for my first ground up build of a motor i chose a 79 4 bolt 350. its .030 over with some silv- o - lite dished pistons. i just put on the heads, they are 081 casting with 1.94 intake valve and 1.5 exhaust. Now the CC volume is the same, i believe stock they are 58cc. Whats pondering my mind is the compression ratio that i will acheive with this combo. my machinist told me about 9.5:1 but after seeing some other combos on here i dont know if i belvie him or not.

so what do you guys think the compression ratio will be roughly?

 

Forgot to add one more thing in, the head gaskets i used was from the fel pro gasket kit, i think the thickness is .035 thousandths?

 

Geeese you don;t want much do ya.

How about some specs or part numbers on the pistons you used and the deck clearance.

Without accurate specs its going to be a guess at best.

9.66:1

 

the deck was not milled at all and the pistons i do not ha ve a part number, only the box. i will go to the machine shop tomm for a part number.

i didnt want to use the dished pistons but they insisted i did or else the motor would gaurantee ping and knock even on 93 octane. so now ill do anything to get the best C/R i can. mayb a copper shim gasket? or possibly shave the heads?

 

You'll be fine right where you are.

 

so 9.66:1 then ? i kinda wanted it a little higher for a bit more powa . But that is ok.

btw how much do you think like about a half a point or a point of compression ratio is for HP.

 

i'd say after puttin the numbers into a formula for c/r depending on how much your pistons are dished ur lookin at around 11 to 1 maybe a little lower somwhere around there, tommorow if u get the number of cc's ur pistons are dished, and if u might know the deck clearence i can just about get u a prisice C/R

 

Therory says for every 1 ratio increase power and torque will go up by 4%.

Unless you run the required fuel to keep the motor out of detonation, all you're going to do it turn it into junk if you exceed the point where the motor detonates.

If you want more power, improve the engines ability to pump air. Pull the heads , fully port them and reinstall with thinner head gaskets.
What fuel will you be using ( every day)?

 

as a daily driver in the summer i will be using premium 93 octane fuel, possibly a higher octane for the track

 

if my compression ratio "guess" is accurate I'd stay pretty well right where you are. Not a lot to be gained.

Air flow, air flow air flow. Did I mention air flow?

think air flow not compression to get more power.

 

i have thought about air flow, money comes into play. i am saving up for some vortecs down the road though. and i would imagine the C/R would drop a lil bit more with those too. 64 cc

 

You could pull your heads, fully port them to flow more air than stock vortecs, then ( with the heads off) after carefully measuring all the factors that affect compression ratio an calculating the true compression ratio of your motor , flat mill the heads and/or switch gaskets to get the cr to what ever you want.
All for a $$$lot less$$$ than a set of vortecs and a manifold.
It will take work on your part thou. The heads you have now have lots of potential in modified form.
(I'd keep it real close to 10:1 max for 93 octane)

It's just a matter of getting in there and getting dirty.

 

ill port the intake and exhaust ports

thanks for the help so far

 

9.62026:1 to be pretty well dead on

If you look for the info you can find it... Only dish piston KB silv-0-lite comes in for a 350 is a 12 cc dish

the std. blue fel-pro gaskets that comes in the kits are 4.166" x.041" thick

The stock spec deck height for a SBC is .025"

I would have went with a 4 VR flat top. That would have put you around 10.16:1 used a cam that matchs that. Something like 216/228* @.050"

But.. You would still be left with a not so good quench height (deck + gasket) best range is .035"-.045" over .050" and any benifits from quench is gone.

Now you can go back and add a steel shim gasket (not copper) to yours and have even more compression and better quench. But you'll need even a bigger cam to keep from pinging.

With those heads, pistons, etc and a 4.100" x .015" gasket your at 10.2698:1 You'll need an awful big cam

But you could go with a 4.100" x .018" gasket and still have good quench (.043") and come up with 10.1937:1 I run right at that, and i'm fine.

I run 10.18:1 with iron heads, a 218/230* @.050" cam (after you factor in the 1.6 rockers), and if I keep timing pulled back to 34* total I can run in the dead of summer on pumped 93 octane.

If I had a tad bigger cam, I could bump my timing back to 36* and still not ping

The tight quench helps to fight off detonation

 

the cam i have right now is a comp 268h advertised duration is 218/218 at .050. 454 lift intake and 454 lift exhaust.

 

part number for the pistons is 1472, so yea a 12 cc dish