GregBBRD

Let me change my previous wording:
"Ring wear" is never an issue in a 928 engine....unless someone bead blasts an intake, doesn't clean it good enough, and the silicon media gets into the cylinders. (If this occurs, you basically need to remove the engine, throw it away, and find another one.)

The only exception is on the GTS engines. The oil control rings will get so filled with "gunk" that they no longer can wipe the cylinder walls clean. (However, the rings will not be worn, just "gunked" up.)

Speedtoys

@GregBBRD

Dumb question coming...

If air pressure is...14.7psi, a 10:1 compression engine should deliver 147-14.7 psi optimally, right?

This is off tangent here..but...??

Cortina

No , they are not (directly) related. Next question ................ (It's to do with valves , and timing of same , and overlap , and ........................)

FredR

Atmospheric pressure at sea level is 14.7 psia and if were compressed at 10:1 the resulting pressure would be 147 psia.

Speedtoys

Then..if this is holds...the compession #s are pretty good..

karl ruiter

If this is correct, keep in mind that your compression gauge reads gauge pressure rather than absolute, so you would have to take the 14.7 back off the 147, and
should expect to see more like 132 psi on your compression gauge.

ramcram

Having been watching this for a while I have to add my 0.928 worth.
I'm with speedtoys, do a leak-down test. It will tell you much more than a cylinder pressure test will.
A good leak-down kit is not expensive, nor is a good automotive stethoscope.
With pressure applied you can listen through the dipstick tube to hear ring/piston bypass, listen in the inlet for leaky inlet valves, listen in the exhaust for leaky exhaust valves.
You can also look in places like the header tank for signs of head gasket leakage.

FredR

For sure it holds unless Boyle did not know what he was talking about.

The compression ratio of the pistons for sure plays a major part in predicting the cranking pressure attainable but it is not the entire story as the relationship of the cams and the timing of them also plays a part in the actual numbers generated. Changing the LSA will change the cranking pressure as will advancing or retarding the cam timing. Advance the cam timing and more cranking pressure will be attained thus explaining why such increases low down torque at the expense of top end power. Increasing the LSA reduces overlap and thus favours increased cranking pressure but does the opposite for top end delivery.

It all boils down to compression efficiency thus why VVT emerged as and when the technology became available albeit a bit too late for the 928 engine design- even so it appeared in the 968 series motors.

In this case the OP's numbers are in the 140 psig [155 psia] range- for a "performance motor" that is not too clever - my S4 motor typically exhibits 185 psig give or take a couple of psi on my test rig. I do not know what numbers the OP's build variant should achieve- the appear somewhat low but not catastrophic.

Before taking the numbers at face value I would want to see what numbers the same rig generated on a known good motor as without calibration one should not take such [absolute] numbers at face value and make major decisions based on such. Thus the numbers on their own suggest something may not be right but they do not indicate what such cause may be. The leakdown test compliments the compression test in that it typically indicates where the problem likely lies as it quantifies how much gas is escaping relative to what can be expected and also would confirm if the numbers generated make sense.

GregBBRD

That seems like it should make total sense and I'm at a loss to instantly respond....need to think about that.
However, I've got 3-4 compression gauges and they all seem to read in the 180-200 psi range for an S4/GT/GTS engine.
I'd expect that a Euro 2 valve engine would be about the same...perhaps a bit lower because of the camshaft profile in a Euro.

fiatrn

"they all seem to read in the 180-200 psi range"

Because when the engine is actualy turning, the flow dynamics of the intake and exhaust systems and the valve opening times pull More air into the cylinder than atmospheric pressure alone. The air is moving through the intake tract with some momentum, and in league with the outgoing air creating a slight negative pressure, more air comes into the cylinder and gives it a tad more pressure. Aka volumentric efficiency.

Speedtoys

Someone outside our gated wall here gave me a thought on this.

Yes, compress 10:1, get 10x.

But the RATE of the compression makes a difference.


At speed, the heating is significant, and also adds pressure to the end result.

I figure...at 400rpm with plugs out, 3-4mph of piston speed during compression is not enough time to compress the air AIR cool the charge entirely into the cylinder volume materials, to relieve the pressure that this heating contributes to the test.

Figure..that maybe....150++ degrees F are added to the gas charge in this scenario.with much of the total heat from compression lost in the metal in the cylinder...and that WILL get you to 180psi or so Im guessing.

This -is- how a diesel engine works

I can see this now...and this is why plugs out, throttle open, and battery on a solid charger during the test is important.

Much like the adiabatic compression of air on a downslope windy day makes it HOT in the valley in Norcal and Socal during fire season.

GregBBRD

I've spent 50+ years improving VE on Porsche engines and optimizing the inertia of the incoming column of air.
However, "inertia supercharging" a cylinder occurs at fairly high rpms.
You think that this is a significant factor at cranking speeds?

GregBBRD

Wouldn't this be more of a factor when the engine is super hot versus stone cold?
I just don't seem to recall that significant of a delta between hot and cold compression testing.

Speedtoys

No..the pressure is the same at all baseline temperatures. 10x 14.7 is 147 (assuming 14.7 is the pressure and zero leaks and any added compression related heat dissipates) in a cold motor, or a hot one, and in any climate.

The added temperature/energy from the QUICK compression of the air is whats bringing up 147, to the test pressure. Not enough time for the added energy in the cylinder from quickly compressing the air, to dissipate into the surrounding metal, and the time is so short that I dont think the temp of the block/cylinders/pistons is all that much an issue.

If the cylinder had zero leaks, and you let it rest at TDC, it would come down to ~147. The compression energy would dissipate, and the pressure would decrease.

Speedtoys

Not sure what he thinks...but I dont. That velocity pulse is helping the atmosphere fall/organize into the cylinder as efficiently as possible and push out old gas during overlap...to make up for the extremely limited time and space available around the geometry of the intake valve.

Tricks of the trade to increase the Ve of an engine.


Ya ever blow down a christmas wrap tube, and wonder why the air keeps moving after you blow?