crankcase compression ratio

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Deleted member 12676

Using the painstakingly accurate method of determining the crankcase compression ratio I outline at I discovered that my reed valved 55cc has a ratio of 1.2 and that a meager ratio of 1.11 is existent with my 48cc piston port engine. This ratio is the ratio of compression of intake charge before its released into the cylinder thru the transfer ports. A higher ratio indicates there is more pressure at the time of transfer opening. 1.2 is for low rpm engines and up to 1.5 for high rpm engines. The reasoning is that at low rpm you don't want the intake charge blasting (via high pressure) into the cylinder too fast and looping around and exiting the exhaust port. But at high rpm you do want a fast transfer since there's so little time for the movement of charge to take place. A ratio of 1.1 is around 1/2 the pressure that 1.2 produces and is too low.
So how can we increase the ratio? By reducing the space that contains the intake charge being compressed. This space includes the space between the two halves of the flywheel, the space under the piston, and the space in the transfer channels. To reduce the space between the flywheel halves we'd have to take apart the flywheel and bolt on aluminum spacers. Too much work for me baby! JB Weld can be used in the other two areas but increasing the piston weight would probably imbalance the crank and cause more vibration unless you drill out the center of the piston pin to counteract. JB Weld can be safely put along the walls of the transfer channels since the channel cross sectional volume is much more than the area of the transfer ports when opened.
What effect can this have? It can increase the delivery ratio of intake charge into the cylinder which increases power throughout the whole rpm range.
Since the total volume that can be added to the transfer channels is small and I don't want to add flywheel spacers, then I'm going to also add JBWeld under the piston and let you know how it goes. (I am well aware that temperature is pretty high under the piston crown which may disconnect the JBWeld there, leaving the side adhesion in effect.) This isn't new by the way. I have read of others doing this. Here's a graph showing the relationship of compression ratio to psi pressure:


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Fiddling around, I found that putting the JBWeld in the piston would occupy only 15ml, and probably the same for the transfers. 80ml total reduction is needed to get the ratio to 1.2 which means the flywheel would have to have spacers (at least 2mm wide each) installed. rats!
The problem exists with the 48cc piston port engines because the movement of the piston from closing the intake port to opening the transfers displaces only 23ml (23 square cm). Installing a reed valve greatly improves the ratio because then the measurement of piston displacement begins from top dead center.
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My reed valved 55cc engine has a 1.2 ratio but only because the reed valve is located inside th intake tract and almost touches the piston.
I have decided to pull the crank and use JBWeld to fix two aluminum plates in place.
Graham Bell had wrote "all things being equal, a higher crankcase compression ratio increases transfer flow and peak horsepower".
Calculating I get these figures:
A plate that is 1mm thick and 10.5cm in diameter occupies 8.65cm2 (square centimeters, millilitres). Looks like 3mm on each inside of the flywheels is a safe amount to add which would take up 52cm2. That would leave 148cm2. (148+23)/148=1.15 ratio
Using JBWeld in the transfers might be enough to get the ratio up to the desired 1.2
I re-analyzed the graph with more precision and I came up with these psi amounts for each said CCR:
1.1 = 1.5 psi
1.2 = 3.0
1.3 = 4.5
1.4 = 6.0
1.5 = 7.5
So my upping the CCR by reducing the compressed volume from 200ml to 114ml ups the CCR on my 48cc from 1.1 to 1.2, and on my 55cc reed valved engine from 1.2 to 1.4 which is basically doubling the pressure released into the cylinder when the descending piston uncovers the transfer ports.
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I have completed "stuffing" the crank to reduce the volume by around 86ml. All the JBWeld is drying now and I will test the engine Thursday or Friday. A good part of the volume was stuffed by filling the space to the left and right of the flywheel, at the cases, the 57mm diameter center space. I reduced the crank volume by 20ml on the left side and 40ml on the right side. Inner plates as well as stuffing the transfers and flywheel balancing holes caused the final 86ml reduction. Of course I need to pour in gasoline and measure it to confirm how much difference I made.
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Pouring in alcohol I found out that the final updated CCR is 1.3 for my 55cc reed valved engine. I probably miscalculated how much volume the JBWeld was displacing. So instead of doubling the transfer pressure I only increased it by 50%. Well, that turns out to be a good sized increase to test anyways. So how did it all turn out? First let me interrupt the final announcement to say that these engines have a serious weakness in their crank seals. Increasing the crank pressure did not give me the boost it should have but instead it is acting like the seals are leaking. That is exactly what happened when I first started using a reed valve. So now I have to split the cases again and replace the seals. If the final power boost isn't significant then I'll probably try to remove most of the JBWeld I put on to return the CCR to how it was. There's no need to make my ride unreliable.
If anyone knows where to buy crank bearings that have a seal on them then please let me know. The ones I have on now do not which puts all the stress on the seals located to the outside of the bearings.
I just went to my local bearing shop, he had proper NSK sealed 6202's, picked up heaps of em, $3 each.
Thanks for the help with the bearings.
I replaced the seals w/o having to split the cases by just prying them out and pressing in new ones. The right seal was worn out but the left seal still had some life left.
After increasing the jet size (making it richer) there was around a 5% speed boost going up hill at 28mph instead of 26 as before. But lower speeds and top speed is less. I need to experiment with the transfers shape/height and the boost to match the new compression ratio.
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