360 pistons 318 block

[Odilon_df]

Hi guys,

These may sound stupid but I want to build a low compression 318 tu be used with a supercharger.. So.. .7 - 8.5:1 will be OK.
I was looking at summit to copression height of 318 pistons .. average 1.84" th 360 pistos are 1.625" . Imagining the 318 is zero deck with 1.84 (yes I can cut the block) Wich compression may I have with 360 pistons???

Will this work ?

I want to sleeve de 318 block to be 4"

[345Dart]

7.80 with 70cc heads and .040 hd gasket

[451Mopar]

I came up with 6.71:1 compression ratio.

Specs:
Bore = 4.00"
Stroke = 3.31" (318 crank)
Rod = 6.123"
Block Height = 9.600"
Piston Compression Height = 1.625" (as stated)
Valve relief volume = 0cc
Head Gasket = 8.77 cc (estimate for a 0.040" head gasket)
Cylinder head volume = 70cc

Piston deck height = 0.197" below the block deck
Cylinder swept volume = 41.59 cid (total cid = 332.76)
Volume above piston = 119.35cc or 7.283 cid

If you mill the block 0.020" (about zero deck with the 318 pistons), and use a 0.020" head gasket, the compression come up to 7.14:1
with a 65 cc head and the milling/thin gasket, you get 7.43:1

On the other hand a stock 360 with a 0.040" head gasket is pretty close to what you want, but with more displacement.

OK, for cheap, use the Silvo-lite replacement 318 piston (SL-1266) with a compression height of 1.741", then use 360 cylinder heads (about 70cc). With a stock 0.020" head gasket your about 8.05:1 compression. With the 0.040" head gasket, compression is 7.73:1

[Locomotion]

I never heard of it being done. But boring a 318 +.090" would cause problems with the cylinder walls and ring seal. Boring it even more to fit sleeves with a 4.00" bore would likely break into the water jackets and/or offer little support for the sleeves. It could also affect the deck surface and head gasket seal.

There are stroker pistons for a 318 that could also be used to lower the compression.
But considering your location, I understand the need to improvise.

[DartGT66]

In reality, the 318 pistons are propably way below the deck to start with. Propably in the .090" range. It's pretty much possible, that your CR is already in the 7's with typical composite head gaskets, so do the measuring and math first.

BTW, we have bored a 318 block for 340 pistons, 4.04" bore. It worked, but the cylinderwalls were very thin. If you plan to supercharge it and achieve high cylinder pressures, you are propably looking for trouble.

[Odilon_df]

Hi...

I liked the 451MOPAR tips... looks good... Thanks all.

[it's all dodge]

the 318 pistins had compression heights of 1.740 to 1.770, with a 9.599 deck, so they are .051 to .081" in the hole, I found that out the hard way, as once a machine shop tried to get me some pistons for a 69 318, and I had somebody else doing the heads, the replacement pistins where 1.720, .101" in the hole, that was about 8.2 to 1 with the Mopar Performance orange head gaskit, .024 - .027" thickness, with I think 68 cc head. And that was no good for sea leval with a slightly bigger cam, and worse off at 6000 feet altitude! 1.821" comp. height would put a 318 pistin at zero deck, w/undecked block Most 318s are 8.4 to 8.7 to 1 ratio, and thats with a thin head gaskit. DART GT66, what year 318 block did you bore to 4.04? Was it sonic checked, what was the minimum thickness, and I wonder what min. is recommended? I have a 68 318 block like that, heard it from a Dodge IMCA modified racer (with a few championships) that it could be run that way, only using the early 318 blocks 340s had comp height of 1.84 for the early ones and 1.74 for 72 and 73,

[gomopar]

Been there, done that. I've also had a 318 bored for a 340 pistons. But I didn't even check what year the block was, neither got it sonic testet. Though it was running pretty strong produsing estimated 380 hp with ported 360 heads, 284 mopar hydraulic cam, Edel Performer, 750 vac Holley. Never had issues with cracking cylinder walls.

[Crank]

i would just put 360 heads on a 318 and you will be around 7.5 to one.

[DartGT66]

The most important thing is to not assume anything. Measure first the deck height, chamber volumes etc before buying any parts. Or do you have used pistons somewhere, that you are going to use no matter what? All the work needed, sleeving, decking, correcting lifter preloads etc. seems quite a lot of work to get a thin wall 334 with cast pistons. But of course this is a hobby, and man does what needs to be done.

[Odilon_df]

I Agree that I will have small gains in displacement.
Brazilian 318 was hevier castings until 1973 and I want let you know that we spend only $50 for a complete backyard engine... But it´s the only engine we have in Brasil... No problem on breakin some block (I still owns around 25 - my threasure)
It´s impossible to find 360 heads in BRasil

It´s VEry difficult to find something bigger than a 318 here... I found a ford 351C- 4V some years ago ... but I really don´t like fords (the connectin rods looks like pushrods to me)

The only way we have is try to over something in a 318.....
The Brasilian good thinks on the 318 are the heads.... small runners like all 318 but, since they was made to run on ethanol they are onli 34cc...
Thanks all .... I will appreciate more suggesntions

[pishta]

351C-4V?!? about 2x the HP of a 318 out of the box. Just run it above 4000 RPM at all times. Connecting rods will hold. What about fly cutting the piston tops, then you can get almost any compression you want considering the meat on top of the piston. Or even offset grinding the rod journals, or bushing the little end lower on the rod. CC those before cutting, all the formulation you get out of a book you can toss, those are blueprint numbers and we all knowhow accurate those are.