[old chevelle guy]

Working on the 496 Buildup. Have some #390 Heads. Closed-Chambered Oval Port. Told that would give a compression ratio of 9:1. Will these heads work for being mostly street/strip? Seems restricted on flow. Also, would getting the compression up to 9.5 or 10 be any better?

Rick

 

[Johnny O]

I would use them, they're an older style closed chamber head. Bigger valves and a little port work and they'll work nicely! It would be nice to increase c/r too, but then you're looking at a piston change too, way more money. 9/1 aint so bad!

 

[old chevelle guy]

John,
Thanks for the help. I've already talked to the builder about some slightly domed pistons. He's gonna look it up and give me the price increase. Then we'll talk about the cam next.

Rick

 

[Johnny O]

Rick, Im sure your builder knows this, but the closed chamber heads require a closed chamber style piston, if it's to have a dome that is....flat tops, it doesnt matter. There are still lots available though. Im having a set of those old heads done at the machine shop as we speak, #215's.

Then we'll talk about the cam next.

THAT will require an entirely new post!! Have you done a search yet on cam choices?? As you probably are already aware, there are tons of posts on that topic here. When you get your final chamber size, after the machine work, and figure out what piston you will use, then you can come with a comp. ratio, and pick a cam based on that. Is he decking the block at all??

 

[old chevelle guy]

Yes, the block is being decked. I'm a little nervous about what the vacuum will be. But the builder is being patient with me, verifying everything before we go to the next step.

Rick

 

[Johnny O]

Yes, the block is being decked. I'm a little nervous about what the vacuum will be. But the builder is being patient with me, verifying everything before we go to the next step.

Rick

Sounds like a good guy! Are you worried about vacuum for power brakes? Just dont go crazy with the cam, you should be fine.

 

[Guest]

And don't forget the old stock type .020" thick, steel shim headgaskets too!

To keep the quench tight!!

pdq67

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[old chevelle guy]

And don't forget the old stock type .020" thick, steel shim headgaskets too!

To keep the quench tight!!

pdq67

Sorry, I don't understand that?

 

[Guest]

Flattop pistons, and 100 cc chambers (approx 390 size) will yield about 10.2 compression on a 496.

 

[old chevelle guy]

Flattop pistons, and 100 cc chambers (approx 390 size) will yield about 10.2 compression on a 496.

Jake,
So, now I'm confused Any idea how he is getting 9.0 Comp. ratio? Lots of difference between 9.0 and 10.2. I haven't gotten a response on cost of domed pistons yet, but I think I would be happy @ 10.2.
Should I ask him if he has cc'd the chambers?

Thanks,
Rick

 

[Guest]

I did a 505 combo using 390 heads. They were right at 100cc give or take a cc or two once the chambers were polished, and heads surfaced to clean up.
They had larger valves installed.

Using a SRP flattop piston with 6 cc valve reliefs it came out right at 10.2-1.

The longer stroke of the 496 combo makes compression get up there pretty quickly. Your guy probably doesn't realize he isn't dealing with a 118 cc chamber. That would be about 9-1 or so just guessing.

You won't need or want a dome for a street 496 combo using 390 casting heads.

You might question your builder and if he doesn't know these things, find another.

 

[Wolfplace]

Jake,
So, now I'm confused Any idea how he is getting 9.0 Comp. ratio? Lots of difference between 9.0 and 10.2. I haven't gotten a response on cost of domed pistons yet, but I think I would be happy @ 10.2.
Should I ask him if he has cc'd the chambers?

Thanks,
Rick

=
Rick,
It depends on how much you deck the block
If you do not deck it & the heads are actally 100cc:
The piston will be .020 down nominally for 9.6
If you deck it to 9.790 or about .010 you will have 9.8
At zero deck or 9.780 it will be 9.9
An SRP flat top has a 3cc pocket
This is assuming an .039 x 4.370 gasket, rings down about .250

 

[old chevelle guy]

Talked to the builder today. He agreed he made a mistake on the stroke and the compression will be higher. So, we're gonna stick with the flat-top pistons.

Rick

 

[Guest]

Headgaskets come in various thicknesses here.

.020" for thin one's all the way up to .040" thick and even WAY above.

You want the flat of the head's chamber to be between .035" to .045" from the flat around the edge of the piston so that a more eff. combustion process can take place.

This distance is called the quench/squish..

Our engines are usually made w/ a piston down-in-the-hole at a stock .025" or so, so therefore a .020" thick shim headgasket will put it at .045!!

Is all..

So if you deck your block so that the piston flat is closer to the head, you can use a thicker headgasket matched to still give you a good quench..

Say at zero deck, a .038" to a .040" thick headgasket would be a dandy..

pdq67

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