Holes in the air intake ?

[georgesgiralt]

Hello !
I read from time to time that one can make a couple of holes in the trumpet of the air intake of post 81 BMW in order to reduce fuel use and increase torque.
Sometimes there is a saying about a BMW note about it.
Could you please tell me the truth about this ? Because increasing mpg and getting more torque is like having a free beer at each refueling !
Now, my '82 R65 has two identical air intake trumpet and I know there are some bikes with one trumpet bigger than it's is sister...
So maybe the two "things" are related ?

[montmil]

Urban myth, it is.

[Bob_Roller]

Some of the larger displacement bikes had intakes with different diameters, or size intake 'horns' .

The R65 didn't .

I wouldn't drill or cut any holes in the inrake, I think about the only thing you may accomplish, is getting water in the intake system .

[Barry]

I've tried to collect all of the service bulletins for the R65 models but I don't have one specifically for R65 air filter trumpets.

Snowbum is your best bet on this one. The bulletin he lists appears to be for R80 models only. I just found it for you.

http://bmwmotorcycletech.info/R80-fuel-mileage-problems.htm

[georgesgiralt]

Well, I've searched a bit on RealOem.
On September 1982, BMW changed (again) Bing settings for our R65.
The main jet became 138 instead of 148 and the air intake cover changed from 12 72 1 337 228 to 13 72 1 337 675.
The needle was set a step higher (3th instead of 4th position).
The new cover has non symmetrical trumpets. So I do wonder if this is a myth or not. And what difference a hole in the trumpet make ?

[Barry]

I just added a service bulletin to my previous post.

I doubt it's entirely a myth. The whole thing about the new air filter box and trumpets is that it's a "carefully designed" tuned intake system so I'm sure the holes will have some impact on the torque curve for better or worse. Maximum torque moved up from 5500 RPM to 6500 RPM and I wonder if the snorkels were an attempt to compensate for what might have been a peaky engine by flattening the curve out.

[georgesgiralt]

Well my R65 had a flat spot on violent acceleration at around 3000 RPM.
So ....
She also had bad piston rings and ate more oil as she should.
Now I've put new piston rings, had the rockers re-polished.
So this week end, I will tune the bike, run it, and see what it gives.
My problem is that once the holes are made..... there is no turning back  
And after buying her new piston rings, gaskets, clutch, filters, oil, .... I'm broke...
Thank you for the PDF !

[bruce_launceston]

My 84 LS came from the factory with 2 slightly different sized air intake trumpets.
I've also read that they are tuned length / diameter and I wouldn't mess with them.

[steve hawkins]

I had heard that on the larger capacity bikes that holes are cut in the top of the air filter box to increase airflow, both in the black plastic variety and the aluminium clamshell variety.  However we are talking about 'souped up' R100s and not R65s, where the need to increase the airflow might not be of major concern.  This mod, on the larger capacity machines, will be one of many much more expensive changes.

I have individual pod filters without trumpets on my R65 and no airbox.  But you have to be careful with those, as if you buy them too small, you will restrict the airflow to the motor at high speeds.  Even so they are just asthetic, making the bike no faster really.

Rev.Light

[Barry]

As the jet sizes are reduced, the tuned effect of the trumpets appears to be to enable the engine to run weaker and as the mid range flat spot is reckoned to be due to an overly rich mixture,  before drilling holes couldn't you temporarily remove the trumpets altogether to see if that helps. It would at least demonstrate if the trumpets are causing the problem.  

[georgesgiralt]

A very good idea indeed, and a clever one !
I'll try it as soon as the bike returns to the road !

[Dizerens5]

You may get oily feet from the breather. Wear a pair of rubbish shoes!

[Ed Miller]

I got one of the intakes with non-identical horns along with some other parts, and put it on my R65.  I can't tell any difference, though I haven't removed it to put the original one back on.  

I think you have to have a lot more imagination than I do, for most tuning tricks to make any difference.