Technical Help w/compression ratio

I’m having a 396 w/023 heads (396/325, slight bowl blending) rebuilt. Pistons are Icon 9948 raised.
Want 10.5-11 compression.

Any way to confirm once engine is built ?

 

What is the deck height?

 

:

That is dependent on your deck height, you need to measure your block to know for sure, there is a calculator for figuring it out.

 

On summit website

 

https://www.summitracing.com/popup/calcsandtools/compression-calculator

 

As Grumpy rolls over in his grave!
.

 

1 Set IC9948.STD/1 Set 4125AM8.STD Kit W/Rings (uempistons.com) Is the info on the pistons.

I've been though five different listings for big block cylinder head casting numbers and can't find an 023 head in any of them. is that a missprint?

I did find an 063 with a 100.987 CC combustion chamber that is going to be a challenge to get 10.5 out of
3931063 1969 396 325, 350 2.06/1.72 100.967 Similar to #3909802 & 3917215 with two accessory bosses above #3 (or #6) exhaust ports for air conditioning rear support bracket. Straight casting flashing.

Found here Head Casting By Year (chevellestuff.net)

 

The confirmation is in the parts selection, measurements, and math calculations before and during the build. Once it's together it's too late.

 

Mr48Chev,

Yes, those are the pistons.

My apologies as heads are “063”


“3931063: ’68 and ’69 oval port, closed chamber, 2.19- and 1.88-inch valves. Used on 396ci, 402ci, and 427ci engines.”

Don’t know deck height, block has been decked previously.

Any suggestions ?

I don’t want to buy new aluminum heads. If I have to, are there recommendations as to something less than full race ? My selections might be limited due to small cylinder bore ?

 

Way to many things left out .

 

You need a lot more info, measurements to possibly calculate a accurate ratio.

 

It’s not real difficult for the shop doing the work to let you know what’s up. Block being “decked” could mean zero decked, then to what style piston? Or was the block just trued up?
Regardless, have the shop tell you where the pistons sit in relation to the deck at TDC.
Once you know that and the volume +\- of the piston, you can figure the CR with your bore/stroke/head CC and see what happens with different compressed heads gaskets.
As mentioned, once it’s bolted together you have what you have.

 

Lots of variables. CC of valve reliefs on pistons, head gasket thickness, ect. If you want accuracy, you need to get all these specs.
For example, have the heads ever been milled? 50 year old heads aren’t necessarily going to CC out to original specs if they’ve been cleaned up a few times.

 

The low dome 325-350hp pistons were only rated at 10.25 to 1 with the closed chamber heads and were lucky to get that.The high dome L78 375hp pistons were supposed to be over 11 to 1 so you might have better luck reaching your goal with those but they have to be the close chamber versions.TRW used to make them years ago but I don't know if they're still available.The icon link above is for the low dome 325-350hp 402 piston hence the STD listing.

 

Old heads won't have hardened seats for unleaded fuels .

 

Pistons have 17cc domes.

“Hardened seats” is a whole different can of worms, no clear answer.

ICON chart shows 9948 capable of 10.5 w/90 cc head.

Perfect world, I’d like 11:1 as plenty of 93 no corn here.

 

Those are fairly small 105cc chambers, so with the pistons you're using, and assuming stock deck height, the engine should easily be in the 10.5:1 CR range, and maybe a bit higher. If you ran a thick head gasket it will lower the CR slightly, or a thin head gasket will bump it up.
You'll want the machine shop to install stainless valves, and hardened seats. I also prefer iron guides to bronze guides, as they hold up much better.
With today's junk octane ratings I'd want to run a thicker head gasket to not have issues with detonation even with premium grade gas.

 

Icon chart shows,
90cc = 10.5
109cc = 8.5

063’s are MOL 101 cc’s

Using ICON chart as a guideline,
looks like a lot of grinding ! ?

Some say steel shin head gaskets wick oil ?

 

I would just put it together like that, with normal gaskets, and run it. It's a great combination for a typical mild performance street big block. I'm planning to build a similar 396 for my truck soon.

Sent from my Trimline

 

Wick oil how and from where?

 

Need at least 10.5
if for no other reason,
cam specs require it.

Steel shin head gaskets rumored
to be wet between head and block.

 

You can bench race this thing all day, but to get to the answer you want, you need to firm up a few things. Assemble enough parts to confirm just how far a piston is down in the hole at TDC. Based on the piston compression height you should be able to figure if, or how much, the block has been decked. But you will want to know where the piston sits to calculate CR and if you are going to include getting to a target quench number while playing with head gasket thickness. There are a lot of choices besides steel shim gaskets. And if you are really want to calculate a true CR number, you will need to cc the heads. They may have been cut at some point, or you may want to cut them.

 

Just wanna know if I’m in ballpark, looks really close !

Re cam: “ Mild Race Cams - Rough Idle, Low Vacuum. Not for Towing. Engine Modifications & Stall Converter Required. Compression 10.5:1 or 11:1. Spring Pressures - 120 lbs. On Seat - 300 lbs. Open”

 

And you picked this cam because....???

 

Prior experience.
Not interested in a pizzing contest over cams, sorry !

 

Is the engine standard or bored? With the information you and others have given, an approximate CR can be calculated. What I have gleaned from this thread is a 396, standard engine, that has been decked, so we will estimate they took off .010. Your heads are 101 cc chambered heads, that have not been shaved. Let’s assume your gasket will be .042 thick.
Are these figures correct so far?

With the figures you gave, as we have no compression height of the piston, so we will have to assume the piston is level with the deck. You CR on a standard engine would be 8.68 to 1, a .030 over would be 8.81 to 1. Remember there are some variables involved here. One is the piston height in relation to the deck, it is probably not zero.

Edit: I missed the thin steel head gaskets, assuming they are 1/2 the thickness of the composite gaskets, .042x1/2=.021. Your CR would be for standard engine would be 9.13, .030 would be 9.26

Bones

 

Bones,

Old machine shop ticket says “resurface heads”. Block decked enough to remove stamped numbers.
This is a 396, .30 over, or 402.

I’m trying to get some info on steel shin head gaskets. They would help restore compression.

Your CR estimates are what I’m fearing.
Agree pistons are prolly not at top of deck.

I need a minimum 10.5 and want 11.
IYO, can I get heads/block cut enough to get to 11 without having intake alignment problems ?

 

How much .020 or .030 taken off a head increases the CR is difficult to determine, due to the odd shape of the combustion chamber. It can be estimated if I had the dimensions of the combustion chamber where it meets the head.
I am not well versed in BBC, the last one I built was many years ago. But my recommendation would be to find some other heads with smaller combustion chambers or get larger pop up pistons. I would do this before slicing and dicing the block, heads, and intake.
Maybe some of our Big Block Chevy Gurus will chime in?

Pistons .020 in the hole and thin gaskets will give the same CR as the first estimate. With the .042 gaskets it will be lower.



Bones

 

BB Chevy are their own can of worms.

BB Chevys seem to need large cc heads, exasperating my compression problems. Most performance heads are 129 cc up. 396/402 head selection is limited as desirable 2.25 valves common on 427’s and up can hit smaller bore cylinders. The good stuff is rectangular ports and I don’t need that much motor or costs.

063 heads, although oval port seem to be a decent compromise offering HP
over peanut port heads.

This is what I gather from what I read.

If car wasn’t documented SS 396 convertible, I’d slap a 383 SB in it and be good to go !

 

I’m a Ford guy, see my avatar! Lol I’ve built tons of SBCs for street and racing, but few BBCs.
From what I remember even the stock BBCs had “ pop up” pistons.
Maybe you could explore high compression pistons.


Pistons with 30 cc pop up and thin gaskets and .030 over would get you about 10.8 to 1.





Bones