Compression question: HD vs Jap
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Tourer
Joined: Sep 2016
Posts: 375
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From: Illinois
Compression question: HD vs Jap
Looking at specs for certain jap bikes raises an interesting question: why can jap bikes run almost 13:1 compression, while HD struggles with 9:1, with many of the newer HDs having even lower compression?
Take the Suzuki GSX-R600 for instance. A stock 2014 model had 12.9:1 compression with 103 bhp. HDs new super 117 inch boasts a whopping 80 horsepower. Ive ridden them. My 93 Shovelhead pulled harder.
Yes, I understand the difference between torque and horsepower: its the difference between a dragster and a Formula 1 racecar.
But my question is valid. How can the Suzuki squeeze 12.9:1, while HDs have to use much lower compression?
Take the Suzuki GSX-R600 for instance. A stock 2014 model had 12.9:1 compression with 103 bhp. HDs new super 117 inch boasts a whopping 80 horsepower. Ive ridden them. My 93 Shovelhead pulled harder.
Yes, I understand the difference between torque and horsepower: its the difference between a dragster and a Formula 1 racecar.
But my question is valid. How can the Suzuki squeeze 12.9:1, while HDs have to use much lower compression?
Popular Reply
May 6, 2019, 06:13 AM
Banned
Joined: Sep 2009
Posts: 3,569
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From: deland,florida
Wow, what are we back in the 40's?
Jap is an English abbreviation of the word "Japanese". Today it is generally regarded as an ethnic slur among Japanese minority populations in other countries, although English-speaking countries differ in the degree to which they consider the term offensive.
Jap is an English abbreviation of the word "Japanese". Today it is generally regarded as an ethnic slur among Japanese minority populations in other countries, although English-speaking countries differ in the degree to which they consider the term offensive.
oh jeez
Seasoned HDF Member
Joined: Aug 2008
Posts: 5,071
Likes: 1,006
a lot has to do with design
then there is engine management
rice usually has smaller cylinder displacement which is a plus. while over-all, the rice displacement mass maybe greater, it is spread out unlike hd which uses two larger sized pistons and larger displacements. the small cylinder by itself is less prone to detonation. if you look at the auto industry, the trend is using smaller displacement, direct injection and boosted intake charge. all of this makes detonation less of an issue and easier to control.
hd is still archaic.
then look at how the engines produce power, aka, long vs short stroke and power pulses per rpm for ex.
can a hd engine produce more compression numbers, sure but at a cost, they have issues with heat as it is and not cost worthy for a street machine. back in the day, a 500cc rice off the showroom floor would smoke anything that hd offered off the floor.
then there is engine management
rice usually has smaller cylinder displacement which is a plus. while over-all, the rice displacement mass maybe greater, it is spread out unlike hd which uses two larger sized pistons and larger displacements. the small cylinder by itself is less prone to detonation. if you look at the auto industry, the trend is using smaller displacement, direct injection and boosted intake charge. all of this makes detonation less of an issue and easier to control.
hd is still archaic.
then look at how the engines produce power, aka, long vs short stroke and power pulses per rpm for ex.
can a hd engine produce more compression numbers, sure but at a cost, they have issues with heat as it is and not cost worthy for a street machine. back in the day, a 500cc rice off the showroom floor would smoke anything that hd offered off the floor.
Road Warrior
Joined: Feb 2017
Posts: 1,528
Likes: 646
From: Saint Charles, Missouri
Also chamber design makes a big difference, Jap bikes are 4 valve per cylinder pent roof (very small valve angle, can run a nearly flat top piston and get high compression, Harleys are old style hemispherical with wide valve angle, and must use a high dome to get compression which messes with the flame front.
Supporter
Joined: Apr 2010
Posts: 2,434
Likes: 292
From: Earth
Wow, what are we back in the 40's?
Jap is an English abbreviation of the word "Japanese". Today it is generally regarded as an ethnic slur among Japanese minority populations in other countries, although English-speaking countries differ in the degree to which they consider the term offensive.
Jap is an English abbreviation of the word "Japanese". Today it is generally regarded as an ethnic slur among Japanese minority populations in other countries, although English-speaking countries differ in the degree to which they consider the term offensive.
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Grand HDF Member
Joined: Nov 2009
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From: Tampa
I've done a bit of research on this over the last few months. The quick answer is that liquid cooling, total ignition advance, combustion chamber design, piston crown design, and cam/injection timing all play a part in allowing the metric bikes to have much more radical CR than the Harley equivalents.
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Banned
Joined: Sep 2009
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From: deland,florida
Wow, what are we back in the 40's?
Jap is an English abbreviation of the word "Japanese". Today it is generally regarded as an ethnic slur among Japanese minority populations in other countries, although English-speaking countries differ in the degree to which they consider the term offensive.
Jap is an English abbreviation of the word "Japanese". Today it is generally regarded as an ethnic slur among Japanese minority populations in other countries, although English-speaking countries differ in the degree to which they consider the term offensive.
oh jeez
