Can anyone shed any light on any relevant calculations to size ducting for an intake of a BEC with about 170hp?

I don't know the calculations, although there is a whole web-full of them out there!

What I do know is:
I connected a 3inch hose up on mine (about 90cm long), which is a GSXR1100 (of the type that you are familiar with!).

I have a megasquirt that has a MAP sensor.

With or without the hose I saw no difference in air-pressure at WOT and max RPM. It only measures to 1kPa, but 98 at half-revs, dropping to about 97kPa when at 10.2k (my rev-limiter comes in earlier than it should).


So my suggestion is that 3inch diameter is adequate.

I will be interested to see whether this is supported out by the calculations.

Matt

That sounds about sensible, I'll keep looking around for info - the original GSXR airbox has a fairly small entry hole...

James

Agreed. The airbox hole is smaller than my ducting (I tapped it over the hole initially).


Actually, you got me interested. Unhealthy as I am supposed to be working but....


have you seen this link?
flow_calculator

There is both theory and a calculator.

Based on a length of 90cm, and a flow rate of 120litres/second (that is 14400rpm from a 4 stroke litre-bike, assuming 100% filling, so an over-estimate of flow rate).

Then the pressure drop with 75mm is only 0.08psi

I have tabulated some other numbers:
75mm 0.08psi
60mm 0.2psi
50mm 0.5psi
40mm 1.34psi
35mm 2.9psi

below 33mm it all goes supersonic! so the calculations don't work!

I agree the entry hole into the GSXR airbox looks a bit small. Measuring it gives:
50mm diameter, and 57mm length.
Giving a pressure loss of 0.38psi at te above 120l/s

At 80l/s which is probably the limit of the old-gixer, this gives a pressure drop of 0.15psi, which may the the source of the pressure drop that I see.

I guess the size of this is to allow the thing to resonate (Helmholtz resonator) but that would be more calculations. Damn I will never get any work done.


Matt

So, FWIW

50mm diameter hole
57mm length (approximate as it has nice radii on it)
10litre airbox (I am no idea of volume but this seems to be a typical volume)

Gives a Helmholz resonator freq of 94Hz.

Multiplying from cycles per sec, to cycles per min. Gives 5700rpm.

Interesting, as the VE table on my engine has a clear dip at 5200 and 6000 (i.e. 85% at WOT) versus the values at 3500 and 6700 (95% at WOT).

Either the air box is there to correct for something (i.e. exhaust) and only manages to partially do it, OR I have the frequency wrong and there is a 4-stroke thing that I need to take account of which means I get a power DROP at this range of frequencies.

I guess I can take off the long rubber inlet (effectively moving the resonance stratospheric) to see which is the case. If only it weren't raining.



Interesting. Sorry to hijack your thread.



Matt

[Edited on 8/11/09 by matt_gsxr]

Excellent Work!

That definately gives me some idea of what size i'll need (I think I will run two pipes as the hole is rectangular and bring air in from the outside of each seat) Having the air intake 4 inches from your left ear makes it painfully loud (if impressive!)

Obviously need to take precautions on picking up water from so low...

James

First point to note is that bike engines will have greater than 100% VE, sometimes up to 125%.

As for the right diameter pipe, as can be seen from the above posts, any pipe provides a pressure drop, thus the biggest possible is best. Shorter is also better. Is this going to the standard bike airbox, if so then what ever size is easiest to fit. In terms of will it be big enough, i.e, if the engine was massive, or revved to the moon, then the best starting point is the combined area of the inlet ports/butterflies.

As for Helmholtz, longer pipe, lower frequency, wider pipe, lower frequency. The volume does include the TB dimensions and all the air volume to the mouth of the inlet pipe, and there will be a dip either side of the peak at the specifed rpm. Unless a new airbox can be made, or the engine is vastly different from standard, then stick to the standard everything. The manufacturers spend alot of money on them.

[Edited on 9/11/09 by brianthemagical]

Brian,

Interesting point on amount of air getting into the engine. Presumably this only goes over 100% at some tuned points in the rev range? Given that I tune on AFR and directly adjust the amount of fuel per squirt the fueling table should give a decent measure of air going in. My fueling numbers drop off above 80% of the max RPM so I assume the cylinder filling drops off there (well torque curve and fueling will be related). But please correct me if I am wrong (I am trying to learn, but there is a lot of mystery out there).

I agree with the general principle of "the airbox has been designed by clever people, so give them some credit". But they do have limitations, the GSXR1100 for example has quite restrictive flow to cylinders 1 and 4, which is caused by the engine being wide and the frame of the bike chassis. This chassis has long ago been turned into quite a lot of beer cans, so it isn't in the way any more. Also there is the issue of noise, which is probably more of a problem on the bike install than the car.

I guess I have got to your "substantial changes", now that I have EFI'ed it and obviously have a different manifold, exhaust and airfilter. Presumably if I can see the amount of air the engine is getting with the AFR/fueling table, then I can investigate the effect of small modifications (for example changing the inlet into the air-box for something longer or shorter, or trying longer inlet trumpets).


There doesn't seem to be much sharing of knowledge on airbox design out in internet land, and I don't have the inclination to do the full Computer Fluid Design modelling of this. I know the Andy Bates airbox is well regarded, and that is based on a short (<2ft), wide (4in), direct (straight line often) intake of undisturbed cold air flow. Personally it isn't ideal for me (price and the need for a big bonnet hole being my concerns, also it probably doesn't fit the GSXR1100).

If anyone has experiences/knowledge of what sidecar teams do (for example) then I'd be interested. On the other hand A.B. has probably used that knowledge in his design, so maybe short, wide and cool are the critical things.

Interesting stuff.

Matt