Head Gaskets & Compression Ratio

[Lextreme II]

Cometic make a .051" stock height head gaskets. It has three layers. I wonder what if some member just use one or two layers of the MLS. I know this will change the compression ratio. Would it affect anything else.

.051" divided by 3 will be .017" each layer. What if we only use two layers and the thickness will be .034" How much compression would it change by the increment of .017"?

The stock US Spec 1uzfe is 10.4:1 with .051" What compression ratio it will be with .034" and .017" head gaskets.

Would this method be beneficial for NA applications? High compression will make more HP. Secondly would the decrease in head gasket thickness affect the valves/pistons interface?

 

[Peewee]

Changing the headgasket thickness is probably the worst way to change the compression ratio.

I realise its a helluva lot cheaper to use thicker/thinner gaskets than to change a set of pistons, but the combustion chamber has been precisely designed with stock head gasket thickness in mind.

Changing the gasket thickness changes the squish? area of the chamber, and thus reduces its efficiency.

 

[1youZee]

I don't know if it would seal properly... from the cometec website..​

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"...The outer layers of the gasket are embossed and coated on both sides with a flouroelastomer rubber based material (Viton) designed to meet the demands of a variety of harsh sealing environments, load conditions and surface finishes.... ...The center or shim layer of the gasket is comprised of an uncoated Stainless Steel layer, which can be varied to accommodate multiple thickness requirements."​

​

like gasket, metal, gasket sealing on the top and the bottom. It seems to me a thinner center "shim" would be the way to go, not removing it all together.​

 

[rarson]

Changing the gasket thickness changes the squish? area of the chamber, and thus reduces its efficiency.

Peewee, I am not doubting your knowledge but I would disagree. How would we be changing the squish area? If anything, we'd be getting more squish due to the tighter space between the head and pistons. But I'm not sure, so that is why I ask.

I would think milling the head WOULD change the squish area, probably make the quench pads bigger so we'd get more squish in a smaller space. Is this a bad thing? I'm not sure. I will say that I think the best way to increase CR would be with pistons. Milling the head or changing the clearance of the head with respect to the deck height will affect cam timing. This isn't too much of a problem with using different head gaskets, but if you mill the heads multiple times, for instance on a high-mileage engine, then further milling or head gasket alterations could significantly affect cam timing.

I've been curious about ENDYN's "Rollerwave" pistons. They are surrounded in controversy, but if you read and study their design, it makes sense. They are designed to basically direct the squish area to the exhaust side of the chamber, to promote swirl, and are supposed to allow higher compression ratios. I know a lot of the Honda guys have bought into them, and while I remain skeptical, I wonder how well they would work on the UZ. ENDYN orders regular pistons from Wiseco in certain configurations and machines them further to their own design. They appear to be very high quality and incorporate a number of design features that I like, and would like to incorporate into my own custom pistons for the engine when I build it.

I think if we modified the head's chamber, we could effectively compensate for the change in squish area, but at the same time, I don't think modifying a head gasket designed a certain way is a good way to go about it.

 

[Peewee]

I fail to see any difference between milling 0.017" off a head (stock gasket thickness), and using a 0.017" thinner headgasket (stock head thickness).

Both ways lower the head in respect to the deck height.

 

[rarson]

If your combustion chamber on the head is curved, and you slice off some of the material, you'll effectively change the shape, whereas if you use a gasket, you'll just get the same shape closer. Let me see if I can do a crappy text illustration of what I'm talking about.

Unmilled chamber
....../-----------\
...../ ..../----\.... \
..../___/....... \___\ <- those (_) are the quench pads

Milled chamber
...../-------------\
..../____/----\____\ Milling the head increases the size of the quench pads

The dots are required to fix the spacing.
This is an extremely exaggerated, and poorly drawn, picture of what I'm trying to explain, but does this make any sense to you?

 

[Peewee]

I see what you are saying there.

I suppose it really depends on the design of the head.

 

[rarson]

Yeah, and how much you mill it.