!979 R100RT more power for side car rig

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

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

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

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

Maybe in 4th.

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

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.

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.

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.

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!

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.

Thanks.

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

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.

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'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.

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

I had some last summer directly from Mega Cycle. Haven't been to their website since, but it linked to Andrews spec charts within Mega Cycle's site.

Poke around a little more. It's in there.

BMW advanced the 336's timing as well. The 308 is such a mild cam it is hard to tell the difference. Most think the original timing would have around a 300 rpm higher peak torque. Same with the sport cam. Maybe a bit closer to 500. But you still didn't answer my question. I have a lot of experience with both 308's and only the advanced 336. Pick either 308 or both. It doesn't matter that much.

Read what most serious tuners say about overlap. You will be surprised. The 336 is not a high rpm cam. Chris at San Jose BMW and Dr Curve prefer the 336 for better performance. If I wanted more lift than the 336, I would grind some off the bottom of a 336. It works! 336 lift is 10.68mm and the 308 has 9.4mm. That's 12% more lift. Big diff! Yes, there is room for more but not THAT much more. Remember, that is the cam spec. Then you have a 1.5:1 rocker arm ratio. The percentage difference is the same but . . . . If I was running shorter than stock rods like B+S did back in the day, I might choose a Andrew's cam but I am not. If anything, I would run longer than stock. Sport cams work with stock springs! That is a big diff IMO for a street bike!

The difference in timing between the pre and post '78 ('79?) spec amounts to exactly one tooth on the timing gear, fwiw.

Okay, for a healthy high speed touring mount (RS or the like) the 336 cam would probably work okay (in combination with some proper compression, ignition and carburation) and definately stress the valve train less than a more aggressive cam.

But I still don't think it'd be suitable for a sidecar tug.

My application of late has been a GSPD, so I'm more interested in healthy midrange gains for squirting around twisty gravel bends. More aggressive lift and shorter duration (shorter than 336) is what I need. Andrews grinds a cam that works for those apps. You can check with the local KTM doodes on that score...

Useful gearing charts here

Based upon stock tire sizes, I'm guessing.

Doesn't consider weight, wind resistance, hills, altitude,etc.

edit: I'm an idiot and misread Anton's graph.

I should have known better and given Anton the benefit of the doubt.

What was I thinking?