I tried searching for hybrid turbo's but couldn't find anything about real hybrid turbo's on the forum. What I mean by hybrid turbo's aren't the turbo's that most people call hybrids where you have different compressor wheels and such. I'm looking for the turbo's that have a smaller volume at low boost and then have some internal mechanism that grows to be able to push more air. I remember reading about it online a few months ago but now I can't find the exact info.

Does anyone know what I'm talking about and if its been done in an RX-7 before? I'm basically looking for a big single turbo that can spool quick even at low RPM for street and track use. I currently have BNR Stage 3's at 17 PSi and like how I can get full 17 PSi boost at 2,800-3,000 RPM and anything above that is almost instant full boost.

Budget isn't an issue, performance is.

thewird

 

I believe what you're looking for is called a twin scroll turbo. S4 Second gen's had them and they quickly learned that they weren't well suited for rotaries.

New porsches use the variable geometry turbos (twin scroll v2.0) which are made by Borg Warner.

 

my dd 2004 ford 6.0 diesel uses this type of turbo, they seem to work good when they work, but biggest pos built. imo yea i've replaced a couple in past 2 years

 

So theres nothing on the market that would work with our cars and still give 450-500 rwhp on a street port with quick spool, even at low RPM?

What would be the alternatives? I liked the capability for power of the HKS T51R but didn't want the lag that came with it.

thewird

 

It's an interesting concept. If money is truly no object, anything is possible. I'm interested to see what kind of ideas come of this thread.

It'd be very expensive and intricate, but what if you tried using the Audi bi-turbo style setup, or a Porsche Borg Warner variable geometry turbo?

 

I believe Variable Geometry Turbo was what I was looking for. Has this been done before by any of the RX-7 community or do I have to start from scratch so to speak?

thewird

 

AKA VGT turbo,

http://en.wikipedia.org/wiki/Variabl...y_turbocharger

no go on an rx7 with current tech as they won't tolerate the exhaust heat created by a rotary engine or for that matter a most gasoline performance engines

 

Yes, this was exactly what I was reading about before...



I did now find some posts now that I had the correct name...

https://www.rx7club.com/single-turbo-rx-7s-23/vtg-turbos-new-911-a-526846/page3/#post5657335

Good for 1050*C (1922*F), that is enough for a rotary I would think.

And then I found this...



Garrett VGT GT45R 102 (mm major compressor). Turbine materials rated to 1050C. Would this be enough for my power needs? Has there been any improvements since then since we are in 2008 (almost 2009)?

thewird

 

If it where controlled properly I think it would work, but if not, if the vanes where too restrictive at the wrong time EGT will go over 2K.

Its something that could possible work, but there are issues to work out:

VGT's are usually controlled by vacuum instead of boost, stock vac chamber might work for this, but you might need an elec vac pump to make the vac source consistent. A stepper motor controlled VGT (which I believe is available) would be 1K times better and far more expensive.

Next a typical boost controller isn't going to work, you need a fully mapped system, which leads to to the hard problem

Tuning, is going to be a big big problem. All the ECU's on the market use boost as the load sensing method, well the problem is if you change the AR in the turbo you change the VE of the engine, to illustrate (made up example) turbo setting 1, 4k rpm 20PSI= 2000 CFM, turbo setting 2 20PSI maybe 3000 CFM.

Now is the ECU has no way of sensing where the turbo is actually at (and it doesn't) this is an issue.

I wouldn't even think of doing this without a MAF or similar sensor of some kind as the load sensing method.

If you really want to do this, call motec, and call a good reputable turbo shop.

 

The variable vein turbine housing has been around for years. The ones that work optimize response and max HP output, but being that gasoline engines burn and leave deposites, the veins seize up and no longer work. Back in the early 80's, there were a couple vehicles this was tried on, a Saab and some detroit diesels. The diesels, didn't have much of a problem with the veins seizing b/c the exhaust gasses not being as hot as gasoline engines. The saabs, they would seize within a short period of time. Either they would seize wide open and be laggy or seize closed and spool fast and choke. Actually there were no wastegates, the boost was controlled by the size of the turbine housing and the opening and the closing of that vein.

Porsche revived this idea. I would think that the variable veins would work with a different fuel such as Ethenol. Since ethenol doesn't leave deposites, it could possiby work with a high success rate.

Just my opinion .

Bryan@BNR
205 640 1193

 

I had a VNT turbo on my shelby CSX-VNT. It was an 80's turbo dodge and one of the first production cars to use one. I think turbo diesels use them now.

 

As far as I know only porsche turbo (997) and the Mitsubishi Lancer Evo 9 are the only non Diesel VTG - turbo cars!

But they are rather small turbos so you don´t have any large gain (in max hp) by using them. + You have to manage the controlling of these by yourself.

VTG-turbos are intelligent components. So you have to send them a PWM signal which position they should reach. And so, you have to develop a box which is like a boost controller. And this box has to send the PWM to the actuator of the VTG - turbo.

My Collegue sitting behind me at work is making the controllers Software for our Diesel Engines @ Merc. But it´s all in the ECU so no extra box which you could use. I think you have to built it for yourself!...

but the Idea is nice.

Much boost and power. No lag. No rats nest!! ...

really a nice to have.
Maybe we can find some VTG- turbo models working with the PWM signal input and which reach our requirements...

If there is huge interest in such a thing maybe a collegue and I could develop such a boost controller-box and try it at my Rx...
But I´m honest. I don´t think that it would be cheap. I think to expensive for a nice to have feature.

The ones who are NEEDING the boost (real track use) don´t care about the lag because they run high rpms ... and when you drive on the street.. why would you need 500hp? ^^ . OK wrong question

 

Yah, it looks like it would be too difficult to do since there aren't components you can just buy to make it work. I think I'm gonna settle on the A-Spec 500R turbo.

thewird

 

The veins on my truck were controlled by oil pressure. My first impression is that it would be a waste on a rotary engine application. On the right application, it works wonders. But keep in mind many diesel racers toss the perfectly working variable geometry units and replacing them with huge regular turbos. The top horsepower potential of the factory units is limited. I was unable to exceed 500 wheel horspepower through tuning, with boost being the limiting factor (the turbo didn't like anything over 30 psi). But the low rpm response was insane. We're talking about 900 foot pounds of torque at exactly 1600 rpm. It would light the 285 wide tires clean up from a roll in 4th gear.

PORSCHE VGT TURBOS

 

Diesel engines don't have the tuning issues with a VGT turbo that Gas engines have, they run essentially as much air as possible into the the engine (oversimplified) and control power output by how much fuel is injected.

The mixture ratio changes with power output and is always very very lean with anything approaching stoic blowing black smoke.

Obviously tuning a turbo which varies the engine VE dramatically across something other than boost and rpm will not be as easy on a rotary engine.

 

Temps have also been a limiting factor in developing turbos like this. Generally speaking the rotary is hotter than the gasoline piston engines that the VVT has just been recently mated to. The materials are steadily improving but it would be a while before you find something that can work reliably in rotary exhaust temps.

But it's not a silly idea - for the ultimate stock RX-7 the problem was reliability and low end torque. The sequential system worked well for that when it worked, but it wasn't perfect. If Mazda did it all again today it would be different. Gordon believes that an NA 3 rotor is the ideal platform. But I wonder if you combined VVT with a technology that burned cooler and cleaner (ethanol? meth injection?) it would be a really interesting option. Definitely worth a few design prototypes.

Dave

 

Acura RDX has a variable turbo but instead of vanes it uses an internal flow valve...

 

look up the shelby csx-vnt from the 80's.

 

Agreed, good posts. The turbo on my truck hated to "overboost" and the shaft was sensitive. Besides being somewhat of a biiiiatch to get to on the D-max, you really had to know how to take them apart and reassemble them right or else it would never be the same. So I wouldn't doubt just the heat of the rotary could tear it up. I also believe in simple turbos, if the ball bearing/vgt stuff can't take the heat, then I don't need it, heat is good. I am sure engine porting and "special" tuning could work wonders for a big snail.....