Krodaddy....I have an idea to run an R75/5 as a 650 in an AHRMA class called Vintage Superbike Lightweight. The problem is finding/designing a 650cc piston. I can take R60 cylinders and bore them out, but there is no prototype high comp. piston I can find. Venolia has not been very helpful on this, as they want to deliver an R65 piston...no good. If I reduce the bore of a typical R75 piston, my valve relief is out past the dome edge, nearly violation my first ring....is there problem here one of trying to use R75 heads with this arrangement? Would I have to use R60 heads too? I'm having trouble finding anyone to advise me on this one, not many seem to have gone this path, am I missing the big reason why not? Thanks
brian,I am by no means a bmw know it all but most guys make better power with short stroke and big bore.TIM
Hi Tim, I know.....I have been in touch with Mark M. and he sent me info on cam grinds and other pertinent info for the R65 motor....but this is soooo easy, if I can resolve this piston design. The class has been dominated by an YAM SR500 two valve single for two years, dropping the BMW to 650cc is real quick and cheap. I'm just having a little trouble finding any recomendations for a hot 650cc piston, Dale at Venolia isn't offering up any prototypes, as I don't think there are any. Taking an R65 to race prep status in the time remaining is a bit daunting right now.... Thanks bds
you will also have trouble with the big heads and small pistons.Every thing is backwards to a good running motor.you need small combustion chamber ,big valves,short stroke ,big bore.good luck hope you can make something work.TIM
Getting pistons made for the combination you want will be expensive and time consuming, I don't think this has been done so you'll be forging into unknown territory, and the piston guys will assume no responibility if it dosen't work. I use highly modified stock BMW pistons in all my stuff. The best way ( IMHO ) to a hot 650 beemer is to start with the R65, use R75 heads, and then use all the other performance mods that are normally done to the airhead motor. Then presto, one yamaha stompin, honda chompin, triumph tossin, bsa beatin,race winnin, bavarian bomb. Barry
Wow That was sure some fancy word combo there daddy, I like anything that has to do with bmw's kickin ass. I would have to agree with daddy. A 1981 or newer stock r65 made the same hp as a r75 according to bmw. So with the r65 crank and cylinders and the r75 heads to help it breath it should work very well. Or for a really quick build just run a full r65 motor with a sport cam and a little head work and maybe duel plugs you know the normal stuff. And the r65 heads have the large spigot for the carbs so it should work really well. So how many bmw race bikes are you going for. I currentley have 3 and I ride my sponsers other one. And there are a few more in my mind for when I have a little more money and time. I think it is some sort off sickness but I am not sure. brad
You are better off sticking with the R65 engine for what you are doing. Bump the compression and fly cut the valve pockets. The previous posts on sport cam is good advise, but the R75 heads are not the way to go. R65 heads actually flow better than the R75 when mildly ported. Also you need to run bigger intake valves, 42mm is good, and small 36mm exhaust valves. Also don't go thinking a 38mm exhaust system will get you anywhere, cause it won't. Stick to the 36mm at least for the first 12 inches, and then step up to 38mm and then step up to 40mm and then to reverse cone. Also 36mm Mikuni would be plenty big enough for carbs, you don't want to over carb the thing, you might even consider 34mm. What are you planning on doing to the chassis?
"Stick to the 36mm at least for the first 12 inches, and then step up to 38mm and then step up to 40mm and then to reverse cone. " I'm not following you the above advice, please explain/claify? This bike won't be ready for the start of the season, it's going to be a work in progress for a bit of time. I'll be running two R75/5s, one in BEARS and one in Sportsman 750.
better exhaust gas flow. commonly called anti-reversion. I thought of something else with the 750 heads vs. 650 heads on a 650 size cylinder bore. The 750 head has a much larger chamber and you will lose some serious compression if you use it with a 650 cylinder. Is that Walt Sweatt's old bike?
Yes that was Walt's bike befored he went down on it at the BMW Invitational at Barber. A couple of weeks after Barber, I checked on how he was doing, and he offered the bike to me. So I'll continue to run it in BEARS, and my other bike ti Sportsman750. I believe we met at Barber, I was up on your pit level admiring your bike, talked with you about some of the mods. I was in the BMW race each day too. The other bike.....
My bad, a little brain fade there. BUT, If you look at the two combustion chambers there are some definate differences, and the 750 head does have more volume.
I believe your right about the combustion chambers. I think there is a differance between early and later R65 heads, possibly valve size and/or port size,and the later ones are the better ones. Please read following Disclaimer: I have not worked with any R65 heads yet and I really need to get over to Brads and take a good look at some, so all my opinions are subject to change.
Krodaddy...Please let me know if that's is the case on different year R65 Heads. I have a guy in Kansas with a set of heads/cylinders I'm trying to negotiate a price for, and I'm not sure what year they are from. Thanks in advance. Jerry Settle used to run R65 Heads on his R75/5, claimed it gave him a higher compression ratio/more torque. bds
Yeah, I gave him all of that info. This is the setup I used to run until I destroyed the heads. I have some early and late R65 heads here at the house, I'll need to take a look at them and see if there are any differences(other than the smog ports) The early ones don't have smog ports (1979-80 only)
Sorry, I'm holding on to all of my engine spares. I've raced the same bike since 1996, built it myself out of junk. Since I plan on keeping it forever, I need what I have and more.
