The Truth about 318s

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LXguy

LXguy

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OK:

I've been trying to perform a 360/5.9 upgrade to my 71 Dart to try to make Pinks All Out in Indy next month. It hasnt been going well.

Being faced with some garage time and no opportunity to proceed with my project, I decided to compare the figures in one of my Mopar hot-rodding books with what my 318 actually is. I measured the depth that the piston sits down in the bore at TDC and cc'd the stock cylinder heads. It was very difficult because even though this engine only has 31K miles, it seems like all the exhaust valves leak a little bit. My measurements aren't going to be laser accurate, but they're pretty darn close to reality.

Anyways. The book says the heads are 60cc. Truth: 68cc
Book says the piston will be in the hole .050 Truth: .094

Combined with a stock gasket thickness of .038, this makes my stock compression ratio........

7.87:1

Sheesh. I'm surprised the thing even runs!

It would probably make more power if I could find some 80 octane gas someplace. LOL!

Needless to say putting any kind of cam on that combination would probably lower the dynamic compression to some ridiculous level that would make it a pointless exercise..

Lot of people talk about building stock shortblock 318 combos. Just wanted to let you know what you're up against.

Steve
 
If I milled the heads .050 and used the skinny .28 head gaskets, I could get compression up to 8.8!

If the gasket would still seal it anyways.
 
Steve,solve all your problems with 1 phone call to Bobby Joseph at 1-770-443-8874,for all the small block needs,call him on monday,mrmopartech
 
That's old iron for ya. It's called tolerance stacking. Everything has a tolerance from the factory and when they all "stack" the wrong way, that's what you get. Ever wonder why some guy's car just beat the crap out of everything else bone stock? All the tolerances stacked in his favor. Yep, it happens both ways.

Nothing some machine work can't resolve. :)
 
My piston is .062 in the hole. I figured with 61cc magnum heads and a .028" head gasket, I'll have about 8.7:1. Oh, the PO had bored it .030 over. I ordered a cam before measuring the piston depth. I got too much cam, unless I mill the heads or get the block decked.
 
A lot, and I mean A LOT of 360's have 7.8:1 compression. Mine has the 8:1 pistons and as a result they sit pretty far down the hole. The magnum heads woke it up a lot since the chambers are more around ~62cc instead of 72-78cc.

With the low compression, it would want more ignition timing and have a little less low end torque. I have to run my initial timing between 10 and 12 degrees BTDC.
 
Yeah, I've got a .030 over 360 and its barely 9:1 with some aftermarket pistons. Also got a 5.9 Magnum and the pistons are down .050 on that as well.

I'm spoiled by the Ford stuff. They don't drop the pistons down in the hole like Mopar, so you can actually get some compression and quench without going to the machine shop.

Oh well, if it was easy and cheap, everybody would do it, right? LOL!
 
This is a dumb question, but why couldn't you use a domed piston, or a taller piston? Do the valves go down so far they would hit it?
C
 
Yeah, they make taller pistons.

From my experience, when you measure deck height, it ends up being too tall in stock form, so cutting the deck is normal when I build a SB. (No intake or valvetrain issues, because the deck was too tall to begin with)

Most I have seen, are .050 or so down, not too bad.
 
Anyways. The book says the heads are 60cc. Truth: 68cc
Click to expand...

This worries me - I thought even open chambered 318 heads were around 57-61cc?

68cc is open chamber 360 head territory - are you SURE the heads are original to the block?
 
Many of the "book numbers" appear to have been put where they are to help racers. The numbers can let you do a good deal of mods in the name of blueprinting, while race officials would have to conclude the mods done are still acceptable within stock categories.
 
Sounds like it is ready made for about a 150 shot of spray. It'll wake that mutha up like you won't believe.
 
Moparmal said:
This worries me - I thought even open chambered 318 heads were around 57-61cc?

68cc is open chamber 360 head territory - are you SURE the heads are original to the block?
Click to expand...

I'm pretty sure these are original to the block. the car only has 30K on it, and it was the classic little old lady car. The casting number is correct for a 71 318.

I've been hearing from the machine shops that the ratings for the 360s are just as far off. What's supposed to be a 68cc will actually measure out ot 74 or more cc.

Unless you want to shell out for machine work or pistons, seems like Mopar left us small block guys with one arm tied behind our back.
 
The best stock configuration 318s from a compression point of view are the 67-70 models with a rated compression of 9.2:1. After that, heads got larger chambers and pistons sunk further down into the block. The deviations from this trend were the 318 lean burn (302 heads) and the 318 cop car motor (360 heads).

The combination of a 68 piston and a 302 head that I have in the Demon gives an actual compression ratio of 9.3:1. Even at this compression ratio it is easy to "over cam" the engine, particularly if stock rear gears are used.

Comp Cams publishes a chart that recommends different camshafts based on compression ratio, vehicle weight, and rear gearing among its factors for consideration. I consulted it after building the engine for the Demon and determined I need more rear gear to get the best out of my engine set up.

Wiesco and Keith Black produce 318 pistons that will elevate compression from the low 9s to the low 10s depending on the combustion chamber cc used. Heads can be milled to decrease combustion chamber cc but sometimes introduces sealing problems with the block and/or intake.

At the end of the day, the 318 is what it is. More power is available for less with a 340, 360, or a big block. (It would be interesting to see how a built 360 compares with a 361 B motor.) The higher initial cost of the larger engine is more than offset by the cost of modifying the 318 to produce giant killer results.
 
The higher initial cost of the larger engine is more than offset by the cost of modifying the 318 to produce giant killer results.
Click to expand...

I agree in some ways with what you are saying - but I am a champion for the stroker 318 cause - and I think you have to consider the following points:

When you start with a 318 car , to go with a BB its not just the engine you have to change -

You have different headers, K frame, usually you have to change from a 904 to a 727, change the intake and the list goes on...

By the time you've done this, the xtra $2000 for some KB slugs, 360 rods, some ported cast 360 smogger heads and a 4" crank pales in comparison.

Iv'e heard a lot of reasons to build a 360 or 340 over a stroked teen, and I agree with a lot of them - but I don't reckon the same argument for a Big block conversion holds water.
 
Our 67 Dart has a 1970 318 that runs like a scalded dog, that someone else installed. #GM 318 R. Does anyone have any insight on the stamped numbers on the block? Was this a special engine from 1970?
 
No clue, they did have a "premium" 318, special valves, rings, bigger cam, but the compression was lower. All my 318's ran good, run good, and will continue to run good. Even .060 in the hole, they run better than the motors of comparable size, 305/302, that is.

The power comparison, for the street, is apples to oranges between the 390, and the 360. 390 runs better than a 383/400 on the low. I just wouldn't spin a 390 7500RPM.
 
72 318 .30
Cam: Hyd-267/276 Duration @ 429/444 lift
Windage tray
Ld4b intake
650 Edelbrock Cam
4027596 Heads milded porting 1:88/1:60 valves
360 Exhaust manifolds
2.5 pipes 18 inch glass packs
This is in a 64 Valiant with a OD manual with a 3000lbs kevlar 10.5 clutch.
I had the car on a Chassis dyno
200-225 hp
300-315 lbs torque
I love 318s, disadvantages, or advantages....
I put this motor together with a limited budget and not max hp in mind. 10 years later and god knows how many mile on the road and down the strip, it is still running strong......I will put some spitfire headers on the car in the future. I built the motor with reliability and it has not let me down, now I need to get rid of the 7 1/4 2:76 geared rearend.........
 
64dart170 said:
Our 67 Dart has a 1970 318 that runs like a scalded dog, that someone else installed. #GM 318 R. Does anyone have any insight on the stamped numbers on the block? Was this a special engine from 1970?
Click to expand...

the R blocks came out in 89 as a replacement for racers on the right bank it should have an R cast into it right behind the alternator just below the deck and next time you have the timing chain cover off check the water inlets if they are round its just a plain jane block but if they are half moons you really have a pricey item on your hands
 
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