360/390 questions
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Tuned
Joined: Aug 2003
Posts: 346
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From: Eastern Virginia
360/390 questions
Ok, i've just got a few more questions here. I'm converting my '75 360 to a 390. I'm using C6AE-R heads which i've been told have a bigger combustion chamber (68cc). I've found out that I can use my 360 pistons to boost compression. I just read a post that says I can also use my 360 rods. How much longer are the 360 rods? Will this work? What will my compression be? I would like to have around 9.5 to 1. Will this require 93 octane? I'm just trying to find as much info as I can so I will only have to do this thing once.
The block will not have to be bored and the pistons and rods from the 360 look good. Should I use them or will the compression be so high it will blow the head through the garage ceiling when I fire it up?
I will be running a C5 intake with ports that match the heads, a Crane 901, a Holley 4160 4 barrell.
The block will not have to be bored and the pistons and rods from the 360 look good. Should I use them or will the compression be so high it will blow the head through the garage ceiling when I fire it up?
I will be running a C5 intake with ports that match the heads, a Crane 901, a Holley 4160 4 barrell.
Post Fiend
Joined: Jan 2003
Posts: 8,600
Likes: 4
From: Iowa
Well, I hate to burst your bubble, but you cant run both the 360 pistons and the 360 rods. You'll wind up with the pistons 0.036" above the deck. That aint good. 
You can substitute 390 pistons, with their 1.680" compression height though. That would put you 0.060" below the deck, which would be ok.
Or you can do what I would do, which is to substitute a 390 crank and rods, and keep the 360 pistons. This would put you .016 below the deck, for more compression.
According to my book, your C6AE-R heads have 71.2-74.2cc combustion chambers.
Figuring 72.7cc chambers, and a 4.375" bore .020" thick head gasket, and 2cc's for the valve pockets in the pistons, here are the compression numbers you should get. The first example, where you use the 360 rods and 390 pistons, would get you 9.65:1 compression. The second example, where you use 390 rods and the 360 pistons, you would get 10.62:1 compression.
The crane cams website recommends a maximum compression ratio of 9.5:1 for the 901 cam. I would recommend Crane 343941 for this setup, since it can handle compression ratios up to 10.75. With the 941 and 9.65 compression, I dont think you'll need premium gas. With 10.62, you probably will.
Good luck!
You can substitute 390 pistons, with their 1.680" compression height though. That would put you 0.060" below the deck, which would be ok.
Or you can do what I would do, which is to substitute a 390 crank and rods, and keep the 360 pistons. This would put you .016 below the deck, for more compression.
According to my book, your C6AE-R heads have 71.2-74.2cc combustion chambers.
Figuring 72.7cc chambers, and a 4.375" bore .020" thick head gasket, and 2cc's for the valve pockets in the pistons, here are the compression numbers you should get. The first example, where you use the 360 rods and 390 pistons, would get you 9.65:1 compression. The second example, where you use 390 rods and the 360 pistons, you would get 10.62:1 compression.
The crane cams website recommends a maximum compression ratio of 9.5:1 for the 901 cam. I would recommend Crane 343941 for this setup, since it can handle compression ratios up to 10.75. With the 941 and 9.65 compression, I dont think you'll need premium gas. With 10.62, you probably will.
Good luck!
Postmaster
Joined: Nov 2002
Posts: 3,447
Likes: 1
From: Littleton, CO
If I may as well... You will need to have that 360 block at least honed out to compensate for where the pistions use to stop at the top and built a ridge. There are also a few other things but consider this, everything that seems to work on paper does not always turn out that way in practice. What I mean is that the formula is a good one, but when you start putting the parts together, double check everything. I have built on paper and found when it was combined, things didn't go as planned and you may have to adjust here and there.
You do have the larger volume heads and if you change over to .040 Felpro gaskets, your CR will come down a little.
--Mike
You do have the larger volume heads and if you change over to .040 Felpro gaskets, your CR will come down a little.
--Mike
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Tuned
Joined: Aug 2003
Posts: 346
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From: Eastern Virginia
Thanks for the great replies! I was looking in a parts catalog today and I noticed 360 pistons (C6SZ-6108-A) were also listed for 66-70 390 4 barrels. I think I am going with the 390 crank and rods and 360 pistons. What was the compression of a 66-70 390 4 barrel?
The block, heads, and intake are at the machine shop and he is going to hone the cylinders. He said that it will not need to be bored. It didn't have much of a ridge, but it will be removed.
This is my first non-stock rebuild and I wanted to get as much info as I can. I'd rather have my bubbles busted now rather than after I get the monster together and back into the truck. Thanks again for the replies.
The block, heads, and intake are at the machine shop and he is going to hone the cylinders. He said that it will not need to be bored. It didn't have much of a ridge, but it will be removed.
This is my first non-stock rebuild and I wanted to get as much info as I can. I'd rather have my bubbles busted now rather than after I get the monster together and back into the truck. Thanks again for the replies.
Elder User
Joined: Sep 2002
Posts: 622
Likes: 0
From: Urbandale, Iowa
Originally Posted by rusty70f100
Figuring 72.7cc chambers, and a 4.375" bore .020" thick head gasket, and 2cc's for the valve pockets in the pistons,
4.375 bore???? That's gotta be a typo.... 360/390 blocks are 4.050 standard and some -- repeat some --- will take a .080 overbore to 428 size, which would be 4.13.....
Post Fiend
Joined: Jan 2003
Posts: 8,600
Likes: 4
From: Iowa
No typo. I was talking about the bore of the head gasket, though. Of course, I used 4.050 for the cylinder bore. I measured some head gaskets I've got out in the garage for my 390 I'm working on. The average diameter I came up with was 4.375. Double checked my measurements, but there was no error there. These were old Fel-pro blue gaskets bought sometime in the late 1980's. My theory is that they were made with the largest possible FE bore in mind, to consolidate part numbers. 
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Posting Guru
Joined: Nov 2003
Posts: 1,888
Likes: 1
From: Iowa
i figured you wernt talking about the actual block bore, i measured the bore of my mr. gasket head gaskets and i came up with 4.325 so not to much difference.
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