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Old 03-14-2015, 01:01 PM
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mapbike mapbike is offline
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Join Date: Mar 2010
Location: Central Texas
Posts: 5,379
Default Re: what did you do to your motorized bicycle today?

Originally Posted by DRBS View Post
I also gave noticed the difference in the cylinders theres even a difference in the cranks ive noticed too. I bought a "high performance" crank and the diameter of the bearing shafts are different only buy a very small fraction buy there is a difference that must be why some shake more than others plus the difference in the ports would explain a lot LOL. ive been building these for going on 4 years and ive seen a lot of really messed up ones and a few that were fast right out of the box I should have kept one of the faster ones for myself LOL
The two main differen es Im seeing between the several type jugs I have is that first of all the jugs we're getting can be designed for three different engine geometries, even at that they're still very similar as far as height between base and deck with the one for the GT5 geometry being the shortest.

main big different is in height of transfers and intake port measuring from the base, the cylinder bore extends between 10mm & 11mm below the base on all the jugs Ive measured which doesn't make any difference, but I measure it and then subtract that amount from the distance between cylinder and bottom of transfer or intake and I have found as much as 5mm difference between jugs and that is huge in determining how the engine is gonna act as far as low down grunt or higher rpm capability.

Im thinking based on what Im seeing here that although port size is important, that isnt gonna matter much if the port timing is way off for what you're trying to achieve with the engine, A smaller port engine with correct port position for where you want the engine to run its best at is probably going to perform much better than an engine with huge ports that has then set way low or to high for the desire characteristics of the engine.

Im no expert on any of this but after close to 6 years of messing with these engines Im starting to dig into them a little deeper to see what parts I need to be looking for to get what I want out of them.
One thing Im definitely seeing is that the 40mm stroke geometry is by far the best, the pjugs just dont set well with these 38mm stroke engines, yeah they'll run ok, but for the average builder they will not run nearly as good as the 40mm stroke engine when its set up right, the PK80 engines have for less port timing issues just as Fred has stated in his videos and Im finding it to be completely accurate.

Ifeally hope Duane @ thatsdax will go back to the 40mm stroke GenIV engines, the cranks are better and for higher performance builds they are the winners above the GT5 geometry for me.
I dont always ride WOT, but when I do..I do my best to keep the rubber side down..Ride safely my friends!
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