Mopar Gen III Hemi COMP CAMS Thumper Cam Specs Incorrect?

turbofreek,

I am aware the observation is ABDC. If intake valve closing point ABDC had nothing to do with building compression, it would not try to 'stall' the piston with opposing pressure by nature of an earlier closing event closer to bottom dead center as you have stated. (BTW I totally agree with you on that) Unless of course, there is no relation in 'compression' to low speed cylinder pressure? I feel like you have to decide that the intake valve closing event does affect compression, or, it does not. Unfortunately, we can't have both.

Overlap scavenging happens primarily when the piston is hovering around TDC, and draws IN the intake charge using the temperature differential, and exhaust waves from the open exhaust valve in overlap. If we let the intake open early during the end of the exhaust stroke (the only other time those two open valve events can coincide) We do get a similar effect from overlap as the exhaust pulse pulls 'cool' intake charge 'across' the chamber at that point as well. Too early, we just blow exhaust charge back up into the intake manifold and suck it back in again as soon as the exhaust valve closes.

That sounds like something the EPA would recommend... Both usually happen.

But we are talking about the intake closing here. Not opening.

I'm not sure I follow the 'exhaust debris' explanation in relation to intake closing? I may have just misunderstand you.

Once again, I am not talking about 'static' compression here. I am talking about dynamic compression which is more or less, a loose assessment of average/peak cylinder pressures.

The cylinder isn't going to start building compression (cylinder pressure) until the intake valve closes. Period.

Ever needed a valve job? No squeeze = No dynamic compression. It's not like someone went into the engine and tripled your head gasket thickness, or shortened your rods. That's mechanical compression ratio. That never changed. We just have a valve that won't close.

Correctly, the intake closing point would be as the piston is coming up from BDC. More low speed cylinder pressure (closer to to ABDC) intake closing makes torque, and detonation (preignition) in the right (non-ideal) conditions. A later, (farther from ABDC) intake closing makes horsepower via greater piston speed as a result of a shorter 'mean' cylinder pressure duration. Greater piston speed will increase velocity of the incoming intake charge. Given a constant N/A intake tract, charge velocity is relative to piston speed.

I say this as you CAN have the same cylinder pressure with a later IVC assuming you have the velocity to cram the charge inducing the ever-exponentially shortening window between BDC, 90 degree crank angle, and/or intake close. Boost works. Other than that, you are relying on revs (exponential gained piston speed as a result of inertia from the prior cycle) to build 'power' Better hope you have an exceptional valve-train to capitalize on that pressure that is built by the 8000rpm intake velocities needed, especially if you have a heavy chassis.

So, yes. Even N/A you probably CAN theoretically make the same average cylinder pressure with two different intake closing points, at two different intake durations. But, your pressure peak would move. The pressure peak, combined with other factors, (usually low speed cylinder pressure) is typically what causes detonation. This is why we 'degree' a cam. To fine tune that relationship.

I believe this is also why V-Tec was invented. So you can have a low, and a high- piston speed cam shaft. V-Tec does't 'kick-in' it 'kicks-out' the low speed cylinder pressure event to help carry gained piston speed.(As you stated) Putting a cam with a large IVC number in a single cam V8 (81 degrees for example..) Is like putting two 'secondary' top-end cam's in your V-Tec Honda. No Bueno. That's for Bonneville. Not Normal-ville.

It's finding the balance of low speed cylinder pressure and mean/peak piston speeds that give you the broadest usable 'power band' during component operation.

We old school N/A guys don't just adjust cylinder pressure events with a knob or a ro-box and a computerator.

We use camshafts.