Bore and Stroke, What is it? Options

[Wishful3S-GTE]

Hey guys, this may be kinda dumb to some of you, but could someone please try and explain what bore and stroke signify. I hate reading things and then somethin like bore and stroke comes up and ruins the whole article cause I don't get it. Someone clarify please? Help would be appreciated.

[GeEkBoY]

bore = piston diameter
stroke = distance piston travels
Top Dead Center = highest point in the piston travel
Bottom Dead Center = lowest point in the piston travlel

[macavely]

[5sfeTurbo]

On the subejct of F.A.Q's, what do (valves do) 2.2L "16 valve"?

[macavely]

you have intake valves that let air in to the cylinder to mix with the gas vapors...
and the exhaust valves let the burn up air and gas mixture out of the engine...

[5sfeTurbo]

[Wishful3S-GTE]

Ok thanks for the help. Now could someone explain how the compression ratio is masured. I know the lower the ratio the more you can sqeeze into a cylinder creating more power right? So uhhh ... what does for example 8.5:1 or 9.0:1 mean? I don't get that too well either.

[macavely]

Compression Ratio: the simplest way to explane that is if an engine has 9.0:1 ration the volume that fits into 9 cubic inches will be compressed into 1 cubic inche.. that makes for a more powerfull burst of eneragy...

that the simple explanation

[GeEkBoY]

I'm 99% sure on this, but if you divide the combustion chamber volume when the piston is at bottom dead center and divide it by the volume at top dead center that should get you your compression ratio.

Another interesting fact you may ask about is long stroke vs. short stroke. A square is when the bore equals the stroke, longer stroke is as it sounds. A longer stroke has better low end torque, but lower reving while a short stroke has lower torque but can rev higher. In most engines toyota is a long stroke while most civice are short or square (I think).

Hope this helps.

[Wishful3S-GTE]

Yea both of those explanations helped. But uhhh ... since you said that whatever the comp. ratio is that's how much is compressed, then would 10:1 create more power than 8.5:1? I always thought that turbo engines have lower compression therefore allowing for more power, but now I'm all mixed up on this. So which one gives you more power, higher or lower numbers?

[macavely]

[GeEkBoY]

10:1 would create more power than an 8.5 in NA (naturally aspirated) form. Turbos are another thing. If you had 8.5 turbo motor turboed at 10 psi (atmospheric pressure is 15 psi) you would be putting in 8.5 x (10+15)/15 = 14.1
note that this is assuming 100% efficiency which is not the case and this is merely an approximation. It would be lower than 14:1, but much higher than 10:1, but the most important part is that there is more air and fuel in the combustion chamber than in the NA setup.

there are 2 primary ways to increase power, get more air and fuel in the combustion chamber or to increase the compression ratio.

by turning up the boost (increasing psi) you increase power by adding for air and fuel as opposed to increasing the compression ratio. The reason they lower the compression ratios in turbo engines is because engines will blow up under too much pressure. If you lower the compression and turn up the boost, you will get more power (and kill your gass mileage btw) without blowing up your engine. If your have been following the guys trying to turbo the 2zz in the new celica and matrix, many people have blown the engines. To turbo a 11.5:1 comp ratio engine is very tricky and not that smart. If they drop the comp ration, they should be able to turbo without blowing the engine.

[jayi12-15psi]

you really are a geekboy

j/k

[5sfeTurbo]

[GeEkBoY]

thanks, I'm strong on theory, just need to get dirty more often.

[Inferno]

Excellent and thorough explanation!