Does any one how to estimate compression ratio if you have the cylinder pressure! I got a 350 chevy, not sure of the compression ratio, was trying to figure out approximately what it is without tearing down. Any help appreciated thanks
If you have flattop pistons and don't have dubblehump heads or 283 heads probably pretty close to around 9:1 I've had 64cc 283 heads on a stock 350 before, that's pushing it on pump gas. cylinder pressure isn't real reliable it varies so much you'd prolly have a wider discrepancy in your calculation than you'd have in the actual heads other than the really small chamber heads...that's my way of saying I don't know with flattops and stock heads it doesn't really matter to me, I've never felt any difference and don't measured hp and et and all that I just build em till they feel good and drive em
Compression ratio cannnot be calculated with the engine assembled. The cylinder head must be removed. The ratio is calculated by the following formula: <DL><DD>, where</DD><DD> = cylinder bore (diameter)</DD><DD> = piston stroke length</DD><DD> = clearance volume. It is the volume of the combustion chamber (including head gasket). This is the minimum volume of the space at the end of the compression stroke, i.e. when the piston reaches top dead center (TDC). Because of the complex shape of this space, it is usually measured directly rather than calculated.</DD></DL>
Timing events dictate cylinder pressure,When the intake valve closes the volume taken from that point is running compression.You can have a 11:1 static and a 9:1 running with the right timing events. 9:1 running is the limit i use for a pump gas motor,as high as 240 psi cranking i have recorded. In reality static is just a number,the whole package is what should be focused upon. Static compression can and should be done with the engine assembled,in mock up stage. pop up pistons,deck hight or vave releifs all play a part so to take out any error a one cylinder pour at tdc will give a acurite Vc.
As already stated, it's impossible to calculate CR with the engine assembled, or by using cyl. pressure as measured off a compression tester guage. The numbers off the guage will however tell you the overall health of the engine If the engine is stock, run the casting numbers off the block and find the factory CR. If it's been built, but used garden variety parts, your prolly looking at a hair less than 9:1. If it's been built with performance components, such as corvette aluminum heads, pop up pistons, etc, CR could be almost anything.
A little more accurate formula; Compression Ratio = Cylinder vol. + clearance vol. + piston Comp. vol. + gasket vol. + chamber vol. divided by Clearance vol. + piston vol. + gasket vol. + chamber vol. Piston comp volume will generally be a negative number, unless it's a dished piston, in ehcih case it is positive (it adds to the volume, lowering compression)
Is the engine in the car or out? Impossible to check with assembled? I disagree. If your looking for a %100 accurate answer your prolly right but for what he may be looking for i`d say get the engine on its side ane with piston at tdc and the rockers loose, fill thru the spark plug a measured amount of fluid. preferably something heavy enuff to not leak past the rings. Then rotate to bdc and add more fluid to fill the cyl. Add the the 2 amounts together and divide by the first amount. %100 accurate? no. but you would be surprised. I have checked my race motors both ways to verify the results.
Yes, but thats what the cr is. The cubic inches of the cyl with the piston at bdc versus the piston at tdc.
Sorry, read your post too quickly, didn't catch the TDC and BDC measurements - my bad Here's another way http://blogs.circletrack.com/6377531/tech-articles/engine-compression-check-made-easy/index.html but it isn't cheap
Directly rather than calculated,As in assembled state rather than individual components then figured. Dome volume or dish volume can be configured with the 1/2 inch down pour method but is a added step that can be avoided by using a mocked up assembled engine. more accurite i may add also. good info here.
Warm engine. make sure battery is charged, Block throttle open. DO a cmpresssion test. Divide that figure by (1.31 X 14.7) and you will have the actual compression ratio. The 1.31 accounts for the principles of Charles Law about heat and pressure and the 14.7 is the average air pressure at sea level. It is reasonably acccurate for this type of work. Static compression ratio is a bit diffferent as it does not allow for the factor of cam timing and therefore is almost useless other than for reference work. I use the following criteria. A good street engine with a stiff cam wil give about 165 PSI. A bracket racing engine that is expected to live a long life will give around 175 to 185, A race engine going for power will have as much as 225 PSI but will require race fuel and higher rpm. Hard to generalize. The cam has such an influence on final compression figures. In fact the relationship of cam timing to compression ratio is the whole ball game when it comes to making cam choice and huge power. It is almost THE speed secret. Reference Design and Tuning of Competition Engines by PH Smith. which is the standard for auto race engine info and planning and design. Warning ! Not a lot of photos.
exactly! static compression assumes 100% volumetric efficiency and is an unattainable number... no real use on a running engine.
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