alright. I'm sure this is diffrent on every engine. I'm trying to get a rough estimate so i can figure the compression ratio of my engine. I have a vortec headded 400 bored .30 over has hypertec 4 valve relief pistons on 350 rods. -6cc. the heads are 64cc and i'm just looking for a general number to put in for deck. thank you for your time

 

there are compression ratio calculators online,but id guess around 9.5-1 from your description

 

yes i know there are compression calculators. thats why i'm trying to get an estimate of how deep the pistons usually are so i can use one. thx

 

Virtually all hypereutectic pistons have the usual extra .020" of deck clearance built in, on top of the usual .025" that comes stock. So figure on .045" "down in the hole" at TDC. Calculate it at .025" as well, just in case they don't; your "true" number is probably close to one or the other.

 

these are the ones i'm running. http://www.summitracing.com/parts/SLP-H616CP30/ any way to tell if they are .025 further down?

 

Sure... measure em. Piece o cake.

 

would if it was bored. guess i'll just have to hope, guess and wait.
thx man

 

so running both numbers makes it either 10.38:1 which i think is obtainable with iron heads and pump gas. lil higher than its suppose to be but i dont think i'll have to run race fuel and the other is 10.83:1..which i'm pretty sure i'm suppose to run race fuel with that. or just get thicker head gasets

 

You have to measure how far down the hole the pistons are, If it is .040 your in trouble, even with a .015 shim head gsk. your quench is .055, .040 or a little tighter makes the quench area work better, more efficiant.

Even on my 67 Chevy 1 Ton service truck with TRW 2256 pistons at .018-.020 down the hole I used .018 shim gsks. which made them .036-.038

Recently I purchased some MAHLE hyper pistons real cheap from Star Performance, smoothed all the valve pockets & weight matched them, did a trial assembly & they were .040 down, I called MAHLE, they said thats the way there made, my mistake for not checking comp. height.

I had the crank offset ground to 3.520 stroke, cost about $120.00, same as decking the block .020, this put the pistons .020 down, now with .016 shim gsks. quench is .036- perfect! The reason for offsetting instead of decking was if I have to spend money I want a gain, now I have a 353 with a stock 4.0 bore.

 

Requiring higher octane is more than just the compression ratio. A well built engine could run on premium pump gas with 12:1 compression. There's just too many factors to say a specific compression needs a specific octane rating. Unless you really need it, race gas will just make your pocket lighter.

Octane is the fuels resistance to detonation. Gasoline has the same BTU energy regardless of the octane. You only need the higher octane if the fuel starts to self detonate under compression before the spark plug fires.

Higher compression or using any kind of power adders increases the chance of that detonation.

Assume if the block has never been decked that the piston is at the lowest possible position below the deck which will drop your compression ratio. Every time the block gets decked, more material is removed and the compression ratio will increase. Zero decking means the piston is right at the top of the cylinder. It's also possible to have the piston above the deck with it going into the opening of the head gasket.

It's always better to have the piston down in the cylinder a little bit. Not going all the way to the deck surface adds some piston to valve clearance for high lift cams.

A guess is just a guess. The only way to know exactly how far the piston is down in the hole is to measure it. Take the measurement at the side over a wrist pin. If you take it at the top or bottom, the piston could rock on the wrist pin and give a false reading.

 

ok so what is too much in the hole i guess i should ask. what is acceptable and wat is unaccetable because .015 extra is to much? to make .025 + .015= .040 down

 

Standard SBC deck height is usually 9.025". That's from the crank centerline to the top of the deck.

A 400 crank has a 3.75" stroke. 3.75/2 = 1.875 from the crank centerline to the rod journal centerline.

1.875" + rod length of 5.700 for a 350 rod = 7.575"

Your pistons have a compression height of 1.425. That's the distance from the wrist pin centerline to the top of the piston.

7.575 + 1.425 = 9.00" which is generally the deck height of a block that's been zero decked.

Since a typical SBC has a deck height of 9.025 and you have a length of 9.00", the piston should be down in the hole 0.025" providing the block has never been decked.

If the stamped numbers are still on the pad in front of the passenger side head then the block hasn't been decked. Decking the block will generally machine off the stampings.

 

so i should figure the compression with the .025 down in the hole. Now what about what sofakingdom said that the hypers usually have an extra bit? I'm running a lunati vodoo 60103 cam. Do you think i'll be able to run pump gas. i realize compression is up and that other factors take place.

 

the block will be going to the shop friday to be bored. once i get it back i can set the crank and piston in it so that i can finally measure it.

 

If you already have the pistons, mic the distance from the top of the piston to the top of the pin bore. Add to that, half of .927" (the pin bore dia); that's the piston's compression height. Check a couple because they usually have some variance.

Now take that, add it to the rod length, and add half the stroke to that. That's the "height" of your rotating assy.

Now subtract that from 9.025". That's a pretty decent estimate of how far DITH they'll end up.

Keep in mind that rods rarely get longer than their nominal length; everything that's ever done to them to prep them shortens them.

Keep in mind also, stock blocks have HORRIBLE production tolerances. It's rare to find one where all 8 are within .005" of the same height. There's often that much difference from the average of one side to the average of the other, or even on ONE SIDE from front to rear.

And don't even get me started on cranks, and how sloppy those usually are and how much tolerance there is in them. Throws that aren't 90° apart, aren't the same distance from the crank CL, aren't parallel to the crank CL, and on and on.

A calculator is only as good as the #s you put in. Garbage in, garbage out. You've already got the best "estimate" you're going to get. If you put in numbers that you just made up because you liked them, you'll get completely useless results that in no way resmble your actual hardware. Only way to know what you've got is to MEASURE. You can post stuff on the Internet about what you "wish" it was and what somebody says ia "acceptable" from now til Doomsday and never get any closer to the truth than you are now.

Go MEASURE everything and come back with REAL numbers and we can help you out further. Until then, it's all a waste of time.

 

sofakingdom, you nailed that one, great instruction, I wish I was that articulate. And I agree about how sloppy production tolerances are, set a straight edge on the rocker studs or valve stems, allover the place.

Like I said earlier, offset my crank .020 with .020 under brgs. my original crank throws were long so they didn't have to go .030 under, no big deal just pissed my self off for not getting all the specs. to begin with- another lesson learned (again).