Father Son 75 CB200T Rise From the Ruins
- Thread starter Texasstar
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Texasstar
Can't is a four letter dirty word
One year ago today we joined the online cafe racer community called Do The Ton. Tim had the vision to start DTT and we thank him and all of DTT for their wisdom, experience, and making the memories possible. "God Pat do you want me to send you a bill" said Kop after we thanked him. I said, "No" because I knew I could never repay him or y'all what it is really worth...
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Texasstar
Can't is a four letter dirty word
Holidays were great! Zeke and I have been busy converting his playground into a place to house our welding gear. Lucky got some new Sava's for the Salt rated at 105 mph and Sonreir just sent us our side decals. Cafe Matty sent us some goodies from Cone Engineering…simo said:
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Texasstar
Can't is a four letter dirty word
Reading Smokey Yunicks Power Secrets. He says by pushing the rings to the top of the piston it allows the pin higher in the piston reducing the overall length of the piston and allowing for a longer connecting rod and that a longer connecting rod is paramount increasing the rod ratio. What I can't wrap my head around is that it doesn't increase the overall length of the piston and connecting rod. He says they Dyno proves that a rod ratio higher than 1.75 to 1 ALWAYS produces more torque and horsepower! "Use the longest damn connecting rod you can fit in the engine." Why? How is the dwell time increased?
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It's all in the trigonometry. Think of the rod and the crank arm as two sides of a triangle. The third side is the distance between the piston and the crank centerline. As the crank is rotating, the piston moves up and down, but it doesn't move in exact relation to the crank because of the changing angles between the crank and rod and rod and piston. As the piston nears top dead center, the speed at which it is moving decreases. The rate of that decrease has to do with rod length.
With a long rod, that rate is slower, so the piston spends more time in and around top dead center than it would with a short rod with the same stroke. What makes this really interesting is that the opposite holds true when it comes time to measure piston speeds on the lower half. So longer rod slows the piston at TDC, but speeds it up at BDC.
This change in piston speed has an important effect on the breathing of your engine. Short rods tend to favor long durations with lower lift while long rods like shorter durations with higher lift (in general). The timing on the cam becomes important as well. If the piston is moving faster in the bottom hemicircle of rotation, then you want the intake events happening later and the exhaust events happening sooner.
I don't have a ton of time at the moment, but I'll get a drawing and some math going a bit later.
With a long rod, that rate is slower, so the piston spends more time in and around top dead center than it would with a short rod with the same stroke. What makes this really interesting is that the opposite holds true when it comes time to measure piston speeds on the lower half. So longer rod slows the piston at TDC, but speeds it up at BDC.
This change in piston speed has an important effect on the breathing of your engine. Short rods tend to favor long durations with lower lift while long rods like shorter durations with higher lift (in general). The timing on the cam becomes important as well. If the piston is moving faster in the bottom hemicircle of rotation, then you want the intake events happening later and the exhaust events happening sooner.
I don't have a ton of time at the moment, but I'll get a drawing and some math going a bit later.
The_Stache
Been Around the Block
Wow. Love this build!
Smokey was a smart dude but his work was a long time ago. Long rods or short rods is a debate that comes up every few years and there are pros and cons to both. F1 cars were flirting with 2.0 ratios a while ago - besides it's all relative. What is the rod: stroke ratio on a CB200? I have a couple of cranks in the shop but it's still not into double digits yet, so I'll wait to go out there to measure them.
What do Vizard and Bell say about long rods?
Take a spreadsheet and enter all the variables and duplicate the columns for a different rod length and then plot the outcome for piston position and you'll see the difference is not huge. It's there, but not as much as you might imagine. The math is pretty simple and any spreadsheet software can crank it out over a range of say 0 - 360 degrees in 1 degree increments.
CB200 came with longer rods than the 175 so it already has a higher ratio than the higher revving 1670/175. Lots of things to think about. But remember that our 175 motor with stock crank and crappy Sloper head makes 21hp If we want more we have to tame the cam drive system and reduce friction. If you use other pistons that lighten the top end and reduce friction there's horses to be harnessed. Just sayin'........
If we could get as much power out of one of these tykes as I think they are capable of, the transmission wouldn't last thirty seconds. We already break gears !!!
What do Vizard and Bell say about long rods?
Take a spreadsheet and enter all the variables and duplicate the columns for a different rod length and then plot the outcome for piston position and you'll see the difference is not huge. It's there, but not as much as you might imagine. The math is pretty simple and any spreadsheet software can crank it out over a range of say 0 - 360 degrees in 1 degree increments.
CB200 came with longer rods than the 175 so it already has a higher ratio than the higher revving 1670/175. Lots of things to think about. But remember that our 175 motor with stock crank and crappy Sloper head makes 21hp If we want more we have to tame the cam drive system and reduce friction. If you use other pistons that lighten the top end and reduce friction there's horses to be harnessed. Just sayin'........
If we could get as much power out of one of these tykes as I think they are capable of, the transmission wouldn't last thirty seconds. We already break gears !!!
Texasstar said:
Draw a con rod and piston. Then draw a longer rod beside it - not crazy longer - say an extra 5 or 10mm and now draw the piston in the same position as the first one and see where the pin end up. It moves up inside the piston by 5-10mm to keep the same height. On a clunky old auto engine that's possible but if our pins moved up more than about 1-2mm they would be in the ring pack and even with thinner rings moved as high as practical, there's not a lot of room to move.
What Smokey may have forgotten to mention is that raising the rings closer to the crown increases power. We do that by machining down our pistons and barrels and we get a decent squish plus higher CR plus higher ring pack. But don't tell anyone. It's our secret right?
Texasstar
Can't is a four letter dirty word
secret is safe and a whole lot easier than changing the bottom end same mental note as before... need more squish and a bathtubteazer said:Draw a con rod and piston. Then draw a longer rod beside it - not crazy longer - say an extra 5 or 10mm and now draw the piston in the same position as the first one and see where the pin end up. It moves up inside the piston by 5-10mm to keep the same height. On a clunky old auto engine that's possible but if our pins moved up more than about 1-2mm they would be in the ring pack and even with thinner rings moved as high as practical, there's not a lot of room to move.
What Smokey may have forgotten to mention is that raising the rings closer to the crown increases power. We do that by machining down our pistons and barrels and we get a decent squish plus higher CR plus higher ring pack. But don't tell anyone. It's our secret right?
just read what Mr. Bell said. If we can eliminate the high dome piston it solves several inefficiencies...and that dang cb200 piston is significantly heavier than the 175! Why did they make it so heavy? Lower compression, lower performance, cheaper material for the piston but heavier?
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