tfbmiata

Hi - Hoping to get advice from you all. I realize I'm probably asking a biased crowd, but I'm curious of opinions from guys who are doing it. I plan to mostly do track days and as far as I can tell a Honda J Series swap will cost was seems to be about roughly double of the TDR kit for relatively the same HP. My buddy and I are debating which would be faster (in a straight line) a rotrex kit or v6 at roughly the same HP range. He says the torque monster and I say whichever makes more HP for longer in a rev range. I've also noticed some people posting about cooling issues, but couldn't find anything related to someone using a cooler from TDR to do that or what. Anyhow, thanks fo rthe advice and sorry for the low effort post. I promise, I've been reading as much as I can on it.

Z_WAAAAAZ

I have a handful of (comparably basic/dumb) questions and remarks to pose regarding this topic, but my main one is this. Why two power adders as dissimilar as a Rotrex and J swap? One is a fairly simple and quick install (in the grand scheme of things) and the other is a pretty serious undertaking that will have the car down for a much longer period of time. Also, round deez parts, an LFX swap is much more common for a V6 swap and you'll find more people on here have documentation of this swap compared to a J series swap. Just sayin'.

Regardless of what you go with, I wouldn't worry about cooling ability of one over the other. In a high power track car, you're going to have to beef up the cooling system regardless (bigger radiator/ducting/hood vents/etc.). Long story short, there's plenty of Rotrex and V6 swapped track Miatas out there that don't deal with cooling issues if done right.

I don't personally feel qualified to tell you what to do. However, I did note that my previous stock '95 motor with a turbo making ~210whp would close the gap on my buddy's ~235whp K-swapped NB in the straights when we ran together on track. I don't have his torque curve numbers but torque was definitely playing a strong role there.

Good luck on your high HP track Miata journey!

Gee Emm

Good comments above.

Starting from scartch, I would plump for a Rotrex for a hard driven street car, over turbo, with swap (of anything) waaaay down the list.

You want a big HP track-capable car? Rotrex does not get on the list, it is either a fully built turbo engine, or a swap. Which swap? The one that works, is trouble-free and does not involve pioneering (which means plenty of documentation/knowledge/experience/parts).

Sure you can track a Rotrex, plenty do. It really depends on how far up the results sheet you want to be - if you want to be at the pointy end of the field, the Rotrex is unlikely to deliver.

Just my 2c, feel free to ignore me.

tfbmiata

I'm going to be honest, I'm shocked at the genuine helpfulness of the first reply to my post. I guess my younger years at the old honda-tech forums have taught me nothing about the miata bros. Anyhow, the reason why I compared the two was the j series swap, so far as I could research seemed someone in the ball part of pricing and total HP (though obviously the curves are very different). With the kit from miatav6 and a few extras that i'd need for a higher HP anyways, they seem to roughly come in the 7k to 9k mark and with a bunch more ponies that your standard k24 or 220-250whp turbo kit swap. My thought was for hopefully less heat based hassle of the turbo and less cost w/ more power than the k24, the jseries and rotrex are "roughly" in the same ball park of costs. I hadn't considered the LFX swap because, unless my google fu sucks, I can't seem to find a complete kit or forum post that doesn't mention a LOT of fabrication and issues with forcing it in there where as the j series swap has a true kit and what seems to be an easy to follow guide, plus Honda, and VTEC y0.


Big HP? I was thinking modest? 250 to 300whp range seemed modest to me. I would definitely care more about trouble-free running and not blowing things up every track day than I would ultimate track time. I had a preconieved notion of the lower torque mid range of a rotrex being a good thing on a light way no TC car to help prevent massive oversteer moments on power and deliver a more "drivable" car.

Z_WAAAAAZ

I’m going to be honest, I blindly assumed a J-swap involved just as much fab work as an LFX swap. My bad there. That being said, still a serious undertaking! Haha. If you’re up to it, it’ll be a sweet project but like the comment above, make sure you’re comfortable with the amount of documentatIon on tap before you pull the trigger. With a turbo/rotrex Miata running a Megasquirt ECU, you can be assured that most problems you’ll run into have been answered on this forum or otherwise. With a J swap, I’m not so sure (although I obviously haven’t done my homework on that exact swap).

In terms of “drivability”, I wouldn’t let that decide one or the other for me. Modern turbo setups are extremely responsive when set up with the right components. My turbo car (Kraken kit/Garrett GT2560r turbo) basically drives like it’s NA. Especially at high RPM on track. You get on the gas out of a corner on track and there’s zero lag, the power is just there. I was pleasantly surprised to find out on my first track day with the turbo that I didn’t have to drive the car any differently than when it was naturally aspirated haha.

Ok maybe I am making a case for the turbo.

tfbmiata

Interesting! That's surprising, out of some slow corners I do from time to time already feel myself having to modulate throttle to prevent wheel slip. Admittedly, I'm on ultra performance tires, not tw200 or anything so that could change. I guess assumptions are really the devil. I was assuming that 220 to 250wtq on tap from 3 to 4k rpm would make it a fit to keep the *** end behind me on the exit of every corner, but honestly, maybe that's where all the fun is anyway??

Z_WAAAAAZ

I think that's definitely where some of the fun is haha.

That being said, you're going to run into wheelspin at low speed corner exits with whatever power adder you decide on. The difference on each setup might be going 1/2 throttle vs 2/3 for a given corner exit, but either way you're going to be feathering/actuating the throttle to some degree. You'll get used to it quickly. My car currently peaks at 185wtq though, quite a bit higher than stock obviously but maybe some of the big HP track guys on here can chime in on their experiences too.

I guess my point is, I was originally going to go rotrex for drivability as well but have had no issue with how the turbo lays down power on track.

highroller101

Just throwing this swap in… what about LE5 ecotec swap? It’s cheap and reliable. Plus turbo kits are readily available to hit 260-300whp range.

not sure if ecotec Miata is still in business though…

gooflophaze

All signs point to no. And my impressions from seeing some of the complaints about that kit was that it wasn't that reliable.

engineered2win

I would throw out the J-swap Miata. The kits are pretty haggard. I spent many years working on J-series OEM development and would love to hear one sing in a Miata, but it's a turrible platform for a RWD setup.
The main issue is the starter is always trans side on the J series, and few RWD trannys have a starter hole. So you're either hogging out a giant hole in the bellhousing or putting it where the oil pan sump needs to be. Or you can get a Quicktime bellhousing and bolt it to a TKX, but then you're in about $5k and still need a trans mount, clutch/flywheel, hydraulics, and custom driveshaft. I've looked at the 2 J-series kits and wouldn't trust either of them.

Gee Emm

Yeah, well if you are content now and into the future with that, the rotrex is an option.

However, last time I looked, HP was addictive, and whether you will remain content with that only you know, or think you know.

Me, I am going from 140rwkw to around 220 with the current build, so you could say that I have found the HP to be addictive, and there is more available if I that proves to be insufficient.

I will endorse the above comments on the response of a properly engineered and tuned turbo - choosing rotrex over turbo for response should not be necessary.

rascal

250-300hp is easy with a c30-94 rotrex kit.
Fwiw my underdriven c30-84 made 240 atw. Gearing it up to max revs at redline and moving to the larger 94 would easily bump that to the 250-300 range.
modern ECUs can tune the torque curve of a turbo kit to reduce the big mid range surge turbos typically give and make it more linear like a nat asp car, but the rotrex does this straight out of the box so imo why bother.
add to this the easy button of a rotrex compared to a turbo kit; no drilling of sump, no need to manage under bonnet heat, no worrying about manifold bolts constantly backing out no matter how many locking methods you use, and in my eyes the rotrex is a winner. YMMV.

HP can be addictive but only if you let it. My car made 240 at 8 psi on a built motor but now runs around 180 at 4 psi on an essentially stock motor due to pulley gearing (and classing reasons) and I have no plans to go back to 240.. At 240 everything just wore out quicker, and replacing brake pads, hubs, clutches, gearboxes is a lousy outcome just to be a bit faster down each straight..

curly

iTrader: (25)

I can’t believe no one’s mentioned noise. I’m currently tuning a bone stock ‘93 (stock header/car/exhaust) and I actually have to wear ear plugs when it idles, the whine/gear noise is so so bad at idle. Only mod from the stock kit is no AFM, made 220 with a 72mm pulley on a 1.6. Id still argue big injectors and a stand alone are “required”, plus you’re still installing an intercooler, at that point I’d argue a turbo is simpler than the stupid bracket and belt setup of the rotrex.

Z_WAAAAAZ

Hood vents and a set of $30 locking nuts for the turbo manifold. Boom. Your underhood temps and fasteners backing out are no longer an issue. Although, I'd still run hood vents regardless of the power adder for cooling efficiency.

The oil pan drilling I understand though.

I'm also on the opposite track as you lol. Just jumped from 210whp to 270 in my track car. We'll see if I get agitated this track season at how many brake pads/comsumables I go through I agree keeping the car in good condition was much easier at stock power level.

poormxdad

I have a C30-74 with a 75mm pulley pushing slightly less than 10 psi on a junkyard engine. I'm guessing about 225 rwhp. My best time on VIR Full is 2:11.37. She's pretty fast, despite my best efforts. I love this car.

However, rascal makes a good point. Sometimes, I have the fastest or one of the fastest Miatas at the track, but depending on what group I go out in, I might have one of the slowest cars in that group. It's frickin' awesome to torment guys with way more expensive cars through the twisties, but if they're ahead of you pulling onto a straight, they'll probably stay ahead of you. A few more horsepower won't change that.

Sooooooo, I have accepted the fact she is only as fast as she is. 10-20 horses won't really make a difference, one way or the other. I plan to limit the power to ~225 rwhp so I don't have to switch to a 6-speed. I hated the 6-speed on track. Too much work.

With regard to cooling... I have the big TSE radiator. It's thick but slightly narrower than a normal Miata radiator, which facilitated routing the plumbing for the Rotrex oil cooler and reservoir. I have decent but cheap ducting and on a recent 95 degree day my max coolant temp was 205. I have resisted hood vents since they mean less airflow past the transmission. Everything is a tradeoff. She is what she is, and I'm okay with that.

tfbmiata

So far I've felt the same way about my 6 speed. I love it on the street, but its an extra shift everywhere compared to my buddy's 5 speed and on top of that, the shift to 5th isn't exactly stellar b/c i like to catch the "corner" of 5th and reverse often.

rascal

ha, I’m the opposite. I much preferred the silky shift of the 5 speed on the street but the 6 speed is quicker on the track and mine is 99% track car so I moved to a 6 speed. Adding a Miata roadster tall angled shifter made a big difference to the shift, and has made it a much better experience.