Ive been looking into various pistons and some of them only list compression height. So how does one calculate compression ratio from compression height?

 

You can't calculate compression ratio from knowing the pistons compression height alone.

Are you trying to calculate the piston deck clearance using the compression height?

 

1.56" is "normal" for a 5.7" rod SBC. So if you have 0 decked the block, then 1.56" compression height will give a 0 for the deck height. If you zero decked it, then swapped to 1.54" comp height pistons, the pistons are now another .020" deeper in the hole, so you have a .020" deck height (or add .020" to your original deck height, if it wasn't 0 decked).
That's all that # can tell you, still gotta know all the rest.

 

For a 350 (3.48 stroke) thats true, but its different for every variation of SBC crank. The 327 has a different pin height, as does the 302, and the 383/400 (with 5.7 rod).

Pin height isn't used in the CR formula I don't think. It won't tell you anything useful, just if the pistons will work with the crank, rods and block that you are using.

 

ah, you got me. What I mean to say, was with a 5.7" rod 350 SBC... anyway, it's just a way to measure deck height, which is used in the CR formula...

 

Well I'm building a 400 and I ran across a few Lunati pistons and they only list compression height. And I'm in the process of deciding whether to run a 5.7" rod or a 6" rod.

I was going to go with a 6" rod but I came across a good deal on those pistons and would go to a 5.7" rod if they will work and save me a few hundred bucks.

67cc heads
Stock crank


...

 

Thats right... you do need deck height, which pin heightcan help you figure out, but its not the only way to figure it out, especially if the shortblock is already assembled.

 

A piston made for a 5.7" rod 400 will have a 1.43" pin height. (some will vary a little) a piston for a 6" rod 400 will have a 1.13" pin height You can't mix the two.
what is the pin height spec of the pistons you're looking at?

A standard sb has a block deck height of 9.025". This is a nominal figure. they do vary. If your's has been "decked" it will be less. You have to measure to get accurate numbers.
You need to also tell us the piston's valve relief/dish/dome volume. -5to -7cc for a typical flat top.

Assuming a +.030" over bored 9.025" block deck with pistons .025" below deck @TDC 7cc valve relief flat tops and a felpro gasket your cr will be 10.42:1.

A little high for pump gas and everyday driving.
Keith Black (UEM) makes -18cc D dished pistons. Using these your cr would be 9.8:1 IF You also have the block "0 decked". Just right for a street motor.

 

I believe the pistons were lunatis with a 1.550 compression height. -4cc valve reliefs.

http://holley.com/1411J2S3.asp


Engine plans so far are.

221cc Brodix Track 1 heads
272/282 Isky roller cam
10.5:1 CR (roughly)

 

I assume that figures you're using "normal" 400cid 5.65" conn rods?

How about more cam specs, that's kinda vague.

 

get your piston to deck height.. should be small.. add half your stroke, plus your rod length, and the compression height.. then take the deck height and subtract the number you just calculated.. thats your piston to deck clearance.. it should be a positive number if you followed what i told you.. then plug in that number and all your other numbers into this calculator..

http://www.kb-silvolite.com/calc.php?action=comp

it requires your head volume, piston dome or dish volume, head gasket thickness and bore diameter, cylinder bore, deck height and stroke.

 

Im guessing youre just curious on the cam not that it matter for the compression question..i hope.

I dont have them saved here in front of me but its an isky hydraulic roller grind, not looking to go huge, I want plenty of lowend.

 

I was just going to advise you, since it sounds like you're hoping to find out your CR, and the cam effects your DCR, which is the # that actually matters.
Too much cam, too big of heads, and you will lose your low end. That cam doesn't sound huge, 'specially in a 400, but i'm just going by advertised duration (I hope), so it's kinda a guess.

 

That mild Isky hyd roller is actually kind of a mismatch to big honkin Brodix Track 1's. .500" lift is just a tickle for these race breed heads. Track 1's need lots of valve lift.
.600" is just a start. You'd want to get a Custom grind 224* single pattern on 107/108LSA Hyd roller and use 1.6:1 rockers. Wouldn't go less than .560" net lift.
If you want a torque motor select a smaller port volume head for your 400, Under 200cc.
A 170cc Vortec head makes a ton of torque.
RHS 200cc Lightnings would be nice too. Dito GM's big Bowtie Vortec head.
The newly released Edelbrock E-TEC 195 CNC heads would rock.
Can go up a bit on cam duration 230+@.050" A head like a AFR 195 Eliminator and a .550-.560" snappy cam would make lots of torque and power (over 500hp)

Again Track 1's make a ton of power but need a bigger cam with lots of valve lift to do it. Think moderate solid street roller and a Vic Jr with these heads. They like the rpms.

Those Lunati pistons are for a short (5.565" 400 rod lenght. I would use a longer 5.7" rod and the same piston style with a higher pin height.