Ping TBC - airflow stuff.

I was reading some interesting stuff last night on a bloke's attempts
to modify an airbox based on resonance & pressure calcs. I'd be
interested in your thoughts on why he failed miserably! The relevant
pages are the four with 'airbox' in the title at the top of
http://homepage.mac.com/rg500delta/TL1000/Randys TL1000S page.html
and the 'Dyno testing spring 2007' link for the final dyno results.

It makes interesting reading for anyone else who's bored.

 

I think I'd have to be far more bored than I am currently, TBH.

 

At first glance, I would think it's because he thinks you `tune' the
airbox to give you a pressure boost (on the intake stroke) but he
doesn't seem to know about the spread of frequency on that boost or on
the effect at other frequencies. Depending on how you do it, you can
have a high boost over a very narrow range of engine speeds, so that
the boost is practically useless or a lower boost but spread out
around some centre frequency. You also have to take into account the
fact that there will be more than one resonant frequency and some of
the other ones may lie in the operating range of the engine and cock
up the response at certain speeds.

I don't know what he means by `fully mapped'. Does that mean he has
remapped with some available map, or has be remapped taking account of
what he has done to the airbox?

 

That did seem to be the conclusion he came to. I.e. a noticeable boost
would be in such a narrow range that ram-air would be a much better
option.

Ah, don't think he mentioned that at all.

Remapped on a dyno to get the right mixture at all revs & throttle
positions.

I suppose the real question is why a bigger airbox resulted in less
power? Shortening the throttle bodies to increase power at higher revs
is valid afaik (it's what MV do with their variable length intake
trickery), so to get less power after doing that & increasing the
airbox size seems a bit strange. The shape of the airbox screwing
things up with turbulance maybe?

 

It might be because the shape of the airbox changed the acoustic
response at the intakes. In other words, there was a boost in pressure
but it wasn't at the same place as the holes in the airbox.

 

That's not necessarily true, especially in these days of emissions &
noise regs. There are some very well proven mods that can make a huge
difference (opening the 2nd pre-chamber on 885 Tigers[1], cutting a
hole in the top of DRZs, removing a chunk from the bottom on TL1000s).
The problem comes when people do that but don't sort the fueling to
match.

Which almost certainly means it was running lean as he hadn't rejetted
it.

[1] This gave an extra 15bhp on mine after jetting.

 

So true. Applicable also to exhaust systems, carburettors, FI systems,
and loads of other things too.

Oh, and cam timing: "If you change the cam timing by one tooth, you get
an extra 10bhp, you know! They make it the way they do for emissions
reasons. My mate did this and..." etc etc etc.

Suspension tends to remain an area where it's quite easy to improve on
the standard product.

 

Makes sense. Would it be easier to design an airbox for an in-line 4 as
you can shape it to give a smooth flow to the back where all 4 throttle
bodies are sat next to each other? I'd guess that would be harder to
achieve with a V-twin where one throttle body's at the front of the
airbox & t'other's at the back.

 

(The Older Gentleman) wrote in

Where the mods are more to do with improving the quality of materials than
changing the laws of physics.

Seats are another area where great improvements over stock items can be
made. WTF can't makers provide comfy seats in the first place?

 

I'm not really sure how you can say that given that pretty much every
bike on the planet will make more power throughout the rev range when
fitted with an aftermarket exhaust & re-jetted/mapped to suit.

If you're talking about people who just drill holes in the standard
exhaust & do nothing to the fueling, then yeah.

 

Oh christ yes. Some of them do it to get more dosh when you buy the
'comfort seat' option, but not all manufacturers even offer that.

 

DING.
I fixed the totally hopeless bloo 400/4 by grabbing an airbox cover
from ebay for a quid. Some previous owner had probably just lost it.

 

Oh, yes, yes, yes. And some manufacturers (hello, BMW) even have the
temerity to offer 'comfort' seats as a pay-extra option.

 

Well, I was thinking that way, yes. Should have phrased it better.

 

Result. I think the last one I flogged fetched a fiver

There again, you've had a few bits off me for nowt, you lucky little
chappie.

 

There's always a case in point: the Bandit 1200, for example. Jetted
to run too rich as standard, you can fit a race can and do nothing else
and expect to see up to 10bhp for your trouble. It won't lean out at
high revs, either. Or you can fit a K&N and see up to 5bhp and a chunk
more torque to boot. Do both without jetting and you're in trouble, so
they say - but do them both, fit a Stage II Dynojet and drill the
dreaded 25mm hole in the airbox and you're way above standard power
without doing anything technical or costing the earth.

 

The factory engineers are aiming at different sets of results, including
fuel consumption, ability to run over a wide range of atmospheric
conditions and altitudes, noise, cost etc etc.

After market exhaust systems are hardly cheap, much of that cost being
research related I expect.

Well, yes.


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