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Discussion Starter · #1 · (Edited)
Just doing a few calculations on a build im doing,

4.000 Bore x 4.000 Stroke, .040 head gasket, .000 deck height,

72 cc Combustion Chamber gives me 11.0-1 Static CR,

Cam Intake Closes at 50 ABDC which gives me a Dynamic CR of 9.4-1,

The Cam is Made for 10.2-1 CR but is that static or dynamic?
if i grind the Combustion chamber out to 79cc that will give me 10.2-1 Static Cr, just not sure which CR im shooting for Static or Dynamic???

Would like to use pump gas? I have twin plug heads and compression releases which will allow me to push it a bit,

Just dont what to have any Deto probs, :flames:

Your thoughts???
 

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DanNZ said:
Just doing a few calculations on a build im doing,

4.000 Bore x 4.000 Stroke, .040 head gasket, .000 deck height,

72 cc Combustion Chamber gives me 11.0-1 Static CR,

Cam Intake Closes at 50 ABDC which gives me a Dynamic CR of 9.4-1,

Will this be ok for pump gas? I have twin plug heads and compression releases which will allow me to push it a bit,

Just dont what to have any Deto probs, :flames:
Some folks like to look at it in terms of cold cranking pressure rather than just the corrected compression ratio. FWIW, my calcs for what you propose were 11.05 Static, 9.65 corrected, 202.5 PSI CCP (at sea level). Anyway you look at it, that's near to the top of the "food chain" for pump gas and an air cooled motor. Many respected builders recommend that CCP should be 185 to 190 max for a trouble free street motor --- some are able to push it beyond that, others have trouble with CCP's that are in the 180 range! It really depends on how well versed you are in the care and feeding of a high compression motor, how the bike will be ridden and what kind of loads it will be subjected to, and don't forget the availability of good fuel if you do long mileage rides --- don't know about you, but I've been faced with "one pumpers" and only questionable 87 available. So, if you don't mind carrying octane booster around with you and are the type of rider that understands exactly what it is you have and don't try to ride it like a stocker or pull stumps in 5th gear at 1500 RPM, yes, it's workable. Don't forget to factor in the climate where you ride and if you will see "parade duty" or heavy traffic congestion --- hot weather, high compression/high power, no air movement can make an air cooled motor kinda cranky. How the motor is tuned can have a big bearing on this as well. If you tune to the ragged edge for max power the motor may not be happy under all possible conditions, sometimes ya gotta back things down a bit for driveabilities sake. Only you really know how you are going to use it and what your real objectives are. My $0.02, FWIW
 

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Hey! No fair editing on the fly while others are trying to answer --- dynamic help line?

"The Cam is Made for 10.2-1 CR but is that static or dynamic?"

The recommended Cr is usually the static figure and is based on what the cam maufacturer feels is a safe cylinder pressure for the application. Based on the intake close and the 10.2:1 recommendation it looks as though the cam manufacturer feels that 185 PSI CCP is what they feel comfortable with recommending to the public at large --- usually conservative figures at not always what others would consider to be the maximum.

"if i grind the Combustion chamber out to 79cc that will give me 10.2-1 Static Cr, just not sure which CR im shooting for Static or Dynamic???"

Yes, I calculated 10.27:1 static, 8.97:1 corrected, 184.4 PSI CCP with a 79cc chamber. As stated before, you have to decide what the end goal is. Once you reach 10:1 CR the incremental gains from increased compression start to diminish. Big difference to be seen going from 8:1 to 10:1, much less gain from 10.2:1 to 11:1 or even 12:1. Increased CR is mostly used to compensate for a late close with big cams and not lose all the bottom end power while gaining power at the top.
 

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IMHO I think an adj ign module will help when comp is high.
Being able to tailor the adv curve will keep you out of the deto prob.
I use one and it works great. I have very high ccp in my bike.
 

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Discussion Starter · #6 ·
Thanks Guys, The Ignition is a mechanical advance- hall effect electric,
Dyna 5.0 ohms Single fire , twin plug per head, Bike is a very light custom FXR, I weigh 85 kgs, No pillion passenger, so called 98 Octane availibe but dont know if i beleive BP on that one, redlineing it quite often,

Whats you Opinion on Mech Adv ignition vs Hi -4 etc? Full advance at 34 ,
 

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I use the S/E adj ign. 4 curves & 4 rev limits.
Worked out real well when on the dyno.
Just flip a switch for a change in curves.
 
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