!979 R100RT more power for side car rig

[MMW Motorcycle]

any ideas for building a high torque engine for a R100 with side car. Would like to cruise at 70 mph, but still have power for passing.

Thanks Richard

[supershaft]

Dual plug/higher CR, sport cam, late model clutch/flywheel, higher final drive ratio. You'll be set.

[Uncle Ernie]

Unless you're hauling MAJOR mass, you should have no problem cruising at 70 in stock mode at around 5K rpm.

[Lornce]

3.20 gears should work well for sidecar use with a healthy liter motor.

[Lornce]

Originally Posted by Uncle Ernie

Unless you're hauling MAJOR mass, you should have no problem cruising at 70 in stock mode at around 5K rpm.

Maybe in 4th.

Stock 2.91 gears won't pull 5k at 70 in 5th.

[bmweuro]

How about a big bore kit Siebenrock 1070cc and a 3:09 final drive from a R90/6. Don't run a 3:20. Way to buzzy at highway speeds.

[Rob Farmer]

Originally Posted by supershaft

Dual plug/higher CR, sport cam, late model clutch/flywheel, higher final drive ratio. You'll be set.

Not sure about the sport cam for hauling a sidecar. My RS has a sport cam and is noticeably down on umph below 3k compared to my bikes with the 308 cam. It goes very well at higher rpm though.

[bmweuro]

Originally Posted by Rob Farmer

Not sure about the sport cam for hauling a sidecar. My RS has a sport cam and is noticeably down on umph below 3k compared to my bikes with the 308 cam. It goes very well at higher rpm though.

I agree. Stay away from the sport cam. No low end torque.

[DaveBall]

You will definitely have to change your rear end gear ration. Not certain of the one to go to, but the stock 32/11 will just not last and be so slow off the line your hack monkey will be out pushing.

Check with some of the inmates over in the Hack section. I bet they can be all kinds of help.

[supershaft]

I think a lot of hackers run a 3.56:1. I would think you would need at least a 3.36:1.

I know I would get some nays on the sport cam. I am surprised that I haven't gotten any on the late model flywheel clutch. Less umph under 3krpm? With my small port I would say under 2500rpm but MMW's bike is a big port. Even then it's barely any less and the payoff in the mid and upper revs are big! Sport cams are not top end cams, they are mid range cams. You shouldn't be working your engine hard below 3k anyway. Especially with a hack! An experienced hacker that I know got all kinds of nays on his sport cam and late model clutch but he loved the setup. Just saying!

[Lornce]

Originally Posted by Rob Farmer

Not sure about the sport cam for hauling a sidecar. My RS has a sport cam and is noticeably down on umph below 3k compared to my bikes with the 308 cam. It goes very well at higher rpm though.

Exactly. Gotta wring it above 5k before you'll realise any advantage from a 336 cam. 5k-8k is where they work. It's a high end "timing" cam. Virtually no advantage in valve lift over the stock 308 degree cam. Rather, increased duration.

[Lornce]

Originally Posted by bmweuro

How about a big bore kit Siebenrock 1070cc and a 3:09 final drive from a R90/6. Don't run a 3:20. Way to buzzy at highway speeds.

Thanks.

I'll heed your advice on that with my sidecar project.

[supershaft]

Originally Posted by Lornce

Exactly. Gotta wring it above 5k before you'll realise any advantage from a 336 cam. 5k-8k is where they work. It's a high end "timing" cam. Virtually no advantage in valve lift over the stock 308 degree cam. Rather, increased duration.

That is absolutely not true on both levels! I would say a sport cams advantage starts at between 2500 and 3000rpm. I am not just talking about my own bike either!

Well, at least you didn't say that the sport cam has no more lift than the 308. I have read that on internet forums and popular websites. FACT: It has substantially more lift. Look up the specs. No, it isn't a super high lift cam because it is not that radical of a cam. It does have a comparatively narrow toe to toe angle. Contrary to what you might think, narrow toe angles (overlap) HELP midrange. Many a tuner swear by it including Yunick and Dr Curve.

I do agree that they work best from about 5 to 8k rpm. Where would you say a 308 works best at? I would say 4500 to 6500rpm. I would say the advantage definitely starts by at least 3k and by 4k it is substantial. By 5 k the difference is a big one and past 7k they are a night and day difference since the 308 comparatively does not work there.

[Lornce]

Q. Where does the stock 308 degree cam work best?
A. Depends how it's degreed.

Fact: '77 RS (and every other airhead except the R60/7) used the same cam profile (which determines valve lift and duration) as '79 and all later airheads. BMW degreed (references the cam's orientation to the crankshaft) the cam slightly differently after '78 or '79 (I forget which year the change was introduced) to improve torque/power production in the lower reaches of the rev range.

Increased overlap tends to improve higher RPM power production at the expense of lower RPM efficiencies. The differences are pretty easy to identify on an airhead twin.

It has substantially more lift. Look up the specs. No, it isn't a super high lift cam because it is not that radical of a cam.

It's been a long time since I looked at the specs of the 308 vs. 336 cam (over 20 years). Memory tells me there wasn't much difference in lift, the difference was simply duration. Do you have the specs available from a reliable source?

My experience with the 336 cam in a '77 RS was pretty forgetable. Andrews cams (now mfg'rd by Mega Cycle) produce more impressive performance gains.

[Hawk Medicine]

Originally Posted by Lornce

Andrews cams (now mfg'rd by Mega Cycle) produce more impressive performance gains.

Mega Cycle appears not to list any Airhead Cams. You got any inside info?