Compression Question BMW/Harley
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Road Warrior
Joined: Mar 2010
Posts: 1,505
Likes: 56
From: Vancouver Island, British Columbia
Compression Question BMW/Harley
My 96" bike got bored and Keith Black 103 flat tops got installed with 204 cams and .030 Cometics. Everything else is stock in the build and I have 215 ccp. Apparently that is high. My friend has a new BMW GTL and he says it has 12+:1 comp ratio. We both run high test and if I don't click down my manual compression releases my motor labours upon start up and sometimes kicks back before firing. I don't think he has comp releases but his bike cranks over in a millisecond and fires effortlessly! He says my starter is weak. Why can't Harley's have 12:1 compression? I hear 10.25 is about the cut off for today's gas.
Thanks for the insight again
Thanks for the insight again
according to the internet, its a 12.2:1 CR stock...interesting read though, i didnt know it was that high
http://www.motorcycle.com/specs/bmw/.../1600-gtl.html
http://www.motorcycle.com/specs/bmw/.../1600-gtl.html
Club Member
Joined: Aug 2008
Posts: 479
Likes: 1
From: In the flatlands!
My 96" bike got bored and Keith Black 103 flat tops got installed with 204 cams and .030 Cometics. Everything else is stock in the build and I have 215 ccp. Apparently that is high. My friend has a new BMW GTL and he says it has 12+:1 comp ratio. We both run high test and if I don't click down my manual compression releases my motor labours upon start up and sometimes kicks back before firing. I don't think he has comp releases but his bike cranks over in a millisecond and fires effortlessly! He says my starter is weak. Why can't Harley's have 12:1 compression? I hear 10.25 is about the cut off for today's gas.
Thanks for the insight again
Thanks for the insight again
Seasoned HDF Member
Joined: Aug 2011
Posts: 11,814
Likes: 5,120
From: SW Ohio
My 96" bike got bored and Keith Black 103 flat tops got installed with 204 cams and .030 Cometics. Everything else is stock in the build and I have 215 ccp. Apparently that is high. My friend has a new BMW GTL and he says it has 12+:1 comp ratio. We both run high test and if I don't click down my manual compression releases my motor labours upon start up and sometimes kicks back before firing. I don't think he has comp releases but his bike cranks over in a millisecond and fires effortlessly! He says my starter is weak. Why can't Harley's have 12:1 compression? I hear 10.25 is about the cut off for today's gas.
Thanks for the insight again
Thanks for the insight again
Basically, it's the same reason why his bike makes HP & TQ higher up the rev range than your Harley. Multi cylinder bikes (Like your friend's 6 cylinder) need RPMs to make peak HP & TQ because the displacement is divided among 6 cylinders. Smaller bangs mean less HP & TQ until you get it spinning fast enough for the delay time between power cycles to be almost non existent.
As for the difference in compression ratios. Simple, his bike is water cooled. It can withstand the heat of a higher compression ratio and resist detonation because it runs cooler. There are other factors that determine maximum compression ratio but the quick answer is water cooled.
As for your starter being weak; I doubt it. Tell your friend that's how it is on something that isn't imported from a country where men carry purses. Now if you'll excuse me, I have some things to stir up in the Victory/Indian threads.
Road Master
Joined: Dec 2008
Posts: 872
Likes: 20
From: Niagara Falls Canada
killed my starter
i had a build with 255 cams and 10.5;1 ened up 208 when it was hot it wouldnt start without decompression releases. a guy the did a dyno tune or supposedly did it forgot my acrs when loading a program. a week of no acrs pretty well killed my starter along with a loose battery terminal. ended up replacing the starter and got the high torque one . later on i pulled out the 255 and put in a cam andrews 32 that is more matched for the higher compression. even if i have 10.5 with a 204 i get 199 for that cam with my mva heads what did he do to get the ccp so high. even a stock motor with stock heads is about 180 with that cam
Last edited by 2009blackpearle; Mar 30, 2013 at 08:53 AM.
Club Member
Joined: Jun 2007
Posts: 45,429
Likes: 2,897
From: Bedford UK
Your friend is wrong! His bike is bone stock and requires lower cranking effort, as has already explained. Your bike is modified. That's it! Your starter is not intended for starting a stoked up engine, which is why it now has compression releases. I have them on my bike, but mine are automatic, so I just press the starter button. You may be able to convert yours, so you don't have to fiddle around under the tank.
My 1980s Jaguar V12 had 12.5:1 CR thirty years ago, darned high at the time, but there is more to tuning an engine than bumping up CR. Because his BMW has higher CR than yours doesn't make his bike better, just because of that!
My 1980s Jaguar V12 had 12.5:1 CR thirty years ago, darned high at the time, but there is more to tuning an engine than bumping up CR. Because his BMW has higher CR than yours doesn't make his bike better, just because of that!
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Road Warrior
Joined: Mar 2010
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From: Vancouver Island, British Columbia
Good point karbar. My bike is a 2011 and spends it's idle time connected to a tender. It cranks over just great when the cr's are opened.
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Thread Starter
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Road Warrior
Joined: Mar 2010
Posts: 1,505
Likes: 56
From: Vancouver Island, British Columbia
i had a build with 255 cams and 10.5;1 ened up 208 when it was hot it wouldnt start without decompression releases. a guy the did a dyno tune or supposedly did it forgot my acrs when loading a program. a week of no acrs pretty well killed my starter along with a loose battery terminal. ended up replacing the starter and got the high torque one . later on i pulled out the 255 and put in a cam andrews 32 that is more matched for the higher compression. even if i have 10.5 with a 204 i get 199 for that cam with my mva heads what did he do to get the ccp so high. even a stock motor with stock heads is about 180 with that cam
