360 compression
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Freshman User
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360 compression
where is most of the compression lost in the 360's....and how much cid. would i gain by putting 390 heads.....
i just put ported and polished heads with all new lifters and rocker arms and valves and pushrods etc...
and another question......my truck ticks and it sounds like it is coming from the rear #4 cyl....and i thing its b/c it has sh!%%y oil that had some antifreeze in it but im going to change it tonight and see of thatis it......i just put it in like a week ago and i drove it for the first time yesterday #3 engine change ive done (i did the first when i was 15 and this one im 16) and i did them all by my self it sucks!
thanks
CMD
i just put ported and polished heads with all new lifters and rocker arms and valves and pushrods etc...
and another question......my truck ticks and it sounds like it is coming from the rear #4 cyl....and i thing its b/c it has sh!%%y oil that had some antifreeze in it but im going to change it tonight and see of thatis it......i just put it in like a week ago and i drove it for the first time yesterday #3 engine change ive done (i did the first when i was 15 and this one im 16) and i did them all by my self it sucks!
thanks
CMD
Post Fiend
Joined: Jan 2003
Posts: 8,600
Likes: 4
From: Iowa
You would gain 0 cubic inches by swapping heads. Heads do not affect cubic inches. In fact, you may actually lose some compression, which in a 360 is already at a very low level. You'd be better off by getting a 390 crank and rods, swapping them out for the 360 crank and rods, and running your 360 pistons. Then you'll have PLENTY of compression.
For that lifter tick, change your oil. Then take it out and drive it, and get it plenty hot. If that doesn't fix it you need to put a new lifter in. Before you do that though, pop the valve covers off and check and make sure you dont have any collapsed lifters. Do this by pushing down on each pushrod. If any of 'em go down, that corresponding lifter is collapsed and needs to be replaced.
It may suck, but at least you're learning.
Edit: 360's have low compression because the piston sits too far below the deck, which is to say the compression height is too low.
For that lifter tick, change your oil. Then take it out and drive it, and get it plenty hot. If that doesn't fix it you need to put a new lifter in. Before you do that though, pop the valve covers off and check and make sure you dont have any collapsed lifters. Do this by pushing down on each pushrod. If any of 'em go down, that corresponding lifter is collapsed and needs to be replaced.
It may suck, but at least you're learning.
Edit: 360's have low compression because the piston sits too far below the deck, which is to say the compression height is too low.
Last edited by rusty70f100; Dec 5, 2004 at 08:46 PM.
Post Fiend
Joined: Jan 2003
Posts: 8,600
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From: Iowa
Yes, it will move them higher into the cylinder. Heres why.
360 rods are 6.54" long.
390 rods are 6.488" long.
360 pistons have a 1.776" compression height, while 390 truck pistons have a 1.680" compression height. 390 car pistons have a 1.776" compression height. --> Notice how the 390 car pistons and 360 pistons have the same compression height? That's because they ARE the same! My old 360 pistons even had 390 4V right on 'em.
The 360 crank throw is 1.75" which is half of the 3.5" stroke. 390 crank throw is 1.892" which is half of the 3.784" stroke.
FE deck height is 10.17".
So the 360 deck clearance is 10.17-1.75-6.54-1.776, or .104". 390 truck deck clearance is 10.17-1.892-6.488-1.680, or .11". 390 car deck clearance is 10.17-1.892-6.488-1.776, or .014". See, big difference! And that also makes a big difference in compression ratio.
360 rods are 6.54" long.
390 rods are 6.488" long.
360 pistons have a 1.776" compression height, while 390 truck pistons have a 1.680" compression height. 390 car pistons have a 1.776" compression height. --> Notice how the 390 car pistons and 360 pistons have the same compression height? That's because they ARE the same! My old 360 pistons even had 390 4V right on 'em.
The 360 crank throw is 1.75" which is half of the 3.5" stroke. 390 crank throw is 1.892" which is half of the 3.784" stroke.
FE deck height is 10.17".
So the 360 deck clearance is 10.17-1.75-6.54-1.776, or .104". 390 truck deck clearance is 10.17-1.892-6.488-1.680, or .11". 390 car deck clearance is 10.17-1.892-6.488-1.776, or .014". See, big difference! And that also makes a big difference in compression ratio.
Elder User
Joined: Nov 2003
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What if he had the heads milled a little bit and used steel shim type head gaskets (assuming that he is using the thicker composite type) and then maybe used some total-seal gapless rings. When a magazine (can't remeber which one) used total seal rings in a small-block chevy, they gained 15 psi cranking pressure.
Post Fiend
Joined: Jan 2003
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From: Iowa
Ok we'll say he took 5 cc's off by milling the heads.
Stock, with a .043" thick head gasket, 70cc chambers, 5cc valve reliefs, and .104" below deck, he's looking at 7.87 to 1 compression.
Now lets take that down to a .020" thick shim head gasket, and shave the head a little and take 5cc's off there. Now we're up to 8.63 to 1 compression. A little better, but still not up to where you can use a big enough cam to make good power. And even if the compression was up to where it needs to be, look at the quench distance! You've got .104" to start with, and then .020" minimum on the gasket. That's triple what the ideal is, of .040". If the compression was up on this motor somehow (more head milling) you'd have an extremely hard time making it not ping.
What the 360 needs is better pistons, and right now they dont exist outside of custom parts.
Stock, with a .043" thick head gasket, 70cc chambers, 5cc valve reliefs, and .104" below deck, he's looking at 7.87 to 1 compression.
Now lets take that down to a .020" thick shim head gasket, and shave the head a little and take 5cc's off there. Now we're up to 8.63 to 1 compression. A little better, but still not up to where you can use a big enough cam to make good power. And even if the compression was up to where it needs to be, look at the quench distance! You've got .104" to start with, and then .020" minimum on the gasket. That's triple what the ideal is, of .040". If the compression was up on this motor somehow (more head milling) you'd have an extremely hard time making it not ping.
What the 360 needs is better pistons, and right now they dont exist outside of custom parts.
Elder User
Joined: Feb 2004
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From: Fort Rucker, AL
I've been running numbers at United Engine and Machine Shop's compression calculator page. My heads are shaved .0020, they are C8/C7 so about 70cc. I use the steel shim gasket .021 thick, and using 7cc for flattop dished pistons (which I will get soon). I get about 8.7:1. How in the heck do you guys get 9.5-10:1 CR out of a 390???
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Post Fiend
Joined: Jan 2003
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From: Iowa
We use flat top w/ valve relief pistons, with a compression height of 1.776". And that dish seems small, after all valve reliefs on flat tops take up 5cc's. OBTW, if their flat top pistons they're not dished. Check that you've calculated deck clearance correctly. You're sure you didn't have .020" taken off the heads, .002" hardly seems like it'd do anything.
Recheck your calculations.
Recheck your calculations.
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Elder User
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From: Fort Rucker, AL
Thanks Rusty, yeah too many zero's. It was .020 off the heads. I know about the flattop vs. dished. I have dished now. ........oh oh, I just reread my post. Yeah, I mistyped. So flattop dished is about 5cc. When I order them and the rods, I'll specify the compression height I want unless they come that way.
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what compression would i most likely have.....the heads are out
of a 69' fairlaine 390gt and my dad had them ported and polished and milled and all that good stuff and pistons crank and rods are stock...the engine is all but new it only has like 10k-20thousand miles on it
and im running a 2bbl holley on it with the intake all bored out (whatever you call it when they smoothe out all the corners and polish it) any ideya on horsepower??
i really appreciate all the help.....and i changed the oil and the ticking went away it was a stuck valve i think
CMD
of a 69' fairlaine 390gt and my dad had them ported and polished and milled and all that good stuff and pistons crank and rods are stock...the engine is all but new it only has like 10k-20thousand miles on it
and im running a 2bbl holley on it with the intake all bored out (whatever you call it when they smoothe out all the corners and polish it) any ideya on horsepower??
i really appreciate all the help.....and i changed the oil and the ticking went away it was a stuck valve i think
CMD
Fleet Mechanic
Joined: May 2004
Posts: 1,436
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From: Montana
You need a seet of C3ae heads from a 390...they have a 64 to 67cc chamber and that would atleast help in the comp department. If its an f-250 4x4 a 390 conversion would be best but if its 2wd A 360 can muster up some fun.....Mine would smoke most 351w f-150's, it was stock with a 2bbl and headersn duals. I can imagine if i had justjut a set of worked over c3ae heads on it to raise the cr It would probobly be a peppy little motor obviously nothing on par with a 390 but still good most sport trucks of today only push 240-260 horse. with 8.5:1 compression. My truck performed well because it was a manual tranny The C6 is a Boat anchor with a very slow front drum speed and lots of rotating mass, Hence..."slush Box". A C6 Will tame most motors which is why in the mustang world people try to convert a c4 to fit a bigblock. 
Post Fiend
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From: Iowa
Originally Posted by Annihilator
I though that a stock factory 360 came with a compression ratio around 8.6:1 and at the lowest 8.0:1???