Any rotrex mx5s lurking in here?

madjak · Postby **madjak** » Mon Mar 14, 2016 2:47 pm

I have plans on a 400+ whp Rotrex build. I plan on low mounting a C30-94 or C38 range rotrex on my existing engine. I would run a 4 port solenoid and a wastegate/BOV/recirc after the intercooler so that I could control boost by gear more so than control total boost... Really the Rotrex put the power up high which is where you want it so that you can protect the drivetrain somewhat. There isn't much point to limit it unless you're struggling with traction.

I'd move my half width Honda radiator over a bit more and run an RX7 v Mount intercooler next to it plummed straight into the throttle body. I'd probably move my oil cooler to the windscreen cowl or somewhere else. The only thing I'd need to do would be to replace my crazy cams with something with less overlap... say 274 degree cams. I'd still run 13:1 compression on E85 and my 3" exhaust.

My plan would be get power up high in the revs (6000-8500 range) and try and max torque to around 280 ft lbs limit of a 6 speed (ie 425whp @ 8000rpm) Given the installation of the Rotrex which would force me out of the stock classes, I'd lighten the chassis a lot, run 245 Hoosier slicks and run some more serious aero. All up I think a 800kg, aerod, 400+whp S/C MX5 would be pretty hard to beat...

When I get bored of my car as it is now, I'll hit the button and do it.

speed · Postby **speed** » Mon Mar 14, 2016 5:33 pm

madjak wrote:I have plans on a 400+ whp Rotrex build. I plan on low mounting a C30-94 or C38 range rotrex on my existing engine. I would run a 4 port solenoid and a wastegate/BOV/recirc after the intercooler so that I could control boost by gear more so than control total boost... Really the Rotrex put the power up high which is where you want it so that you can protect the drivetrain somewhat. There isn't much point to limit it unless you're struggling with traction.

I'd move my half width Honda radiator over a bit more and run an RX7 v Mount intercooler next to it plummed straight into the throttle body. I'd probably move my oil cooler to the windscreen cowl or somewhere else. The only thing I'd need to do would be to replace my crazy cams with something with less overlap... say 274 degree cams. I'd still run 13:1 compression on E85 and my 3" exhaust.

My plan would be get power up high in the revs (6000-8500 range) and try and max torque to around 280 ft lbs limit of a 6 speed (ie 425whp @ 8000rpm) Given the installation of the Rotrex which would force me out of the stock classes, I'd lighten the chassis a lot, run 245 Hoosier slicks and run some more serious aero. All up I think a 800kg, aerod, 400+whp S/C MX5 would be pretty hard to beat...

When I get bored of my car as it is now, I'll hit the button and do it.

That would be pretty amazing! :twisted:

jp928 · Postby **jp928** » Mon Mar 14, 2016 6:01 pm

The GT2560 turbo is capable of delivering 18psi and 320hp, so with our set up limited to 8psi, its able to get near that early, which delivers a very flat torque curve, compared to the Rotrex car. If you go to a bigger Rotrex than strictly necessary to get the same effect, dont forget its taking power out of the crank all the time, and if you dump excess pressure with a boost controller, its wasted effort. Ask around to see if its been done.

jp

madjak · Postby **madjak** » Mon Mar 14, 2016 7:34 pm

jp928 wrote:The GT2560 turbo is capable of delivering 18psi and 320hp, so with our set up limited to 8psi, its able to get near that early, which delivers a very flat torque curve, compared to the Rotrex car. If you go to a bigger Rotrex than strictly necessary to get the same effect, dont forget its taking power out of the crank all the time, and if you dump excess pressure with a boost controller, its wasted effort. Ask around to see if its been done.

jp

There really is no need to limit the Rotrex with a boost controller. Given the boost climbs linearly with revs you will never break the drivetrain with a Rotrex unlike a turbo which can build too much torque down low. The only reason to do it would be for traction issues.

We don't need to turn this thread into a Rotrex vs Turbo debate... there are lots of those already. The two setups are very different in terms of drivability and the choice of which depends on the style of power you want. I will say this though. In theory I think a Rotrex can be faster than a Turbo car given the torque limit of a stock 6 speed box.

For me, whilst I agree the newer turbos have less lag and I have not dríven an MX5 with one of NitroDann's lovely torque curves, there will be and always will be lag. What the Rotrex can offer is a smooth power delivery that will be highly controllable on the race track at the limit. There is a throttle response issue with turbos that will only ever be fixed when e-turbos become commonplace.

3gress · Postby **3gress** » Tue Mar 15, 2016 10:12 am

Looking forward to reading another well documented build log madjak.
You mentioned a blow off valve and recirc valve. Are you thinking of using both? Bov for reversion management and recirc for idle quality or such?

madjak · Postby **madjak** » Tue Mar 15, 2016 1:39 pm

3gress wrote:Looking forward to reading another well documented build log madjak.
You mentioned a blow off valve and recirc valve. Are you thinking of using both? Bov for reversion management and recirc for idle quality or such?

It won't be for a while yet! I haven't figured it all out the best way to do it, but here are my thoughts on running a very large Rotrex on a BP.

I think you need a large re-circulation / bypass valve to open on IM vacuum (ie when throttle body is closed). So it's bypassed at idle and on overrun. Because of the size of the Rotrex, selecting the right sized valve / spring here is critical to prevent the compressor from surging on part throttle. I don't know how to size it right, so a bit of trial and error here might be required. Ideally a dual throttle body setup would work better, but it adds complexity.

Then you have option of running a waste gate that vents to atmosphere or re-circs that is controlling max boost. I think in my case I'd want it hooked up to a 4 port solenoid and controlled via the ECU. This could be handy for launch control / wet weather running / boost by gear etc. I really don't think there is any reason to limit Rotrex boost with a restrictor or WG unless you are trying to protect parts of the engine from too much power. If you are going to the trouble of big power with a Rotrex you'll have a built bottom end anyway... so just run max boost.

In both cases a re-circ valve helps if it's after the intercooler so that your are releasing cooled charge air into the intake. I have to research this more but I think when the Rotrex is off boost it can get hot and this re-circ air helps keep it cool.

Postby **NitroDann** » Tue Mar 15, 2016 2:19 pm

You'll need to run a wastegate to control wheelspin.

If you don't need to then you should have gone Turbo.

madjak · Postby **madjak** » Tue Mar 15, 2016 2:31 pm

NitroDann wrote:You'll need to run a wastegate to control wheelspin.

If you don't need to then you should have gone Turbo.

Maybe I'm dreaming, but I was hoping with good aero, wide slicks there would be enough traction on the track to control wheelspin with just throttle. I would install PWM control of boost at the start... even if just for the knowhow on setting it up. I'm sure it would come in handy wet weather and launch control anyway. Dialing down boost with a dial on the dash would be nice.

Traction control would be a whole next level again... I think you need the latest ECU's to do it effectively.

3gress · Postby **3gress** » Tue Mar 15, 2016 6:45 pm

http://audisrs.com/ftopic5305-0-asc-285.php
I've found some promising results on this thread using jenvey throttle bodies and a rotrex c30 94.
Some interesting dyno results are shown and also neat photos of a gtir plenum and custom baseplate to enclose the velocity stacks.
If only e85 became readily available around the northern rivers area. I'm building my engine with e85 in mind and hoping local suppliers will consider the potential demand.

Postby **NitroDann** » Tue Mar 15, 2016 8:11 pm

madjak wrote:
NitroDann wrote:You'll need to run a wastegate to control wheelspin.

If you don't need to then you should have gone Turbo.

Maybe I'm dreaming, but I was hoping with good aero, wide slicks there would be enough traction on the track to control wheelspin with just throttle. I would install PWM control of boost at the start... even if just for the knowhow on setting it up. I'm sure it would come in handy wet weather and launch control anyway. Dialing down boost with a dial on the dash would be nice.

Traction control would be a whole next level again... I think you need the latest ECU's to do it effectively.

Traction control usually is just a bunch of misfires, having a throttle pedal that does what its told is the solution whereas misfires is just a bandaide

ralphk · Postby **ralphk** » Sun Jun 05, 2016 12:44 am

I am having one fitted to my ND, should have it back next week.

Postby **NitroDann** » Sun Jun 05, 2016 1:09 am

150-ish hp?

StillIC · Postby **StillIC** » Sun Jun 05, 2016 9:41 am

madjak wrote:The only thing I'd need to do would be to replace my crazy cams with something with less overlap... say 274 degree cams. I'd still run 13:1 compression on E85 and my 3" exhaust.

Really? Your car is currently normally aspirated, right?

My thinking is that if you can reduce your cam duration and boost the inlet pressure without changing the compression ratio, then you don't have enough compression ratio now. But with 13:1, I am not sure that is a problem, even on E85 (although I don't know your current cam specs).

By reducing your inlet cam duration *and* increasing inlet pressure you are doing two things to increase your chances of knock and detonation. If I had a well built normally aspirated engine (and I do!) and wanted to supercharge it, I would be decreasing compression ratio and/or increasing cam duration.

Postby **NitroDann** » Sun Jun 05, 2016 10:40 am

He doesnt have enough compression right now.

You're on the money.

Look at my recent E85 builds, for example PressurePack's car.

11.5:1, cams, 25psi, not even close to knock limited.

Dann

madjak · Postby **madjak** » Sun Jun 05, 2016 1:29 pm

My engine was built for 98 octane... running E85 N/A frees up heaps of headroom for more compression, but you'd also need some special pistons to do it. My head is decked about as far as you can go (2mm) and I'm running wiseco 10.5:1 pistons. To get more than I'm running you'd either need some custom pistons with shapped tops or weld up the bowl in the head. Maybe with less cam you could deck the head a bit more.

Cams for superchargers are a little different to turbo builds. With a turbo's back pressure the exhaust duration needs to be shorter. Plus you're less concerned with overlap as the turbo can push more air. But generally you are aiming for mid range power so there is no need to run big cams. With a Rotex you can run longer duration n/a style exhaust cams and use the exhaust scavenging effect just like N/A. The problem is the overlap leaking boost at low rpm and so loosing power and fuel especially as the Rotrex has a set volume of air it can pump per rotation.

Dann what sized inlet cam do you think is doable? I'd be targetting power from 6000 upwards and have no concerns with fuel efficiency or low end power. The honda boys run 300+ duration with rotexes for big power up high. They have the advantage of vtec down low to help stop boost leakage. I was looking at high lift (10mm) 274 degree cams but reading some of the honda rotex build they are seeing the biggest power with 310 degree cams and big overlap similar to my current cams.

Is it worth even trying a Rotex with my current cams or is it just a waste of time and $?