oddy

Hello Everyone,

I converted a mitsubishi 33cc

After a lot of experimentation, I added a walbro carb from a DLE 50cc and a tuned pipe of a 60 size engine and presurize the tank and the engine runs great, never had to move the needles again, great idle, great transition and getting 7,500 RPM with a 18x6 wood Turnigy prop.

I live in Mexico city so I fly at a very high altitude and I have received the recomendation that raising the compression ratio of theisse engines would be of great help specially on this altitude

Could anyone tell me hoiw to raise the compression ratio.

My objective would be to be able to use a 19x8 prop on this engine maintaining arround 7,500 rpm to get a better performance out of it

Thanks everyone.

speedracerntrixie

My Feedback: (29)

You would replace the cylinder gasket with a thinner one.

av8tor1977

My Feedback: (6)

Here is a thread I started about Homelite 25cc modifications. A lot of it will probably not apply to your Mitsubishi, but the part about lowering the cylinder will.

Click here: http://www.rcuniverse.com/forum/engi...c-hot-rod.html

Read through all of it so that you understand that lowering the cylinder to increase compression will decrease your exhaust and transfer port timings. They will need to be ported, or at least the exhaust will, to gain back the lost timing. I often leave out the cylinder base gasket to increase compression, and just use a good sealer instead of the gasket. You must be sure to check the piston to head clearance though.

Whomever told you a higher compression ratio would help at high altitudes was correct. You can also try advancing the timing a couple of degrees.

AV8TOR

oddy

Thank you very much, I will try leaving it without the gasket.

For advancing the timing If I recall correctly, I need to open up the exhaust on the top of it righ

Thank you very much.

Víctor.

av8tor1977

My Feedback: (6)

Yes, take material from the top of the port, and do it a little at a time and keep measuring to make sure you don't go too far and take too much. A good timing figure is usually around 150 to 155 degrees. Using a tuned pipe, you might experiment with a higher exhaust port timing though. Make sure to chamfer the edge of the port so it does not have a sharp edge to damage the piston and/or ring after modifying the port.

AV8TOR

irocbsa

My Feedback: (12)

Is it necessary to pressurize the fuel tank? I would think that with a pumped carb this would not be necessary.

av8tor1977

My Feedback: (6)

Both the Walbro and Zama carbs have fuel pumps in them, and should not have the fuel tank pressurized.

AV8TOR

bluejets

Technically, by lowering the barrel you will be increasing compression.
Compression ratio remains the same, as it is a measure of volume at bottom of stroke compared with volume at top of stroke.

av8tor1977

My Feedback: (6)

You contradict yourself. Lowering the cylinder (barrel) does increase the compression ratio, as it reduces the volume at TDC. As you state, the compression ratio is a measurement of the ratio of the total volume of the cylinder, including the head area, at bottom dead center, as compared to the volume of the total area above the piston at top dead center. Lowering the cylinder, using a piston with a dome, using a thinner head gasket on an engine with a removable head, using a head with a smaller combustion chamber, etc. are all methods of increasing the compression ratio on various engines.

AV8TOR

bluejets

Compression increase yes but not compression ratio increase.
When the barrel is lowered, volume at bottom dead centre is also less ....so I stand by my original statement....go look it up sometime.

av8tor1977

My Feedback: (6)

I don't have to look it up, as I have been calculating the compression ratio on race engines and such for 40 plus years. It is a simple mathematical problem. Try it for yourself.

Example

Total volume at BDC = 1000cc
Total volume at TDC = 100cc
1000 divided by 100 = 10
Compression Ratio = 10:1

Next:

Total Volume at BDC = 990cc (Combustion area decreased by 10)
Total Volume at TDC = 90cc (See above)
990cc divided by 90cc = 11
Compression Ratio = 11:1

Math don't lie.....

http://en.wikipedia.org/wiki/Compression_ratio

AV8TOR

bluejets

That would have to be some gasket to reduce the volume by 10cc's.
Maths are correct but you can quote any number to justify your calculations.

av8tor1977

My Feedback: (6)

Whatever. Those are the facts, and you can check them with any engine expert you like.

AV8TOR