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1972 R75/5 racer mod ,

Posted: Mon Apr 04, 2022 9:46 pm
by Efonlupt
My apologies if I am in the wrong thread. But here goes I have been building European bikes for 15 years, vintage focus . I have raced at Bonneville and Maxton several times . Anyways I have never owned BMW’s before but after a nasty divorce , I lost my shop and inventory. Some of my racing buddies gave me two 1972 BMW R75/5 . One I am restoring. But the other I am building with Bonneville in mind for 2023 yet I am also wiring it to be be street legal so there is a balance there . Again I am new to BMW but not building . So what kind of mods would you recommend ? I have built a new rear loop and built a new seat , tearing down everything down to the studs and inspect all parts . The Current valves are in good shape and seat properly ( checked with blue die) . Should I change the springs ? Pistons are good and the cylinder sleeve are in great shape , any recommendations on changing the pistons ? What trick if any can be done on the crank ? I am thinking of using a 336 camshaft and have a kickstart 5 speed gear box , any mods in the clutch ? Oil pump ? I will be adding the extended oil tank to increase oil volume , dynatec III IGNITION , I have Mukuni Carbs , the breaks will remain stock for now but thinking of shortening the front forks 1.5” . I would like to get a digital charging system and use a small antigravity lithium battery . I know it’s a lot of questions and I am grateful for any bits of information. Oh last question if using a 336 camshaft and the 5 speed gearbox what final drive ratio would you recommend. ? 34/11 ? What are the racing and competitive racing ratio that would work best . Cheers Emmanuel

Re: 1972 R75/5 racer mod ,

Posted: Wed Apr 06, 2022 6:49 am
by johnlacko
I don’t know if you’ve done any work on the frame yet, but I have an R75 frame that was raced by R&K BMW here in NJ during the 1970s. It’s pretty heavily braced and modified. I too had toyed around with the idea of building a race/street bike with this one, but I got two race frames from that closed shop, and one is more than enough. Let me know if you have an interest and I can send more photos. I also have some K100 fork legs and tree that are modified for racing on the earlier frame. A popular upgrade in the early 1980s.

Re: 1972 R75/5 racer mod ,

Posted: Thu Apr 07, 2022 3:39 am
by Efonlupt
Hi , that is a cool mod on that frame . I already made a new rear end loop for my frame and new seat pan . My frame and engine match and have a clean title . Your frame is tempting …….

Re: 1972 R75/5 racer mod ,

Posted: Mon Apr 11, 2022 8:49 am
by Ericandchi
Those are all good questions. The only limit on modifications are your time and budget. You didn't mention which class your going for at Bonneville as that might dictate some of the mods.

To list just a few potential engine mods...
Lighten flywheel
Dual plugs
Re-angle and or shape intake ports
Roller cam
Shorten stroke
Oil cooler
Titanium hardware
...The list goes on

Lighten flywheel

Re: 1972 R75/5 racer mod ,

Posted: Tue Apr 12, 2022 12:00 pm
by Efonlupt
Thanks Eric ,

I want to lighten the flywheel , do you know what grams can be taken off ? Or a size pattern I can machine off ?

We usually run the bringwhatyoubrung , I have raced in the 500cc vintage class .

As for time , I figure within 6 months .

Do you know if I use the dyna III electronic ignition if that would accommodate dual plugs ? I can check on that …

Compression ratio for pistons ?

Can you share a link for the Roller cams ?

I was gonna reshape the intake ports , any insights?

I truly appreciate you and your input . Thank you

Re: 1972 R75/5 racer mod ,

Posted: Wed Apr 13, 2022 9:04 am
by Ericandchi
I suggest you read all of Snowbums web pages within this site to get yourself calibrated. Here is a link with photo of a lightened flywheel.

As for a roller cam, well that's a totally custom thing, you'll need to see if you can get someone to grind you a cam. Then you need to source the lifters, old HD were used. Crane did the old race bike cams, but I am not sure if they would be able to help these days. I'm guessing you could count the number of roller cam engines built on one hand, maybe two hands. One of them is in my profile pic, a few in museum at factory. Another guy named Rudy has one in Germany that he rides a lot.

There are plenty of electronic ignition available that work with the dual plugs, dynatek, EnDuraLast, Boyer, etc.

There is no magic answer for compression. Its all going to depend on what your goals are, what kind of fuel you want to burn and 50 other parameters....etc.

As for port shape, again, not something we can hand you a cad drawing and say go do this. If your gonna re-angle the carbs to be out you have a lot of welding work. What size valves will also dictate port shape. Having a flow bench will help. If your sticking with a stock head then maybe just clean up the rough spots with your porting tool and move on.

Maybe best approach is build a mildly tuned engine and make tweaks from there as you learn what works. Look at it this way, if you went to a local Friday night car show and asked every chevy owner there how to build a kick ass motor, everyone of them will give you a different answer. Only difference with the BMWs are there are fewer folks around these days.

Re: 1972 R75/5 racer mod ,

Posted: Wed Apr 13, 2022 11:57 am
by Efonlupt
Thanks Eric,

You are right , the variables are numerous , so many elements and yes build up and fine tune as we go . Elevation , gas , materials , oil flow etc . I have built a few very special engines with fantastic results and have over built with catastrophic results . The Gnosis is half the fun in this realm , thanks for the reminder .
I will continue and update as I go along