long stroke vs short stroke

[mimmo66]

Forgive my ignorance but, I've been thinking about this for awhile now and never asked before.

Can you turn a short stroke r65 into a "normal stroke", and what would be the results / consequences.

Just wish to lower the rpm a tad for long slab transfers, although I love to blast it from 5 thousand all the way to the yellow zone!

"It is much more fun to go fast on a little bike than on a big one"

[Bob_Roller]

'Easiest' and quickest way, would be installing a 1000 cc engine instead of bolting on other larger displacement parts .

The R65 engine doesn't take too well to being 'modified' .

[DgM]

Replace your final drive.

[montmil]

Generally speaking, short stroke engines are revers. Long strokers offer more torque.

For example, air-cooled boxer-style aircraft engines, be they 4, 6, or 8 cylinders are l-o-n-g stroke. The popular Continental O-200 has a take-off limit of 2750 rpm for five minutes. The classic P-51's Rolls Royce Merlin, a liquid-cooled V-12, had a 2600 rev limit at 12250 feet ASL.

Increasing an engine's stroke is usually accomplished by using a different crankshaft; often referred to as a stroker crank. Attempting to bolt in longer -this is a relative term as to rod length discussion- custom con rods could be asking for trouble with valve interference. Either way, results would most likely be minimal. The BMW R65 has been the victim of hot rod techniques many times in the past. I have yet to hear of anyone's quantified success.

Swapping the final drive is probably the best option. I would think the higher gears would provide a potentially higher cruise speed only if the bike's load out weight was carefully managed. Load the tank bag with a map, clean t-shirt, shorts and socks, you're good to go.

Lots of R100s out there for sale. [smiley=bmw_smiley.gif]

Monte  

[nhmaf]

You'll lose some of the already modest grunt coming off the line, but several final drive swaps are possible.  If you have a 32/9  (3.56) rear end you can swap it out for a 31/9  (3.44) rear end to drop the highway RPMs by ~ 150 RPM - not alot of difference but it may help slightly and you will not sacrifice a noticeable amount of take-off acceleration and your speedometer/odometer won't be off by more than a couple percent.

You could also look for a final drive from an R80  from the same time period as your R65   (if your R65 is an 81-84, get an R80 final drive from the same period, or if 78-80 get an R80/7 final drive from the same period if you want to keep the external appearances the same).
The earlier R80 bikes came with either a 37/11 (3.36) final drive or a 32/10 (3.2) final drive.    Either of these could also work but of course will reduce the acceleration capabilities a bit more and your speedometer/odometer will be "off" by as much as 11% unless you send it out to get recalibrated.   They would drop the RPMs at interstate speeds by around +/-400 RPM.

You cannot easily change the R65 engine to use the type 247 engine jugs - the short-stroke type 248 engine has steeper pushrod angles and smaller engine case openings at the bottom of the cylinders - you'd end up having to machine and replace so many parts (cylinders, heads, pushrods, pushrod tubes, seals, connecting rods, etc. etc. it would be MUCH easier and cheaper to find a used R80 engine and drop that in.    Then if you still wanted more grunt you could swap heads, cylinders and pistons with an R100 engine.

[bjamesw]

wow, coincidence.   I searched.   Then I posted about replacing the ring and pinion.  Then I open the next thread out of curiosity about "short vs long" stroke and voila!  

sorry bout that. I'll leave the other post up and follow both of these.

[Bengt_Phorqs]

Lots of R100s out there for sale.  

I got one for sale!

[Motu]

The mono R65 has a 37/11...3.36:1.I don't find it slow off the mark,in fact I find 1st gear a bit short and get into the red too soon.It probably runs out of puff sooner too - but the R65 is not exactly the bike for cruising at 160kph...or not in my neck of the woods anyway.As mentioned by nhmaf,the R65 has a different pushrod angle because of the stroke...as well as machining,it will need the long stroke cam too.

All engines make torque -  a long stroke engine will give ''hanging on'' torque,like a diesel.It will just keep slogging away.A short stroke engine will rev better as piston speed is lower,and it gives low end ''punch'' torque.This is something noticed on trials bikes,the old long stroke singles could slog up a hill,but it is the short stroke engine that gave the trials bike the ability to lift the front wheel at a snap of the throttle.

[nhmaf]

The mono R65 engine was retuned by the factory with the newer exhaust 'collector box' and  and different carb setup to yield a slightly lower peak HP (48 .v. 50 of the 81-84, if that makes muuch difference), but the torque peak from the retuning moved the  from up above 6000 RPM down to 3500 where it was more effective at typical cruising speeds.
I think the torque peak at lower RPMs partially offset the difference of the slightly taller final drive, and this also allowed the R65 mono and R80 mono to share the same speedometers with the same final drive setup, at least those that were delivered with the 37/11 final drive ratio - another cost cutting measure.

[Motu]

I think part of the retuning was to make the R65 not feel so much like a big bike with a small engine.With the lower rpm torque peak it can be ridden just like the R80...just slower.

[Altritter]

Increasing an engine's stroke is usually accomplished by using a different crankshaft; often referred to as a stroker crank. Attempting to bolt in longer -this is a relative term as to rod length discussion- custom con rods could be asking for trouble with valve interference.

Also, here's a consideration from my ancient days hanging around four-wheeler shops: sometimes a stroker crank required a deeper oil pan. Additionally, my anxiety-prone mind start nagging about some things that I haven't learned (yet) about the R65 — e.g., would the engine geometry tolerate longer rods? (Otherwise, a different cylinder jug/piston/rod design might be required.) One soon gets into metaphysics with this stuff.

[mimmo66]

Short Stroke it will be then!

Just been back from some Skyline riding, lots of 3rd and 4th gear with original setup.

Don't want to loose the GRUNT!

At freeway speeds I'm at 70mph at about 5100-5200 RPM, I was just hoping to give the engine a little rest

[Rob Valdez 79 R65]

5200 RPM is not stressing the engine.

Heavy loads at low RPM is much worse.


I used to run mine at 85mph (indicated on an optimistic speedometer) all the time, until a judge threatened to take my license away from me.
I thought 85 was the "sweet spot".  Then I mounted a GPSr and found out how slow I was really going! LOL!