In the middle of my 318 build and am wondering should I deck the block or mill down the heads, to raise compression. I roughly am calculating a 8.85 to 1 CR, showing my figures below. This will be an all street cruiser/no racing, just for fun car, want to run pump gas. Here are some stats: 71 ebody, auto 42rh built up, 318 at .30 over (original block), sidewinder alum heads (waiting to order), .028 compress. thickness head gaskets, H814CP30 Speed Pro hypuerectic pistons, Lunati retro roller hyd. cam 270/279 .515/.530 lift, Thermoquad '73 4 brl carb., no intake selected yet but alum., prob. use 1 5/8 headers.

Here is my calculations for CR using Diamond Pistons website, I did an educated guess on top of piston ring to top of block of 7/16":

Enter the number of cylinders of your engine 8
Bore 3.94
Stroke 3.31
Rod Length 6.123
Gasket Bore 4.14
Comp. Gasket .028
Block Deck Height 9.60
Top Ring Down .4375
Chamber Volume CC 63
Dome/Dish Volume 0
Piston to Deck .067

dimension results:

Total Volume 745.57
Cylinder Volume 661.34
Clearance Volume 84.23
Gasket Volume 6.17
Top Ring Volume 1.68
Deck Volume 13.38
Piston Top Land 3.902
1/2 Stroke 1.655
Compression Ht. 1.755
Cubic Inches 322.85

For the given dimensions, your compression ratio = 8.85


Do you see any errors in my calculations? With these calculations, would recommend to deck the block, mill the heads, both, or neither?

Would you recommend 1.5 or 1.6 rockers?

Thanks!

I don't use the top of the ring lands to determine the piston height in the bore, try using the top center of a piston on each side at TDC and used those numbers on United machine, KB pistons, web site on their compression calculator tab to see how your motors compares compression ratio with Diamond calculations
The true mechanical compression will be a tiny bit less using the center of the piston for the deck height compared to the actual swept volume of that cylinder down to the rings at TDC but I don't figure on a pump gas motor that volume is enough to worry about
Anything under 10.5 to 1 with aluminum heads on pump gas on the stret is good enough for me

Cab...I think you're confusing the measurements here. Diamond's calculator uses the distance b/t the top of the top ring land and the top of the piston. It figures in the volume of the area b/t the piston and cylinder wall above the top ring and below the piston top.

It also uses the same measurement every other calculator uses...the top of the piston at TDC to the deck.

Thanks for sharing that
I use a needle tip grease gun to seal the top of the pistons at TDC to check actual cylinder volume on a "serious" motor or on any Hemi motors I build(push the piston down enough to get the top below the deck on these dang Hemiroid motors first before measuring it ) to get the compression ratio based off of the dome or dish volumes

At .067 in the hole I would deck it

Very likely an old block will benefit from at least enough of a cut to ensure a flat surface.
Aluminum heads will allow a bit more CR, of course, 9.0:1 would be fine for pump gas, might even get away with 89 octane, or push CR to 9.5:1 for 92/93.
I'd lean toward the Eddy Perf318 intake; actually the T-quad will dictate intake.
I'd stick with 1.5 rockers.
I like double springs for a safety margin.
Sounds like a nice street build.

I built a 318 bracket motor years ago. I cut the deck over .060 to achieve .000 deck. Then I cut the 340 pass headed.060 to achieve .045 piston to head clear. It had trw forged replacement pistons. I think it worked out to around 11.5 cr. Ran mid-low 11s w home ported heads and a streetable roller.

Engine will run good at 8.8 to 1. I would deck it to even it from front to back and side to side. I would not cut more than .02 from the block or .02 from the heads. I just assembled a 340 last night. #1 piston is zero deck, #7 piston is above deck .013, #2 is .002 above deck, and #8 is .004 above deck. Factory un-cut deck. KB 243 piston will be .026 above deck when block is at the blueprint deck height. KB has a piston for 318 that will get you to zero deck at blueprint deck height, which would get you 9.5 to 1 to 1 CR, perfect for premium pump gas.

X2

Qwk, are you talking about the KB167? I did the calculation with that piston, puts me at .012 under stock deck height w/ CR of 9.45 to 1. I think that is what I am going to do, change out the pistons. That should be cheaper than milling the block or heads. That should put me right where I want to be.

If I were to either mill the block or head, does the intake need to be milled some? Also, what about pushrod length?

You have .095" piston to head, way too loose. If you don't deck the block, you'll have no quench regardless of chamber shape.
You want the piston .007-.012" down @ TDC (.035-.040" P-H).

Though mine is a 360. 9.0-1, ported crap casting mid 80's heads, 224@.050 hyd roller, LD 340, 650 Holley, TTI headers and 2 1/2"exh, 3.55 4 speed (2.47 1st). Drag radials 12.70's @107 on 89 octane. 73 Challenger all stock street car.
Doug

I cheated and had a forged crank offset to 3.45 vs the 3.31 which netted me an add .070 comp height along with a kb399 mini dome piston but im at 4200 ft as well.

Have you actually measured how far in the hole your current pistons are and what your current pistons ch is, not on paper but actually putting a piton at tdc and measuring.

Blocks vary so so much its almost hard the believe at times.

You can get the mr gasket .028 for $50 a pair if you take the time to hunt them on ebay, most want 2x that.

Ive two untouched 318 blocks that had over .040 difference in deck height.

What bore dia. are you going with, Ive a boat load of kb and other teener pistons,ect.

Sounds nice DVV, close to what I am trying but you have more cubes.

I would take .040 and that would take 8 cc out of the equation put you at .055 quench. Intake Should be mocked up before milling either the head or intake. Same with the pushrods. Mock it up for the correct geometry. I assume that you haven't checked the deck, so even with new pistons it could require decking.

Or

Run it. However low compression 318's don't like very much cam if you require driveability. Stock cams were what I recall some thing like 250* so your cam your looking at is maybe 4 steps bigger than stock.

Correct, KB167. They used to be inexpensive. Price has gotten up there near forged price. Would be a good idea to have the block sq. decked to get it so all pistons are zero deck or -.012, where ever they land. If you have it decked set them at zero.