compression vs. compression ratio and fuel

[JACK STRAW]

how much compression can you have and run on 93 octane and what is the max compression ratio you can run on 93 octane. what builds more power compression or the compression ratio
thanks

[wilkin250r]

what builds more power compression or the compression ratio

The two are related, but there are too many variables to nail it down to a specific answer. I can't lay it out as simple as "11:1 compression ratio will give you 180psi of compression", because it ISN'T that simple. It will be related to the height of your specific ports, the size of your dome, the size of your bore and stroke, and a variety of other factors.

Why are you asking? Do you envision some way to increase the compression ratio without increasing the compression reading?

[JACK STRAW]

The two are related, but there are too many variables to nail it down to a specific answer. I can't lay it out as simple as "11:1 compression ratio will give you 180psi of compression", because it ISN'T that simple. It will be related to the height of your specific ports, the size of your dome, the size of your bore and stroke, and a variety of other factors.

Why are you asking? Do you envision some way to increase the compression ratio without increasing the compression reading?

I am having trouble wording the question. I know what compression is and I know what compression ratio is when you have all the factors. I guess I should word it as this. what is the highest compression I can run on 93 octane with 10% ethanol.

[wilkin250r]

I don't really have an answer for you, but I can help clear some of the confusion on the terminology, and how it all works.

I'll be honest, most people don't refer to compression ratio, because there are so many "ifs" and people cannot agree on all the factors. Is it the total displacement of the piston, or just the trapped volume when the exhaust port closes? Too confusing.

The 4-stroke guys refer to "compression ratio" because it's very simple, and they buy pistons with specific ratios that have been calculated based on their size and volume. For the 2-stroke guys, especially the 250r guys, compression is all about the size of the combustion chamber cut into the head. And many are using designs with interchangable domes, so they just refer to dome size. For example, one of my engines has a 20cc dome in it, which is a little high on the compression, but I'm at high altitude and I can get away with high-octane pump gas.

[wilkin250r]

The engine size plays a significant factor. You can often get aways with a 21cc dome with 93 octane on a stock bore/stroke, but a 21cc dome on a 330 motor would be incredibly high compression.

Compression reading itself is unreliable, because it depends so highly on the condition of the rings and the seal against the cylinder walls. A stock bore can read anywhere from 145 to 180psi depending on the condition of the rings, so a compression reading isn't a reliable measure of compression ratio and need for octane.

Add in the factor that exhaust port height can significantly affect compression reading, but it has very little to do with the actual volume of fuel/air trapped in the engine when it's running. This is because the returning pressure wave from the pipe actually pressurizes the cylinder above normal atmospheric pressure, but it's almost impossible to calculate the exact total volume, and it's not something you can ever measure by kicking it over with a compression gauge attached. So the actual conditions of the engine (the most important part) are more dependent upon the PIPE than the compression reading you get in your driveway.



So in the end, I think you might be able to get away with a 21cc dome, possible a 20cc, but that might be pushing it.

[JACK STRAW]

I tried a squish gap with 3/32 solder and couldn't make a dent. I do have a 3 piece head gasket on it and was going to try a .010 cometic head gasket to see if I can get a squish. if I can is that considered a "poor mans" cool head or is it worth investing in a cool head.

[wilkin250r]

Give us an idea of what you're trying to accomplish.

Squish is important, if I were you I would be looking at a simple head gasket change to get your squish where it needs to be rather than the expense of a cool head, and certainly before the expense of an ignition system.

If you plan on building your motor in stages, then get the cool head now. Your next step would be good, proper porting and matching pipe, and I would do them before an ignition system (unless you're having ignition troubles right now).

[JACK STRAW]

It is a mildly ported on a stock bore and he just bought a Paul Turner Eliminator pipe. He had a FMF Fatty. He also bought a 38 PWK air striker that will be installed in about a week or so. He is running stock 400ex rear tires with stock gearing.V Force reeds and a uni. We were told a '89 CR250 intake boot would be an improvement because it is shorter and moving the carb closer to the head would help performance , so the man we bought the carb from gave us one.I do have to figure out spacing if I move it closer. I do want to try the head gasket 1st ($15 )before he buys a cool head unless he gets one cheap somewhere . So is using the cr250 intake worth the trouble or just heat up the stock boots and install the carb? His goal is to beat some of the local 450Rs around while having a 15 yr old kids budget. thanks