I'm trying to sort out how to do my air intake / filters
(2.1 Pinto with R1 carbs)
The way I see it I have 2 options:
1- Foam filters (/sausage filter) directly onto the carbs
2- An airbox (/plenum?) with a cone filter
Which should I go for ?
If I go for an airbox I'll have make one from scratch - anyone know how to design/ size these ?
quote:
Originally posted by mcerd1
If I go for an airbox I'll have make one from scratch - anyone know how to design/ size these ?
Airboxes are tuned. You have to match the inlet tube size to the volume of the box to get it to resonate at the right engine speed.
Once you get a few hundred RPM above the resonant speed, the airbox doesn't resonate any more. It is just a box of filtered air. The box is
useful for containing / diffusing intake noise. To stop the box being restrictive in term of air flow, you should make the inlet pipe to the box have
a bigger crossectional area than the carb venturi tubes. 2 - 3 times bigger. You don't need it to be as big as all 4 tubes as each one only
breathes for 1 stroke out of 4.
At the resonant frequency, the air box will self presurise slightly giving an extra push to the air / fuel mixture. You should get a bit more torque
at this speed and a few hundred RPM either side.
At 2/3 the tuned speed the opposite will happen. The airbox will have a slightly low pressure and torque will be a bit worse. At half the tuned
speed it will pressurise again and at 1/3 it won't...
The bigger the air box in relation to the size of the cylinder, the less the tuned effect. If you have a 1 litre engine and a 10 litre box then at
full revs and throttle, you'll pull 5% of the air out on each induction stroke. The partial vacuum will pull that 5% of air back in and will
"overshoot" giving almost 5% more pressure before it rebounds back out of the box. A 5 litre box will have a 10% effect.
RPM = 19100 x SQRT(116.5 x A(cm2) / V(cm2) / L(cm))
Where A = area of tube, L = length of tube and V = volume of enclosure.
If there is a flat spot at around 1/3 of engine revs then you should tune the airbox to resonate at this speed, to fill in the gap.
My R1 engine is pretty flat from 3000 - 5000 RPM and then is like a startled cat from 5000 - 12000. I am building a tuned enclosure at 4200 RPM. it
should start to produce an effect from 3500 RPM will the full effect from 4000 - 4500, making the car more drivable around town. The down side is
that engine pull will be a bit less from 2000 to 3000 but it never worked well at this speed anyway.
quote:
Originally posted by smart51
Airboxes are tuned. You have to match the inlet tube size to the volume of the box to get it to resonate at the right engine speed.
A telescopic pipe under ECU control would give the "tuned" effect at all revs. The most ideal solution. The problem is that the length of tube is not linear against engine speed.
Ok, so the inlet pipe should be arount the 3" to 4.5" dia.
but it sounds like I'd be best to wait until I know how the engine behaves before designing an air box
ps - if anyone wants a good R1 airbox with a filter, but without the rubber joiner bits or clamps (sorry smart51 
) - I'll have one spare
shortly
[Edited on 16/10/06 by mcerd1]
the venturies on an R1 carb are 41mm in diameter. 2 - 3 times the area is 50 to 70mm diameter or 2" - 2.75"
Going back to the physics and maths, does the volume of the enclosure (V) include the volume of the inlet tube?
opps - I should have been reading more carefully (area not diamiter - doh)
quote:
Originally posted by Richard Quinn
Going back to the physics and maths, does the volume of the enclosure (V) include the volume of the inlet tube?
quote:
Originally posted by smart51
quote:
Originally posted by Richard Quinn
Going back to the physics and maths, does the volume of the enclosure (V) include the volume of the inlet tube?
The volume is analogous to a spring and the air in the tube to a mass. The mass and spring are what resonates. I would guess that the air in the tube doesn't count as the volume of the box.
This is the way I understand it. I could be wrong.
An inlet port opens and a cylinder full of air is sucked out of the airbox. The pressure in the airbox falls a bit. This low pressure draws some air
in through the tube. The pressure is equalised but the are rushing in has some velocity. It keeps moving in, increasing the pressure in the box.
This pressure slows the inrushing air until it stalls. The pressure then expells some air through the tube, Again the velocity of the air causes
some overshoot and the air keeps going leaving a low pressure in the box. This keeps happening until friction damps the oscilation. I belive that
the volume of the box is the volume as the volume of the tube cancels out some how.
The trick is to tune the box so that the next inlet valve opens when the pressure is a bit higher than it should be.
Above the resonant frequency, the next valve opens before the first lot of air has finished coming in. The air flow in is drawn in by the low
pressure in the box but because it is all happening so fast, the airflow into the box is almost constant and the tube becomes "accustically
opaque" and the induction noise is kept in the box, provided the box is rigid enough not to vibrate.
Ahhh! A light comes on (I think)! So the column (mass) of air in the inlet tube acts like a sort of piston?
So, if my 2.0 16V dips in torque a little at about 3900 rpm and I am putting a box together using a fairly standard 3" neck cone filter on a
fairly short tube, I would need an airbox of about 13 litres.
[Edited on 16/10/06 by Richard Quinn]
IF you wanted to tune it to 3900 and use a 3" ID tube, a 6 litre box would need a 210mm long tube, an 8 litre box would need a 160mm tube, a 10
litre box a 130mm tube or a 12 litre box a 105 mm tube. The bigger the box, the less the magnitude of the effect. All calculations are aproximate.
Note that the effective tube length is a bit longer than the physical tube length.
I would however caution against tuning the box so high with a car engine. With resonance at 3900, you'd have dissonance at 2600. It would be
resonant again at 1950 and disonant again at 1560, resonant at 1300 and dissonant at 1110 RPM... You've have a dip in torque between 2300 and
3000 or so which would be bad news.
My bike engine does nothing below 3000. It is flat from 3000 - 5000 and is hugely fast above 5000. I chose to make mine resonant at 4200 to bring
the 5000 thresold down a bit. I don't use the engine below 3000 RPM so the dissonance at 2800 doesnt concern me. It revs to 12000 and 4000 -
12000 is a wide band. I could have chosen to make it resonant at 3000 to boost the "low" speed running ability round town (3000 = 30 MPH in
top) but there is little point.
For your car engine, look to tune it a little above the minimum revs that you want to use. Let's say that you don't want to use revs below
2000, as per the police driving handbook. Tune your air box to 2400 RPM. The airbox will provide some "boost" from 2000 to 2700 RPM maybe.
It will be neutral at 1900 but will run worse from 1400 - 1800. Better still, work out what revs your engine starts to pull well and set ti to
resonate a little below this to improve the "working range" of your engine.
[Edited on 17-10-2006 by smart51]
Sorry, this is my alter-ego (long story!).
The airbox is going on my autograss car so the police driving handbook doesn't really come into it. The limiter is set to 7700 and is working
most of the time however I do understand and accept your points.
The volume of the airbox seems physically large whichever way you look at it. Much larger than the airboxes you see offered for sale, even for full-on
competition engines. As the effect is greater with a smaller box am I correct in saying that there is a balance to be struck here beween the length of
the inlet tube and the volume of the airbox itself? Does the length of the inlet tube include the length of any additional tubing (and I'm
guessing that this would have to be rigid, smooth walled stuff to keep the effective diameter/area the same) used to remotely site the actual
filter?
Cheers
Dear alter ego.
OK, for racing and with a 7700 RPM limit, filling in a gap at 3900 RPM is just the kind of thing that you want to do.
The length and area of the tube are related to the volume of the box. If you have a smaller volume box, you need a bigger tube to match the
frequency.
Some commercially available airboxes are tiny - 2 litres maybe. with a 50mm length of 75mm diameter pipe, they would be resonant at 11000 RPM. As a
guide, Yamaha's standard airbox for the R1 is about 8 litres in volume.
I've edited the table of volumes / lengths in my post above. You might like the figures more. BTW you can have the filter in the box, it
doesn't have to be on the end of the tube.
[Edited on 17-10-2006 by smart51]
IMHO save yourself the hassle and use a set of Piper Cross or ITG foam filters. They offer virtually no resistance to airflow, but still give good
filtration.
I dont see what benefits an airbox will give you. And will probably give you more problems than anything if you dont get it right...
David
quote:
Originally posted by flak monkey
I dont see what benefits an airbox will give you.
This picture might illustrate teh effect of a tuned air box. The blue line is the torque curve without and the red with the air box. Its a bit like
fitting a reflex port to a speaker.
Rescued attachment torque.GIF
quote:
Originally posted by flak monkey
IMHO save yourself the hassle and use a set of Piper Cross or ITG foam filters.