[ducatithunder]

Originally Posted by cmcfaul@aol.com

Hmmm, any way to determine CR (compression ratio) on and installed engine? I thought it would be as simple as measuring the amount of pressure (compression) in a cylinder.

Time for some schooling.

Chris

There is a tool out there that measures CR with an installed engine. Its called the Whistler.

http://store.katechengines.com/whist...ster-p174.aspx

It pumps are into the spark plug hole and compares the tdc vs the btc volumes to get the "real" CR. No calcs needed. It is pricey but Im sure that some one, ie dealer or shop in the area should have one.

[ducatithunder]

http://www.instant-g.com/Projects/T39/

[cmcfaul@aol.com]

OK so who's got one other than Kurt who likely has two

I still don't get why you can’t just measure pressure. It's a 2.4 l motor. Wouldn’t the pressure of an E be greater than a T, etc (just like it's CR). I would think there would be a chart of the pressure inside a 2.4 at TDC on the compression stroke and how that translates to CR.

More pressure = higher CR....right?

At the end of the day, really looking for how much pressure I have and how that corresponds to the likely CR. The only important thing is too much pressure leads to detonation (at a certain temp) Also, at what pressure & temp is detonation likely to occur with various plug heat ranges.

If a T has a pressure of X and an E or S has a pressure of X The CR of a T, E or S is known. Is there a graph that shows what the effect of pressure has on CR.

Anyone follow my logic.

Thanks,

Chris

[smdubovsky]

Valve timing affect pressure. Valves neither open nor close at dead centers so the pressure you measure does not correlate to CR. T E and S all have different cams so the pressure comparison mean nothing. FWIW, The S will likely measure the lowest compression pressure as it has the highest overlap & duration cams. That does NOT mean it has the lowest chamber pressure once its running or even the lowest CR.

The time to measure CR is when you are building the motor.

[Vicegrip]

Originally Posted by smdubovsky

Valve timing affect pressure. Valves neither open nor close at dead centers so the pressure you measure does not correlate to CR. T E and S all have different cams so the pressure comparison mean nothing. FWIW, The S will likely measure the lowest compression pressure as it has the highest overlap & duration cams. That does NOT mean it has the lowest chamber pressure once its running or even the lowest CR.

The time to measure CR is when you are building the motor.

Ding!

If that ship has left the port the Whistler is your next step. it is a very clever device that uses harmonics to measure a volume. Handy for checking motors at C.R. too.

[Dandelion]

One more reason that a simple pressure gauge won't work: when compressed, air heats up, so unless you can ensure that the air in the cylinder is the same temperature at minimum and maximum pressure, then the ratio in pressures won't correspond to the ratio in volumes.

ed

[HoodPin]

Not to mention that any leakage during compression testing (valve seat, rings or headgasket) can affect the compression measurement, which can distort mathematical attempts to determine a compression ratio.

I wasn't familiar with the WHISTLER; I can see it being a useful tech tool at CR's.

[cmcfaul@aol.com]

OK I get it. Not as simple as I like to make things (over lap, VE, etc). I am surprised the Whistler is not more mainstream with tuners.

Chris

[N0tt0N]

Sounds like the way to go at this point is to enter into a Club Race and then challenge your own motor. They'll check it for you!

[cmcfaul@aol.com]

I have no doubt my car makes good power. My concern, having burnt two pistons in the past, is that it's not making too much power.

It should be right on. Temps are lower than ever, I did do all the math (filled the heads with fluid and measured to add to formula) to confirm CR when doing the rebuild. I run it a bit richer than before. Use colder plugs and always use the highest octane. Also made sure one cylinder does not run leaner than the others. Installed an AFR (air fuel ratio) meter just for triple checking everything.

Just curious at this point

Chris