Tuned intake runner lenght?
 

So, I want to make a tuned runner for my 2011 Honda CBR250R motorcycle with it's fuel injected single. Has anyone seen a web site with a good calculator? This bike already has a huge airbox and air filter and a pronounced tuned runner extending into the clean side of the air box. It might already be as good as it can get for fuel economy but I have no idea what the tuning frequency currently is. It might be as high as 8,600 rpm where a big power peak develops whenever an after market exhaust is fitted. I might like to extend it to get the tuning to match my 65 mph cruise rpms of 6,000 and accept whatever reduction in peak hp at higher speeds that result.

wow! 1 square inches by 14 inches long for 6,000 rpm. It looks like the stock tuning is indeed closer to 1.5 square inches at 9.8 inches long for a peak of 8,700. I would have to make a major mod to find room for 4 more inches of length. Would probably have to add a 90* elbow. I wonder what the gain in torque at 6000 would be if I bother to try.

The throttle body is actually 1.77 square inches. 38mm diameter. His calculators don't allow to fix the area and calculate just the length vs rpm. I assume that the stock 1.77 square inches will still fairly closely follow theory for a 14 inch runner resonating at the desired 6,000 rpm. The over sized cross area only serving to broaden and round off the peak slightly.

...look here too: http://ecomodder.com/forum/279769-post37.html

...also, this book: Two-Stroke Tuner's Handbook, by Gordon Jennings, 1973, H.P.Books (ISBN 0-912656-41-7).

your elbow would have to be a GENTLE curve or it will screw up the reverb - look at the exhaust pipe, not the header, for an example.

also, tuning the intake ONLY works at WOT. Do you need more power at a lower RPM?

And you can get second and third harmonics to work as well.

I'm trying to get the highest efficiency at cruise rpm which would often be 25% throttle so there is some disruption at 4 inches from the valves. But it would also be nice in competition to lower the first torque peak from the current 6,000 rpm and get the most efficiency for 90% load pulse and glide either side of 6,000 as opposed to the power band from 6,000 to 8,700 as it is now. I can also play with the cam timing as the gears are taper fit and can be timed anywhere. Shaving the head to raise compression from the stock 10.7:1 is also not out of the picture eventually. Will a closed pipe have a second harmonic or only odd order harmonics of 1,3,5?

Is the frequency of the intake determined only by the length, independent of the cross section for a normal range of areas which have apparently been optimized at 1.7 square inches for red line 10,500 rpm.

That's a good link. Up to now the only information I've found on intake tuning was highly technical. This calculator might be a bit general, but it is certainly a good starting point. I didn't see any mention of incorporating the runner length and diameter with a plenum size to take advantage of the Helmholtz principal. Has anyone seen any similar type calculators that will give ball park figures for that kind of set up?

with part throttle, you cannot get intake tuning, or exhaust tuning for that matter.

Tuning an intake for part throttle cannot be done, and should not be a goal as you are missing the point of it all.

If it were me, and I wanted better mileage, I'd put longer gears in it to where I can lower the rpm and use more throttle.

I have half of the longest gears I can fit installed with a +1 ft sprocket. The -2 tooth rear is late in production due to the flooding in Thailand a few months ago. this will drop my cruise rpm at 65 mph from 6,500 rpm to 5,900. I should be able to fit a half circle of 38mm plastic hose inside the stock air box to get a 15 inch path length to tune the full throttle pulse and glide.

W.O.T @ drag racing equations.

your engine peaks not only at 8700 rpm, and also at 6450 rpm.

tuning also depends on valve area, and hence port area. waves happen at all rpm, at all throttle positions.

6000 rpm wants a 380 mm intake runner, as measured from the valve to the plenum. the plenum can be as large as 2L, but slightly smaller (~1.5L) will help throttle response.

increasing flow by porting, in other words reducing pumping loss, will boost economy.

imo the biggest thing is the large cam overlap designed for 8700 rpm power. you want minimal overlap so you're not sucking raw fuel out the tail pipe. roughly in the 200* @ 0.050", 112* LSA.
hth

I'm curious as to how you do your runner and plenum calculations.

i made up a spreadsheet with formulae derived from decades of (someone else's) empirical research. it's more for analysing engines, than for predicting power output. the above numbers are actually very decent starting points. fabbing an intake can be a wonderful learning experience - and minimal economy benefit. can you even get "eco" cams for the 250?

fwiw, 1D simulators like engine analyzer pro aren't accurate or useful. i just got a quote today for professional software that costs $18,000.00 for a 1 year license! (or buy 2 get 1 free :) the amount of information you need to input is staggering (and what it can do is mind blowing!). not many professionals have this software, and nobody in Canada does. I would love to buy a license this year. It amazes me how much most OE and pretty much all aftermarket miss the mark.

I suppose that obtaining a copy of your spread sheet is out of the question then?

If you were to plug the intake runner cross sectional area, length, and volume of the plenum, could it calculate the RPM at which the system would be tuned? Would it also calculate how much less of an effect would be felt with the harmonics? Would it give you an idea of about how many revs either side of optimum would still receive some benefit?

I dug up an old post of mine (2004) showing the relationship between runner length and harmonics.
Edit: never mind you need to be registered on that forum to view attachments, so I just re attach it here.
In certian RPM ranges its like having free turbo or super charger boost.

I have found a lot of runners like to be right around 24 inches.

Fascinating! Thanks for posting that.

So, If I'm understanding the chart right a 28" runner will peak at ~5100 RPM with the 2nd harmonic at a strength of 10%. A 14" runner will peak at ~3600 RPM with the 3d harmonic at a strength of 7%. A 7" runner with the 4th harmonic will peak at ~2750 RPM at a strength of 4%.

% of what?

Keep in mind I am discussing my motorcycle engine which is an FI single and has the throttle body in the middle of the runner as pictured in the earlier post so a plenum isn't really present as the air box is running with the dirty side lid off right now so it should pretty much always stay at atmospheric regardless of any pulsing.
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The % is the amount of extra air that can be forced into the cylinders.

I have found on quite a few trucks regardless of make modle and year the OEMs like to keep the intake runner length right around 24''.

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?

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.