As for the M8 engines having larger pipes I wonder if part of that has to go with the 4 valve heads? They need to breathe different from our Twin Cams? I don't know enough about this to be able to offer any experiance or knowledge.
Thanks for all the feedback and real world experiance.
Stock M8 heads flow better than stock twin cam heads. In fact a stock set of M8 heads flow very close to as good as my twin cam heads that were ported by Don at HD street performance.
I am still trying to learn about what is need for exhaust on a built M8 motor. I know some where there is a youtube video comparing M8 cams and two exhaust systems, one being the Fullsac again. It did very well. 2.25 baffle and his MX head pipe.
Steve has done a build with a M8, with his MX head pipe and 2.0 baffle and it was 114 HP and 150 torque.
So not sure yet that the M8 requires big exhaust either.
https://www.cvoharley.com/smf/index.php?topic=118283.0What I have learned is there is no magic cam, or exhaust or intake or so on. With a twin cam, its critical to select parts designed to work well with each other, cam and compression have to be matched, heads and intake have to be able to support the air flow needed, exhaust has to have enough back pressure.
I know enough to be dangerous, but am smart enough to ask professionals advice, and recommendations. Like many will point out the the HD-259E cam is no good in the HD 117 kit for the 110 motors. Its actually a very good cam, but the kit compression is way to low. I built a 110, not bored over with that HD-259E cam and 10.8 compression, it was 121.8 HP and 121 torque. Again Fullsac DX head pipe and 2.25 baffles.