I purchased some good condition 2.8 heads from the junkyard to be cleaned and ported. I was also thinking about having a shop deck the heads to increase compression, but I have some questions about that. How do you calculate the compression ratio after shaving a given amount off the bottom of the head? For example, if i wanted a 9.5:1 compression ratio, how much do I remove from the heads? I was also thinking that this might cause some reassembly issues because the dimensions of the head will have changed.

How to figure your compression ratio

http://www.youtube.com/watch?v=OGuOMItsCXk


If you shave the heads, at some point you will need to deal with the intake mating surfaces, and the intake itself. The machine shop should know what needs to be done and how much needs to be adjusted to the intake to match up to the head milling

I thought the 2.8s already ran a 9.5:1 compression ratio stock?

No... 8.9:1 per 86, 87, 88 owner man

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thanks, but it occurred to me that all you need to do it divide 2800 by 6 (466) and then divide that by the compression ratio (466/8.9) to get the head volume (56). That was easier than I thought...

so now i will determine how much the head volume needs to be reduced in order to bring the ratio up to my target of 9.5

so thats 466/x = 9.5

which works out to x = 49

and 466/49 = 9.5102..... pretty close

I don't think decking the head is the correct way to gain compression. You usually take the bare minimum off to get it into spec. If you want to raise the compression, get new pistons.

Okay so I figured that I need to lose 3 cubic centimeters from the head volume to bring the ratio up to 9.5

were my methods in the last post correct?

Aside from any issues of bolting on the intake manifold, here's a good compression ratio calculator.
http://www.60degreev6.com/content/CR_Calculator

To find the combustion chamber volume as a function of how much you shave from the head isn't going to be easy.

But, for a rough estimate, I would paint the bottom of a head, and then press it on a piece of graph paper.

You'll be left with an unpainted area on the paper from the combustion chamber.

You can then integrate (just like in the youtube video) the unpainted area on the graph paper, and then multiply the area by the thickness you want to shave to estimate the combustion chamber volume reduction.

[This message has been edited by pmbrunelle (edited 03-09-2010).]

so, using that formula, if you bolt the stock 2.8 head onto a 3.4, you get 9.4 compression ?

hey it actually does. The 2.8 and the 3.4 use the same heads and the 3.4 has a stock compression ratio of 9.54 : 1.

cool observation

Take off too much and you run the risk of valves slamming into pistons too. Some engines have pretty tight tolerences.

Yeah i dont want to make this into an interference motor cuz that just makes me nervous. i'm nowhere near finished with the math and figuring so all of this business is still tentative.

The Chevy Power book says to not take off more than .030" due to the coolant passages but one buddy took .050" or .060" off (I can't remember the exact number) and didn't have any problems. The lower intake had to be corrected too.

The head gasket thickness of the various manufacturer's is all over the map too so you need to select a gasket for your calcs and then use it.

The cylinder bore is 89mm nominal. the chamber is the same diameter for quite a few thousands till you get past the edge. But there is that flat area that will decrease the volume of a pure cylinder: (you can see the lip at the top)


in a pure cylinder with 89mm diameter, .001" will decrease volume by 1.6 cc, since I cannot measure the loss from the flat area without removing the heads, I will swag that that number will be more like 1.3cc, so a .003 deck will give you what you want, but I think your number is wrong.
stock is 8.5:1 - 8.9:1 from the 88 fsm, I believe the Cali motor is the lower c/r.
the head volume can be determined from the displacement of the motor, but not the 2.8l number, you have to calculate that, which is simply bore area x stroke, in the 2.8 that is 89mm and 76mm, which if you do volume as pi x r squared x Height = 472.80 cc per cylinder, or 2836 cc total engine volume
So if the c/r is 8.9 estimated head volume by your method is 53 cc or 56 for 8.5 so a .4 change in c/r is 3cc. For you to go from 8.5 to 9.5, the number is 49.7cc, or 6.3 cc decrease.

So without knowing which c/r pistons are in the motor, you can't accurately determine how much decking needs be done.
To CC a head you can look it up if some has already figured it out, or you install a plug, and put a piece of glass or flat plastic over the chamber with a hole in it and using an accurate syringe, you fill it with water until there is no air left, and that will give you your true combustion chamber volume.
Even knowing the chamber volume tho, since the shape is not really uniform, it could get very complicated if you get past that small area where it is just a flat sided cylinder.

Now that I said all of that, an sbc and 3.8 buick was about .010 per .5 increase, (larger pistons) so I would say you can probably do it with a .015 deck using the stock flat metal headgasket, but more if you use the multilayer gaskets like the permatex. the thicker the head gasket, the larger the volume of the combustion chamber.
Even .030 max will more than do it, but you will need both the mating face of the lower intake and the bottom of that manifold milled by the proper amount, a shop has simple charts for that.
Your time would be better spent porting/polishing and fixing the exhaust logs.

[This message has been edited by tjm4fun (edited 03-10-2010).]

If the engine is out of the car, shave the new heads the minium to insure they are true and have the blocked decked to do the same. There's not much to be gained by just a tiny compression increase, if you want a noticable HP increase you need to go from 8.5 to 9.0 or even 9.5 to feel the dif. Like somone said, look to other areas for HP gains like porting or replacing those awefull headers etc. I don't think head work is worth the effort without lots of -even bolt-on- mods first.

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I was talking to my engine machinist last year about potentially doing the same thing. He advised me that decking the block is the better route.

I didn't deck anything though. I got thinking about all that high test gas and the price difference from 87 octane. I'm not racing so I'm keeping my compression ratio stock on my new 87 motor. You can also bump up compression without decking by putting in some total seal rings. They are a little pricey but they do the job. No loss down the cylinder walls because there is no ring gap.

Just some thoughts

Arn

If the heads are off, and being ported, then why not get custom flat-top valves, oversized of course. They will be lighter, and you will reduce the combustion chamber 3cc.

dave

quote Originally posted by tjm4fun: stock is 8.5:1 - 8.9:1 from the 88 fsm, I believe the Cali motor is the lower c/r.

the c/r is different in california?? well i got my car in florida

alot of good ideas in this thread. I think i will hold off on decking the heads and prolly just get the high compression pistons like you're "supposed" to.
but the flat top valve idea is pretty darn good.

Stroke it. Put a 3.1 crank in it.