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Old 09-03-2012, 11:54 PM   #31
Twin-shocker
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Quote:
Originally Posted by Wirespokes View Post
I'd like to increase the compression on my 92 R100R from 8.5 to at least 9.5. I know one solution is to get 9.5 pistons, but I'm wondering why I can't just shave .020 off the head, which should do the same thing.

Anyone do this or have the reason it's not a good idea?
Its not a good idea simply due to the fact that without other alterations carried out at the same time its not going to make that much difference! Changes to the intake and exhaust systems should probably be carried out first, and when you are ready to fit a longer duration cam, cc the combustion chambers and adjust compression to suit the cam at the same time.
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Old 09-04-2012, 02:48 AM   #32
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The reason why it can be a good idea is that if this is done properly by shaving the heads, you get a squish band that will promote proper combustion which will maximize power and Minimize pinging. The big pistons and head shapes of airheads are not really the most efficient.

I would also agree that this work combined with some port work will maximize the results so you get an engine that shows noticeable improvements throughout the rev range which is always very sweet on the road. I don't think that there are any downsides, the extra power produced is well within the normal range of airhead engines.

I'm someone who is happy enough doing general work on my bikes, but I don't have the tools or experience to do tuning work, so I got one of the most experience UK based tuners to work on my engines, I went to him wanting a 336 cam etc etc and he said why not go down the head and piston tuning work first and see if you like it, I took his advice and as I have written before, this stage one tune was the most cost effective 'tuning' for my bikes, I have gone full hog with a 1070 cc engine with dells etc etc so I can speak from experiance.

Dual plugging will give you a smoother engine which is easy to start and will run high compression ratios on poor quality fuel. dual plugged engines will give you better combustion and can be jetted leaner than stock, it's fairly easy to do nowadays as there are various aftermarket ignitions that will provide you with an optimized ignition map.
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Old 09-04-2012, 11:39 AM   #33
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I made my mods in just the opposite direction. I put a 336 in first. Everyone told me that I had to raise compression and put carbs on and whatnot. I did none of it. I just put the cam in. The difference was night and day! Tons more midrange and top end. Before the 336 I had to rake the gearbox going through traffic on a local interstate mountain pass. After the 336, I could just leave it in top gear and throttle through traffic. Even bigger gains after I put some Dellorto's on and rejetted them.

I just dual plugged it, raised compression, put bigger intake valves in, remapped the ignition curve, port work, valve job, 4130 push rods, and replaced my hammered titanium valve spring keepers that I put in when I put the 336 in. Big diff all over again. The thing motors!

I don't think dual plugging effects jetting that much. Yes you can lean them out after you dual plug them but you can do the same leaning before you dual plug them because they are a little to a lot too rich on the mains from the factory. Nobody mentions that so here I am mentioning it.
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Old 09-04-2012, 12:00 PM   #34
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SS, that's the strongest testimony for a 336 cam I've heard yet. As with most mods, it requires a combination of very well thought out improvements to net out the whole package. One thing in isolation tends to yield only compromise.

Good on you for describing the outcome so clearly!
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Old 09-04-2012, 01:32 PM   #35
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You need to listen to people that have actually used one in order to here good things about them. There are a number of us here on ADV that love theirs. I don't understand how all the 336 BS got started but there is a ton of it out there. When I put mine in the owner of the well respected and busy dealership I was working at and three of his mechanics said that they had them in race bikes and that they made no power below 6000 rpm, wouldn't idle below 2000rpm, wouldn't work at all with my stock 8.5 CR. I had to change this and machine that. The horror stories went on and on. While I was working there the owner had his best guys try to get one of his personal friend's bikes with a 336 in it running right and they couldn't. It was the cam they said. I was right next to them working while they worked on that bike. I knew better. Turns out NONE of it was true. I love mine as do quite a few other inmates here that actually have one. I would never go back to a 308 now.

The funniest part of that story is that right after I put just the 336 in, my girl friend and packed up and went motoring around the Sierra and camped out at a local rally. We were excited about the way my bike was running and a couple of people wanted to hear it run. Duane Ausherman, a real good friend and business ally of the owner of the dealership we had just left earlier that day, was one of them and he told the ten or so people listening that I had installed a 308. It idled too well to be a 336. We didn't say anything but my girl friend and I about busted out laughing. She had just helped me put the 336 in herself and had just got off the bike with plenty of beforehand seat time on it.

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Old 09-04-2012, 01:40 PM   #36
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I have a brand new 336 on the shelf gathering dust.. One of these days...
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Old 09-04-2012, 05:03 PM   #37
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Quote:
Originally Posted by R-dubb View Post
I have a brand new 336 on the shelf gathering dust.. One of these days...
would you line me to install it for you ? On my bike that is.........
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Old 09-05-2012, 03:14 PM   #38
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Quote:
Originally Posted by Bernd Bauer View Post
Hi,



in order to raise the CR from 8.5 to 9.5 you have to shave 1.1mm off the heads.
With this PDF you can do all the calculations: Compressioncalculator

When you shave off more than 1mm then you should check the valve/piston clearance.
But 1mm is ok due to the big clearances our airheads have.

On my GS I installed 9.5 pistons and shaved 1mm off the heads.
Even with the 324 camshaft which has a higher lift than the stock cam
there is still enough room between the valves and pistons.

Regards

Bernd
So what is the minimum valve to piston clearance and squish band clearance? I've read it's up to 2mm. If you can go smaller, just how small is still safe at 6000-7000 rpm with stock valve gear?
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Old 09-05-2012, 03:36 PM   #39
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It depends on which valve. The piston meets one coming and the other going. Valve springs and valve train make a difference too. In my experience .040" is plenty. I don't think I would go past .020". If I remember right my intake valves are at .020 right now. My 336 is advanced 3* so my exhaust valve isn't of too much concern.

According to my tach, I rev mine past 8k with no problem. Stock springs. No shims. My stack is a little short according to spec anyway. I do run titanium retainers and I agree with most that it gets around another 500rpm before the valves float.
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Old 09-05-2012, 07:53 PM   #40
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Quote:
Originally Posted by supershaft View Post

Kai Ju, remember that CCing the head is still just a CR guess. How are you measuring the rest of the combustion chamber and calculate the gasket? Your CR sounds as if it was calculated based on the stock CR. Someone here actually measured CR and found BMW's specs a lot higher than what it actually measured out to be. That didn't surprise me because I find that other BMW specs are in dream land all the time. CR is like HP. Once the factory original stretched story is out there, the truth is hard to sell.
That might have been me.
For the brave among you, here's the thread. Amongst all the dross, there's lots on squish band etc.

http://www.advrider.com/forums/showthread.php?t=716776

When reading this, keep in mind that the R100S's, RS's etc that were delivered to Australia were billed not to suffer the CR decrease that the US bikes did. All the BMW literature here claimed that our early 80's bikes were still 9.5 CR. But the thread above proved that this was a crock of shite (At least with my bike). I have gone on to check a couple other so called 9.5 CR bikes here and found the same thing. So really, it was far more than BMW guilding the specs a tad...

For those intersted in calculating CR, I've written up a method tailor made for airheads in a blog including a spread sheet calculator you can download. The calculator has a variable called 'Proposed thickness to shave off head' so you can check the exact effect on CR:
http://www.cafebeemer.com.au/blog/Co...ion_ratio.html

Back to the thread..

@Wirespokes: as already mentioned here, one of the chief reasons for increasing CR is to optimise turbulence and mixing caused by the squish band. If you really want to maximise this effect, the number that keeps popping up here for minimum squish for an airhead is 0.8mm. To do this, you need to take 1.2mm (thats 0.047", not 0.02") off the stock R100 head which is close to the max you can shave before starting to cut meat out of the neighbouring cooling fin and gives you a CR of around 9.6 (from CR of 8.5 as Bernd Bauer pointed out). I haven't done this yet but on my list... Got to finish building my inertia dyno before I continue.... Also, before taking these numbers as gospel for your particular engine, you'd best double check your stock squish with standard gasket - should be close to 2mm. Then of course you need to check valve to piston clearance etc.

The upshot of all this is that with stock barrels, pistons and head, physical constraints dictate that the max CR you can ever get from head and/or barrel base shaving alone is 9.6. (notwithstanding the issue of pinging of course)

Look for @lornce posts in the thread link above that describe how all his squish clearance is provided by 0.032" (0.81mm) copper gasket.

Oh, and to ward off the inevitable argument from the usual suspects: we are talking about static physical CR, not dynamic or effective CR due to cam profile and valve overlap.
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Old 09-06-2012, 12:51 AM   #41
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Thanks Adrian. I do remember your thread and was following it as it progressed at that time.

Had you noticed any negative effects due to the reduced rocker height? I can't imagine 1mm making much difference.
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Old 09-06-2012, 03:01 AM   #42
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Originally Posted by mattcfish View Post
So what is the minimum valve to piston clearance and squish band clearance? I've read it's up to 2mm. If you can go smaller, just how small is still safe at 6000-7000 rpm with stock valve gear?

I run 30 thou squish clearance on my racer and I think it's a little tight. There;s some evidence that the pistons touch the head under certain conditions. I would think 40 thou squish is a safe figure.
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Old 09-06-2012, 05:01 AM   #43
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Quote:
Originally Posted by Wirespokes View Post
Thanks Adrian. I do remember your thread and was following it as it progressed at that time.

Had you noticed any negative effects due to the reduced rocker height? I can't imagine 1mm making much difference.
As mentioned, haven't shaved the head yet because I want to finish building my dyno so I can take a 'before' run and compare.
And I don't think 1mm will make much diff to rocker/pushrod geometry - haven't heard of anyone making other adjustments to compensate.

@pommiejohn. We've been talking in context of street machines (at least i have). Guess if you're continuously red-lining at 8500RPM in race situation then I suppose you'd want some more slop in your squish!
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Old 09-06-2012, 05:29 AM   #44
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To measure combustion chamber volumes accurately a burette http://simple.wikipedia.org/wiki/Burette is required. Using a syringe will provide a rough figure, but checking volume properly takes a while to do, so its best to get it as accurate as possible.
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Old 09-06-2012, 05:35 AM   #45
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To measure combustion chamber volumes accurately a burette http://simple.wikipedia.org/wiki/Burette is required. Using a syringe will provide a rough figure, but checking volume properly takes a while to do, so its best to get it as accurate as possible.
Thats for sure. and you want a 100ml burette with minimum 0.2ml graduations. Combustion chamber volume at TDC for R100 (8.5 CR) is around 88ml or cc's
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