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Old 01-31-2012, 08:08 PM   #21 (permalink)
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That's what I thought, but getting into that 2nd harmonic might be difficult on a slow turning engine.

Another thing. Lets take the 24" length as an example. At ~6000 RPM you get a 10% boost, at ~4500 RPM you get a 7% boost, at ~3400 RPM you get a 4% boost. What happens in between these peaks? It would seem that if pressure peaks, it should also valley in sort of a sine wave. Perhaps there would be negative pressure at ~5250, ~3950, and ~3000 in decreasing amounts. It might make for an interesting torque curve.

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Old 02-08-2012, 12:34 AM   #22 (permalink)
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Quote:
Originally Posted by E4ODnut View Post
That's what I thought, but getting into that 2nd harmonic might be difficult on a slow turning engine.

Another thing. Lets take the 24" length as an example. At ~6000 RPM you get a 10% boost, at ~4500 RPM you get a 7% boost, at ~3400 RPM you get a 4% boost. What happens in between these peaks? It would seem that if pressure peaks, it should also valley in sort of a sine wave. Perhaps there would be negative pressure at ~5250, ~3950, and ~3000 in decreasing amounts. It might make for an interesting torque curve.
I'm impressed that you hypothesized about the troughs. That's exactly what happens. There are always 4 peaks, and 5 troughs. It's the job of the exhaust system tuning to fill in those troughs. 2nd wave is the strongest, and also the peakiest. However, there is actually a small difference between 2nd and 3rd wave peaks. As I said before, a street car tuned to 3rd wave at 6300 rpm has the 2nd wave at 8500 rpm.

That graph is a little funny. They don't tell you that nobody uses 2nd wave tuning below 7000 rpm or so. Otherwise the runner becomes too long, has more resistance to flow, and hurts your VE at higher rpm. Think L98 Corvette v. LS1 v. LT4 - 2nd, 3rd, and 4th wave tuning, respectively. The LT4 had the flattest torque curve, and highest peak hp of the three. Race engines use 2nd wave because they peak at such a high rpm that the runner length is fairly short. See how a little bit of knowledge is dangerous?

I won't share my spreadsheet. runner (in inches) length is approx. 123,000/rpm. I can't predict power curves...software that does that accurately costs $18,000 per seat per year!

Sendler, in your case the runner length would be from the valves to the trumpet - the sudden expansion. A plenum does provide some supercharging - helmholtz resonator tuning. If the plenum has a 2" hole or so (larger than the runner), it will always be at atmospheric pressure. That's the benefit of moving the throttle blade into the runner. What difference have you noticed with the lid off?
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Old 02-08-2012, 07:37 AM   #23 (permalink)
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What difference have you noticed with the lid off?
I never did any full throttle dyno testing and would need an MPGuino to really measure what is happening at part throttle cruise speeds. removing the air box cover was one of the first things I did to the bike so I don't even really have any comparison with fuel economy. Don't know if it helped me, hurt me, or just allows more noise to escape. Looking at the cut away photos of the engine again, it is a long way down through the throttle body to the valve. With the stock runner that is there, it may already be 14 inches total with the throttle plate near the middle. I will have to stick a piece of wire down in there to measure.

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