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787B Exhaust Pitch

Old 12-09-13, 10:27 AM
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787B Exhaust Pitch

I've wondered about this for a while now...
What makes the 787B racecar's exhaust note so high pitched?
Is it the exhaust construction?
Maybe the number of rotors?
Port style?
Special header?
Any info would be helpful!
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Old 12-09-13, 11:50 AM
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Old 12-09-13, 12:40 PM
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lmao, i'd say the same thing, the 787B is like a flat plane crank v8, 90 degree firing intervals, but with no adjacent cylinders like an american V8.

for some reason you can hear the difference between 180 degree firings (4 cyl/2 rotor), 120/60 degree (6/12 cylinder) and 90, v8.

the american v8s have two cylinders that are next to each other physically and in the firing order, so we hear these cylinders as one big one, in an irregular interval, which is why they sound like they do.

the 787B exhaust really is normal, its just a 4-1 header into a straight thru muffler. header looks to be in the 30" range, pipes look fairly big, but they are probably around 2" OD, my fist doesn't quite fit in the tailpipe, so ~80mm pipe.
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Old 12-11-13, 09:06 PM
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That's actually really interesting to me.
1. Do you think if most American V8's were 90 degree cranks, they'd have a smoother operation? Less mass/TDC/cylinder= more balance at higher RPM?
2. I'm completely smitten with the high pitched sound... is there any way of replicating or creating a similar pitch with a stock 13B with or without custom piping?
3. A long shot... since the 13B reaches its TDC at 270, 810, etc degrees (making it 1.5x longer than a traditional piston engine) is there any functional way of tuning that down to 90 degrees?
4. These questions are all theory so the more info the better! I'm working on a sort of "dissertation" on the rotary engine by itself and I've become completely obsessed lol.
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Old 12-12-13, 11:06 AM
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Originally Posted by BloodbathRX7 View Post
That's actually really interesting to me.
1. Do you think if most American V8's were 90 degree cranks, they'd have a smoother operation? Less mass/TDC/cylinder= more balance at higher RPM?
not that i know much about v8's, but it seems like the american way is smoother, it just sounds weird.

2. I'm completely smitten with the high pitched sound... is there any way of replicating or creating a similar pitch with a stock 13B with or without custom piping?
i'm not sure why the ear can detect a 120 cycle vs a 180 vs a 90 cycle so easily, but we can. you can run slightly smaller pipes, like a pipe organ, pitch and diameter are related.

as an aside a 13B at full tilt makes a happy noise

3. A long shot... since the 13B reaches its TDC at 270, 810, etc degrees (making it 1.5x longer than a traditional piston engine) is there any functional way of tuning that down to 90 degrees?
hmm i don't see a way, but that doesn't mean there isn't one

4. These questions are all theory so the more info the better! I'm working on a sort of "dissertation" on the rotary engine by itself and I've become completely obsessed lol.
yeah its pretty neat stuff, and its pretty well documented lots of SAE papers and things
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Old 12-12-13, 11:44 AM
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Excellent. Yeah I've printed off 8 or so service manuals for the rew, the msp, and the first gen modification manual. Great stuff! Thank you for the answers, I'm sure I'll have more soon.
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Old 12-15-13, 11:57 PM
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Originally Posted by BloodbathRX7 View Post
That's actually really interesting to me.
1. Do you think if most American V8's were 90 degree cranks, they'd have a smoother operation? Less mass/TDC/cylinder= more balance at higher RPM?
2. I'm completely smitten with the high pitched sound... is there any way of replicating or creating a similar pitch with a stock 13B with or without custom piping?
3. A long shot... since the 13B reaches its TDC at 270, 810, etc degrees (making it 1.5x longer than a traditional piston engine) is there any functional way of tuning that down to 90 degrees?
4. These questions are all theory so the more info the better! I'm working on a sort of "dissertation" on the rotary engine by itself and I've become completely obsessed lol.
Most American V-8's are 90 degree cranks. Very few street cars have the "flat plane" 180 degree crank. The only one I'm sure of is the 430-458 Ferrari.
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Old 12-16-13, 02:58 PM
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Mmm Italians :P
Since I can't afford a Ferrari I'll stick with my Mazdas lol. Although, the Mondial goes for about $20g nowadays. Anyway, how much power loss would one experience if they installed smaller diameter exhaust pipes? Significant? Mediocre? Minimal? Somewhere in between?
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Old 12-17-13, 01:12 PM
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Originally Posted by BloodbathRX7 View Post
Mmm Italians :P
Since I can't afford a Ferrari I'll stick with my Mazdas lol. Although, the Mondial goes for about $20g nowadays. Anyway, how much power loss would one experience if they installed smaller diameter exhaust pipes? Significant? Mediocre? Minimal? Somewhere in between?
it depends! header primaries are 2" OD pipe, usually, and this makes the ID something like 1.73" which is just about ideal, so we'll leave these alone.

after the collector, depending on power level we'll collect into a pipe that is anything from 2" OD to 3" OD.

my car is collected into a 3" pipe, and it lets a lot of low frequencies out, kind of boomy, just to drive it around i have a resonated exhaust tip i put on it, and its maybe 2x 1.25"? this tip kills all the low frequency noises. the primary effect is that it seems quieter, but its mostly through a change in pitch.

its not in ferrari territory though…
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