Which 13b
 

I'm considering converting my E30 BMW track car to a rotary power plant. You could say I'm getting tired of changing timing belts and adjusting valves. I'm considering a carbureted naturally aspirated 13b for simplicity. Something similar to an EP motor. I'm not limited to any rules or limitations due to the fact that I race in a NASA (GTS Series) and we are classed based on power to weight. Upon doing some research, I understand that there are various versions of the 13b, 4 port, 6 port, turbo etc. What is the best comination of rotors, rotor housings and side plates to use. I'm not looking for an exotic engine, just something that will put out about 220 - 240 reliable horsepower. Thanks

get a 13B-T. build it and run it with a standalone. you'll get 240 all day long with proper cooling.

Stock RX-8 motor. 238hp (if you believe Mazda), 9000rpm redline, 6 speed transmission, multi-stage intake manifold for broad powerband. You'll probably need a standalone though, as there's probably issues with all the chassis sensors that'll prevent you from using the stock computer. Megasquirt will probably work just fine there.

Or if you want to build it yourself, get an S5 (89-91) NA motor, that's a 6 port with 9.7:1 lightweight rotors, 8000rpm redline, stock 160hp, but in EP trim it'll run about 200hp to the rear wheels. That'll be with a montser port though with little to no bottom end and it'll need to be revved a lot (9-10k maybe?). If you're serious about doing this talk to some of the experianced EP engine builders. One that comes to mind is Mazdatrix, Dave Lemon runs a fairly succesful EP 2nd gen convertible in EP.

In the end the RX-8 and standalone option will probably cost less, should last longer and will be easier to replace if/when it wears out or breaks. They'll likely make about the same power in the end too.

well, ultimately the choice is yours, but i would think the turbo motor would have a more ... ahem ... "tuneable" ... torque curve, which would not require designing and fighting through intake tract demons and not require as much RPM.

on a total aside, why not use an E36 powerplant? even the regular (non M) powerplants should be able to give you simpler and better improvements, perhaps with less stress.

The crisp throttle response of a N/A motor is more fun to drive than dealing with turbo lag, if you like to make the rear end hang out in the corners. Yeah, yeah, all the turbo guys say "their" system doesn't have any turbo lag, but they ALL have turbo lag - just some are slower than others.
The turbos do make more torque than the N/A motors.
I have been road racing rotary N/A motors for 13 years now.

I know one thing about turbos that can be tricky around a road course is that around a long sweeper at constant throttle you can actually be accelerating unintentionally as the turbo builds boost, and it can be less predictable and harder to control due to the non-linearity of the power curve.

He also mentionned NA specifically, so that's what I stuck with. On the street for those power goals then yes, turbo is better.

Stay with the BMW motor. If you don't like to take the time to change timing belts and adjust valves, you'll truely hate the time it takes to sort out a 13b transplant with aftermarket ECM. And you'll REALLY hate your decision after your second blown engine. If you are respecting the redline, have good general racing habits and perform careful regular maintenance, your BMW should be stone reliable and a joy to race. This opinion comes from 35 years of racing... the last four with my 13b RX7. Don't get me wrong, I like the RX-7 package I have, I just think you also have a great package just like it sits.

I have an EP 1st gen Rx-7, a fairly stock E30 track day car, and a Pro-3 (E30 325iS spec series race car) under construction so I have some pretty direct comparisons.

A rotary with a relatively aggressive street port like those used in EP really needs a light car to work it's magic. Rotaries are torque challenged by nature, and it is going to be really hard to get an E30 down to a weight that would give it comparable performance with the stock motor. Perhaps a Rx-8 motor would be a good compromise, but keep in mind packaging issues. Rotaries have the output shaft coming fro the middle of the motor instead of the bottom like a piston motor. That means the transission will have to be mounted higher likely meaning some serious modification and enlargement of the tunnel, , or the motor will have to be mounted lower which compromises ground clearance and interferes with suspension function. If you love fabricating you might find it rewarding just to accomplish such a project, but it is going to take hundreds of hours of inventive design and fitting to do this properly. Every single system needed to make this car lap the track is going to need to be "invented" all over again.

In addition, although I sympathize with your costly breakages, BOTH motors can be expensive if the proper parts, planning, and maintainence aren't observed. Local guys in the Pro-3 series sometimes find some problems breaking the stock rockers which are required for the series, but that can be repaired at the track. If you have a mechanical problem with a rotary of any kind, you are going to have to disassemble it just for take a look. No matter how small the problem, a major disassemble, reassembly, and a seal kit are going to be needed just to assess the problem. Don't get me wrong, intellectually I love the challenge of projects like this and I have 1st gen Rx-7 with an aluminum V-8 and a t-5 that mounts to all of the stock holes. On short notice, I could lift this engine, front crossmember and motor mounts, and t-5 transmission and bolt it directly into my EP racecar chassis using the stock holes - and don't think I haven't considered it!

well put and i agree wholeheartedly. for as much as i am not a huge fan of the E30 motors, i respect them for what they were in their day. i've worked on them, in fact my very first engine swap was on a 325e (a guy got the 325is motor for it) and i've seen them make power. they are "almost" Toyota reliable with good maintenance.

although i still vote for an E36 swap. :)

^Probably, but recently I've heard of a number of E36's endurance racing that can't keep their heads on. Each engine has it's issues that need to be addressed and respected. It will be easier to swap a BMW engine in though, and it'll probably give you better resale value if you ever do want to sell it.