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