[Vettezuki]

Originally Posted by 94cobra69ss396

There isn't any boost.

I meant when the engine is running. Or is it like this

Engine off, turning manually, the pressure in the cylinder is "cranking pressure."

Engine running, the pressure in the cylinder is "cylinder pressure."

enkei's motor is boosted, 15psi I think.

Que?

[94cobra69ss396]

You get cranking pressure by pulling out a spark and installing the gauge. Then you crank the engine using the starter motor until you reach the max psi which is usually after few revolutions. What you are checking is to see that all the cylinders are in the same range.

Like I said before, cranking pressure doesn't tell you much because cam timing can change cranking pressure. I read an article once written by an engine builder who was using the cranking pressure to demonstrate how to choose a cam. He said that for a street engine he doesn't like to see more than 210-215psi cranking pressure. The customers engine he was writting the article on had 250-260psi with the cam that was in the engine. He had a custom cam ground and it dropped the cranking pressure down where he wanted it at around 210. I can't remember what difference in HP it made but it did make more power. I'll see if I can find it online somewhere.

[enkeivette]

That's the gasket Cobra, and it was a 4.2 bore, my old gasket was a 4.16. That must be the 1/10th difference in compression. I'll bet if you guys do it again it'll be right at 9:1.

Ben, cranking pressure, you got it. By the time the motor is idling the pressue will increase. So I guess you could make the argument that a better starter would increase the pressure. It's really not a good test for anything other than comparing cylinders to look for leaks. But the numbers themselves tell very little.

[Vettezuki]

Originally Posted by enkeivette

That's the gasket Cobra, and it was a 4.2 bore, my old gasket was a 4.16. That must be the 1/10th difference in compression. I'll bet if you guys do it again it'll be right at 9:1.

Ben, cranking pressure, you got it. By the time the motor is idling the pressue will increase. So I guess you could make the argument that a better starter would increase the pressure. It's really not a good test for anything other than comparing cylinders to look for leaks. But the numbers themselves tell very little.

I see, it's just a comparison test, nothing so absolute. Wouldn't a compression leakdown test give the same diagnostic information (i.e., identifying trouble cylinders.)

[94cobra69ss396]

Originally Posted by enkeivette

That's the gasket Cobra, and it was a 4.2 bore, my old gasket was a 4.16. That must be the 1/10th difference in compression. I'll bet if you guys do it again it'll be right at 9:1.

Ben, cranking pressure, you got it. By the time the motor is idling the pressue will increase. So I guess you could make the argument that a better starter would increase the pressure. It's really not a good test for anything other than comparing cylinders to look for leaks. But the numbers themselves tell very little.

The best way to do a compression test is with all the spark plugs removed and crank the engine until max psi is reached. That way there isn't a load on the engine caused by the compression stroke of the other cylinders.

Ben, a leak down test is better but is also more time consuming. It is quicker to do a compression test to see if there is a low cylinder. If there is then you can do a leak down test on that cylinder.

[Leedom]

Can someone explain to m what a leak down test is?

[94cobra69ss396]

Originally Posted by Leedom

Can someone explain to m what a leak down test is?

You pump a measured amount of air pressure into the cylinder, and the gauge will tell you the percentage of leakage the cylinder has.

Read this article done by Car Craft. http://www.carcraft.com/techarticles...ter/index.html

[BRUTAL64]

Originally Posted by 94cobra69ss396

You pump a measured amount of air pressure into the cylinder, and the gauge will tell you the percentage of leakage the cylinder has.

Read this article done by Car Craft. http://www.carcraft.com/techarticles...ter/index.html

OR--the ole driveway leak down. Pump compressed air in a cylinder, make sure that both the intake and exhaust valves are closed and listen for leaks.

I've done dozens of these--quick way to find broke rings or bent valves.

[enkeivette]

On another forum it was recommended that I run a 26 first and 24 second gap.

Doing the math on the chart that came with my total seal pistons, I get 22 first and only 14 second. So what's up?

[94cobra69ss396]

Originally Posted by enkeivette

On another forum it was recommended that I run a 26 first and 24 second gap.

Doing the math on the chart that came with my total seal pistons, I get 22 first and only 14 second. So what's up?

Call them and tell them what you have and get their recommendation. Boosted engines need a larger gap than a N/A engine does.