Autocross: XB miata engine build
 

Hey guys,
After years of reading on this forum, I'm looking to add power (force induction) to my autocross miata. I want to run in XB (street tire, unlimited engine mods basically), have a reliable, easy to drive, and fast(ish) car. No plan to win nationals anytime soon btw, just have fun locally! I drive it to and from the events, no trailer. 91 octane available here, no E85.
Traction will be a factor since I need to run on street tires (most likely 245 CRS on 15x10 for now), so I'm not looking for ultimate power and torque. Kswap is out of the equation for now, just too expensive, too intrusive, and I think a boosted BP would be more than enough. For now, I want to keep the stock motor (around 220 whp, 190-200 ft-lbs), and see what I can do with that before adding more.
I have narrowed it down to basically to options:
- TDR rotrex c30-84 kit
- G25-550 turbo kit with boost control to keep everything safe (transmission, rods, etc.).

I know that the turbo kit has more overhead potential, but I'm a bit concerned about heat management, reliability on a stock motor. It's also more complex to install and need more costum fab that the bolt on rotrex kit. Root blower would probably be great also, just seems less easy to integrate, cooling the air charge is a challenge, not a lot of bolt on options aside from the FFS kit.
Throttle response of the rotrex, driveability, low heat, relatively low end torque, relatively low cost, all seem good things when I look at my goals, for a street tire autocross car. Is there anything you guys see that I am missing? There is a lot of great info on this forum with very knowledgeable people, so I thought I would ask and see what you guys come up with.

Thanks for time!

Traction won't be an issue on a stock block with your tire setup, regardless of the power adder. Plenty of guys on here running 220-250whp on 225s w/ 15x9s or even smaller. Hell, I ran 205s on 15x7s at 210whp for one track day and it was completely manageable. This was at a higher speed track though, might've had a little more wheelspin at an autoX event.

G25-550 sounds like it might be a little overkill if you're not planning to go built motor? You could get quicker spool with a GT2554 or GT2560r if you go turbo.

Heat management on a turbo system is easily mitigated. Add a set of hood vents and a basic heat shield between the turbo and firewall and you're taken care of. You could probably get away without the hood vents for autoX, but for a couple hundred bucks and a couple/few hours of labor it's a no-brainer.

I'm a little biased toward turbos so hopefully one or more of the rotrex guys will chime in too.

Yeah G25-550 is what my builder recommended since I could easily get more serious power if I ever build the motor, so if I go turbo, I would do the turbo setup only once.
Already have hoodvents, reroute, aluminum rad and ducting in place.
I know people on this forum have a turbo bias haha, but a lot of people that I race with have had their share of issues with turbo setups on various cars, so that's why I'm being a bit reluctant to add a turbo setup to my car...

Thanks for your feedback, great stuff

Ah ok, gotcha. Yeah, built motor changes everything haha. I too thought I'd be content with sub-250whp but after I built my motor, I immediately made plans to swap my GT2560r with a GTX2860. Good on ya for considering you'll want to up the boost in the future. Many guys (myself included) have thought, "200whp will be plenty forever!" only to get intoxicated by the power and want to add more horsepower later on.

Right on, if you already have all the cooling system upgrades there, I'd only add a heat shield for the turbo and you'll be good. Lots of dudes on here running that same cooling setup and not running into underhood temp issues on track.

Glad to be of help! Again, I'm only one dude. Hopefully some other guys chime in too and help your decision.

Good luck on the quest for more power!

C30-94 E85 rotrex here.

I have casually done a few autocross with my setup. I preface this with i did not build my car to be a autocross car. and if i were to. it would of been a turbo. in low speed second gear corners on a 5 speed and 4.1 rear end i was Power limited coming out of corners. A turbo setup would undoubtedly be faster.

On the contrary, my opinion is that super charged 4 cyl's sound better and are way easier to put together. i think a child could assemble the TDR rotrex kit. If you are willing to put the time and money into a nice turbo setup, do that. If you want your car to have more than double the power in 2 weekends of casual building, go with rotrex.

Just to make sur I understand: With the rotrex, you were traction limited coming out of 2nd gear corners? I have a 5 speed with a 4.3 rear end right now, raised the limiter to 7500rpm for more top speed (and my engine still makes good power up there suprisingly).

Ease of installation is one of the main advantage I see with the rotrex, thanks for confirming that!

Sorry, typo. i was POWER limited.
this was on 205 wide maxis RC-1's and a torsen rear end

I have had a c30-84 on my NB1 track car for the last two years. I absolutely love it. Drives just like it did when nat asp, except more power everywhere, with a very addictive pull towards redline.
Mine is 90% TDR kit and was super easy to fit, and has been zero maintenance since fitted. If I was to build another track car, I would go rotrex again in a heartbeat.

not so sure if it would be the best thing for auto cross, as it doesn’t have the turbo’s low down surge you would use a lot out of the slow corners, but then that was something you said you wanted for tyre mgmt and this would work well. You wouldn’t be top time of day but again you said that’s ok.

for time attack/ track days on full size circuits I find it the perfect setup, as I’m never low enough in the rev range to notice the relative lack of torque to a turbo guy, and it’s super reliable and fast everywhere else. But like the previous poster I’m biased towards what I have. (though I did do looots of research to determine which option (turbo or rotrex) would suit me and decided rotrex path and thankfully have had my usage confirm my decision) ymmv

Given your purpose is autox and your power target is sub 250hp, I'd recommend a rotrex supercharger.
Nice linear powerband, very minimal boost response time, and lower torque to keep the stock bottom end alive.
Even a EFR 6258 isn't going to have the torque response a rotrex will.

Thanks for the great feedback guys, happy to see that the rotrex kit has good reviews. I get that a well sorted turbo would probably be faster, but it comes with its ''risks''. And then you see Mr. Anderson and his LaFawnda win in about every class it performed at the solo nats with a (huge) twinscrew... haha, such a sick car.
So many options to make power on these platforms!
Realy appreciate your feedbacks! Keep'em coming!

Ben

Took me forever but I responded to your DM :D. If you have a built motor I'd go 94 at a minimum for your SC. 84 is just a waste of potential unless you're doing an intentionally low HP stock block with no real ECU. I'd even say it's worth considering a c38-61 if you're feeling adventurous, but a c30-94 will be absurdly easy to set up and will make simple reliable power.

A c30-84 will still make 300hp if desired so hardly a waste, and OP has a stock block.

Also a c30-94 can start getting into gearbox harming territory, which OP also was wary of.

imo, a c30-84 is the sweet spot for enough power potential from a stock block without the constant thoughts of needing a gearbox backup plan.

I'll agree to disagree on this. No reason to stay with an 84 at all when a 94 is the same cost and doesn't really offer any downsides, just more headroom and not having to push the setup as hard. I'd only go smaller than a 94 if you had no standalone or are building for a specific class/power limit.

Power delivery on these is so soft that I'd be shocked if a trans was ruined in a premature manner without some form of negligent abuse.

These don't make part breaking torque - you can't compare hp/tq figures for failures on turbo cars to rotrex setups. Look at thepower delivery vs RPM. I've seen high power turbo cars keep gearboxes alive far past what they are "supposed to explode" at simply by keeping boost low at lower RPMS (like a rotrex powerband).

Anyone wanna buy my 74 headunit? Been looking to upgrade to a 94 ever since I helped another guy with his setup :rofl: