OK, let's do it.

[Le singe]

I've finally made the decision to modify the R1100GS engine unit. On Wednesday,I sourced and bought an early R1100RS in Germany,which will be the donor unit for all the engine parts required,ie cylinder heads,camshafts,higher compression pistons plus the barrels.
The plan is to fit these parts plus modify the heads,different valves,gas-flowed,etc.
I would like to keep the present mid-range drive but also extend the power curve upwards for another 2000rpm after the point at which it starts to fall away now.
The RS heads will be removed,stripped and the ports dimensioned next week,then I'll decide what valve/port ratio will work best with the 'S' specification camshafts.
I will to dyno the GS as it is now to give a before and,later,an after,but dynos in Morbihan are rarer than hen's teeth,so finding one locally will be a project in itself.
I agree it could be argued the bike is perfectly OK as it is,but as most people on this Forum would admit in a truthful moment,the 1100 is the best GS ever made and an 1100 with 1200 power would be the ultimate

 

[(RIP) Kaister]

Excellent post - keep us informed.

How will you do the ports and what decides on the best option - maximum flow or maximum velocity ?

 

[Le singe]

I'll have to map the cams before I decide on the best port/valve ratio,I need to know things like lift of each valve at TDC overlap and the opening and closing rates of the valves.
There isn't a great range on the ratios,so it's just a case of deciding which end of the range to start with.
Maximum flow or maximum velocity? Can't answer that as I don't know!
For instance,the Japanese bikes run relatively huge ports but cam accordingly,hence their power is always high-rev related,(we're talking 4-cylinder here).
All the single cylinder engines I've worked on would be a memory at those type of revolutions,so it's smaller ports and different cam profiles,which also give fat torque curves for better drive,for an engine like the BM,this,I think,is more important than an 'empty' peak power figure.
It's going to be interesting to measure the BM's ports.

 

[Deleted account 220427002]

go for it!!!!

and take plenty of pictures


any chance of fitting a turbo

 

[Taipan]

I've subscribed to this thread. Well done for doing this. I've always wanted a bike with a spare engine that I could play with. Thi is the next best thing I suppose. Seriously I'll follow this with interest. Please update us every step along the way.

 

[Le singe]

The engine is now on the bench and one cylinder head has been removed and stripped for measuring.
I chose an earlier engine because it has 36/31mm valves,rather than the later 34/29mm valves.
Port wise,the inlet valves need to be a minimum of 38mm. I would rather use 40mm valves,but this would mean re-pitching them and modifying the inlet rocker arms.I've already made the decision that,with this first level of tune,to keep things sensible.
So two-off 38mm inlet valves it will be.
The exhaust ports are limited by the inside diameter of the two small pipes feeding the larger exhaust pipe.Although the port I.D. will be less than the theoretical optimum size,past experience has shown this shouldn't present too much of a problem.
The weld penetration inside the pipes needs serious attention!
It might be worth looking at an 'S' manifold later if the present set-up needs improving upon.
It must be remembered that the Motronics built-in rev limiter will set a peak in terms of usable rpm.There's no use in modifying the engine for peak power at 9K rpm if it can't reach those rpm!
The next job is to map the camshaft and see what this reveals,I'm not expecting any surprises.

 

[Le singe]

While I'm sorting out everything necessary to prepare the heads,I've decided to fit the RS camshafts plus the slightly higher compression ratio RS pistons to the GS.
BMW claim an extra 10bhp for the RS,the extra power being produced after the GS flattens off.
Fortunately,both my barrels and those off the RS are graded 'A',so I'm using the RS pistons with my GS rings in my GS barrels.
I'm waiting for a couple of gaskets to arrive,then it's testing time,it will be interesting to feel how the engine performs.
It could be a cost effective alternative to buying a 'chip',just hope it doesn't turn into a 'beast' like some of the chipped GS's I've read about

 

[DrAlf]

just hope it doesn't turn into a 'beast' like some of the chipped GS's I've read about

Well of course it will - I thought that was the aim of the game Did you know that bb power do a range of serious engine modification products such as HC pistons and bigger valves? Well worth checking out.....

BB power stuff

Keep us updated and good luck I'm tempted

 

[Le singe]

I've seen the BBPower mods already,their Stage 3 is almost identical power and torque wise,to a standard RS engine,The RS has a different camshaft and slightly higher compression pistons,costs a whole lot less as well.
I don't think 90bhp in a bike that weighs the same as a Hummer will ever be a 'beast',

 

[Deleted account 220427002]

pics please

 

[DaveGSCymro]

I will to dyno the GS as it is now to give a before and,later,an after,but dynos in Morbihan are rarer than hen's teeth,so finding one locally will be a project in itself.

Don't worry about the dyno, what a beautiful area you live in.

 

[Le singe]

Pics of what?
I'll take a pic of the GS and RS piston crowns side by side which will show the differences clearly,but the camshafts all look identical.

 

[Le singe]

What a beautiful area,are you taking the p1ss?
It's been drizzling for about 4 days now,it's the rain which starts the boredom which leads to the projects

 

[DaveGSCymro]

What a beautiful area,are you taking the p1ss?
It's been drizzling for about 4 days now,it's the rain which starts the boredom which leads to the projects

So what, it rains here in North Wales. It's only water for fecks sake. Get some 020's, so what about the rain........

 

[Le singe]

A few photos,should be self-expanatory,the main one being the differences between the piston crowns.

 

[Le singe]

Another...

 

[Le singe]

The pistons....

 

[Deleted account 220427002]

a thread is always nicer with some pics

since i've never had a GS apart it's good to see the bits in the flesh and the steps involved.

now, i have to say thats a nice garage

or is that your living room?

 

[Le singe]

I've now completed the GS to RS conversion and ridden the bike.
Initial start-up was,as expected,no different to normal. I let the engine 'settle' for a minute or two then blipped the throttle,pick-up is a lot more rapid and it can instantly be heard that the engine's been modified.
Riding it was fantastic,yes,I know it'll never be a Fireblade,but what a transformation. The old set-up was positively asthmatic by comparison.The slight drop in mid-range torque is not noticeable,but the extra 'go' above 5K rpm is really good.
At 4.5K rpm there is a slight lull then the engine starts to pull really strongly. Because of the lack of go on the standard GS,short-shifting was the norm,but with this set-up,a conscious effort has to be made to change at 7K rpm as the motor is still pulling strongly.
At 160kmh,(100mph),the engine is still ready to go,and yes,it will pull 7.5K rpm in top gear!
FWIW,I think the RS set-up should be standard.
So,it's on with the next stage now,I've seen an1150RT/RS motor for sale and I'm led to believe the barrels and camshafts fit the 1100,the voices are telling me to do it........