Backpressure Research PDF Print E-mail
Written by BMWVMCA NEWS   
Thursday, 31 July 2008

Backpressure Research



By Markus Bittcher



Ser Wijenberg writes:   

“The section on R24/25 mufflers gives detailed info on the various tests with different hole sizes or number of holes.  

These modifications are then run through a computer model and Markus told me that it would take 14-18 hours to complete one model calculation.

The pressure reduction is calculated for every option. 

There have been actual tests with modified exhausts, results are good.  

Info can be found in
the thread on this subject.

Hope this helps”

Markus Bittcher submitted the following article to the ‘Einzylinder-Zentrum Forum’, which now has a section for English speaking fans.

The article is reprinted here with both the author’s and the owner and moderator of the Einzylinder Forum, Karl Spajzer’s permission and blessings. 

Karl asked me to emphasize that this article and all other information found on the ‘Einzylinder Zentrum
Forum’ is protected by copyright.    

Markus began with a letter to the distributors of vintage BMW parts in Europe, which we read here translated by Ser Wijenberg:

“Please inform the after market builders of the following: if the diameter of the 13 holes in the round internal dampener plates is increased from 6mm to 7mm then the pressure reduction will be, with 751Pa, almost identical as that in the original muffler.” 

“The holes shouldn’t be made much bigger because a muffler with 13 8mm holes already has a pressure of 577 Pa.”

“We, the owners and drivers, would be aided with this small modification, because our BMW-singles then would again operate and function as planned by the BMW company.” 

“For any further information, you can of course contact me.”

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Hello everybody,

I made some tests and compared the top speed of my R25/0 with a replica muffler and a modified replica muffler, as described on my site.

Here's a list with the results:  

Unmodified Replica Muffler:
1. Gear> Vmax. 10 km/h
2. Gear> Vmax. 40 km/h
3. Gear> Vmax. 65 km/h
4. Gear> Vmax. 78 km/h

Modified Replica Muffler (with5 additional holes of 8mm diameter):  

1. Gear> Vmax. 30 km/h
2. Gear> Vmax. 60 km/h
3. Gear> Vmax. 80 km/h
4. Gear> Vmax. 85 km/h
and more; (engine needs to run a while)      

It's quite interesting: such a small modification has a great effect

Kind regards,
Karl Spajzer
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Note: I enlarged  the existing holes on an R25/3 aftermarket muffler to 8mm, and got results similar to Karl’s. The engine does run leaner after the change.


Cutaway view:  Original R25/3 muffler



Cutaway view:  Currently produced aftermarket muffler.  Markus suggests opening the holes in the baffles from 6 mm to 7 mm diameter to reduce back pressure to match that of the factory originals.  Read about Karl's findings on Page 3.



Flow diagnostic showing pressure differentials under normal operating conditions.  OEM muffler (R25/3)

Flow diagnostic showing pressure differentials under normal operating conditions in aftermarket muffler. (R25/3)



Last Updated ( Thursday, 31 July 2008 )
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