lets do a compression test

[cals400ex]

by doing a comression test, can you tell a true compression ratio your running or do you just know the cylinder compression in psi for example. i guess what i am trying to say, how can i tell if i am running 10.5:1 or whatever i am running?? a compression test will just tell how many psi or pascals the bike is at. for example, 125 psi or 800 kPa or whatever. this may be a simple question, but this compression confuses me a bit. thanks

www.rosspistons.com go there and do their compression test...it will tell you your true compression

[JOEX]

Originally posted by cals400ex
by doing a comression test, can you tell a true compression ratio your running or do you just know the cylinder compression in psi for example. i guess what i am trying to say, how can i tell if i am running 10.5:1 or whatever i am running?? a compression test will just tell how many psi or pascals the bike is at. for example, 125 psi or 800 kPa or whatever. this may be a simple question, but this compression confuses me a bit. thanks
I really don't think you can find out the compression ratio with a compression tester, I think you already knew this;)

A compression tester can be used for doing a leak down test, don't ask me how to do that!:p Or to compare compression of cylinders on multi-cylinder engines to find problems such as an internal blown head gasket.

Joe

[cals400ex]

yeah joe, i understand a leak down test is not the way to tell what compression your running. i don't know how to do a leak down test either. i don't think i am really interested in that. i just wanted a way to tell if i was running a similar compression as the piston states. i have a feeling its lower, but i just wanted to see how much if any. the ross website is down for a little bit. i guess they are re-doing it.

[JOEX]

I think the only way to determine the compression 'ratio' is to actually measure the difference in volume of the cylinder from BDC to TDC. You will probably need to take into account the volume of the head, they call it cc'ing in automotive work as well as gasket thickness.

I don't think there is a consistant PSI that a specific piston should read but some of the pro engine builders might have a range of what it should be.

Joe

[wilkin250r]

Most people aren't willing to do a real compression ratio test, because it involves pouring liquid into your cylinder, usually a motor oil.

If you want to find your compression ratio, you start by knowing your displacement, which is calculated from your bore and stroke.

When you have your displacement, you put your piston at TDC, and pour a measured amount of liquid (like motor oil) into your cylinder until it is flush with the bottom of your spark plug hole. Let's say you start with 100cc, and when you fill your cylinder you have 64cc left over. That means you have 36cc inside your cylinder, which means your combustion chamber is 36cc.

If you're using the stock bore, you have a displacement of 397cc. Divide the displacement by your combustion chamber volume to give you the compression ratio.

397 / 36 = 11.06

So you have basically a 11:1 compression ratio.

Obviously, a higher ratio will give you higher readings on a compression test, so it's possible to approximate your compression ratio based on a compression test, but it's not exact, since there are many other factors involved in cylinder pressure.

[cals400ex]

Originally posted by wilkin250r
Most people aren't willing to do a real compression ratio test, because it involves pouring liquid into your cylinder, usually a motor oil.

If you want to find your compression ratio, you start by knowing your displacement, which is calculated from your bore and stroke.

When you have your displacement, you put your piston at TDC, and pour a measured amount of liquid (like motor oil) into your cylinder until it is flush with the bottom of your spark plug hole. Let's say you start with 100cc, and when you fill your cylinder you have 64cc left over. That means you have 36cc inside your cylinder, which means your combustion chamber is 36cc.

If you're using the stock bore, you have a displacement of 397cc. Divide the displacement by your combustion chamber volume to give you the compression ratio.




397 / 36 = 11.06

So you have basically a 11:1 compression ratio.

Obviously, a higher ratio will give you higher readings on a compression test, so it's possible to approximate your compression ratio based on a compression test, but it's not exact, since there are many other factors involved in cylinder pressure.



damn dude, how do you have an answer for every technical question on here?? yeah, thats seems like it would take time and it also seems like it would be hard to be precise when you are dealing with cubic centimeters of oil.
by the way, thanks for the help everyone.

[wilkin250r]

Originally posted by cals400ex
damn dude, how do you have an answer for every technical question on here??

My reputation as a nerd is growing. :rolleyes:

[cals400ex]

i think i rather be a smart nerd rather than a popular dumb ***. :D

[wilkin250r]

Funny thing, as much of a nerd as I am, you wouldn't guess that I was Captain of the football team AND wrestling team, would you?

[PHIL_B54]

dumb question of the day....how would you get the oil back out?

[hondafox440]

A straw. Don't you like motor oil?

What I would do is suck what I can out with a syringe, then remove the head and clean the rest up with kerosene and paper towels.

[cals400ex]

Originally posted by wilkin250r
Funny thing, as much of a nerd as I am, you wouldn't guess that I was Captain of the football team AND wrestling team, would you?

there is nothing like a stud with a brain. you don't see that too much. there is no sense calling yourself a nerd then.

[wilkin250r]

Originally posted by PHIL_B54
dumb question of the day....how would you get the oil back out?

If you are truly determined to use this method, you take the motor out of the frame. You need to rock the engine back and forth to make sure there are not air bubbles trapped in the head.

When you are done, I suppose you just dump it out into a container, and probably rinse it out with gasoline. I wouldn't know, I've never actually done it, I just know the method.

[Guy400]

To get the oil out just take a lit propane torch and stick it down the spark plug hole:D

Whenever I've built small block engines I've always figured the compression ratio before I assembled the motor. I had a piece of flat plexiglas with two small holes in it. I left the spark plug in the head (since it's going to be in their when the motor's running) and then used a small beaker to pour measured amounts of water into the combustion chamber until it came up to the plexiglass. Now you'd have your combustion chamber cc. Most head gaskets will tell you what their thickness is when crushed to a certain torque setting. Now you've got the head gasket thickness. You then must find out your swept volume, deck clearance, and the dome or dish volume of the piston. After you got those there's a few quick raps on the old calculator and you've got your static compression ratio :eek:

If you want to do the math yourself this is the method I always used:
swept volume + combustion chamber volume +/- dome/dish volume +/- deck height + head gasket volume DIVIDED by combustion chamber volume +/- deck height volume + head gasket volume +/- dome/dish volume

[ewalker302]

take the motor out of the frame???

just crank it over a few times with the plug out that will blow all the oil out.

maybe put a rag or something a few inches out from the hole to stop the splatter from being too bad.

That or take the tank off & just turn the quad upside down for a min.

:bandit:

[LS@GtThunder]

When I do this I do it with the engine on the bench. I first lock the crank with the piston at top dead center with a wooden wedge at the flywheel. I then seal the piston to the cylinder with a heavy grease and wipe off any that sticks above the piston. I then install the head and head gasket and torque to specs. Using a graduated buret (SP?) I fill the head thru the spark plug hole with mineral spirits. I then dissassemble the top end and clean everything out. From there its all math. This may not be the best way but it is the most consistant way that I have found that will account for gasket diameter and domed pistons without making molds of the piston to check the cc of the dome.

LS