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Old 11-28-2013, 05:52 AM   #1
lancas OP
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Compression bump up ????????????????

I'd like to bump up the CR on my R100 and am wondering what might be a safe amount to have shaved off the heads without having to do the calculations. These heads have never been machined before by the way.
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Old 11-28-2013, 06:17 AM   #2
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What year and model R100. There were different pistons and compression ratios thru the years?

May also include some note about what result you'd like, are you going with any other "bigger" parts like cams or carbs and any budget ideas?
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Old 11-28-2013, 08:00 AM   #3
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I am very interested in this discussion . I have a set on nice 1980 R100 heads that have been rebuilt and dual plugged and wanting to put them on 91 mono . I have a set of Omega ( MS) pistons ... Just not sure if they fit
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Old 11-28-2013, 09:52 AM   #4
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Originally Posted by lancas View Post
I'd like to bump up the CR on my R100 and am wondering what might be a safe amount to have shaved off the heads without having to do the calculations. These heads have never been machined before by the way.
It all depends on what model you have and/or what pistons you have in your bike. Personally, I am very conservative about CR. I grew up in and around a BMW dealership in the '70's. I started working there as soon as I quit my paper route. I have heard a LOT of complaints about airheads pinging. I have seen tons of airheads that came stock with 9.0:1 CR get their CR lowered with thicker base gaskets for pinging. I had to detune my own R69S for pinging. My LS with 8.2:1 CR didn't ping. My '92 R100 with 8.5:1 CR didn't ping on regular until ambient got around 80F. Then it needed premium to not ping and that is WITH a 336 cam that lowers effective CR for a later closing intake valve. That is cutting it pretty close IMO. I think 8.5:1 is about perfect for single plugged airheads. Personally, I would not raise my CR above 9.0:1 without first dual plugging it and re-mapping the ignition curve.
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Old 11-28-2013, 11:58 AM   #5
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Does fuel vary from country to country as I run all my bikes on 95 including the race bike and a Ducati ST4 and no issues with pinging ever.
Even my race bike with 1mm off the heads runs on pump gas.
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Old 11-28-2013, 12:03 PM   #6
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Does fuel vary from country to country as I run all my bikes on 95 including the race bike and a Ducati ST4 and no issues with pinging ever.
Even my race bike with 1mm off the heads runs on pump gas.
Yep. Octane ratings differ. RON, MON, or AKI are used to calculate the octane rating depending on the country, so numbers from one country aren't necessarily directly comparable to numbers from another country.

http://en.wikipedia.org/wiki/Octane_rating

Our premium in the US which is typically 93 by the AKI method is roughly equivalent to 97 by the RON method which (I believe) is what you kiwis use.
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Old 11-28-2013, 02:22 PM   #7
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Originally Posted by supershaft View Post
It all depends on what model you have and/or what pistons you have in your bike. Personally, I am very conservative about CR. I grew up in and around a BMW dealership in the '70's. I started working there as soon as I quit my paper route. I have heard a LOT of complaints about airheads pinging. I have seen tons of airheads that came stock with 9.0:1 CR get their CR lowered with thicker base gaskets for pinging. I had to detune my own R69S for pinging. My LS with 8.2:1 CR didn't ping. My '92 R100 with 8.5:1 CR didn't ping on regular until ambient got around 80F. Then it needed premium to not ping and that is WITH a 336 cam that lowers effective CR for a later closing intake valve. That is cutting it pretty close IMO. I think 8.5:1 is about perfect for single plugged airheads. Personally, I would not raise my CR above 9.0:1 without first dual plugging it and re-mapping the ignition curve.
Standard cam, 44mm in, 42 mm ex, twin plugged, currently standard pistons but would like it to be flexible enough so a Siebenrock upgrade could be fitted at some stage. I agree with the idea of being conservative which is why I only want a modest increase. I could also be swayed to stay with standard CR.
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Old 11-29-2013, 09:54 AM   #8
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Originally Posted by Airhead Wrangler View Post
Yep. Octane ratings differ. RON, MON, or AKI are used to calculate the octane rating depending on the country, so numbers from one country aren't necessarily directly comparable to numbers from another country.

http://en.wikipedia.org/wiki/Octane_rating

Our premium in the US which is typically 93 by the AKI method is roughly equivalent to 97 by the RON method which (I believe) is what you kiwis use.
So does specific gravity and boiling point.
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Old 11-29-2013, 09:58 AM   #9
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Originally Posted by lancas View Post
Standard cam, 44mm in, 42 mm ex, twin plugged, currently standard pistons but would like it to be flexible enough so a Siebenrock upgrade could be fitted at some stage. I agree with the idea of being conservative which is why I only want a modest increase. I could also be swayed to stay with standard CR.
Siebenrock upgrade with 9.5:1 pistons?
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Old 11-29-2013, 01:42 PM   #10
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YOu can only go far with head gaskets and whatnot, right? custom pistons are the answer, after that?

HEy SHaft, i do have the 9.5's, and I run 87 octane through them on single plugs, no problem.

My R60/6, CRwhoknows pings like crazy, unless its getting the pretty wine.
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Old 11-29-2013, 02:02 PM   #11
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YOu can only go far with head gaskets and whatnot, right? custom pistons are the answer, after that?

HEy SHaft, i do have the 9.5's, and I run 87 octane through them on single plugs, no problem.

My R60/6, CRwhoknows pings like crazy, unless its getting the pretty wine.
I have heard your story many times Sh but I have heard my story many times over that. I think I have a pretty good sample of such things growing up around and in a BMW dealership and working at a few more after that. Plus I have my own personal experience. Some people say they never hear their airheads pinging. Some people say R1200's don't ping because they have anti-knock sensors. I reqlly wish I could say that and so do a LOT of other people!

You don't hear your airhead ping? Some of it depends on how you use the throttle. Some of it depends on your hearing. I think a lot of it depends on SO many people timing their airheads retarded from spec for looking down at the timing marks versus lining them up correctly.
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Old 11-29-2013, 02:18 PM   #12
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HMm. OK, I will keep checking. THere's always a lot going on, and my hearing aint the best.. and honestly the old girl aint running right, offhand

I do trust your opinion, so my own experience is always interesting to compare.
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Old 11-30-2013, 12:14 PM   #13
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HMm. OK, I will keep checking. THere's always a lot going on, and my hearing aint the best.. and honestly the old girl aint running right, offhand

I do trust your opinion, so my own experience is always interesting to compare.
Thanks Sh!

Since I got my bike running a LOT stronger with a major CR bump and port job you can really notice a power drop for pinging versus pinging going on with less power to start with. I hope that makes sense?
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Old 12-02-2013, 05:21 PM   #14
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9.5:1 pistons

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Originally Posted by supershaft View Post
Siebenrock upgrade with 9.5:1 pistons?
That would be the ultimate set up but it may be a year or 2 before that happens. In the mean time my aim is to avoid any pinking on 97 / 98 RON fuels and improve the combustion process and performance. No plans for anything radical in the ports at this stage but if you have any photo's of your porting work that would be great to see.
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Old 12-02-2013, 09:11 PM   #15
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Originally Posted by lancas View Post
That would be the ultimate set up but it may be a year or 2 before that happens. In the mean time my aim is to avoid any pinking on 97 / 98 RON fuels and improve the combustion process and performance. No plans for anything radical in the ports at this stage but if you have any photo's of your porting work that would be great to see.
We still need to know year and model and hopefully what pistons are in it now?

Pictures? I did buy a camera a few years back at nearly 50 years old. It's the first one I have ever owned but I still don't take many pictures. Especially while I am working! I learned my general port mods while doing valve jobs on airhead race bikes with heads flowed by Branch flowmetrics, Kenny Agastini (SP), and the like. They all did pretty much the same thing to them. Pretty much right out of a Smokey Yunick book only a little different.
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