I am going to check the compression on my 98 Banshee and was wondering what the best method for doing so was?

In the past, I took both spark plugs out, put the compression gauge on one cylinder, put the bike in 3rd gear and then pushed for about 30 feet. I then obtained the gauge reading.

Is this an accurate way to measure the engine's compression? Is there a better, more accurate and/or easier way to do it?

About riding 12 hours ago, the engine was ported, polished, bored once over and Wiseco pistons installed. What type of reading is good/marginal/poor?

Any help/experience would be appreciated!

 

That would be an interesting way of doing it. I really don't see anything wrong with it. If you use a gauge that has a needle valve(valve core) in it then it will keep the highest pressure and that is what you are trying to determine. Static pressure is going to be independent of all else. I always make sure that I hold the throttle wide open so that it can freely intake air. I think a good gauge is far more important than slight variations on getting the static compression. I would only reccomend using a Snap-On gauge, and with teflon tape on the threads to ensure a tight seal.

As for what a good reading on a ported Banshee, can vary from anywhere around 155 to 185, sometimes higher, but 185 is "generally" considered "plenty" of compression in a twin. How it's set-up will depend on who did it and what they did, but "generally" look for numbers over 150. "Generally" understand what I'm saying..."generally" speaking.

 

Kick it over with the throttle wide open. Kick it until the needle on the gauge stops.

 

I checked the compression as I described above (both plugs out, pushing quad in 3rd gear) and came up with 163 and 165 compression.

Is that OK for a recently rebuilt engine?

 

Sounds great!!!

 

I don't understand why you would remove both plugs at once? When you check compression on a car engine, you don't remove all 4, 6, or 8 plugs. I would believe by removing both you are altering a true read out. Because one piston will affect vaccum and/or compression on the other. Especially if you want to check proper seal (piston rings) I would definitely do one at a time.

Putting the bike in gear and pushing it, is a great idea!

Generally speaking 160psi is going to be around 12:1 compression ratio, maybe even a tad more. 120psi is around 8.5:1 Someone correct me if I'm wrong.

 

You pull the plugs because it is easier to turn the engine over. Each piston is independent of one another, therefore what one piston is doing won't affect what the other piston is doing.

Compression pressures are a better way to gauge the overall health of a motor. As parts wear, tolerances increase and you start to lose cranking pressure via blow-by past the rings and piston. Determining compression ratios via the cranking pressure by far not the best way. Compressions ratios(the uncorrected compression ratio in a two stroke) are determined from the volume of the cylinder at BDC, versus the trapped volume at TDC, not cranking pressures.

It can be used a general ballpark to determine possible octane requirements, but part wear and a myriad of other factors makes it too vague to be able to say compression reading A equals compression ratio A definatively.

Example my Honda, the correct compression ratio is just a tad over 8:1, whereas the uncorrected compression ratio is 15.014:1 right now, cranking pressure is 258 lbs.

 

Backcountry:

You are way over my head.....

What would you say is good/moderate/poor compression for a Banshee as I described in my first post in this thread?

 

I was trying to explain more about compression ratios to the Kawi-kid.

A quick breakdown of compression:

Stock Banshee's: 120

Good compression on a ported motor would be right where you are at: 165

Moderate compression: 145

Poor: 120 or lower.

One reason to increase the cranking pressure in a ported motor is to help offset the low to mid RPM lack of throttle response and power due to the enlarged ports. The now larger ports have a lower velocity at sub-powerband RPM's(usually under 5500 RPM or so). So you try to create a way to tune the engine to create decent(rideable) power at sub-powerband RPM's and the easiest and most common way is to increase the cranking pressure.

I would say "generally" from the cranking pressure that you gave--your motor is "generally" doing well. Keep track of your compression readings and time spent riding and you can begin to develope a plan of when you need to rebuild your top-end(rings or complete) by the drop in your cranking pressure readings.

 

Now you guys know that there IS a reason to pay attention in Math.

Those who achieve exceptional performance did so because they took the time to learn how.