- The rotary engine does have a reputation for breaking and that reputation isn't necessarily
always 100% justified.
There are some reasons why this is seemingly so common.
We're here with Rob Dahm to talk a little bit more about rotary engine strength, reliability
and tuning.
So Rob, for a start, what do you see as the main reason why rotary engines traditionally have had
this reputation of breaking?
I mean we've all seen the memes, boost in one side, apex seals out the other side.
- Yeah so I would like to preface that by saying, because I'm not a traditional builder, I rarely
get to see the actuality of what a typical person sees the rotary as and so I've got all
the variety, the two rotor, three, four and I got the six in production and so I see a lot of
different misnomers or things that people think about the engine or have come from the history
of the motor that incorrectly characterise it now.
And so there's a lot of different things we can expand on but I can definitely say that
poor motor, it gets the reputation it shouldn't get and it's an honour that I see a lot of people
call me Rotor Daddy when there are people like Pac Performance and others that have
really set the stage for me to succeed, both with tuning or building and supporting that motor
so it's an honour to be in this spot, speaking as almost an authority or a subject matter
expert when really I'm one of the newer people to the scene but I certainly have learned a lot
and built off of the existing.
- I think where you differ though from some of those shops, and obviously not to disrespect
Pac or any of those who are achieving, the issue we see is obviously they're doing it commercially,
there are a lot of players in the market and they want to keep their secrets, as they call them,
to themselves, which is understandable so they kind of don't really share too much of what's
going on inside the engine.
On the other hand, you're really not doing this for a living, you want to share this information
so you get to see what's going on.
Now I want to really focus here on two separate elements and I want to start, which is more my
specialty, which is the tuning side of things.
And I think, in my opinion, what I've seen, a lot of the rotary engine blow ups or failures
are actually the result of poor tuning with a tuner who's perhaps had little to no experience
on rotary engines and is trying to apply what they know works on a piston style engine.
Doesn't always work, let's hear your take on the differences.
- So I will tell you, the motion of anybody getting into tuning a rotary engine is as simple as how
people perceive using E85 on a rotary engine.
There was that common consensus that it was bad, that you were going to ruin the motor
and I heard of a guy or a friend that did it this way and had problems and so there's a
lot of people that treat these engines with that sort of anecdotal information and so they go
into tuning the engine wrong, instead of looking at the facts and I think you and I very much share
that diagnostic approach and cause and effect and see it for yourself and having that experience
I think is what's given me a competitive advantage over many people and of course even as simple
as just making tonnes of power which is one type of challenge but then also making that
power reliably which is the other axis of challenge.
- The reliable is the key part here.
So there's two separate elements, there's a lot we could dive into here but just to keep it
relatively brief, there's the fuelling and the ignition timing.
So let's start with the ignition timing and the rotary engine, what's important for viewers to
understand is that they're a sensitive little soul.
They really need to be treated with a lot of TLC and where we could potentially on a piston
engine, run the engine into light levels of detonation on the dyno, obviously it's not
preferred but it can happen, the engine's not instantly going to fall apart, we can find where
that knock threshold is, where MBT is when we're optimising the timing.
That's not a real good approach on a rotary engine is it?
- No I mean I've done that approach on a rotary engine and I paid the price and detonated
the motor and I mean it's the same reason I don't, some people do but I don't run knock
sensors, because the knock is basically the moment the engine fails.
- That's a really good point, I've actually had a lot of people reach out on our YouTube channel
and on our forums as a result of my rotary tuning worked example which is in one of our
courses and I specifically make the point of I do not believe in knock control, this would
be controversial, have a lot of people here saying I'm wrong but I do not believe in knock
control, obviously for the same reason.
I'll talk about that in a little bit more detail, when we're validating or tuning a knock
control strategy, it's not magic, we actually need to teach it what frequencies to look for and what
the noise level is which is normal background engine operation vs knock and the only way
to do that is to make the engine purposefully knock.
Now again coming back to what I mentioned, that's not something I'm prepared to do on a
rotary engine.
Almost any level of knock can be enough in some instances to cause damage so we want
to stay away from that.
Alright so that's our ignition tuning, conservative is really the aim of the game.
Fortunately, we don't necessarily see such a steep relationship between torque and ignition
timing with a rotary engine so we can kind of be a little conservative and we're not leaving
bags of power on the table are we?
- No I actually rented a hub dyno recently with my rotary corvette and went from about
6° timing all the way up to 22 and I did see obviously improvement, increase in power but
it was actually so subtle.
You're expecting 50 horsepower more and that's not 50 horsepower more and so I
believe let's keep it more conservative and then just add boost.
- Yeah OK so that's one element and then the other as I mentioned, the fuelling.
And again I think people try to apply what they know works on a piston engine.
I mean I hear crazy air/fuel ratio targets being discussed with rotary engines in the low 10:1,
maybe into the 9:1, I haven't found any need to run that rich but generally I will use a little
additional fuel to help with fuelling, how do you approach it?
- Completely agree, I share all of this in my videos but I run in lambda as you often do and
I found that I was running more lean than many of my friends who were saying hey you're going
to blow your motor at that and I have not yet and so I run on ethanol, I run a 0.82 lambda
and what's fascinating about that is, we spoke briefly off camera is that injector timing I think
especially with large injectors is a deal breaker on a rotary engine.
I found that myself is that with one simple button click my injector timing, whether it
is the beginning of the time of injecting or the end, at about 50%, 30 to 50% of injector
duty is extremely critical, especially when your secondaries are coming on and there's
26 100th for example, 'cause you've got ethanol and you want to really dump that in there.
I found in my rotary corvette that I was still meeting the same air to fuel ratio but it was
actually circumventing the motor and blowing by through the intake to the exhaust, igniting
and still showing the same air to fuel ratio but it was not going through the motor and as a
result, the motor went lean.
- So your actual air/fuel ratio inside of the engine which is what counts was leaner than what you
were registering, that's what you're saying?
- That's correct, and so that's where I was like OK well if you're tuning for a specific air/fuel
ratio, that's not the entire picture if you have your injector timing off.
- Yeah absolutely I think people don't understand some of the tuning parameters that can actually
affect what we're seeing.
Alright we'll park the tuning side of things there, there's a bunch more we could talk
about but that's a high level view of some of the elements that I see as problems.
Let's dive into the engines now, the other thing I sort of see is that the rotary engine has always
been sort of discussed as being simple, in terms of it doesn't have as many moving components
as a typical piston engine.
Absolutely of course that's correct and I think the knock on effect of that is, a lot of young
enthusiasts think that well it's so simple, I can build it in my back shed, maybe with
a bit of sand blowing in the door, because you're living near the beach or something
like that and it ain't no thing.
These engines still need precision when it comes to assembling them, we need to work
in the same clean environment that we'd be building any engines, the tolerances, clearances
inside them, absolutely every bit as critical as any other engine?
- Incredibly critical.
The best place for any beginner, getting into rotary engines to start is Mazda's spec, Mazda's
parts, Mazda's side seals, Mazda's, even, you could even say Mazda apex seals but to
start, using their tolerances is a phenomenal place to be, so I would go, on a budget I would
go to Amazon and get a set of callipers, a micrometer, all the different things to measure
and make sure that your seals are in spec or the rotor is the correct width compared to your rotor
housing, all of those things are pretty critical to prevent catastrophic failures and after that's
all verified, then yes I think that the engine assembly is simple but that's like the home
stretch.
- Yeah that's the last probably 10% of the actual job, all of the hard work's already been done up
to that point correct?
- Yeah 100% now I do like to test, like on my 3 rotor, I purposefully have non balanced
rotors, so I let the motor, I want to see OK what's the effect, and that motor surprisingly
has worked way better than I thought, one of the rotors are dented.
And so there is some level of backyard, shade tree mechanic room to give on a
rotary engine but it's only in certain areas and you have to be aware of it.
- Alright let's talk about what are the key upgrades that should be applied when building a
performance rotary engine and maybe what order they should be.
I'm guessing there's a lot of people out there who are pushing a factory engine beyond where it's
probably safe and likewise there's probably a lot of people that are building a hugely
expensive long motor with parts that they don't really need for the power they're
aiming for.
- Yes so I understand that a shop needs to survive by making money, that's an absolute
necessity so that puts me in a very unique position that I am not a shop and so my job
technically at the end of the day I'm an entertainer, I'm not an engineer but that's
how I think of myself 'cause I'm engineering these things but my job is to entertain and
so by entertaining, I love to share information.
So what I've noticed is a lot of shops will add on things that again they're hearsay, anecdotal
but make them money to stay in business.
Something as simple as lightening rotors, anecdotally makes sense like yes less rotating
mass for sure but is that at 400 horsepower on a 2 rotor going to make a big difference?
No, price per value, and especially if say you do have a failure in the motor, now you're
having to pay all this extra money to replace a very customised part so my job in my mind was
to use a stock rotor, roughly the same weight as the other rotor, stock seals and then just push
that to see its limits with minimal adjustment to the motor to see OK when fans ask me, hey I want
to make a 400 horsepower 2 rotor or a 500 or 700, at what point certain modifications I feel
from my experience are justified.
- Alright let's talk about some of those common modifications, so stud kits, dowelling, the housings
to the plates, those are two of the more common modifications, then in high end race engines
there's clearancing of the rotors as well, what, if any of those are necessary?
Let's say we wanted to shoot for maybe 600 horsepower, nothing crazy in a 2 rotor turbo?
- Yeah you almost nailed it perfectly from what I've found, is starting with the idea of using
ethanol to prevent a lot of carbon build up in the motor so you don't have that dieselling
hot spot which I've experienced, that's one.
And then two is dowelling, I'm a big fan of dowelling, that was kind of shown to me, it
wasn't like I discovered it, is that when I originally purchased this motor, it was very
controversial as to whether it would run well or not but it was built with actually Jeff from
Precision Engineering and made these 16 mm which are the same diameter as the stock tube dowels
but solid dowel bolts so they were like a stud that was also the dowel and my findings were
that that was a phenomenal solution for the torsional challenges of the motor and so I'm
a firm believer that keeping it simple, I tell fans if they want 400 to 500 horsepower, a stud kit and
then adding some dowels which does take some machine work but the stud kit first, maybe buy,
Pineapple Racing sells a solid dowel kit which is just the upper and the lower one, and at the very
least the lower one alone, strengthening that side to side.
- The torsional rigidity, increasing that.
- Yeah so that's the very first thing to attack in my opinion.
Especially as a street car where sometimes you're slamming on it, sometimes you're not, if it's not
a purpose built motor, that's what I attack first and then as you go on, I have found that stock rotors,
especially at high high RPM, do make love to the side plates and you'll see the scuffing and the
discoloration of the tip of the rotor, not the apex seal, not the seals, but the actual physical rotor
and I've gotten away with not catastrophically destroying my motors but pulling them out and
going OK you do need to take that 5000ths off the tip, if you're looking at the rotor spinning
like this, it's the tips, you almost have to put it on a lathe and just take all the corners down,
that seems to be a good next step once you start going really high RPM.
- So that's more RPM vs actual outright power which you can understand, essentially at higher
RPM, the rotors tend to want to walk about a little bit inside of the rotor housings and just
make that contact.
I'm interested, with that process, is that more race only, is there downsides in terms of increasing
blow by etc or does it not really make a big difference?
- On a traditional motor, this is not a traditional motor 'cause it has a dry sump,
so there's a vacuum in the crank case.
In the vacuum crank case situation, I don't think it makes very much difference at all
because all the seals are almost being sucked to them, I can see that it wears, the more the
vacuum, I have a vacuum regulator on this car because the stronger the vacuum is,
the more I was wearing down my side seals and corner seals and so on a traditional motor
where there is no vacuum holding those seals, assisting those seals, yeah I definitely see
more blow by on my 3 rotor, it fills that catch can pretty quick.
- OK so I mean there's compromises that we need to understand and be prepared to take.
One of the other obvious questions here is apex seals.
And I mean most people building a performance rotary will jump to an aftermarket apex seal,
there's literally dozens of options out there all with their own sets of pros and cons.
Then there's also the Mazda factory seals as well.
My personal take is that people probably jump away from the factory seals maybe a little bit
too early and the factory seals, when tuned correctly are pretty strong and do a great job
of not beating up on the housing so where is the sort of line in the sand do you think where
aftermarket seals are non negotiable.
- Yeah so I will definitely say, and because my adventures are very public, you'll see the old
traditional rotary engine builders say that you're doing something wrong if the stock apex
seals break.
Now that is objectively true, you're doing something wrong but I think that the better way to approach
the motor as an enthusiast, not as a obsessive builder but as a person that's in their garage and
doing it with a reasonable budget, an aftermarket seal does have some benefits that outweigh the
trade offs associated with those benefits.
And so a softer seal tends to be, which I found the soft soft seals tend to warp under intense
moments but they never take out the entire motor, you just lose compression and you
said off camera, that almost forces you to have to rebuild the motor before it's
catastrophic.
- Yeah I mean it's obviously never ideal when you lose compression and the engine either won't
start or is really hard to start which coincidentally is one of the big red flags
with rotary engines.
If it's difficult to start, chances are, start looking at what's gone wrong but of course,
the failure mode there is bending and losing compression as opposed to catastrophic
destruction and when one of these apex seals lets go, there's not a lot left of your engine
and also your turbocharger if it's forced induction.
Look Rob I think we'll finish it off there, we could talk for probably another couple of
hours on rotary engines but there's a few tips hopefully for our viewers.
If people want to follow you and see what you're up to, where are they best to do so?
- Rob Dahm on all the different platforms.
- Perfect and we'll drop a link in our description there so people can find you easily.
Thanks again for your time Rob and enjoy the show.
- You too.
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