I saw on the yamaha site that the 00 banshee has 6.5:1 compression. I have seen that most mod banshees have at least 8-10:1, and in "macdizzys" terms I believe, he says hp increases from say, 40 stock, to
6.5/10= 1.53* 40=61.2 hp, so with this increase in compression, does it give a reliable source of hp?
...if so, why doesnt yamaha up the compression?

btw, according to atm, 16 being atmospheric pressure,
6.5*16(i THINK)
that would make 104psi, and as with the above, it makes more hp with more psi.

so you tell me, does 6.5:1 sound too low for compression?

 

No. Most manufacturers use the Corrected Compression Ratio to define thier set-ups. The corrected compression ratio is the ratio of the volume of the cylinder from when the exh. port is closed to TDC. Whereas the UCCR(UNcorrected CR), which for most engine builders, is a more accurate reflection of what's going on in a high performance engine, is the entire swept volume from BDC to TDC.

What drives them to use the CCR method. Hmmm....Beats me....there could be alot of reasons from their engineering staff functions to marketing....

Compression readings in general should be used to gauge a motor's overall health. Readings taken from the initial start-up, thru break-in as the pressure will plateau as the piston and rings wear in and seal properly, to deterioration of compression thru the "life-span" of a piston assy. help give a idea of what is going on in a motor. Compression can be used as a "ball-park" guesstimation of what a motor is doing or has the potential to do, but factors of the head's set-up really dictate what it does. Squish clearance, mean squish velocity, flat plate volume (CC's), pocket shapes(and a few others) all add up to the equation what a certain head set-up will create. More compression can be used to help offset lack of low range to lower mid range power in motors that have had massive port modifications to make the power up higher. It's all part of the big "equation" when modifying or even designing motors to get you where you want to be...

Compression ratios are just that, the ratio of the volume at BDC versus TDC. So it's not linear in that you can multiply it and get the cranking pressure. Higher compression builds heat. Basically all a motor is a heat pump. If you compress the mixture into a tighter area at ignition the mixture will "explode" harder creating more power. Compression also is a tad harder on engine components like rod bearings and main bearings, as the forces acting upon them are now greater.

 

Good topic, higher compression is a very good mod for HP gains. Compression ratios will be lower on 2 stroke engines. They have twice the power strokes and rev much higher for there power bands.
Compression builds heat which makes a more complete combustion. Also atmospheric pressure is 14.7 psi @ sea level...

 

Also, with higher compression, low end and midrange normally have greater power figures, but at higher rpms climb, the increased compression can actually lower peak power if an engine is not set-up properly. Example: Check around on the 6 horsepower Banshee pistons advertised in various magazines. You will find out that top end power is actually lost. Check with the guys with Alba that sell Alba Banshees and other yamaha atv's, they seem to be pretty knowledgible on how far to increase compression without hurting top end power.

 

thats right, I should have remembered it was 14.7psi. Very informative posts, thanks guys!