Valve Train Noodling

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Feb 22, 2011 | 10:54 AM

#11

BlackFatBob64

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Thanks djl for taking a good look at this for me. I'm in SW Florida so our riding conditions are pretty similar. I think I'll go back to the CR calculator and play around to see if I can pull the corrected CR down to your recommended range of 9.3 before I spend money on the dyno. Maybe I can do something with the head gaskets by adding 0.005 or see how many cc's I need to add back to the combustion chamber. Just a random though, I wonder if I could machine a pocket into the face of the pistons to help bring the CR down enough? I really want to try to keep the 76cc heads as they increase the proportion of combustion area in favor of the piston face.

Thanks again

Feb 22, 2011 | 11:31 AM

#12

BlackFatBob64

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Thanks djl for looking at this in detail. I'm over in Sw FL so we probably have the same hot summer riding conditions. I think it might be a better bet to bring down my corrected CR down to your target before I go spend money at the dyno. I'm not sure how good the two tuners within 50 miles of me actualy are on v-twins.

According to the calculator on Big Boyz, I need to add back 8cc's to the combustion chamber volume to get down to a 9.35 corrected. I'm thinking if I turn a 4.828 cm x 1.65 mm pocket in the center face of each piston, I should get there. This would leave a 10mm squish shelf around the entire perimeter of the piston.

Any thoughts on this approach?

Thanks again

Feb 22, 2011 | 11:42 AM

#13

djl

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From: san antonio

Quote:

Originally Posted by BlackFatBob64

Some are comfortable with a corrected CR between 9.3 and 9.6; I am not that adventurous with friend's builds where I assist. Cometic is now making a .036" MLS head gasket which, if deck height is near 0.00" should work OK. That gasket and +4cc of chamber volume would bring your corrected CR down to 9.4, which is manageable. Even with a .030" head gasket, corrected compression will be about 9.54.

A good head guy can open up the valve reliefs on the pistons and generally rework the chamber to pick up the additional volume. It's a liittle more work but the valves can be dropped a tad in the seats as well.

You do need to use a calculator and do the math before doing anything.

Feb 22, 2011 | 11:56 AM

#14

BlackFatBob64

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Thanks DJL, I guess there is some slight difference in the calculators, but that doesn't matter. I'm fortunate to have a lathe and mill in my shop so basic machining including valve reliefs only cost me my time.
I did set up my deck heights at dead nuts nothing on the front and -0.002 on the rear. So maybe just going to the 0.036 cometic would get me close?

Feb 22, 2011 | 12:07 PM

#15

BlackFatBob64

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Nope, 0.036 head gasket alone doesn't do it. I think I'll settle on the 0.030 head gasket and add 5cc's to the chamber volume to get a 9.64 corrected per Bad Boyz.

Thanks one more time DJL! My bike is always a science project anyway!