Higher Compression & Engine Reliabiity- FM 10
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Road Master
Joined: Feb 2007
Posts: 781
Likes: 35
From: St Louis
Higher Compression & Engine Reliabiity FM107
Thanks guys for the great and thoughtful responses to my earlier questions about 107 build ands head work. I've settled on Fuel Moto (seem like great people) and have one other question: Anyone have any problems with engine reliability yet with their FM 107 and a 10.5 CR? I'm wondering how much more stress a 10.5 ratio puts on the lower end/compensator/etc. than a ratio in the low 10's?? It seems like it would be minimal but I'd love some input. I have ACR's and it's a 2011 103".
Road Captain
Joined: Jul 2012
Posts: 687
Likes: 1
From: Texas
Thanks guys for the great and thoughtful responses to my earlier questions about 107 build ands head work. I've settled on Fuel Moto (seem like great people) and have one other question: Anyone have any problems with engine reliability yet with their FM 107 and a 10.5 CR? I'm wondering how much more stress a 10.5 ratio puts on the lower end/compensator/etc. than a ratio in the low 10's?? It seems like it would be minimal but I'd love some input. I have ACR's and it's a 2011 103".
First, I am not aware of a higher percentage of engine failures of 107" builds with 10.5 CR's. In fact, I have not heard of many 107 failures at all (just a couple for varied reasons). FM seems to get slightly higher TQ & HP numbers than your average 107 build and maybe that is due to 10.5 CR that i have seen with most of their 107's lately. I have a 107" BB with a 10.3 CR and no bottom end work. The builder who did mine feels like this is a proven, reliable build with stock bottom end and did not recommend going any higher compression. FM may feel the same about their build @ 10.5 so it will be interesting to see what others have to say. Personally, I would not push it too far without bottom end work.
Last edited by Charlie Fogg; Mar 12, 2014 at 04:33 PM.
Ultimate HDF Member
Joined: Mar 2012
Posts: 6,129
Likes: 143
From: Santa Cruz
Any time ya add power (and use it) there is more stress on engine and drive-line components. Lets say a stock 103 makes about 70 hp and FM 107 about 110+hp, so ya will be adding nearly 40% more stress potential. In reality few folks actually use that potential very often. So it really boils down to your right wrist how much reliability ya will lose.
Tourer
Joined: Sep 2013
Posts: 297
Likes: 1
From: Ok
I'm newer at the bike scene but coming from many years in drag racing . I'd think the difference in 10.5 to say 10.2 compression would be so minimum that you wouldnt even notice a difference. I've never known of any durability or dependability issues due to high compression . Just have a proper tune and good octane fuel . Maybe if you were talking 12:1 compression then I'd say that's not going to be as easy for the street .
Road Master
Joined: May 2008
Posts: 1,196
Likes: 14
From: connecticut
I don't consider 10:5 to 1 a high compression engine. If all the components work together and is tuned properly. There is no reason it cant be used for 100000 miles. I personally beat the hell out of my bike and usually put 15000 miles a year on. My 2013 cvo 124 inch engine@11:2 to 1 compression will be 140 -150 hp and used for long distance touring. Theres a lot of factors that come into play for a combination that runs cool and doesn't ping on pump fuel.
I'm running a 107 at 10.8 compression. It's about 1-1/2 years old, and there's been no issues at all. There's a little more top end noise than on a typical stage I or stage II build, due to the more agressive cams and forged pistons.
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Intermediate
Joined: Jun 2011
Posts: 26
Likes: 0
From: Minnesota
I have a FM 107 build on my 09 Ultra, the build was done late Feb of 2012. The bike had around 22K when it was done. It now has just over 50K on it, and still as strong as the day it ws built. Not a single issue with the work or build, and it isn't babied. Depending on the cam you select you will notice some more top end noise, I run a Woods 777 .. or maybe not depending on your exhaust DB's. I run a very stock sounding 2-1-2 exhaust sysytem. They did a fantastic job, and the numbers are great 111.5 HP and 122 TQ, with stock heads a ThunderMax TBW ECU. I am considering FM's head upgrade with compresion release, with the right (or wrong) conditions it can be a little slow to crank. It's just a little red sleeper. 
with LED's all the way around.
Thanks to Jammie and the guys at FM !! Nice job
with LED's all the way around. Thanks to Jammie and the guys at FM !! Nice job
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Road Master
Joined: Oct 2006
Posts: 1,122
Likes: 23
From: Mishawaka,Indiana
You have to consider the dynamic compression ratio, static ratio for 10.2 and 10.5 could have same dynamic ratio (every cam is different) most newer cars run 10-11 to 1 without issue. But, you always run the risk of failure when increasing power. Mike
