850cc to 1100cc conversion.

[Deleted account 111026]

Has anyone fitted 1100cc barrels to an 850cc engine unit leaving the 850cc cylinder heads in place?
I realise the compression ratio will be around 12.9:1, but, with the smaller inlet and exhaust valves, would this give better mid-range torque than the standard 1100cc cylinder head, (with either 34 or 36mm inlet valves)?
Both engines use the same camshaft profile.
An R850R rider asked the question today and it would be interesting to see if anyone has done the conversion.

 

[Paul Young]

Why not go even further & use 1150 barrels & pistons, then fit 1100S heads. That is where the 1150's came from (strictly speaking, the barrels/pistons were from the 1200C). The exhaust headers can be retained & the cams swapped over so you get the better breathing, too.

 

[Deleted account 111026]

Unfortunately, for the price of that conversion, he could exchange the R850R for the R1100R!
Performance isn't the ultimate aim, the idea was to see whether anyone had done the conversion, and it's effect on the torque curve.
By the way, the 'S' heads are the same as the 1150 heads.
Thanks anyway.

 

[DrFarkoff]

I realise the compression ratio will be around 12.9:1, but, with the smaller inlet and exhaust valves, would this give better mid-range torque than the standard 1100cc cylinder head, (with either 34 or 36mm inlet valves)?

It would spend it's life pinking it's head off especially with the quality of modern fuels

Also I would presume there would be issues with clearance of liners going into the block

I have a Damaged 1100S engine with good barrels, pistons and heads (inc good exhaust studs and cams and cam carriers) if your mate is interested?

 

[Paul Young]

Unfortunately, for the price of that conversion, he could exchange the R850R for the R1100R!
Performance isn't the ultimate aim, the idea was to see whether anyone had done the conversion, and it's effect on the torque curve.
By the way, the 'S' heads are the same as the 1150 heads.
Thanks anyway.

I've got a pair of nearly new 1100S t/spark heads that were a very good price, as it's the cams that really cost. They aren't quite the same as 1150 heads as they retain the older exhaust studs, so the 850's headers should be correct for them.

eBay (.de) is your friend

 

[Deleted account 111026]

Aware of the twin-spark ones, the porting is just the same as the earlier ones. There is a major problem with the standard porting on all these heads, unfortunately it's not worth the cost sorting it out.
I've dyno-ed the engines with flowed heads, bigger valves, different compression ratios, all variants of the BMW specification camshaft, different advance curves, etc.
The conclusion being, if I wanted a real gain, I'd buy a 1200 or Japanese variant, pounds spent against performance achieved, the BMW is not your ideal bike.
Set everything up to it's optimum, exhaust, etc, then leave it be.
(Unless the bloke in the pub knows better!).