jeff32

we sometimes hear a compression ration of let's say 8:1
sometimes we hear 10:1
what makes the difference and how can you tell what compression ratio when you already have it in your boat?

and when we take a compression test, and we get let's say... 150 lbs of compression on the gauge. How do you know if ths engine would be supposed to have 165, or 180, instead of 150?

how does those 2 compression reading relates to each other ( comp ratio and comp test)

thanks for teaching!

Crude Intentions

iTrader: (1)

Don't think they actually relate. In doing a compression test your really looking for the cylinders to be as even as possible. I believe 10% is an acceptable difference from highest to lowest. So if your highest is 150 you wouldn't want any lower than 135. But consistency is the key.

auto_werks 3.6

Compression test numbers will mean more to compare cylinder to cylinder as ICU mentioned. If you test the engine from new you can track loss of compression over time, but that's obviously impossible to start at this point... unless your engine is new. google static compression ratio and dynamic compression ratio, and you will learn some other info on topic. Keep in mind that due to the change in dynamic compression, you should lose craking PSI if you take a stockish engine and add a cam that has more overlap. This is because both valves are open longer during the time when compression is supposed to occur.

These are also reasons why a leak-down test is really the only way to evaluate a used engine. It will tell you exactly how loss (leak) each cylinder has, and where the leak is going... Compression test will only give you an idea of damage, leak down will tell you if its a valve (and which one) or if its the rings / cylinder.

SB

iTrader: (1)

A compression test is actually a cranking pressure test. That's where the confusion can play. The terms.

Biggest influence on Cranking Pressure test is the engine's compression . A super close 2nd is the camshaft.

I've built 10:1 350's with small cam and had 210 cranking pressure.
I've built 10:1 350's with snotty cams and had 150 cranking pressure.

In fact, shhhhh, speed secret, a long time ago we had a compression rule for one of our circle track cars and the track officials checked cranking pressure to do this. Bullschit ! So, I got smart and put in a slower cranking starter which reduced the cranking pressure. LOL.

Actual physical compression ratio is Swept volume vs volume left when piston at TDC. Basically, how much air a cylinder can hold vs what space it is compressed into.

Here's an article on what compression ratio is and how to compute it:

http://aftermarket.federalmogul.com/...0Calculate.pdf

auto_werks 3.6

When I worked for SCCA we had a "Whistler" which measured the acoustics of the cylinder and could read out compression ratio digitally. it was reasonably accurate, but we most likely wouldn't throw someone out based on that machine, unless they weren't willing to pop their heads off.

Black Baja

iTrader: (2)

I work on a 16:1 motor it has 125lbs cranking pressure. It really has do do with how soon you get the intake valve to close.

Rocco

Compression ratio is figured on the gross combustion area in the cylinder,when the piston is at bottom dead center, divided by the unswept area, which is the area above the top of the piston when its at top dead center