outboard compression ratios

[keefallan]

Gentlemen. Since I have been messing with outboards..........which isnt that long ago.........I have never heard of compression ratios of these motors ever been brought up. We always hear about street engines/ race engines/ turbo engines etc. and their compression ratios. Bit I have never heard of any outboard compression ratios been mentioned. It doesnt really matter to me. This is more of a post about me being curious? What are the compression ratios of all the factory outboards? When I mess around with my flatheads(attn: Chaz), I never even give it a thought. So, I have finally given it a thought.............

If anyone knows......post away.

[KIRCHNER]

I think with a 2 stroke you have a ststic c\r and dynamic c/r. guessing 8 0r 9 to 1 not sure if it calculates same as a automotive engine, swept volume divide by chamber vol..

[canuckr]

For some reason in certain parts of the 2 stroke world people are all caught up on exhaust port heights and whether you measure total cylinder volume or the volume from the top of the exhaust port, up. And it can create some pretty interesting debates.....
I too would like to hear what ratios guys are using, based on total cylinder volume. It makes it easier to compare to things I already know, based on other 2 stroke engines and the automotive world.

[rock]

3.5 bore and 1.6 port height and 34cc chamber is app 8.4/1. Exhaust port height matters (EPHM)
cuz there's no compression till port closes.
Rock

[patchesII]

3.5 bore and 1.6 port height and 34cc chamber is app 8.4/1. Exhaust port height matters (EPHM)
cuz there's no compression till port closes.
Rock

Yep. It’s not really a cylinder till it closes

[FMP]

That's a mechanical ratio from ex roof up but as far as dynamic pressure increase its occurring. Exhaust blowdown pairing timing (acoustic compression wave from one cyl's opening ex port to the next closing ex port) is making sure of that. At optimum is arrival is around 30-35° prior to exhaust port closing and continues to within the blowdown ° interval (not the event but the range° on the upstroke), which is when the transfer is closed. So during the remaining interval when the exhaust is the remaining open port the cylinder pressure is increasing. No other way you'd have a 6-8:1 CR make the power per c.i.
The effect is more advantageous during high rpm when arrival range shifts from unfavorable detrimental at low rpm, too early prior to exhaust closing at 45-55°+ , to 30° at high rpm, as a result of increasing crank piston speed.
But with the higher temp comes earlier arrival, this at very high rpm can maintain the arrival within the optimum range and reduce the over pressure situation within the longest travel pairing which can result in arrival at less than 20° prior close, as an example. And again all a function of port duration , height.
Back to ceramic coatings
Grab the popcorn

[FORBESAUTO]

Trapped compression ratio is what you have to be concerned with when building engines. That’s what you can measure and control. You will have to have a knowledge of what ratio works well with the particular engine design. Good idea to check what it is stock to get a good idea. A lot of Outboard’s are actually around 5 to 5.5. Here is a great link to an online calculator
http://torqsoft.net/piston-position.html

[FMP]

Outboards are two-strokes the rest are hondas

[keefallan]

[QUOTE=FMP;3234051]Outboards are two-strokes the rest are hondas[/QUOTE How true that is