What makes the rotary go "brap brap" instead of "hmmmm?"
 

Ok here's the stupid question of the day. I've heard some rotaries run quiet, like in the old "and the Mazda goes hmmmm" ads, while others have that heavy "brap brap" sound. Also heard some built engines sounds loud, but without the brap quality.

So, here's the question: what makes these engines go BRAP?

porting

overlap

hmmmmmmm comes from all the muffling of the stock manifold + cat/reactor.

brap brap is the Wankel uncorked.

The exhaust gases just rush through the port after combustion without waiting for valves to open, that's the "popping" sound.

yeah headers should make the brap a little more noticeable but not much at all, porting really brings it out.

My stockport with RB exhaust and Sterling carb still hums along nicely. Overlap will bring out the brap alot more. Not that brap is what your after when porting. it depends on your needs with the engine.

what do you mean by overlapping?

and the monster ported rotary goes brrrraaaapppppp hmmmmmmmm *nothing* apex seals gone!

Overlap- the span of time when both the exhaust and intake ports are open to one face of the rotor. This can be adjusted through porting by moving the exhaust trailing edge up, or the intake leading edge down.

Similar to overlap in a piston engine when both intake and exhaust valves are open at the same time.

Rotaries are naturally loud, but the stock restrictive exhaust system is able to quiet them down a lot. Uncork it and the volume goes way up.

As the others have said, brapping is caused by overlap.

Broccoli

The brap comes from large porting where leftover exhaust gasses enter the next intake phase, causing a near stall effect, but the way the rotary works, it basically won't die...unless tuning is off. Since there are two rotors, each with 3 phases (intake, burn/compression, exhaust.) it keeps the motor running...put a large P-Port will not idle smoothly at all...it will jump around 1700rpm-2000rpm.

if your familiar with piston engines, porting a rotary is just like installing a bigger cam. the rotary doesnt have to deal with valves, only port timing. that is the amount of time that a port is open to ingest or expell air. when you enlarge the ports, you're making the port open earlier and close later. when you advance the opening of the intake port and delay the closing of the exhaust port enough,you have overlap.

overlap is the amount of time that the exhaust port and the intake port are open at the same time. on a stock ported rotary, overlap is non-existant. that is the hummmmm. on a large street port, half -bridge, bridge-port, monster-port, j-port, and peri-port, the port-overlap steadily increases with each style of port. exhaust gasses are allowed into the intake charge contaminating it and causes a slight to moderate "stumble" or "brap-brap". the more overlap, the higher the idle rpm, the more brap.

one of the drawbacks of overlap is less low end torque and less streetability. the benefit is increased upper rpm range and higher horsepower. hope this helps.

Thanks folks.... answers the question!

Stock manifold and nothing else... (as in no exhaust) and mine braps. I hope to end that problem this coming week.

idk i kinda like the brap sound, but i didn't know it was an actual problem lol. i always just thought that that's what they're supposed to sound like. well,..learn something new everyday lol.

well not so much the brap is a problem, but I meant I hope to end the excessive loudness. The "brap" is actually kinda cool... but not when it's open-manifold loud.

BTW: http://youtube.com/watch?v=NjEL1N4f8qw

"Piston engine goes "boingo boingo" Mazda engine goes HMMMMMM"

The basic sound pattern is from having 2 plugs in one chamber so your hearing 4 combustions. Or 2 double taps. The modded engines and exhaust just of course make it sound better and louder.

I really understand porting a lot better based off of your answer. Thanks

You might also want to know about porting compared to a cam:

The length / height (around the peanut) of the port determines the "duration"
The width of the port determines the "lift"

It's not true that stock ports have no overlap. The Renesis was the first Mazda rotary to not have any overlap at all.

The BRAP BRAP BRAP I am thinking of sounds WAY too low a frequency to be individual combustions.

Lets say you idle at 750 rotations/minute which equals 12.5 rotations per second. Any 2rotor engine will have a combustion event 2 times per rotation. So,
12.5 rotations/second * 2 combustions/rotation = 25 combustions / second (at 750 rpm).

That would be 25 "BRAPS" per second based on the technical description in this thread. I am acoustomed to hearing something more on the order of 5 BRAPS per second at idle.

If I had to guess, it seems like the engine speed fluctuates for some reason (without intentional external input).

Could it be that the speed oscilates becasue of an imbalance between air pumping losses and BMEP???

so at 740 rpm the pumping losses drop, allowing the engine to speed up, then when it hits 760 rpm, the pumping losses catch up resulting in the engine dropping back down over and over again? I don't think a typical rpm gage would be accurate enough to show you this fluctuation at this low of a speed. Maybe the CAS and ECU can't tell the difference either, so there is no way to counteract this effect electronically. Do 13B-REW engines that use "crank trigger" instead of a CAS also go "BRAP BRAP BRAP"?

doesn't matter what engine or ignition system used, it's all in the port timing and the amount of overlap between intake and exhaust.

i challenge your theory

If you slow the "brap" down, what you're actually hearing is six pulses, followed by silence for the same amount of time. All three faces of each rotor fire once, then misfire once, then fire once, then misfire once, and so on.

yeah the brap is when its firing, depending on port and exhaust it might brap every 4th chamber event (you have to look at each chamber individually)

if you would like to learn more, the book 'rotary engine' is out on the web

IIRC, Mazda's rotaries have a 3:1 ratio of rotor to e-shaft speed. :scratch:

Whoops, the White One goes hmmmmmm, not brap. Is there something wrong with it?

http://www.youtube.com/watch?v=3bcwNT6XoX4

That was a very strange video ray...I'm not entirely sure what to say.

what made you guys say its "brap" "brap" "brap" thats Aus interpretation. Us in the US must make our own.

In comparison to a dog, a dog bark is interpreted by every country differently. here's a good example.

# English - woof, woof; ruff, ruff; arf, arf (large dogs and also the sound of sea lions); yap, yap (small dogs); bow-wow
# Afrikaans - blaf, blaf woef, woef keff, keff (small dogs)
# Albanian - ham, ham
# Arabic - hau, hau; how how (هو هو)
# Armenian - haf, haf
# Basque - au-au, txau-txau (small dogs), zaunk-zaunk (large dogs) and jau-jau (old dogs)
# Balinese - kong, kong
# Bulgarian - bau-bau (бау-бау); jaff, jaff (джаф-джаф)
# Catalan - bau, bau
# Chinese, Cantonese - wow, wow (汪汪)
# Chinese, Mandarin - wang, wang

+1 on the weird video, but non the less cool.

mine just goes purrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr rrrr

Mine goes: *cricket* *cricket*

God rest my little motor.....

At idle outdoors near traffic, you have to listen close to even hear if mine's running...

Its really called 3:3 ignition misfire by Mazda.

This from Kenichi Yamamoto's book.

Barry

Attachment 705225

My 50mm P-port can idle as low as 500rpms but i keep it around 1500.

ZOMG!!! A bridgeported Skyline!!!

http://www.youtube.com/watch?v=Cxg3Llt4cU8

Same principle.

^^ fuck yeah lol

Ok I read through that part of the online book, but the preceding paragraphs explain the situation a little better. Basically, when the intake manifold is at high vacuum it will suck exhaust into the intake chamber via the overlap. It will want to do this because the intake is at high vacuum, whereas the exhaust chamber / exhaust system will be a bit above atmospheric pressure. When the air in the intake that has been polluted makes it's way to the spark plug, nothing will fire because it is too diluted with inert chemicals (CO2, H2O). Of course it will also contain the full dose of vaporized fuel and some fresh air. When this chamber comes around the exhaust port again, the same thing will occur, but this time when the over-lapped intake chamber sucks crap out of the exhaust chamber, it will contribute to a mixture that will experience combustion. So on a chamber to chamber basis, each chamber will skip an ignition due to dilution, that succeed in an ignition the next time. I understand why each rotor skips 3, then succeeds in 3, but why is it that both rotor (in a 2rotor) will have success at the same time, and failure at the same time.

My next question is:
Could an ECU theoretically cut fuel on the 3 intake events that are destined to fail anyway (and then give a little more fuel to the ones that will succeed to make up for the fuel that would be lost in carry over). I know this only would help at idle, but you could probably greatly improve fuel economy at idle. This isn't something that most people on these forums would care about, but I'm sure Mazda tried something like this. Is this how the stock ECU works?

Max

Series 4 injection timing:

https://www.rx7club.com/attachment.p...1&d=1242406735

Series 6 injection timing (FD):

https://www.rx7club.com/attachment.p...1&d=1242406735

Series 2 Rx-8 injection timing ('09 spec):

https://www.rx7club.com/attachment.p...1&d=1242406735

I actually understand it :)

Hmmm, not sure what you've been smoking lately, but I want some!


Nothing wrong with the car, but if you listen closely to the exhaust note after she warms up, you'll hear about 10 stumbles per second (average for a stock 12a). They are more pronounced with an open exhaust, but they are still there from the factory. This is because, as PercentSevenC pointed out, ALL mazda rotaries have overlap. Even the renesis, which is the reason mazda decided to put valves in the thing (that's what I heard, I haven't ever studied the renesis design).

Also explains why bridges and other "wide" ports have a bigger problem with this; the port is not just exposed earlier, but MORE of the port is exposed earlier in the overlap.

The width of the port doesn't just perform the function of "lift" in a conventional engine, it also changes the RATE of opening... e.g., the "slope" of the cam-face, which governs how rapidly the port opens.

With a valved engine, there's a mechanical limit to how fast the port can open, because there is a mechanical limit as to how much slope-per-degree the cam and follower can tolerate without jamming. The rotary can be ported to open much more rapidly in comparison.

Also, the drawing leaves out the indentations in the faces of the rotors; makes it look like there's only a tiny path from port to port when overlapped, when in fact it's a sizeable volume that joins the two ports during overlap.

Nope, the Renesis doesn't have any overlap, nor any valves besides the auxiliary port actuators. That's the beauty of side exhaust ports.

Ah, then I stand corrected. Just goes to show that I should do my own research instead of trusting another.

I looked at the OP and thought what a stupid question. I have actually learned a bit about porting by reading all the posts. Thanks OP... for asking a question which is common knowledge ;D

you probably could. if you keep reading the book, mazda's solution is to inject the air pump air into the exhaust ports.

my PP has a scary amount of overlap, i put a degree wheel on it and spun it a couple times, its pretty obvious why its going to be angry...

That would not be a problem to me. Mine braps too stocky, I like it.