Liberator21

Is the guy doing the test correctly?
All plugs out, throttle plates open?

cuda

Someone correct me if I'm wrong, but no matter what cam you are running, at some point both valves must be closed, and the compression records the highest point.

Holy Smokes

That's EXACTLY what I've always thought. This idea of "cranking compression" makes NO sense to me. But hey, neither does alot of things!

blue thunder

If the intake valve closes sooner less compression is bled off. You can actually build compression somewhat by timing cam so the intake valve closes sooner.

BT

tpenfield

Regardless of what they dealer and mechanic say about the comprssion numbers, the REJECT METER is reading pretty high on this one.

The numbers just don't make sense and the explaination is BS.

mike11

To answer the cam lobe center angle questions, there is a point of valve overlap in every cam which is when one valve is almost closed and the other is begining to open this is valve over lap when both are open at the same time. The theroy is to have the over lap at the same time as bdc to allow what is called "scavenging" which is when the exhaust gasses still exiting the manifold create a vacum effect in the cylinder aiding in the flow of fuel in. This can be adjusted slightly by degreeing the cam. Ask him if he had done that.

rmbuilder

There are two mechanical parameters that will alter cranking pressure. They are
1) Static compression ratio or the swept volume of the cylinder.
2) Effective or dynamic compression ratio, which is determined by the Intake Valve Closing angle. (IVC) Altering the LSA or overlap does not affect cranking pressure unless the changes made move the IVC angle.
Because the intake valve does not close until well into the compression cycle, you cannot build significant cylinder pressure until that point of closure. That means the total swept area is not compressed, only the volume above the piston when the intake is completely closed. An early IVC will increase cranking pressure and a later event will decrease it. Be careful not to confuse cranking pressure with running cylinder pressure. Advancing or retarding the ICL can also accomplish this after the cam is ground. Matching the IVC to your static compression ratio is critical in determining the torque curve you arrive at.
In this case the IVC point would not alter your cranking numbers to the degree shown in your results. I hope this helps.
Bob

Lmarth

Bob- I was hoping you'd chime in on this one. I was trying to PM you this morning but was having problems with the site. Thought of you earlier this week as I was burning some 93 octane! Lou

Gary Anderson

Stupid question, but did he have the throttles opened?
I hope you didn't pay for that compression test because it tells you nothing except maybe the engine is worn.

rmbuilder

Throttle open/closed +/- 10#. Warm/cold +/- 10#. This could swing the delta approx 20# in either direction. Not the correct way to perform the check, but it wont alter the results by 100#. This cam (396271/281-14) specs at:
270*/280* Adv
221*/232* @ .050"
.542"/.565" valve
114* LSA
This cam set @ 114* ICL will have a IVC value of
69* ABDC
112* ICL
67* ABDC
110* ICL
65* ABDC
Bob