Modified Carbs and Airbox Mods.

[koko64]

[/url]2013-10-30 21.34.41 by Tony Kokonis, on Flickr[/img]

2013-10-30 21.34.41 by Tony Kokonis, on Flickr

[koko64]

20170803_163214 by Tony Kokonis, on Flickr


So by taking about 20mm off the rubber stacks and "porting" them, and then raising the air filter to the very top of the airbox, there is a large increase in sub filter volume and a larger gap to the 1-2 litres of airbox volume at the front lower section. You effectively unite the airbox volume available to the stacks for each intake stroke.

At present I am testing with stock CV carbs with just a jet kit and have found the power quite ok with this configuration. Not as good as FCRs, but better than with just a stock or open airbox. 165 mains, 37.5 pilots, Factory Pro needles on #5 out of 7 and stock springs.
What's good is you can do this yourself on the cheap.

[koko64]

[/url]2013-10-31 11.30.20 by Tony Kokonis, on Flic2013-10-31 11.30.20 by Tony Kokonis, on Flickr

Enlarged airbox on a modded 900 with FCRs before and after with no jet changes on the same dyno (which by the way only get's stingier over time).

Sorry for the Izzidity, but its a pic of the old paper print out from way back.

[koko64]

You know you can hotlink pics so they appear on the forum...right?

Im a bit slow on the uptake but I got it in the end.

[koko64]

While working on some oem carbs it really struck me how bad the lip and angle are from the bellmouth to the carb throat. The carb body is cast and then bored from the engine side out to give the venturi size which is advertised at 38mm. However, on most oem carbs I have seen, the bored section stops a mm or so before breaking through to the as cast bellmouth which tapers down closer to 36mm restriction creating a turbulence inducing lip and angle to airflow. It's a built in restrictor caused by cost cut manufacturing imo.
I will post an exaggerated illustration to show this.

[koko64]

https://www.flickr.com/photos/150482584@N03/shares/4X16yB


Series of pics and crude drawings in an attempt to illustrate what I'm trying to describe.

[koko64]

[/url]20170909_174100 by Tony Kokonis, on Flickr[/img]

20170909_174121 by Tony Kokonis, on Flickr

[koko64]

20170909_174100 by Tony Kokonis, on Flickr


It's a 1mm lip where the boring bar stopped in the transition area from bellmouth to throat. I believe that and the sharp edged slide face produce flow limiting turbulence and slide flutter. The digital calipers confirmed it. Maybe there were production runs like this and the other batches don't have it. Let me know as I'm curious whether it's a mistake or intentional for some theoretical reason I don't understand. If there was a theoretical reason for this I don't believe it worked in practice.


20170909_175130 by Tony Kokonis, on Flickr

[koko64]

20170909_175100 by Tony Kokonis, on Flickr

[koko64]

Clearly answering a question no one is asking in this age of EFI, but for us Luddites with carbed bikes it's what us older members grew up with.

[greenmonster]

 

[koko64]

Cheers Gert. It's not interesting to most people, but it is to me . While I occasionally play with Power Commanders, it is satisfying to deal with something so concrete, tangible and analogue.

[greenmonster]

How much were the gains in the chart above? Looks quite impressing!

Know what you mean, promised myself never fiddle w injection or suspension,
lesson learned, leave that to those who know...

[koko64]

The chart above was from airbox mods as covered in that thread. It was a surprisingly good gain (4-5hp). The carbs were FCRs, JE pistons, ported heads and the only change to the bike between dyno runs was a significantly modified and reorganized airbox.

I did some more testing with ported carbs today. They helped smoothness and response,  but I dont expect the power gains or instant response from well tuned flatslide carbs. They are still a 38mm CV carb and surge rather than snap like FCRs, but felt good.

Oh another thing. Initiatially, I thought the modified airbox would only work on ported heads. The owner of one bike tested said the bike wasnt ported and the dyno test showed no improvement. I found out later from the engine builder that the motor was ported but that the dyno also read 10% less due to software/firmware changes.
What this means is that I still dont know if the modified airbox works well with unported heads, but the good news is that me mate's bike (944 hi comp, FCR's) makes 10% more power than he thought. The changes to the dyno I use confused my data (as did incorrect information).

[ducpainter]

I had an email conversation with Doug Lofgren about this. By the time I did he was doing FI and had no interest in the BDST Mikunis any longer.

He would address the very areas you're noticing Tony. Go for it...

or just let the moths out of your wallet and get some FCR's.