Recently I have been pondering the differences between these two model's pistons. The K75 3 cylinder has an 11.0:1 compression ratio whilst the 8 Valve K100 four gets by on a 'mere' 10.2:1. They both use a 67mm bore and 70mm stroke, and share 34mm intake & 30mm exhaust valve diametres.

There is a forum member I know of who is reported to have K75 pistons in his K100 motor with good result. I'm wondering if anyone else out there has experience with this 'upgrade'. Also, has anyone got decent quality photos of a K100's piston top? I'd like to get a visual idea of the differences in valve cut-out area, offset, if any, and squish band, required for valve to piston top clearance.

I fully appreciate that it is a relatively minor 'bump' in compression from stock but it's from little things that big things grow. I went up from 9.5:1 to 10.0:1 pistons on an R100 Airhead and the mid-range to upper RPM 'punch' was quite noticeable, accompanied by cylinder head dual-plugging, an issue not so easily remedied on a K100.


Motobins have these pistons for less than AUD$80 a piece, and you'd then want to use the pinned version of piston rings.

Thinking, just thinking...


Auguste Rodin's The Tinkerer...?

Hi there, ain't this good enough?
Best regards
JAP

They certainly have more on top so you will get a bigger bang for your buck.
I have never checked it out but I wonder if the cams are a bit lumpier as the 75 really rocket when the cams come in.

I dont believe the cams are any lumpier, I believe the bike gets up and goes because it's getting over the 120% timing of the power pulse only 90% for you on yours. the 11:1 evidence is in the photos. have a close look at the two and you can see the raised chamfer and the greater cut out for the valves. I don't believe there is any difference in gudgeon pin to upper ring land length such as you would find between a 202 and 186 engine in a holden of that ilk. Its all on the crown. however I will go through all I've got to see if anything suggests a cam profile change.

nice idea gith

the only things i could offer to the upgrade conciderations might be ....

1> replace with better than stock (k100 )valve springs ...or new ones ( the tolerances are just too close to frig around with old springs in an interferance motor ...with higher compression )

2> the fuel mapping would change a little bit .....might be a good idea to observe fuel mixtures with the proper tools ..we dont want to leen out at high revs and cook the piston tops , plugs ad infinitum


3> concider the firing timing ...with higher compression you might have to retard the advance somewhat ...depending on fuel and actual compression acheived


dont forget that the k75 uses single fire mapping for fuel injectors and coil firing ...not that it makes a whole lot of difference to the k100 ...but obviously there are benefits to sequential injector firing ...rather than synchronous alternating injection

As much as I could search through the stuff I have on the k's I cant find any difference on the pistons other than the crown. The rest of the stuff appears the same, also both have 284 duration, all have a common 9.3 and something equal for inlet across the board. also exhaust. The 75 makes 75hp the 100 makes 90hp so changing the pistons should mean an extra 10hp being all else equal. the thing with our pistons is they come in triplicates all weighed to the microgram so to get four you will need to have the extra one finished off to the other three weight wise, apart from any other things done to your bike a stock bike can gain 10hp or killawattsits and that 10 is a lot!

More than 10% increase in power..... :cyclops:

The big question is why BMW didn't do for the 4-cylinder what they did for the 3. Increased vibrations??

I have just bought a set of 1200 cams so I will be finding out if they work as well as I have been told when they go into an 1100.
Wouldn't mind getting some higher comp pisstins for the 1100.

ReneZ, I think they were into that whole ultimate cruise machine thing, so didn't chase the ZX10 or V4Honda1000 type power. Maybe it's the heat thing, they would need a 5 core radiator! Well, maybe 2 radiators. RickG, it's kinda infectious isn't it?

Sure is. To get half the gain as last time you pay twice as much

just a question .....is it the later k75 with the um ....3 core radiator ?

has anybody got a good shot of the head .....i want to look at the ridges (joggles called by trd )etc that might need a smooth out in the squish zones .

um wouldnt a service grind of the head ...maybe .25 mm give much a similar result ? or along that same thread .....a thinner head gasket ?

charlie99 wrote:um wouldnt a service grind of the head ...maybe .25 mm give much a similar result ? or along that same thread .....a thinner head gasket ?

Think of what cam timing issues might arise and have to be dealt with. There's no easy way to compensate for that.

I say stuff the holes with higher domed slugs (checked for proper clearance & fit), then the suck, squeeze, bang & blow becomes more robust & healthier.

yep could be right ...its that valve overlap period that would be the valve bender huh ?

i guess with deeper fly cuts its sorted !!

You sure right about that. The harder the suck the more squeeze and better blow makes for a real good bang.
I like what Stu Hilbourne said "injection is nice but I'd rather be blown"

Charlie the 3 core radiators came from the 83 and 84 K100s

Guest in the House wrote: Also, has anyone got decent quality photos of a K100's piston top? I'd like to get a visual idea of the differences in valve cut-out area, offset, if any, and squish band, required for valve to piston top clearance.

here is a pic for you..84 k100

They say a picture says a thousand words, but does anyone else hear oval pistons coming out of this pic?? nothin beats cubic inches they say Some nsr technology for the K me thinks. Anyway definatly more crown showing in gith's pic of the 75 slug

reading an article yesterday in the trader mag for bikes, they were looking at the 1600 K series BM and had the origional K1 along as its origintor but they quote the K1 as being 11:1, so one wonders is there any truth to that???? and could the gith go that way????

The K1 was a four valve... so no love there.