Bottom End & Top end Match?

[dave2large]

Can you bolt up a set of 1000CC cylinders & heads to a R80 bottom end?

[tkpinsc]

Can you bolt up a set of 1000CC cylinders & heads to a R80 bottom end?

Yes*


* In the worst case, depending on the year of the cylinders and and crankcase you may need minor machine work.

What year top and bottom are you working with?

[dave2large]

They were already done by a professional BMW mechanic and I was thinking of buying the bike. It's a 1979 R80 w/1000CC cylinders so it's kind like a /7 with more low end but less top end speed.

[Na Cl K9]

it's kind like a /7 with more low end but less top end speed. I'm curious how you came to that conclusion?

[535is]

I'm curious how you came to that conclusion?

Same here. Stroke is the same, so adding the cylinders and pistons makes an R100 engine.

[jforgo]

I would think the top end is lower, because you are still using the R80 lower geared rear end

[Na Cl K9]

A stock 79 iron barrel R80/7 was delivered in the US with a 1:3.20 final drive. It made 55hp @ 7000 rpm with a compression ratio of 9.2:1. Top speed in fifth was listed as 111 MPH. Stroke is 70.6. Listed torque: 47 Ft Lbs at 5500 rpm.

A stock 79 iron barrel R100/7 was delivered in the US with either a 1:3.20 or a 1:3.09 final drive. It made 60hp @ 6500 rpm with a compression ratio of 9.1:1. Top speed in fifth was listed as 115 MPH with the 3.20 FD. Stroke is 70.6. Listed torque: 54 Ft Lbs at 4000 rpm.

From experience and looking at the torque figures an R100/7 will out-pull an R80 in roll-on (low end?). They top out (top end?) within 4 miles an hour of one another (the larger displacement being slightly faster), however as you can see, the R80 is busier than the R100 at that speed. I think the important thing to consider is that at 4000 rpm, the R100 is in the lower half of its power band. At the same RPM, the R80 is in the upper half.

Interesting looking at the charts again...I spent a sick day once calculating the top speed for each displacement using the same 5 speed transmission ratios and all the available final drive ratios...That was when I realized I was really sick :-)

Buy it, ride it, enjoy it...more power is ALWAYS mo-betta!

[Rapid_Roy]

It's my bro, and he has found a couple of actual 1000's to ogle now too, with lower miles.

[Na Cl K9]

The R80 once converted by adding the 1000cc top end effectively becomes an R100. There is no real discernable difference except when you open the throttle and at the insurance company. If it is titled as an R80, usually it is cheaper to insure due to the smaller displacement category (798.2cc's)...but, I wasn't here and I didn't tell you that and this message will self destruct presently...

[lkchris]

I would think the top end is lower, because you are still using the R80 lower geared rear end

That's correct, and there's more as the R100 engine doesn't like to rev like the R80 does and this can become an uncomfortable ride.

Most R80 to R100 conversions are just half done.

[Na Cl K9]

Most R80 to R100 conversions are just half done.What then is the other half of the equasion?

[tkpinsc]

What then is the other half of the equasion?


Better matched rear drive ratio.

[Na Cl K9]

So, if the 1979 R80 came to the US originally equipped with a 1:3.20 rear gear set, then what final drive ratio would you recommend as a better match for an R80 to R100 displacement change?

[tkpinsc]

So, if the 1979 R80 came to the US originally equipped with a 1:3.20 rear gear set, then what final drive ratio would you recommend as a better match for an R80 to R100 displacement change?

Depends on use.

2.91 for reduced rpms when cruising at speed.

3.00 good for all around performance

3.09 better acceleration and sporty riding in the twisties

3.20 for trailer pulling

3.36 for sidecar

3.56 for sidecar with trailer

each step is not that different so 1 step either way will not be all that noticeable.

[lkchris]

The 79 R100s came with 2.91 ratio. As did they all through 1984.

The lower-powered R100/7 of '77-'78 came with 3.00.

The lower-powered '88-on models came with 3.00.