Combustion Ratio Difference between Head Gaskets

DieselJeepLuvr · **Posted:** Tue Apr 11, 2023 12:15 pm

Just in case anyone wants to know how putting in a thicker head gasket affects the combustion efficiency.

Stock Combustion Ratio 17.5:1
Eg: zero hole to one hole 17.2:1
Eg: zero hole to two hole. 16.9:1

The higher the CR, the higher the combustion efficiency. Similar to boost pressure via a turbo. Also heat retention helps with combustion efficiency but having an aluminum head (which dissipates heat quickly) makes this option counter productive. Increased combustion Ratio/ boost puts added strain on traditional IC design and can cause loss of tension of head bolts/studs and head gasket failure. The unfortunate design flaw of the VM Motori intake cover does not allow retorquing of the head bolts/ studs.

layback40 · **Posted:** Tue Apr 11, 2023 7:21 pm

Interested to know how you calculated these numbers. Dis you use swept volume + void volume all divided by void volume?

DieselJeepLuvr · **Posted:** Tue Apr 11, 2023 10:15 pm

Same way you do on any other motor. I rounded to the nearest tenth but you get the general idea

WolverineFW · **Posted:** Wed Apr 12, 2023 2:52 am

But do you understand why there are 3 different head gasket thickness?

I do agree you should be using the correct one.

DieselJeepLuvr wrote:

Just in case anyone wants to know how putting in a thicker head gasket affects the combustion efficiency.

Stock Combustion Ratio 17.5:1
Eg: zero hole to one hole 17.2:1
Eg: zero hole to two hole. 16.9:1

The higher the CR, the higher the combustion efficiency. Similar to boost pressure via a turbo. Also heat retention helps with combustion efficiency but having an aluminum head (which dissipates heat quickly) makes this option counter productive. Increased combustion Ratio/ boost puts added strain on traditional IC design and can cause loss of tension of head bolts/studs and head gasket failure. The unfortunate design flaw of the VM Motori intake cover does not allow retorquing of the head bolts/ studs.

Sent from my SM-S906W using Tapatalk

WWDiesel · **Posted:** Wed Apr 12, 2023 4:04 am

I have read that Head Gasket thickness is determined based on the clearance between the piston and the head. Piston to valve clearance may also play into the equation.

From my files:

Quote:

The following information regarding cylinder head gasket selection on 2005-2009 Jeep 2.8L diesel engines.
Selecting and using the wrong head gasket on this engine could cause engine damage.

Jeep offers three different thickness head gaskets for this four-cylinder VM diesel engine. The proper gasket to use depends on the amount of piston protrusion present. When measuring piston protrusion, make sure that you measure each of the four cylinder locations. The gasket to use should be based on the average of the values recorded. Tools used to measure piston protrusion are available from Jeep or VMdiesel.

CAUTION: DO NOT Rotate the engine with the cylinder head off without first installing the cylinder liner retainers VM1076. Failure to do so may result in the cylinder liner moving and a false piston protrusion reading. A false piston protrusion reading may result in a wrong cylinder head gasket selection.

Obtain and record the maximum value of projection for each of the four pistons. Establish the thickness
of the steel gasket by averaging the four protrusion readings and determine the gasket grade using the
information in the chart below.
Gasket Grade Projection Value Thickness Grade Part Number
A (NO-HOLE) .018-.024” .052” A 5072675AA
(.460-.609 MM) (1.320 MM)
B (ONE-HOLE) .024-.028" .056” B 5072676AA
(.610-709 MM) (1.420 MM)
C (TWO-HOLE) .028-.032” .060” C 5072677AA
(.710-.810 MM) (1.520 MM)

DieselJeepLuvr · **Posted:** Wed Apr 12, 2023 11:05 am

WolverineFW wrote:

But do you understand why there are 3 different head gasket thickness?

Yes.

One thing to consider is, in WW 's reply it refers to the piston protrusion from the liner NOT the deck. And it says to use the average clearance NOT the least which I disagree with. I've observed depressions in the head where the sleeve has hit hard against it. This would give a lower compressive strength to the head gasket and possibly increase the chance of head gasket leaks, warping and/or cracking the head. That's why we always say to use the 2 hole gasket. Better safe than sorry.

WWDiesel · **Posted:** Wed Apr 12, 2023 2:02 pm

DieselJeepLuvr wrote:

WolverineFW wrote:

But do you understand why there are 3 different head gasket thickness?

Yes.
One thing to consider is, in WW 's reply it refers to the piston protrusion from the liner NOT the deck. And it says to use the average clearance NOT the least which I disagree with. I've observed depressions in the head where the sleeve has hit hard against it. This would give a lower compressive strength to the head gasket and possibly increase the chance of head gasket leaks, warping and/or cracking the head. That's why we always say to use the 2 hole gasket. Better safe than sorry.

If you study the FSM, it covers piston liner protrusion from the top of the deck and choosing the correct liner shims accordingly.
But I think what they are most concerned with is how close the top of the piston (piston protrusion above top of liner) will be to the bottom of the head?

From the FSM wrote:

NOTE: When installing liners for protrusion measurement, remove all O-rings and discard original shim.
2) Measure the liner recess relative to block deck with dial indicator.
NOTE: The cylinder liner reading will actually be a negative number.
5) Then select the correct shim thickness to give proper protrusion (0.00 - 0.05 mm, liner protrusion above top of engine deck)

Without knowing the ramp curves and lifts for the cams, we don't know exactly how close the tolerance (clearance/gap) is between when the intake & exhaust valves at full open when the piston is close to TDC. We know this is an "interference engine" and valve timing is crucial and can and will allow valve contact to the top of the piston if the cam timing gets too far out of the required specifications. So head gasket thickness might or could actually play into this clearance when the piston is approaching or leaving TDC?
Certainly something to consider...

Comments or thoughts more than welcome, as this is always a good discussion subject and helps to understand some of the nuance's of this very unique engine!

DieselJeepLuvr · **Posted:** Thu Apr 13, 2023 11:30 am

Just by eyeballing the cam profile it would appear the cams would have to be 12-15 degrees off to start getting close. I could put a dial caliper on it and mic it but I have more important things to do with my time right now.

Combustion Ratio Difference between Head Gaskets

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