My new 360

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Nick M.

Nick M.

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Just picked up a 360, complete carb to oil pan. Casting # 4045601-360 dated 8-11-76. Did some teardow to see what i had to work with, as expected it could use an overbore (currently std dished pistons) as it has a ridge at the top of the bore. Took some crude measurements of deck hieght (feeler gauge and straight edge) and found the piston to be 0.093" in the hole! I know this is not the most accurate method of measurement but wow, is this the common for this era 360?
I plan to use KB-107's on my tight budget rebuild and was hoping to avoid the added expense of zero decking the block, but i am afriad my comp ratio will still be low even with the new pistons. I will be re-using the stock 915-A cyl heads that came on the motor. What can I expect (ballpark) for a compression ratio with an uncut deck? Any ideas how much it costs to zero deck a block? >
 
Yes, that's normal... and why you read about recommending the stock pistons in 318s and 360s (and most 340s..lol) be tossed and replaced as a first step... If you really want to see where reality is, cc the head cahmbers and figure the static compresion ratio... If it's any higher than 7.2 or 7.3 I'll be pleasantly surprised...
 
I'm in the same boat, Nick M. I'm tearing down a '77 360 now and I also want to end up with more compression. I'll be looking for at least 9:1. If the block and heads check out OK (and I think they will) my crank and connecting rods are staying but will be balanced, the block will be square decked, cylinders honed but only bored if needed, new pistons, bearings, seals, etc., heads will be stock but resurfaced, with a valve job and some bowl work, new cam, springs, lifters, push rods, maybe some new valves...I'm looking for a machine shop now and have some very good leads. Please PM me if you have some references and if you would like my references. I'm just north of you in Murrieta.
Thanks,
Forrest
 
Nick my 79 360 was about the same stock. I used KB107's and didn't deck the block and they ended up at .018~.020 in the hole. With 67 cc chambers if I remember right the compression figured out to be about 9.8 to 1
 
Fishy, what would it be if the head cc's were 72 and a .039 X 4.1 gasket were used? (Approx bore of the Fel-Pro gasket)
 
Thanks everyone for the info, finishing my teardown this week hope to have the block crank and heads at the machine shop by the end of this week. I will be using Speed Specialties in San Diego have used them before with no problems.
 
rumblefish360 said:
Fishy, what would it be if the head cc's were 72 and a .039 X 4.1 gasket were used? (Approx bore of the Fel-Pro gasket)
Click to expand...

Rumble I came up with 9.32 on my calculator. I used 4.18 for the gasket bore though cause that's what fel-pro's are if I'm not mistaken. If they are 4.1 it isn't much diff. at 9.38
 
Great info on the calculated comp ratios. Sounds like i will be fine with the stock deck, now if i can just find someone who has the thin 0.028 mr. gasket headgaskets in stock. How much compression can you get away with on our crappy california 91 octane anyway.
 
Mopar has head .028-.024 head gaskets available thru the dealer and at a reasonable price. Then again, it was early 2008 when I got mine.
 
Nick M. said:
Great info on the calculated comp ratios. Sounds like i will be fine with the stock deck, now if i can just find someone who has the thin 0.028 mr. gasket headgaskets in stock. How much compression can you get away with on our crappy california 91 octane anyway.
Click to expand...

I you use Mr.Gasket brand make sure it's the top line gaskets. I used their cheap ones and they were pretty crappy. Even though I checked the block with a straight edge and it was only .0015 off in the middle and the heads were freshly planed and I used new Mopar head bolts one of them wouldn't seal up. I don't think I'd go over 9.4~9.5 to 1 max on 91 octane. My 360 is 9.8 to 1 and it has to have 93 so it won't spark knock running just 33 degrees of timing. That said I think you'll be better off using the standard Fel-pro's cause if you use the thin gaskets you'll be way up in the 9's on compression. Probably too much for 91 octane with open chambered heads.
 
thanks, i wish i could get 93 here, I will keep it in the low 9's then. Should have info on the condition of the block etc.. first part of this week.
 
Ok so I was playing with the compression ratio calc (above link) and the numbers I am getting are way off, which of the numbers I am entering are wrong? Bore 4.030 Stroke 3.58 HG bore 4.18 HG thikness 0.039 Comb chamber 72cc Piston dome volume 5.00cc (kb-107) Piston deck clearance 0.020
This gives a calculated ratio of 10.359:1. What is the cc of a smog era 360 head? What is the compression hight of a stock 360 piston?
Thanks, Nick
 
Nick the bore is 4.03 at .030 over and the stroke is 3.58 for a stock 360. The std. Fel-pro HG thickness is approx. .039 and the HG bore is approx 4.18. 72 cc's is pretty close for a stock head chamber. A stock 360 piston ranges from about .060 to .080 in the hole. Most are closer to .080 in the hole. The KB pistons usually end up about .015-.025 in the hole depending on the block height so the .020 # you used will be close but it is not a dome piston. It's a flat top with reliefs which are 5 cc's so you need to enter than number as -5. Using the above #'s I come up with 9.32 to 1 with the KB pistons. With stock flat tops it's only about 8.3 to 1 figuring the pistons at .08 in the hole.
 
That must be where my error was, works out just as you said with -5.0cc piston dome volume. I could swear both summit and the keith black website lists them at +5.00 piston dome volume which really doesn't make sense for a flat top piston.
 
Nick - Thats so when its entered in the KB United Engine calculator its actually a "subtraction" field for the software program - so it "expects" you to enter a + figure.

The idea is it subtracts the piston dome from the total volume of the chamber.

If you happen to use that calc - just enter the dome volume as a + figure...and the calc will automatically subtract that from the total volume.

Beware - If the dome fig is entered as a "minus" , the calc will assume its a "dished" slug - and subtract the minus figure - which of course = +.......and it will add the 5cc to the chamber volume - and your comp will read much lower than it should.

Its nuts but thats the reason.
 
Now it all makes sense, pistons with a dome are described as having a negative volume, which is correct, the dome subtracts from the total volume of the combustion chamber. Think i might have been confused by the above listed comp calc as it requires you to enter - in front of your piston volume to indicate a depression in the head of the piston
 
Think i might have been confused by the above listed comp calc as it requires you to enter - in front of your piston volume to indicate a depression in the head of the piston
Click to expand...

Thats because you are calculating available volume, and subtracting two negatives makes a + or increase in volume.
 
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