Switch to 9.5s while I am at it?

[jbcollier]

I am doing a bunch of work to my R100R powered sidecar outfit and I have sent the heads off to get dual plugged. I am sitting there staring at the 8.5:1 pistons and wonder if I should spring for the 9.5:1 pistons while I am in there. It has the 42/40 heads with 40 Bings. Good idea or...

[PAULBACH]

$00.02

Seems like a lot of work for a sidecar rig. You will get more bang for the buck and time by badding farkle to the side car.

[widebmw]

Heat, lots of heat :cry
Watch your valves.

[Stuff2C]

I put a lot of $$ in a 1980 R100. Bored 2 over, hi-compression pistons, dual plug, flat slides, ported, lightened and balanced...$$$$$$$$$$$ even with a friend doing all my machine work for beer.

All I got for the $$$ was a R100 that was faster than another R100...spend the $$$ somewhere else, like a lower final drive...they're cheep in comparison.

.02 :D

[srb]

cheap 87 octane with 8.5 vs. high test gas with 9.5

[rinty]

An inexpensive alternative is to shave the heads while they're off. If you take .030" off an 8.2:1 head, you'll end up with a compression ratio of 8.7:1, which is still conservative. But I don't know if .030 is safe, with respect to valve clearances, for an 8.5 head. You could check with Anton Largiader or another airhead specialist to see if it can be done.

Rinty

[Bigrider]

What's a source for 9.5 pistons. Siebenrock in Germany is one source, but they are expensive. I also understand China now makes pistons. Any domestic locations known?

Dave H
San Antonio, TX

[jbcollier]

You can order them from BMW. Part number is 11 25 1 337 175.

[tvrla]

You'll get better mileage, more power, and a smoother running bike with dual plugs and 9.5 compression. With dual plugs you can also run regular gas without pinging. Another trick to prevent pinging is to radius all the sharp edges in the combustion chamber - the idea being that energy radiates from sharp edges. Just take the sharp edge off the combustion chamber - like around the plug opening, piston top, and where ever else you find them in there. It'll make a big difference.

jbcollier posted a number that I'm assuming is the correct one for the stock bmw european pistons. Only question is whether they're for the nikasils or cast iron liners - they are different pistons with different tolerances.

[AntonLargiader]

Those are the right numbers for Nikasil. I have the 175s and 176s (second and third wear levels, respectively) sitting here on the shelf and I'm in the middle of putting the 175s into an R100. I'll let you know the results. The 176s will probably go into my own R100R.

[jbcollier]

Thanks for all the replies. I am going to be going ahead with the dual plugging and will order the 9.5s as well. The heads are 42/40s so I will also pop a 44 intake in. It already has 40 Bings so that is all I will do.

[pmdave]

If you're running stock diameter wheels, I'd strongly suggest you drop in a 37/11 final drive. The engine will spin a little faster, staying up farther on the power curve. And airhead motors like revs not lugging.

While you've got it apart, I assume you'll also convert to the latest valves and seats.

There is only so much power to be wrung from an airhead--around 65 rear wheel HP for a strong engine. It's marginal for superslab crusing, however much you breath on it. And the more you try to squeeze out of it, the more it begins to resemble a grenade.

If you feel the need for speed, it's time for an oilhead, or some big UJM inline-4. If you absolutely MUST have a BMW for the sidecar rig, the K bikes are very strong, but of course require some extensive rework. (complete lower subframe etc.)

pmdave

[jbcollier]

Good points but I am happy with what I have. The only mod I really wanted was the dual plugging and that only to reduce pinging and make for easier cold starting. The rest is happenning just because I am already in there. Going to 44 over a 42 is not that big a jump. The old pistons look fine but going to 9.5s will pep it up a bit and make it a bit more efficent. It is not that radical really. Heck I run 11.75:1 in my Ducati single (dual pluged too). No chair mind you...

[28796]

I installed the Siebenrock 1000cc kit w/9.5-1 pistons in my 87 R80 & have had no proplems with pinging at all running 91-93 octane with the single plug heads. I think bumping up to the 9.5 pistons with a sidecar rig would be a good move. One thing I remember from my sidecar days is that you can't have too much power.

[Bill Burke]

It's marginal for superslab crusing, however much you breath on it. And the more you try to squeeze out of it, the more it begins to resemble a grenade.
Can you elaborate on this statement please? Thanks pmdave-
:lurk

[28796]

Lets not forget that BMWs came with 9.5 pistons (R90/S, R100/S, R100RS) back in the 70's & they were not known for blowing up as a result. I'm currently installing 9.5 pistons in a customers 88 R100 GS as well as having the heads dual plugged as he will be riding this in areas where premium may not be an option. IMO the low comp Airheads of the 80's/90's were dogs that were known for lousy gas mileage & lackluster performance. Bump the compression back up, give them a real valve job(most needed this right from the factory), dual plug if you feel the need and you will have a better performing bike with an engine that runs much more efficiently.

[rinty]

...they were not known for blowing up..... bikepaintr

I agree, 9.5 is not an aggressive compression ratio, and the engines are easily up to it. And by bumping a 1980 and later engine's compression up, you get the benefit of the Nikasil cylindes to boot.

Interestingly, if you the bump the compression of a Lycoming 360 aircraft engine from 8.7:1 to 10:1 (by installing Lycoming helicopter engine pistons), the cylinder head operating temperature drops by about 20 degrees. I would imagine that some airhead tuner has done a similar measurement on airheads, and it would be interesting to see the result.

Rinty