Thread: 6.5 GM Diesel

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I've got a GM 6.5 diesel engine which I am rebuilding and I am considering installing a thicker head gasket to loose some compression in order to give it more boost. I would go with cometic gaskets but I'm wondering if diesel engines have the same concern with quench as does gas?

What I'm asking is, I wonder if I could go with .200 thickness head gaskets and what would my new compression be. Stock gaskets are .050 and I have 21.3 to 1 compression and I would like 18 to 1.

The bore size is 4.055.

I used to have a compression calculator but I couldn't find one on line. Thanks for the help.

Originally Posted by TyphoonZR

I've got a GM 6.5 diesel engine which I am rebuilding and I am considering installing a thicker head gasket to loose some compression in order to give it more boost. I would go with cometic gaskets but I'm wondering if diesel engines have the same concern with quench as does gas?

What I'm asking is, I wonder if I could go with .200 thickness head gaskets and what would my new compression be. Stock gaskets are .050 and I have 21.3 to 1 compression and I would like 18 to 1.

The bore size is 4.055.

I used to have a compression calculator but I couldn't find one on line. Thanks for the help.

Another area to think about is your valve-train.
At .200 thick it may change a lot.
Cometic head gaskets are very good just make sure you fully prep your block and heads to run them.

Shoot, I hadn't even thought about push rod length, so Erik, what would my compression ratio be, do you have that ability to calculate that?

Originally Posted by TyphoonZR

Shoot, I hadn't even thought about push rod length, so Erik, what would my compression ratio be, do you have that ability to calculate that?

Yes,I do but If you surface the block and heads which you will have a different set of numbers.
Where are the injectors located at?

I am purchasing new bare heads rather than resurfacing, pressure testing etc my old ones. The price difference is only $80.00 each. I am not decking the block other than cleaning it up. My current compression ratio is 21:3 stock, so what gasket thickness do I need to bring that down to close to 18:1? Thanks in advance.

Originally Posted by TyphoonZR

I am purchasing new bare heads rather than resurfacing, pressure testing etc my old ones. The price difference is only $80.00 each. I am not decking the block other than cleaning it up. My current compression ratio is 21:3 stock, so what gasket thickness do I need to bring that down to close to 18:1? Thanks in advance.

I can't find all my formula's.
You will need the inside dia. of the head gasket.
3.14 X radius which is 1/2 the dia. of the dia.of the head gasket squared=area X gasket thicknness.
To convert cubic inches to cubic centimeters.
X 16.387.
One other are to think about is the injector spray pattern and how will the cone spray pattern change and also pressure loss because of the great distance it will travel.

The inside diameter of the holes in the gasket are 4.115. Hopefully that helps.

Is this a true rebuild? If so buy the lower comp pistons. Gasket thickness is quite hefty (3/16{.200}) for flame thru depending on how much boost and fuel increase this thing is going to go with.

Originally Posted by nitrowarrior

Is this a true rebuild? If so buy the lower comp pistons. Gasket thickness is quite hefty (3/16{.200}) for flame thru depending on how much boost and fuel increase this thing is going to go with.

Yes, I am rebuilding my engine, I have purchased .040 over pistons which are destroked by .010. The destroked pistons compensated for the larger bore and the fact that one will deck the block by at least a few thou.. and so by lowering the pistons in the block, at least we won't be raising the compression ratio.

From my calculations, if I thicken the head gaskets to .085 from the stock .050, my compression drops from 21.3:1 to about 17.9:1
I will be using Cometic head gaskets, so the thickness is not a factor cause they can with stand 80 lbs of boost and I am only shooting for 15-20.

Sorry, when I first posted .200 head gasket thickness, I was just throwing out a number, I had no idea that all I would need would be in the area of an extra .035.

Past 15psi on a 6.5 is flirtin with disaster, I have mine set to 12 to be on the safe side, kinda found of the old girl.

Originally Posted by viking

Past 15psi on a 6.5 is flirtin with disaster, I have mine set to 12 to be on the safe side, kinda found of the old girl.

I hear that, but isn't that when you have a compression ratio of 22:1? I thought that as you lower the compression you can feed it more boost.

By lowering the compression (swept area) there is more area for the boost charge to fill (volume) so more power

More boost pressure will also adds power but past 15psi the 6.5 lower end gets over stressed and ...................

The others , D-Max, Cumm, P Stroke can use lots more boost than we can, upwards of 50psi in some.

But I still like it, 80% of a D-Max at 20% of the price.

But even at 15 psi of boost and 18:1 compression, it will have plenty more power compared to the stock 7 psi with 22:1 compression, yes? Add to that, it should be less stressful to the block.

From my calculations, if I add .030 thickness to the head gasket, plus destroking the pistons by .010, decking the block .003, I should end up with 18:1 compression. Cometic makes a .080 thick head gasket. I ordered a set yesterday.