Technical Head gasket effect on compression

Rebuilding a mopar 3i3 magnum.
Only changed being a cam with a little more lift (.506) and a bout the same duration. 267 advertised .The book says that the stock head gasket was .021
The rebuild kit I am getting has a gasket that is around .040.
Compression height on the new pistons is same as stock (1.92)
How much will this head gasket lower my compression from the factory 9.5-1?

 

I am ***uming the factory gasket was a steel shim type.

 

there are lots of compression calculators on the internet. I use the one that pops up on Summit when you're shopping for pistons there.

plug in the numbers, see what happens.

 

The stock 313 bore was 3.88" if memory serves.

Your difference in gaskets is .019".

The area of a 3.88" circle is 11.82". Multiply that by .019" and you have a volume of .225 cubic inches.

That is 3.687cc's. The old fellas used to tell me that 10cc's is about one point of compression.

If that holds true, you lost about .3687:1 in compression, which would take you from 9.5:1 to 9.1313:1.

If that small change bothers you, then have the heads milled (possibly the manifold, too).

 

I believe you are right on the steel shim head gaskets. I had to go do some homework to find out what a 313 Magnum was. Found it on Allpar can't get the link to work .
Somewhere in a serious performance engine builders guide book it probably says how much .019 will change compression with that bore. We no doubt have some guys on here who do have the formula and can figure out but you may not notice a lot of difference if it is a mainly street engine but it may mean a few tenths if it is a race engine. it may mean a few mph if it is for Bonneville and the lakes.

 

It is not only lower compression, but your quench area will be larger. I would look for the exact thickness that was original.

 

48 is correct. It could be two tenths in the quarter mile, but if you don't race the vehicle regularly, I doubt you will feel the difference.
Of course , I had a guy tell me one time, he could feel his 350 Chevy was stronger with 4 bolt mains.

 

Less vibration at 10,000 rpm? LOL! Gene

 

That was a typo.
It is a 383 magnum.

Will be 4.280 bore

 

I am building for the most torque I can get. Not racing. But like to smoke the tires.

 

Okay, so a 4.280 bore has an area of 14.39 square inches.

At an increased gasket thickness of 0.019" you have a volume of 0.27341 cubic inches, or 4.48cc's.

So you're going from about 9.5 to about 9.05.:1, give-or-take.

Sure, steel shim head gaskets would be the hot ticket, but unless you have a NOS hookup, or feel like having a custom set made, I think you're stuck with modern ones.

 

How would thicker gaskets affect intake manifold alignment? [I know milling the heads does]
Cometic gaskets [0.027"] are $86 each.

It would probably better to deck the block 0.021" to restore the heights while its apart.

 

Mopar performance still makes the steel shim gaskets.

 

As the piston area is a constant throughout the calculations, you can work everything out from the stroke. This is useful if you don't have an advertised chamber volume. Let's call the piston area 'A'.

Stroke is 3.375", so displacement is 3.375 x A cu.in.
Stock CR is 9.5:1
CR is (volume at BDC) : (volume at TDC)
Displacement is (volume at BDC) - (volume at TDC)
Therefore (volume at TDC) = (3.375 x A) / (9.5 - 1) = 0.397 x A

You're adding .0190 x A to that, so working back:
Volume at TDC = 0.416 x A
Divide 3.375 x A by this: A's cancel out, so 3.375 / 0.416 = 8.11183
Add 1 to get new CR = 9.11183:1

 

You might notice the diffrence in fuel economy. Perhaps that doesn't bother you in the states, your gas being so cheap, but around here it would be very much worth taking into consideration.

 

Getting the steel head gaskets is no problem. They list them in summit for only $29.95. I am getting the master rebuild kit from them anyways. They may even just swap out the gaskets in the kit for me.
Thanks for the info. I don't really want to give up the half point in compression so I think I will give the steel ones a shot.

 

The engine is no virgin. It had composit gaskets when I tore it down. Also a .020 crank that is perfect. But std. Bore. Am boring it .030.

 

My old 1970 383 Magnum rattled on premium so use the thicker gasket. I had to add water injection but gas was **** in early '70's so don't know how compares to now.

 

Most 383s had non quench heads anyway. Summit's calculator figures 1/2 a point difference. 383s 4 barrel motors in the late 60s were 10 to 1 motors with out good quench that's too much for todays fuel without a knock sensor.

 

The problem is not loss of compression it is gain in quench. Where the problem arises is that MOPAR heads depend on squish ( for lack of a better term) to breath properly and the tighter quench of the steel shim gasket makes a difference. It still probably won't be anything that you can feel in the seat of your pants but it will cause the heads to be a little lazy.

If it were me and I wasn't goin racin wouldn't let it bother me much, but if performance was my plan I would spend the extra cash and get the steel shim gaskets.

 

Kind of a catch 22 I am told that
More quench area also causes preignition

 

Yep someone who understands.

 

You replied with a question pertaining to a comment that I made in a comment different to the one that you quoted.

You quoted and questioned:

You will note that there is no mention of machining anything, or intake manifold alignment in that post.

When you should have quoted:

You will note that I mentioned milling the heads recover the lost compression points, and subsequently the manifold, so it is possible for it to seal against the heads that have been moved in alignment by milling.

 

I know that, but I don't spend my day in a catalog memorizing whether or not steel shim head gaskets are still sold for every single engine.

For several engines I have been through recently, they simply are not.

If he can get them, then that is what he should do.

 

Cometic has many SBC thicknesses of head gaskets. I currently lowering a IMCA alcohol dirt Modifed engine from 14.7 to 13.9 to run gasoline. A little smoothing of the pistons and a .051" gasket will do it. Your typical Felpro is .040 to .042 crushed. It's easy to move to something in the .020to.025 range. The are MLS so you need to take care with your block and head surfaces..

 

Curious how the actual formula, and the backwoods, back of an envelope, old-dude-rule-of-thumb formula came out about 0.06 different.

 

The best/easiest formula is on the United Machine page. KB Pistons.

 

I know you didn't mention any machining. It was just a question about a situation that is opposite to what is normally done

My question was aimed at the consequences of putting a 0.040 crush gasket where a 0.019 gasket was.
I've had heads milled done before, and this required milling 3 sides of the manifold [BBC] because of manifold mismatch
so I was asking what would happen if everything was "shimmed out" with thicker head gaskets

The reason I suggested decking the block is because less meat needs to be milled off [bore area] compared to the chamber area of a wedge head.[ literally the same amount milled off as the increase in gasket thickness ]

 

That 31e quote was a typo.
It is a 3i3 and will have a 4.280 bore.
I don't know yet if the heads need milling yet. Waiting to hear from machine shop. If they don't need milling I will be going with the stock .020 steel gaskets.