BTH COMPETITION MAGDYNO

[andrew zac]

Hi . to all , i have a new BTH competition magdyno fitted to my VH . i used it at SELLBACKS BEACH in 2017 . the motor, and the rest of the bike ,wheels,bearings,rims,etc .was completely rebuilt and prepared for the event , pre race testing at a remote beach location on a measured race distance allowed me to select the correct gearing. i restricted my revs on the race week/end to 6000 . the magdyno is set at max 65000. however i know the motor will a taller gear and i am looking to increase my top speed from 80mph to a possible 95mph a gearing change and an increase in max revs should achieve this. my question is HOW do i reset the BTH to increase the max revs from 6500 to 7000 . i have contacted the supplier in the UK , he has never been asked the question and is not sure . see pics of my bike before and

now in prep for 2019 regards Andrew

[Eero.Korhonen]

Wow that is great! High piston, what is compression ratio? What is the size of that Concentric?
Br, Eero

[Simon.Gardiner]

Hi Andrew

Can't help with the problem myself but Tony Harris (who owned BTH until 7 years ago) looks into the forum occasionally, he might be able to help although as he hasn't jumped in on this so far maybe he can't....
(See eg in this thread for some of his 'history'
http://forum.arielownersmcc.com/viewt ... bth#p42180 )

Have you contacted the supplier - or the manufacturer ?

Mighty fine looking steed, by the way!

HTH in some small way.

SG

[andrew zac]

ok Simon thanks for the reply . i am running H5 cam , 1.3/16 amal smooth bore , piston is a JP 9 to 1 hi comp . i am going to replace the gudgeon pin with a tapered one as it is a little lighter. i have other improvements in the pipeline as i begin to resemble thee complete bike in prep for the beach race events in feb 2019

[Brian_Walker]

Hi Andrew,
Bike looks real nice I was thinking of running a JP 9 piston in my VCH but I don't have an estimated weight for it, the picture below is my current one which weighs 485 gm and the pin 113gm. Would be interested what yours weighs as my old piston (the one with the hole was weighed when we still had ounces I don't remember, what it was which was shame as I balanced the crank for that piston, bit late now it has been lightened in an orthodox manner)
I did have a BTH mag and have copies of the service manual which seem to indicate the gear ratio is the limiting factor for rev range. I now have a Lucas SR1 and it ran fine at 7500 rpm, I don't what the specs are but maybe the lower speed limit for the BTH comp is based on testing where they look for a maximum number of "missfires by the magneto" on a test rig of 5%." these may not equate to engine missfires and at 100mph with a rigid frame you don't seem to notice much anyway.

Below is my two pistons, would also be interested how large the dome and valve cutaways were on the JP piston.
Cheers,
Brian

Keep us posted hows it runs on the beach.

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