Guys,
I got a problem. I'm rebuilding my 390 engine now. My heads are C8AE-H with 2.02 intakes and 1.65 hardened exhausts. I got the Comp Cam 268H Kit ( springs gears and such ) and I ordered the Silvolite 1130 pistons with moly rings. The carburation is a new Street Avenger 570cfm with a 66 S code intake. The rods are the C7AE-B rods.
Here is my problem. The pistons claim to be 9.4 to 1 compression but the calculators on the web sights say it'll be 8.3 with the Fel-pro .041 gaskets, 8.8 to 1 with the .020 steel gaskets. Is the Claim by Silvolite wrong, or do I need to use .020 gaskets to get my compression up.
P. S. What would you do if the web calculators are correct.
Thanks,
Jim Nolan

 

What is the CC for your heads and how much deck clearence do you have?
What are the pistons you have? Flat top or dish if dish how many CC's?

 

Well ya also need to know how far down in the cylinder the piston sets to the deck of the block, Head cc's, and what the pistons dish is in cc's..or it maybe a flat top piston?

Russ

 

I got 8.8 to 1 using:
68.5cc head
19cc for piston
4.4 gasket bore
4.08 bore
.041 thickness
.03 deck clearence, should be .029

The 1131's will get you 9.5!

 

LOL..Randy, I guess we were typing at the same time!!

Russ

 

Randy and Russ you guys need to step aside and let the professionals like ummhhhh me do the replying.

You guys just have more questions than answers.

 

gotta have acurate numbers.
garbage in/garbage out.

 

Mr. Matt, The Floor is all yours SIR!!! LMAO!!!

Russ

 

Yep, we need:

1. Head CC's
2. Head gasket(s) you are considering
3. Piston dish volume

I found the compression height of the Silvolite 1130's at least, it's 1.76"

Then, and only then, can we accurately calculate compression. I suspect it wont be too high, but get the info and we can go from there.

 

Russ and all,
Compression height=1.76, Dish of piston=19cc, Heads are 68cc. And like I said, I could use the .020 gaskets with a 4.2 bore or a .041 gasket with a 4.4 bore. The block and pistons are standard 4.05.
Thanks
Jim
PS. If I use these pistons at 8.8 compression, will the cam I bought be OK.

 

Ok, now we're in business!

I'm getting the following:

.020" gasket: 9.25:1
.041" gasket: 8.79:1

If you use the .020" gasket, it'd be perfect for that cam. The .041" gasket would not only lower the compression ratio, but kill any quench advantages you had with the .020" gasket. I calculated .030" deck clearance with those pistons, giving you .050" quench space, which is ok, not the best, but a lot better than .071"!

With the .020" gasket, I think you have a good torque motor combination. I say go for it.

 

randy and Russ, you guys know i was just messing around. Kurts 8.79 is pretty close to my garbage in=garbage out.lol. I need to read some of these quench articles to see why this lower quench/higher compression works out.

I am wondering does anyone have combustion chamber cc's? Everyone says the chambers are bigger than advertised. The same with the edelbrock heads. I assume ford could get within their 3 cc range for cylinder heads but i guess not. I also have seen instances were ford overclaims the cr of a motor when they have all the numbers to know accurately. It seems like 11:1, back in the day was a sales pitch or something.

 

That was a good guess on head CC's, I'll give you that!

On the quench:

I got most of this information from www.kb-silvolite.com, in their articles. I may use some of their examples.

The basic thing to understand is this: When you run tight quench, like .030" to .050", as the piston approaches TDC, it squirts out the air/fuel mixture at an extrememly high velocity from the flat part between the head and the piston into the rest of the chamber. Since the spark has already ignited the mixture and the flame is still not very big, the effect is that the flame instantly becomes huge. Think of filling a coffee can with gasoline, and lighting it. Slow, lazy flame. When the piston approaches TDC with proper quench, it's like hitting the coffee can with a baseball bat. Instant explosion. Thus, you dont need to run a lot of spark advance. This lets you get away with higher compression ratios than you could if you had an open chamber (think Ford 400). With the less advanced spark advance, you have less negative torque losses during the piston's upstroke. You also get turbulence in the combustion chamber which keeps hotspots from causing problems. When you run larger than .050" quench, you effectively have an open chamber, and lose ALL the benefits. Might as well be running open chamber heads at that point. To go back to KB's coffee can example, it would be like hitting it with a pencil.

Edit: Take a look at this article. They explain it a lot better than I can.

 

Thanks Guys,
Makes me feel better about what I have spent money on. I can live with 9.2. The quench lesson was extra helpful.
Jim Nolan

 

Yes Quench is very key...alot of builder's dont even know about it...Go to your nearest garage an ask one of them to explain head quench to you..I bet they look at you with a "what" look on there face!! LOL..

And Matt, as you can see in my reply..I knew you were Joking... Mr. CR...LOL..

Russ