hey guys i just decked my head and a total of .020 has been taken off my head i had orderd a .020 over head gasket and fire ring kit from pure diesel do i actually need to order a stock head gasket and fire ring kit i just read somewhere that the only time i need to change head gasket size is if i deck the block is this true and if it is will the loss in compression by useing the .020 over head gasket make a big difference?

 

One thing to watch out for is valve protrusion. Though the head is flat unless the seats were ground interference *might* become an issue.

There are benefits and drawbacks to lower compression. At low load the burn may be slightly less efficient, and it might make the engine slightly harder to start in very cold weather, but on the other hand, you can push more boost more safely due to lower cylinder pressures.

However, by my math the .020" gasket would drop the compression about 7% to in the realms of 15.24:1 versus 16.3:1 which IMO is pretty excessive unless you are building a monster twin turbo drag truck. Probably pretty hard to start.

Hopefully someone else will check my (off the top of my head) math, but I'd say bad idea.

pi(4.02/2)^2x4.72/16.3=3.675 Comb ch
pi(4.02/2)^2x.020 =.254 ci additional chamber volume for 6.9% more.

16.3/1.069=15.24:1

Even if there's a goof in there, I'd say still probably a bad idea. Stock head gasket and measure valve protrusion to check for interference with pistons and slap it together.

Oh and one last note. With the stock head gasket the compression will probably still be very slightly higher due to the valve protrusion taking from the compressed volume this protrusion will be somewhat offset by the wear on the valves and seats, though.

 

That's what i was wondering and I know the machien shop I took it two re ground my valves so that shouldn't be an issue. But I called pure diesel and they told me know matter what I take material off eather the head or block I need to make it up in gasket size so I'm all confused

 

Most heads have a domed combustion chamber which becomes smaller raising the compression ratio if you deck the head. But since your head is flat, there is no dome to cut into. The only increase in compression is from the valves sitting lower encroaching .020" minus whatever was taken off the valves and seats.

Maybe there is something I am missing here. One thing I am fairly certain of is that you want to dial in the compression ratio to your application, which for the *vast* majority of us is stock ratio, and very small changes in the combustion chamber make fairly large changes in compression (unlike gas engines) due to the starting high compression ratio.

Were I in doubt, I'd use a graduated beaker to pour oil onto a piston at TDC to measure the volume, add that to the loss of volume from the two valves (protrusion x pi r squared for each valve) and add that to the *compressed* thickness of the head gaskets available to figure out proper compression.

What you were told was erroneous. Any meat taken off the block is made up for with the same thickness on the gasket, but with the head, combustion chamber shape must be taken into account.

Just think about it, the head is flat and you machine it. Does the flatness drop encroach on the combustion volume? No. But the valve seats have moved down .020" so the valves do stick down .020" more. But the seats/valves were ground, so if they (combined) were ground .020" you are EXACTLY where you were before if you use a stock gasket. But if they were ground less (which is likely) they will protrude slightly more raising compression ratio.

I've done plenty of IDI overhauls, even as far as manufacturing new guides and seats, but I must admit I've not yet done any internal work on a DI engine. My intuition is to *always* verify valve protrusion though since the consequences are pretty drastic, and will only contact after the head gasket has been compressed and the engine turned over. You do NOT want the piston to be able to kiss the valves-better to verify before assembly.

 

I am amazed that no one has a direct yes or no answer especially here and will a 7 percent increase in cylinder volume really hurt my fuel milage? Also on top of my turbo I want to add a litte water meth injection so in that respect is the added volume a good thing. I never realized how many variables could go in to this project

 

The lower compression (assuming my math was right) is significant. Direct injected engines already are pretty low compression (versus idi's which usually are in the range of 22:1). But the main issue is generally not efficiency, little will be lost. The main issue is the ability to create enough heat on the compression stroke to ignite the fuel. In short, such low compression primarily would be a problem getting the truck to start. This kind of low compression is chosen in high performance applications to allow crazy boost without cylinder pressures getting destructively high. Such machines are generally tough to start, and smoke a lot until the turbo spools, even when idling.

I really hope someone else does weigh in on this, but two things to consider are that most folks don't ever need to get inside their engine, let alone take .020" off the head.

Diesel overhaul is a little more complicated than gas simply because the tolerances of the combustion chamber are so tight. Most gas engines don't even HAVE different thicknesses of head gaskets.

I personally wouldn't consider 15.25:1 for a street machine. I'd rather lean towards slightly higher compression from the valve encroachment. I'd sooner go a point higher to 17:1 (though that's probably pushing it for a non-steel top piston) than lower to 15:1.

 

12v's are already at 17:1 compression. The SO 24v is the oddball, at 16.3:1.

 

Naturally aspirated idi engines are in that 22:1 range. Like the 6.2l gm.



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Ok so now in theory could I just bump the timing up a little more wich will raise cylinder pressures and increase efficiency

 

No. Set the engine up to have the proper compression ratio for your application. Take the measurements, do the math, and figure out what you've got. Choose the appropriate head gasket.

Cylinder pressures will be completely unaffected by timing until the engine is actually running, so the *main* issue (starting) with low compression will not be adressed.

BTW Tate's correction shows my math wrong due to using 16.3. But it still will drop compression a similar amount percentage wise.