ZenVirZan

Hi all

Equal length headers are cool because they allow equal spacing between pulses, which gives the smoothest sound and often the highest pitch. When a runner (or more) is longer than others, it causes the pulses to be slightly uneven. Our ears interpret each pulse as a contribution towards the pitch of the exhaust note, and when they are uneven they typically add harmonics but also drop the perceived pitch (see: all UEL wrxes)

A two rotor has two ignition events per 360 degrees.
A three rotor has three, a four rotor has four, etc.

This is why a two rotor sounds similar in pitch to a 4cyl engine of the same RPM. The engines have their own unique sound, but the pitch is in the same ball park. A 4 stroke 4cyl has four ignition events across 720 degrees, or in other words two ignition events per 360 degrees, the same as a 2 rotor.

We're in the fortunate situation with the renesis that we have two exhaust ports for a single cylinder. Why is this a big deal? Well, usually when you have a UEL header, it delays an entire cylinder's exhaust pulse event, which causes a change in the sound but ultimately the same number of pulses end up at the tail pipe, just with some exhaust pulses slightly delayed. But with the renesis, due to the siamese exhaust port, in theory we could both have delayed pulses AND normal pulses at the same time!

What does this mean?

If someone was to increase the siamese port runner length to be double that of the front/rear runners, there would be pulses arriving with twice the frequency.
Pulse order

• EL Stock (two distinct pulses per 360 deg):
  • Front/SiaF
  • Rear/SiaR
• UEL Modified (four distinct pulses per 360 deg):
  • Front
  • SiaF
  • Rear
  • SiaR

This is four pulses per 360 degrees, closer to that of a four rotor. Even if it's not perfectly offset to be even pulses compared to the primary front/rear runners, any additional pulses will count towards an increase in pitch.

Has anyone tried playing around with the center runner length? I'd imagine the runner would need to be tuned for the RPM it should be singing at, but if it turns out it just needs to be double the other runners or something then hey even easer. It may not make any extra power, but if it makes a Renesis sound like a 26B at redline I'd make that trade

TeamRX8

iTrader: (25)

should I laugh or should I cry … 🤔
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ZenVirZan

Which part do you take issue with? I know you know your stuff, and I'd genuinely love to hear why you don't think it'd work. It probably doesn't hence why no one does this, but I'd be curious to know why.

ZenVirZan

Crunching the numbers with respect to a frequency of 8000 rpm:
8000 rpm / (60 / 2 rotors) = 266.66 Hz
1/266.66 = 0.00375 seconds between pulses
0.00375/2 = 0.001875 seconds offset required

The pressure wave travels at the speed of sound, 343m/s.
0.001875 * 343 = 0.643125 metres.
So the siamese header would need to be 65cm longer than the front and rear runners to achieve this effect at around 8000 rpm.

So it's a ridiculously long header, yeah, and will require crazy fabrication, but it's not impossible. For a Proof-of-concept you could use a spiral of narrower diameter pipe as the pressure wave isn't impacted by diameter (exhaust gas flow is of course, but we're not aiming for power here). I'm FI so I don't have the space for it, but I'd be fascinated to see if any fabricators could make it work.

EDIT: the speed of sound changes based on temperature and air pressure. This could be 500m/s or more, which means it may need to be an additional 94cm or longer

EDIT2: numbers updated as I didn't calculate for two rotors initially

TeamRX8

iTrader: (25)

let me guess; 20 years of information is on this forum, yet you made no attempt to search and see if this subject ever came up before in all that time. Like nobody here ever had such an idea before, right?

but let me get you going in the right direction. The Renesis engine has zero-none-nada intake-exhaust timing overlap. That’s right; none. That alone throws out all the time you wasted even thinking about the subject. Because the theory you’re using doesn't apply at all for that particular circumstance. .
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ZenVirZan

What does timing overlap have to do with what I'm talking about? I'm not talking about scavenging at all, port timing and overlap has nothing to do with this thread.

TeamRX8

iTrader: (25)

I always said that nobody on here ever did anything dumber than ever I did, but you might actually have me beat with this idea
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ZenVirZan

Thanks for the insight

TeamRX8

iTrader: (25)

hoping you end up with the best sounding 160hp engine out there
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