I'm in the middle of the fuel moto 107 up grade and sounds like there are two options for pistons on this build one configuration nets a 10.25-1 compression ratio and the other about 10.75-1 ratio what are the pros and cons with these two choices?

All changes included are:
- Stage 1 AC
- PC-V tuner
- 107 jugs and pistons
- Fuel Moto's level B head work
- V&H Monster rounds
- Jackpot 2-1-2 head pipe
- TW 777 Cams and directional lifters

Also does anyone suggest doing any clutch upgrades right away on this build or am I good for a while?

2011 RKC with the 103 Thanks Ken,

 

An expert will be along shortly, but my guess is that one CR works with some cams and the other CR works better with other cams. I would have thought FM could advise on what would work best with your chosen cam.

 

I'm sure they will just thought I would get a jump on understanding this prior to their call, my parts should be showing up in WI about tomorrow or Monday

 

Patience! There may be a more useful reply soon.

 

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Don't have much of a choice

 

My 106 has 107hp and 115 tq on the dynojet and had roughly 16000 miles on the stock clutch before I went bigger.
I now have over 3000 miles in it and full blast rolls don't slip,yet.
I've read a stiffer spring will help but I havent needed it yet.
I can't help you with the cr. Sorry

 

Thanks for the info!

 

The low compression pistons are the CP flat tops, the higher compression are the CP 3cc popups. The flat tops work well with the tw777 or tw555 cams (or Andrews 54, etc). Probably a better build for a touring bike than the higher compression build.

I put the high compression pistons in my bike. They need a longer cam, I've got the tw8 cams. My CCP is just over 200 psi both cylinders, and I have automatic compression releases. I haven't had the bike dyno'd yet, but it pulls HARD off idle, and is a big jump up in performance from the previous 103/SE204 build that I had last year.

However, with the higher compression comes more noise and heat. I'm still tuning the bike, maybe some of that will be mitigated once it'd dialed in, but in some ways, the lower compression 103 build I had last year was a friendlier motor for daily use.

I think if I were to do it again, I'd probably go with the flat tops and the tw777 cams. Basically, I have a build very similar to what FM calls their "Hellraiser" 107. It's probably more suited for a bar hopper type bike like a hotrod dyna.

 

Get the round one's

 

10.25:1 vs 10.75:1.

Higher compression usally is good for power...but will generate more heat...if you are using forged pistons...the higher heat is easier to deal with since forged pistons transfer heat to the outer cylinders easier and better than cast.

Higher compression may result in higher octane requirements. I'm at 10.5:1 calculated and can run 91 and 93 gas. So that probally means I am really at a lower actual compression ratio.

Startup def needs MCR's with 10.75:1. Spinup on startup with MCR's open is a longer spin duratiion.

with 10.25:1 you can staty with 6R12 plugs (stock) which is good if you have stock ECM's becuase it will keep your anti-knock feature working.

with 10.75:1 you will need 10R12 plugs

I would watch the low idle RPM with either 10.25:1 or 10.75:1 especially the hot low idle RPM. If you don't watch this, your crank will spin so slow that you will hear your crank chirping on each revelotion. not very good for cran bearings.

watch your case vents with 10.75:1.....you might have crank pressure issues and find that you have seeps at your upper push rod seals...let it seep for a few hundred miles and it might settle as the seals mate over time and heat. If it never settles...you might need to drill out you crank case breather bolts at the stage 1 intake.

what would I do and why....I would go with the 10.75:1 under the assumption its a theoretically number and if actually tested after the build it would probally be less. you can always change the head gaskets to move from 10.75:1 to a slightly lesser value but you can't use even thinner head gaskets to try to increase compression even more with out risking valve to piston clearence issues.

what your high lift cams, sometimes they will hit the rocker covers and harly say play giggle wiggle during the install to prevent that....I used two head gaskets to get an extra .010 of an inch clearence so I would not have to play giggle wiggle.

for either...the power range may not be exactly where you would like it. You might need to go to 30T/68T to move the RPM range closer to the ideal power range. engine will spin a few hundred RPMS faster and that might help move the begining of the power window more into your driving needs. I say this since most high lift cams have late power. If you are in the 10.25:1 and 10.75:1 compression realm..your probally running very high lift cams. if your not...go with the lower compression choice.

clutch question

before 60mph/3000rpm I seem to be fine with the stock clutch plate arrangement. But if I gun it after 60mph/3000rpm...I sense a clutch slip..so I need to go in and do the clutch plate upgrade.

hope this helps.