Rotrex 400+ Do I or not?
 

I have a potential deal on a C38-81 that is hard to go past. It's probably a size too large for me but I'm thinking I could make it work. I've been toying with this idea for years so maybe I should just do it.

It could either go directly onto my current 200whp high compression race engine or I could build a new head, or an entire new engine. Either way it would have a target HP range of 400-450whp. Rev range up in the 6000-8500. I'm not worried about drivability, fuel usage, noise etc etc... this is a track only car. I'd be running a 4.1 rear diff with my close ratio sequential dogbox that has a 1:1 top gear, so revs will be required to hit top speed. I plan to mount it low under the intake manifold so that the weight is low and it can directly feed into an intercooler and then out into the throttle body. I have already moved my alternator to the hotside to make room.

Q1. Intercooler sizing. Can I run a 300 x 300 core or do I need bigger? My plan is to sit it next to my half width radiator so that it gets nice clean airflow. I can compensate the restricted area by using a thicker core but where it will be placed should be fairly efficient. I'd start with a high quality core and make custom end tanks to fit. My current engine runs nice and cool even with the half width radiator so I think it will work.

Q2. Should I experiment with my current cams to see what happens? I'm running big 300+ degree cams with something like 55 degrees overlap. I'm willing to chuck it on the dyno for science purposes. I know there will be boost and fuel wastage galore, but at some point in the rev range it should start to work right? Some of the Hondas are running 300+ duration with the Rotrexes but I think they are more dyno queen than track cars. Maybe a VVT head is required to stop fireworks out the exhaust at low revs. If my current cams aren't feasible I'd probably switch to something like 284 intake, 274 exhaust... still big lift and large for a FI BP.

Q3. Compression too high? I'm running somewhere around 13:1 compression at the moment on E85. I could see if that works or build a new undecked head to drop the compression a little. My race head flows really well (it must do to hit 200whp n/a) so I'm hoping total boost won't be too crazy. Maybe 15-16PSI? With the big cams the dynamic compression will also be reduced so maybe that will also help? or maybe not. I can also run an intake restrictor to knock the top off the boost if I have too.

Q4. BOV? What to run?... I've read with big cams that don't produce vacuum you can get some nasty side effects like fluctuations and delayed opening which the Rotrex's dislike. I can use a low vacuum butterfly valve BOV like this Low Vacuum Bypass Valves

I don't need to be told to go turbo... I know, I know. The Rotrex is ideal for my driving style and if this works it will be epic. I'm still not sure if I want to do this, but first step is to see if it's worth trying. I haven't built an engine for big boost before I'm not sure where that line between sanity and craziness sits. I want to be right on it!

this question may be better asked on: slowboringnotorquemiata.com

1) I have no idea and you're smarter than I am so I say try it and see what happens. It's an easy fix if it's not big enough.

2) Please test it as I want to see how huge cams work with FI on a miata. Run ALLOFIT for science.

3) Since you are running E85 I think you should stick with high compression pistons you have currently and monitor closely for det. Best case it's absolutley glorious. Worst case you start to hearing signs of det with the det cans and back off. Then you have to build another motor anyway. You just might save a lot of extra work and expense trying what you have first.

4) Absolutely no idea to be honest.

I LOVE your build and pretty much everything you post so I want to see what other crazy shit you come up with.

He's starting with an n/a miata that's faster than your last turbo one was so.........


Come at me, bro!

1) 300x300/12x12 is fine for 250, borderline at 300, and I'd call it too small for 450. I'm guessing you'll need to make it 4-5" thick
2) they will probably work. not ideal, but you already know what to expect
3) no such thing as too high on e85 :)
4) you're gonna struggle with idle and driveability either way. #racecar

I don't think the bp is going to really like revving to 8500 all the time. You're gonna go through top end parts quite a bit.

It sounds absolutely awesome to me, and I agree with Ryan_G (including the part about you being smarter than me).

I am coming around on rotrexes bigtime. My favorite Miata I've ever driven was Oregonmon's rotrex'd VVT'd NA. Just awesome. No, not as much torque. but the linearity was so satisfying. To each their own.

Lulz.

am I way off?
how often do you rev yours out to 8500 on track

I too have found N/A stuff, or things feelings like it, to be extremely satisfying, but then for normal street driving, leaning into things at 2k and making 5psi by the time I get to 50% throttle is quite nice.

But I have to ask, why not just a huge rotrex on a stock VVT head or similar and run ALLOFIT to a more noramly 7-7.5k redline?

Never, but i have a stock motor. He doesn't. And hasn't, for years.

He's been doing exactly what he says he's going to do to that motor, for a long time.

I'll read up on intercooler sizing. I thought it was based on PSI rather than power as you are basically removing the heat from compressing the air. Did Tntuba run a layout like this? I'll have to go search.

My current rev limiter is set to 9000 and its up there every shift on the track. So I was being conservative at 8500 :) given the rods will see the extra HP as well as being flung around.

Revs are important to reduce torque. The whole idea is that a linear 450whp up to 8500 rpm should be perfectly fine on a stock 6 speed. Sure you don't get that mid range torque of a turbo but that is what destroys boxes.

1. I think it can work, but a larger IC that cooled the air better would improve reliablity and HP. I'd keep an eye on air temps. I have a 20x12x3 inch core intercooler in my car, plan to swap in a significantly larger on in the future. But I run high boost and want the lowest AITs possible for more HP/more reliability.
2. Try what you have, go from there. If they work on a NA car, probably work pretty good with a Rotrex.
3. I wouldn't worry about it, but I would run a cold spark plug.
4. Dunno.

I have the 12x12x4.5 garrett core on my car its rated for somewhere between 350 and 450 hp. With the stupid speed sensor port open on the efr it was hitting about 145 MATs at 12psi on a 90deg day on the dyno with it, I'm sure that the 1/4" boost leak fix it would be more efficient.

Because of the way the Rotrex builds boost I focused on getting best cylinder pressure possible in the mid range by shortening " seat to seat" duration times. This along with 11:1 CR helped give me great torque in mid range. I then looked to improve flow with larger +1 valves and greater duration in the upper areas of valve lift. In other words steeper ramps with much larger window at higher valve lifts. This is a 99 head.

I am running the C30-74 rotrex at about 9psi of boost at 7800 redline on E85.

I think you'll have too much cam duration and compression with the current head/cam setup. Even the 284/274 combo seems a bit much. In your shoes I'd run slighter hotter cams than I do (+10 to +20) for the higher redline. With 15psi I'd try and keep CR around 11:1.

Lastly I can't imagine wanting 400+HP at the track. I am sitting on a C30-94 and opted not to use it after getting almost 300whp out of the C30-74.

I think if I was building a motor specific for a Rotrex I'd target 11:1 but still go for the larger cams just because I want the power north of 7000rpm. Having the power high in rev range brings the torque down and given my c/r dogbox I only need a really peaky power curve to be in the power band. The rev drop is tiny at around 1200rpm 3-4 and 4-5 so when I'm shifting from 8800 the revs sit above 7500 on the next gear.

Still I think it might be worth running it on my existing engine even just to see what will happen. I'd only do it on a dyno so I can control the test. Half the tuners and engine builders I speak to say it definitely won't work, but then the others think it might... I think the Rotrex is a little different to what everyone else is used to so they don't really know. Most BP builders say 300+ degree cams on a N/A is stupid...

This pretty much sums up my concerns too, but then I watched this: It's pretty much 450whp Rotrex in a 900kg car. I think I'd be happy with 300whp and i'm sure the car would be more controllable, maybe even quicker in certain conditions, but then I just wonder if I'll want more. I think I'm driven to do this just to see what will happen... I love to experiment!

I actually drew up a billet V8 block the other day... 2 x Mazda heads destroked to 3.2L... 9500 rpm and 380 whp N/A. That would be interesting.
But it would be a lot of work for only 9500rpm and the billet was too big and expensive. So I switched to 2 x GSXR 1000 heads for 13000 rpm and 2L in V8. It would probably only be 350whp if that but it would sound glorious. I could do most of it on my CNC machine myself so it wouldn't cost a lot. Maybe I should be bolting the Rotrex to that instead.

Benny and Jimmy understate their engines WHP so that people think theres extra clever stuff happening with the car setup rather than just K24+boost= very fast. It really makes 600hp. Source: I built their MR2s turbo setup which is currently the fastest 2wd import in australian roll racing by a HUGE margin and they also heavily understate its power figures.

I dont know if that info helps, but I hope so.

Dann

Forget static compression and think about dynamic compression. Target 17:1-18:1, with your current cams how much boost can you run before you hit those corrected CR numbers? Next look at the shape of your current power curve. The Rotrex will add to it disproportionately on the higher end. If you're targeting 7,500 to 8,800 (I think 6,500 to 8,880 is better focus) and torque curve is currently pretty flat now it will have a much bigger bend upwards after the Rotrex. That's why I like smaller cams. They help flatten the torque cure.




When I did my research on building a motor for a centrifugal blower the newest thinking is to build what you would want NA (cams/heads/intake/exhaust) and go from there. Corrected compression still needs to be looked at though.


In my humble opinion build something the leaves headroom on the table. Right now 300whp is good enough for me but once I go sequential that C30-94 may end up on the car. I have no doubt a 400 whp Rotrex will be drivable. I am personally not ready for that just yet.

I look forward to following the madness!

Headroom is key to having more fun driving and less time wrenching, I'm having a blast with my C30-74 w/ E85 @ 9.5psi on a stock block. In for videos with a sequential set up with a C30-94.

For the BOV, indeed you'll want to run a bypass valve which will release pressure immediately. I've been running a Bosch style replacement from GoFastBits for a while and it works great, I threw a filter on it and VTA. I've street driven the car and I do not have any idle issues with it.

Sub'd. And thanks Ed.

In for 400whp version of your car.