manufacturer show a perfectly constant slope. I.e. Say the mass flow was 10g/s at 1000 RPM, it is 20g/s at 2000RPM, 50g/s at 5000RPM, 100g/s at 10000 RPM etc. (numbers made up to illustrate the point).

Hence once the pulley arrangment is decided upon (speed of blower relative the the engine), the RPM is irrelevant to boost.

Remember, all this is at no restriction, i.e. Wide Open Throttle (WOT). In practice we must use a throttle. We also have the bypass valve, which is controlled by manifold vacuum. This effectively means one can control the boost by the use of the throttle. Put your foot down = full boost, instantly.

One of the big things that needs to be managed carefully in this application is excessive combustion pressures at low RPM. That is where the experience of the tuner, and the capabilities of the engine management system come into play. Thanks Grandad.

Cheers,
Hymee.

PS - Turns out to be a rather long post, considering I started out with "for simplicity".

KrisD1

So, when are we going to see the first available kit( for the manual 4port)?
I can't hardly wait.

KrisD1

Thanks for the explenation.
Ps, I am already putting some money away for your kit.

Spin9k

Hymee would I be correct in calculating like this for max airflow: (effective) 2.6L air /rev x 9000 rpm = 23400 L air /28.316 L/cu ft = 826 cu ft air at max rpm

rkostolni

No. At 9k rpms and assuming a VE of 100%, the engine will flow 413 cfm. You then need to account for actual VE, account for boost level, and account for temp increase due to boost.

At 8psi, 100% VE at 70degrees ambient temp, 75% compressor efficiency, the compressor will flow 555cfm.