Making a Rotary Engine Better

I use to be all things RX7.  But for all the reasons that this video gets in to I eventually moved on. I new some of this but there appears to have been a lot more research and fix's that have been developed since I got out of the rotary game to make these motors much better.

I didn't know that bit about e85 keeping the rotors free of carbon buildup, I always thought I was doing my do diligence in avoiding buildup by running the motor to redline often.. 

pretty cool seeing the numbers climbing higher and higher with all the development carried out by the aftermarket.

It'd be interesting to see how reliable a 700hp 13B would be in a road race. I know there are some time attack RX-7s making those numbers.

E85 and the discussion about the bolts I did not know about. The other thing that I thaught was interesting was taking a couple thou off the sides of the rotor up by the tips. 

The redlining is supposed to keep the carbon buildup in the apex seal slots to a minimum, I don't think it's supposed to keep the gunk off the rest of the rotor. But using E85 does seem to be an interesting way of keeping things cleaner.

I appreciated that video, mainly because he actually put a few things together that I already knew about, but they made a lot more sense in this context.

Of course now I have an urge to get another rotary...

BoxheadTim said:

Of course now I have an urge to get another rotary...

Yup. I was now having the same thoughts. I may have a 12a or a 13b in the back of my shed. I am now going to have to look. 

In reply to dean1484 :

I know I have a 12A in the back of my shop, mainly because my dang BRZ blocks in the Maxton .

A late JDM FC or and FD would be nice, though. Wouldn't kick an RX8 R3 out of the garage either.

I never knew about the wobble that occurs with the rotors. That was really interesting. I also was fascinated with the loading on the bolts that hold the motor together. The need to make those fit tight makes perfect sense.  The last bit about putting solid dowels in and doing the mods to the oiling system was a lightbulb moment for me.  That is brilliant. 
 

These are all little mods/engineering upgrades that by them self are not much but together I can see making a huge difference in reliability of these motors. 

I for some reason want another FB. That was the car that I had the most fun with on track and on the street. My FC was a better car but it did not have that sort of happy feeing. It was a more serious car and the FD was a true modern sports  car that really was the next level car in terms of speed and handling. 

In reply to dean1484 :

I had only learned about the rotor wobble from a few videos from another shop not too far away from here. They had a few examples of it, and their rotor balancing guy showed how badly some of the rotors wobble. IIRC he was able to correct for most of the wobble coming from a slight imbalance on the rotor, but they still ended up shaving the corners just a tad, too.

It's still amazing to me that together with the simplicity these engines embody, most of the higher power improvements are also pretty simple.

Although I would dread having to pay the E85 fuel bill for a 700hp 13B.

In reply to dean1484 :

I've owned one FB (an Elford Turbo), one turbo FC and one FD. I agree that the FB was probably the most fun car, but the parts situation seems to be pretty dire for the FB and to a certain extent, the FC. But I agree, the FC is certainly more serious, and even my mildly breathed on FD was a serious weapon (and threat to one's license).

I assume the wobble is only prevalent on really high horsepower engines running lots of boost? My rotary interests are all n/a, I'd really like to build an n/a 20b when funds allow. Rotaries seems not always the most cost effective motors to make power, but to me they're certainly the most exciting..

According to Mazdatrix the wobble also occurs at high rpm and recommends side cutting for engines spinning above 8500rpm.

I remember 15 years ago 20Bs were all over ebay for $6000. It seems everybody blew theirs up looking for 1000hp.

all the old rules still apply for NA engines with modern injection feeding them better.

Hah, I just ran across that video also.  It's nice that he spills the "secrets" that most seemed to keep to themselves in the past.  Unfortunately it seems like the  days of cheap engines to play with seems to be gone.  Although, I am sure there a number of people with pretty large stashes.

The notes about how detonation is a killer are interesting.  It's really the smaller parts going through the engine that is the killer of course.  I can imagine even a small piece of an apex seal could destroy an entire housing (and maybe a rotor also).

I suspect that one aspect of running E85 that should have been noted (although probably obvious for anyone sophisticated enough to actually do it) is a rather serious fueling upgrade.  Probably not a big deal on FI engines, but I imagine you could run into big block V8 size carbs on an NA motor!  Of course, some big bore Webers are probably a good way to go.

I personally find the engines fascinating, even if they aren't terribly practical in many cases.

Doing some snooping around, I had no idea this existed:

A now defunct marine diesel engine:  https://drivetribe.com/p/this-116l-turbocharged-big-block-TTyBaDvUTjawADBpK4bedQ?iid=Zoh2esqzSriNNFn2oZcBKg

dannyp84 said:

I didn't know that bit about e85 keeping the rotors free of carbon buildup, I always thought I was doing my do diligence in avoiding buildup by running the motor to redline often.. 

E85 makes for HORRIBLE carbon buildup because the only ethanol-friendly premix oils are castor based, which makes a nasty tarry varnish everywhere unless you premix the fuel the day you use it.

dean1484 said:

I also was fascinated with the loading on the bolts that hold the motor together. The need to make those fit tight makes perfect sense.

It's also wrong, though. Mazda's research showed that there needed to be some disconnect between the combustion area of the rotor housing and the rest of the engine, because of thermal expansion effects.  So they use coolant seals that can move around a bit, and they dowel the engine together bracketing the combustion chamber, so that area can float around.  On race engines they would even cut the ribs on the inner side of the rotor housing for maximum thermal flexibility.

j_tso said:

According to Mazdatrix the wobble also occurs at high rpm and recommends side cutting for engines spinning above 8500rpm.

I remember 15 years ago 20Bs were all over ebay for $6000. It seems everybody blew theirs up looking for 1000hp.

all the old rules still apply for NA engines with modern injection feeding them better.

You can make more power with a 2 rotor, though 

The rule I had heard was that the rotors will kiss the rotor housings at over 10,5, a figure that I can confirm.  8500 is the point where 3mm iron apex seals will destroy themselves, they start bouncing at 6200 and it either hits a resonance or something at 8500.  A figure I can also confirm 

Stationary gear loading also goes insanely high over 8000rpm with old nine pin rotor gears, which is why they used to have hardened stat gears and they would circlip the rotor gears to the rotors to keep them from walking out.  The 12 pin gears have none of these problems, the gear loading is much lower and it doesn't climb exponentially over 8k either.

Pete. (l33t FS) said:

dannyp84 said:

I didn't know that bit about e85 keeping the rotors free of carbon buildup, I always thought I was doing my do diligence in avoiding buildup by running the motor to redline often.. 

E85 makes for HORRIBLE carbon buildup because the only ethanol-friendly premix oils are castor based, which makes a nasty tarry varnish everywhere unless you premix the fuel the day you use it.

Interesting.  I wonder what the video guys response to this would be.  Based on the HP he is talking about, I would suspect he mixes on the day (drag cars?).  I also wonder what difference running E85 with the stock oil injectors would do carbon wise.

If carbon build up is a primary concern, water injection might be an easier fix.  Of course E85 with a turbo has some serious advantages.

There's now a sort of part 2, talking about doing the mods and additional HP gains- 

In reply to Pete. (l33t FS) :

More power from a 2 rotor vs 3 ?

aircooled said:

Pete. (l33t FS) said:

dannyp84 said:

I didn't know that bit about e85 keeping the rotors free of carbon buildup, I always thought I was doing my do diligence in avoiding buildup by running the motor to redline often.. 

E85 makes for HORRIBLE carbon buildup because the only ethanol-friendly premix oils are castor based, which makes a nasty tarry varnish everywhere unless you premix the fuel the day you use it.

Interesting.  I wonder what the video guys response to this would be.  Based on the HP he is talking about, I would suspect he mixes on the day (drag cars?).  I also wonder what difference running E85 with the stock oil injectors would do carbon wise.

If carbon build up is a primary concern, water injection might be an easier fix.  Of course E85 with a turbo has some serious advantages.

I have noted zero carbon buildup when deleting the MOP and exclusively premixing at a rate of 60-100:1.

I take anything drag racers do with a grain of salt.  A drag engine has to run at WOT for six-ten seconds.  It doesn't have to idle or cruise or tolerate high temps for extended periods, if ever.  Yes, it is amazing what can be done to shore up what was originally a 100hp engine from 1967, but most of those things done are contrary to engine longevity.  (I wouldn't want to drive a jacketless V8 on the street, either)

dannyp84 said:

In reply to Pete. (l33t FS) :

More power from a 2 rotor vs 3 ?

The eccentric shaft is more stable and isn't two piece.  Even in street trim that becomes a liability, and that is before you get into the cooling issues.  20Bs are notoriously hard to keep cool on the street.

There are 2 rotor doorslammers in the 6s.

Pete. (l33t FS) said:

... Mazda's research showed that there needed to be some disconnect between the combustion area of the rotor housing and the rest of the engine, because of thermal expansion effects.  So they use coolant seals that can move around a bit, and they dowel the engine together bracketing the combustion chamber, so that area can float around.  On race engines they would even cut the ribs on the inner side of the rotor housing for maximum thermal flexibility.

In the second video linked above, he talks about how the stock dowels where actually cut down at the ends so they fit a bit looser into the side housings, seemingly for the reason you stated, to allow them to move a bit.

It does make sense though, the thermal expansion rate of aluminum and iron are clearly different (AL about twice the amount) and forcing them together seems like it would lead to bad things, like cracking.  Unless they used alloys that could avoid or minimize that (?).  I suspect the dowels themselves provide some amount of heat transfer but I am not sure if that is good or bad. The rotor housings probably have less heat transfer area in the combustion chambers than the side housings(?)  Of course aluminum transfers heat about twice as fast....

I am not sure I am using the right numbers, but it seems like the size difference between the housings from 70f to 250f could be rather significant (depending on the specific types of aluminum and iron of course):

https://www.engineeringtoolbox.com/linear-thermal-expansion-d_1379.html

In reply to aircooled :

The thermal expansion is why Mazda used a lot of fairly thin tension bolts at fairly low torque.  Spec is 23-29ft-lb on undercut 10x1.0 bolts, so they are more like springs.  ("Tension bolt" isn't just a clever name )  I usually torque to 23 because I am more concerned about the engine being able to tolerate high coolant temps.  Super tight bolts (some drag racers torque the OE bolts to 50!) means there is no "give" to them if the engine gets hot, so the rotor housings get crushed (aluminum is weaker than 17-19 bolts) and then the coolant seals blow on the next cold start because now the whole assembly is relaxed and not under tension.

I saw my temp gauge peg Friday night, halfway point is 220F so I'll let you extrapolate from there, and I was finishing rallycross runs this weekend at 230F, and engine is still perfectly happy, because I torqued to 23.  I'M not happy that it's running that hot, but much of the course was 7000-8000rpm in 2nd gear and, per Racing Beat, the water pump stalls at 6000 unless you underdrive the water pump.

Different priorities have different solutions.... I sure wouldn't want to try to run 30 pounds of boost through it, for more reasons than one, but I also wouldn't want to take one of those overtorqued/studded drag engines and bolt the throttle to the floor for minutes at a time, either.