rx7tt95

I ran into a second gen at a Florida rotary meet that's running a single turbo setup with what looks to be a fairly large compressor with a REALLY SMALL hot side, maybe the size of a large orange.

I "heard" a few things while at the meet but I didn't get a chance to confirm or get more information. I heard it was Chrispeed's old turbo (so perhaps he can chime in), that the car is making over 500RWHP but the turbo is "good for much, much more. (I'm assuming this is on race gas).

My question is, how can a turbo with such a small a/r produce big numbers like that? Sounded like it spooled about 4.5K rpm. Rotaries hate backpressure so how's this thing making power?

Here's a link to pics. It's the orange car about halfway down...The hp numbers seem to be legit according to a few people I trust. Just leaves my head scratching (and thinking I should be letting Chris look at my car!)

http://www.nopistons.com/forums/inde...howtopic=34414

Would post the pics here, but it takes me 1/2 hour per pic on this forum :-)

carx7

iTrader: (31)

What size W/G's is he running? Open air or not? what size DP?

allrotor93

iTrader: (126)

what are the specs on his turbo?

Zero R

iTrader: (1)

Big numbers on smaller housings are possible usually at the expense of higher egt's, needing to run higher boost and higher octane gas all of which wouldn't have to work as hard on a larger hotside the downside of the larger hotside is a more narrow powerband but in a drag car that isn't too bad.

-Sean

fdracer

you can still make a ton of power on a small hotside, it's just very inefficient. if you look at all the fwd drag cars, their turbine housings are very small relative to the compressor, cause they launch at a very low boost level and they need it to spool up as fast as possible after the initial launch.

carx7

iTrader: (31)

I belive the assumption of a smaller hotside limiting top end power assumes it's the only path out of the engine and is a choke.

What if the exhaust has a more free flowing path?

Small turbine housing for spool and then partially bypass the turbine housing for all out flow?

crispeed

What's exactly your idea of a large compressor and a small turbine housing?

fdracer

yeah it's called a wastegate

carx7

iTrader: (31)

Right.

So why not run a small turbine housing for spool and then a LARGE WG to dump the extra exhaust at higher RPM?

fdracer

it's all about the amount of boost you want to run. you still need to feed a certain amount of exhaust gas through the turbine housing to spin the turbo to make x amount of boost. the more boost you want to run the more exhaust has to go into the turbine. at higher boost pressures the sheer amount of exhaust going through a small housing is gonna cause some serious backpressure. you can't just vent this through the wg and call it a day. that's why matching the turbine (wheel and a/r) to the amount of boost and hp you're planning to make is so important. you don't want excessive lag going too big and you don't want to choke top end power going too small.

carx7

iTrader: (31)

Why not? If the turbine housing is a restriction the exhaust will take the path of least resistance and go out the WG. Therefore the turbine housing won't be a bottle neck.

Ranzo

What is being said here is true but I think the common sizes used on Rotaries are a little oversized. Using a larger A/R in the hot side can produce higher peak horsepower numbers in many applications but running a smaller A/R will result in a broader powerband which in turn will be a faster car in any situation besides an all out drag car.

Zero R

iTrader: (1)

Exactly

fdracer

again it's because of the amount of boost you want to run. you open the wg at 20 lbs. it will divert flow but then you can only boost 20 lbs. what if you want to run 30psi, or 40, or 50? then you'll have to feed 50 psi through your tiny turbine housing.

rx7tt95

Chris,
It looked as though I could completely cover the turbine housing with both my hands, cupping it like an apple. I'd need six hands to cover the turbine housing on a T78 which is a fairly popular "high" hp turbo.

I'm just curious as to what the benefits of running a turbo that small and the disadvantages as well. This used to be your turbo, correct? I've just never seen anything like it to be quite honest :-) Also, the IC piping on that setup was HUGE, along with the front IC core. It looked too big. But if it's drag only, I guess that isn't a problem. I never thought of say 2.75OD piping being a restriction. Looking for a bit of insight here...

Sean, maybe I should have done a .84 on the 40R :-)

IGY

I say the common sizes used on rotaries are a little undersized. Using a smaller A/R on the hotside can produce quicker spool up in many applications but running a larger A/R will result in a broader powerband which in turn will be a faster in any situation period.

Say I have a ported motor that makes power all the way to 9000rpm's and I put a turbo with a small housing that chokes the motor out at 7000rpm's but spools at 4000. Take the same motor and put the big housing that doesn't choke the motor but spools up at 5000rpm's. Who has the broader powerband.

It's not all about bigger or smaller it's about what will match your motor and the application.

Ranzo

What you say about matching the motor is true, however the average rotary will not rev to 9000RPM's reliably. High boost + High Rev= Blown engine. For Drag racing where you see those high revs for only seconds at a time this might work but for people who road race or on the street you cannot maintain that RPM.

Say I am driving in a stock geared RX-7 and my power band comes on at 5000RPM but at this RPM I am doing 125-130kmh.......way to fast for the 42R curve ahead of me. Downshifting would put the RPM's past 9000 and the power would come on way to fast to control realisticly. I would say that not only the broadness of the powerband but its placement into a useable area is more important.

There is alot to choosing a turbine to match your application as well as the right motor. There is also a lot to be said about a large housing and free exhaust. As turbines get better with ball bearings and faster response times the larger you can go with the A/R and still have low end power.

IGY

High boost + high rev does not = blown engine. High boost + high rev+ NO tuning = blown engine, but so does low boost and low revs. So say I run the bigger housing with less boost and make the same amount of power and I know that I can't be in 3rd gear at 5000 I have to be in 2nd at 5000. I still have a broader powerband, I am running lower boost and I make the same amount of power. Or I spent the $8000 on a transmission that is more suited to my power output.

rx7tt95

And rotaries hate backpressure..it kills rotaries...obviously something on the orange 2nd gen works and Chrispeed isn't saying a word about it...I mean this car is putting down mid-500's. There somehow must be a balance/rhyme/reason to what's happening between this turbo and motor. I'd like to know what it is!

Ranzo

Well I think the Rhyme and Reason is on the compressor side of the turbo. If you look at flow maps they have no connection with what exhaust housing you are running. For example a TD06 20g makes between 350-400 hp even on a SR-20 but the exhaust housing on a SR will be signifigantly smaller than one for a 13B. Rotaries don't have massive amounts of volume but the PRESSURE in the exhaust is high this allows rotaries to run larger housings on the exhaust side.

If you port the living **** out of your rotary you will need a larger housing to compensate for the added volume and Pressure. If you have normal or small ports you can run a smaller exhaust housing....the compressor will still flow the same amount of air at given boost.

Heath

iTrader: (11)

Interesting. I hope we find out more.

RICE RACING

Thats the way people do it, my "only" concern is that with excessive bypass thorugh the WG system exhaust manifold pressure increases, it does not decrease. Some engines are more sensative to this than others.

Basically if you have a large compressor like the one on that turbo for a given pressure ratio you may need around120bhp worth of energy to drive it (at a specific pressure ratio), if you put on a realy small rear housing and "waste" half the available exhaust exhaust flow, you have to make up this energy loss by doubling the manifold pressure (since you have half the flow of a larger AR rear housing). The available energy to drive the compressor is a function of heat and mass flow rate, mass flow rate being a function of pressure ratio and volumetric flow rate.

It is easy to see for this that you approach a point of diminishing return, the question is how much back pressure is too much and how much extra exhaust manifold pressure starts to effect turbo and engine reliability ? Best answered by your engine builder, I am sure crispeed does it for a reason (a good one at that too). that would definatley be running a Q trim exhaust wheel (or equiv) as on anything smaller that housing would not work at that power level on a 13B. Remember with the bigger turbine wheel you can run a smaller AR (to a point) as if you try to stuff too big a wheel in a small housing you can get into the scroll area and change the aerodynamics so badly that it will actually flow less not more

Roro

Thanks Peter. That is the "scientific" explanation that I was looking for. I was trying to think through the energy equations last night and what applied etc before I replied.

So it does work... just a function of how far is too far with the point of diminishing returns etc. It would be hard to find exactly where this point is without a great deal of time and or money in experimentation.

That was the theory that I tried to play with on my system, but maybe not to the extent that crispeed has done. I'm running a.84 divided turbine housing with a GTQ wheel. 66.7mm GT compressor It makes 15psi by 3500 and 19psi by ~3900 in second gear and pulls all the way to my chosen 8kRPM shift. This combo was good for 470RWHP and I have a few MAJOR flow issues that I found. I'm confident once those are fixed I'll be over 500RWHP. Oh and this is with a 10.8AFR across the board. Considering this is my first ever turbo setup and I choose the parts and designed the manifold, I"m pleased with the outcome.

Anyrate, this is a great thread!

-Chris

Roro

^ Just realized I wasn't signed in above

-Chris
CARX7

93 R1

Do you care to elaborate on the MAJOR flow issues any?