Got some cheap low-mileage magnum heads, going to slap them on a 318 in my 1970 Satellite 2-door.

I know about the swap and it seems pretty straightforward - special length pushrods with oil passages look to be the biggest thing I need to worry about - but the cam choice is puzzling.

The regular 318 rocker arm ratio is 5:1, but the Magnum head rocker arm ratio is 6:1, so all of the advertised 318/340/360 cams will lift a higher than advertised specs.

I assume I can't use magnum cams because I'm using regular hydraulic lifters and not the roller lifters magnums came with... I probably wouldn't want too anyway because they're really expensive.

I'm looking for a regular small-block hydraulic lifter LA cam that will take this motor to the 300 HP range, maybe a tad more.

It's an automatic car, BTW, doesn't have a high-stall converter yet but that's part of the plan.

I'm considering the Comp Cams XE256H...

Any cam suggestions by people in the know?

Thanks!

- Don  

With stock 360 mag heads I'm thinking the 260*-268*w/1.6 fac ratio=.488
adjustable rockers would be nice too [with what ever cam you chose].
No offense but,
I'm not joking when I say I'd rather take your 318 heads & shave, big valve, and port
them then do what your talking about doing.  

I hear you. I don't have the time or money to properly play with some 302 casting heads, so I'm going the bolt on route. For me this 318 is kind of a toy to play with but not invest too much into, I just want to see what it can do. If it pops I have other plans for the car.

The XE256H is 256/268 duration and 447/455 lift; that's the advertised spec with the 5:1 factory ratio though, I'm not sure how to calculate the 6:1 rocker ratio that will bump it up a bit, but it looks like it might be a decent choice.

Thanks 3404spd!  

Right on, been there.
good luck.  

the magnum heads are a very good choice. there are a few things you must consider. they are a bolt on but you need quite a bit of parts to make em work. an intake manifold with the magnum bolt pattern with magnum gaskets. the hollow pushrods with AMC lifters or other lifters with provisions for the oil through pushrods. Magnum head gaskets. Magnum head bolts. stiffer valve springs and better retainers. magnum valve covers. you can't run much cam lift/duration with the stock valvesprings. I agree with the suggested cam selection since a dual pattern/split duration cam works best with magnum heads. You need more lift and duration on the exhaust with magnums. Another thing is that magnums are prone to cracking in between the valves in the valve seats and the only way to check thoroughly is to remove the valves. the factory did not design the magnums with much casting material in the valve seat area. To run a decent cam you will have to change to high performance springs anyway, you might as well take these heads to a machine shop, have them dissasembled, cleaned, and checked out(especially the valve seat area). Or you could just spend all that money on parts to install your magnum heads, simply bolt them on a run a very small cam and not know if your heads check out all right. On a positive note, Magnums outflow X heads. Magnums will increase your compression on a 318.  

Another thing is and I'm trying to remember so don't quote me on this.  Around .510 lift on a 318 with stock pistons the valve will hit the pistons. A big plus with the magnums is that you don't have to run insane cam duration to make decent power because of the 1.6 ratio. It will make a cam with .450/.455 lift about a .480/.485 lift. So keep your cam about 218/222 duration at .050.  

Actually magnums heads have a chamber volume of around 60-62cc while 318 heads have a chamber volume of around 60-63cc so the compression increase is BS.

No they don't flow more than x heads hell stock mag heads flow 190 or so cfm  while stock  xheads flow any where from 199 to 219cfm if you do big valves then mags can get in the range of xheads or so, of corse mags have smaller stems hence any flow increase over xheads, so put small stems in your xheads and we still have a winner!  

The comparison was magnum compression to x head compression, not magnum to stock 318. Magnums are a 60cc closed chamber and X and J heads and later model 318 heads are a 70cc. open chamber. You will have about one full point of compression increase with magnums over LA's. Check out HUGHES engines.com for flow numbers for X heads, magnums, magnum R/T's, and their new Iron Ram magnum head. Also magnums have no heat crossovers which is another +. On a side note, I have a 67 Coronet with a 318 with 57cc heads since the factory used 273 heads for the first year of the LA 318.  Stock Magnums make more HP than any stock LA head and that's what matters. Or maybe the Dodge Motorsports engineer who wrote "How to build Big-inch Mopar small blocks'' is wrong.  

I did more research in "How to rebuild you small block Mopar" and checked their cylinder head page.      
68-74 318 60cc
68-71 340 63cc
70 TA 340 65cc  

I didn't mention the 318 '234' head cause it obviously  has a smaller chamber than all of the production heads & besides the 1 yr all 273's have a different intake bolt angle and don't apply [my point was made].
I don't search the net for info, I flow test heads instead, so I know what flows what.
I also know that every bench varies in results with the same head, so f what hughes says, thats their bench which I've already seen a 19cfm difference on when comparing a certain after market head to it's advertised #'s from the manufacturer.
I haven't read 'how to build big inch small blocks' [And I build big inch small blocks] probably like a lot of people who also have built 'big inch small blocks'.
I tell you what,
If you like to read and sharpen your wit then read speed secrets by smokey yunick.  
You'll thank me. good luck  

Hey fellas!

Just wanted to point out the cheap heads I'm getting are very low mileage off of a 320HP 5.9l crate motor; the beauty for me is that they already have stiffer valve springs, and the mileage is low enough that I'm not going to have them reworked. Yes, it's dirty and loathsome what I'm doing, but like I said - cheap and easy is the order of the day.

I know about the oil-through lifters and pushrods, the biggest pain in the ass expense is the intake manifold which you can't really find cheep in a 4bbl form...

As far as the chamber volumes I'm not going to sweat it too much, I just don't want to lose that much compression if I can help it. I'm even OK with something around the stock 8.5:1 because I don't really know what shape the bottom end is in; Some of the cylinders tested with piss-poor compression but compression didn't increase at all after a few squirts of oil in the cylinder, leading me to believe (fingers crossed) the problem's in the head or gasket and that my pistons/rings should be OK.

I removed the old heads today and didn't see anything in the cylinders that would make me worry, I know that doesn't count for much but it's enough to keep forging ahead with this low buck/low effort hotrod attempt.

So my plan is to pick up the heads, custom pushrods, and magnum head gasket, put them on the motor, and check compression again; if it looks good I'll invest in the cam/lifyers/intake/4bbl carb, if it doesn't I'm out the cost of head gaskets but I still have a cheap set of 320HP crate motor magnum heads which I can sell... or maybe I'll pay to have the block rebuilt and do the whole thing properly with zero-deck pistons. We'll see.  

Thanks gents!  

Just an update:

Turns out the correct number to multiply to get the correct lift from an advertised 5:1 rocker arm to convert to a 6:1 rocker arm is 1.067

This in mind, and also considering the engine's probably going to have close to the stock 8.5:1 compression ratio with the magnum heads, I think my cam choice will be the cheapo SUM-6901 cam from summit: 276/286 dur, 441/441 lift... with the 6:1 rocker arms in the magnum heads that lift becomes 470 - pretty ideal considering all the factors.

Dulcich coaxed 300HP from a bone-stock 318 long block with an XE262H cam 262/270 duration and 262/270 lift. I think the magnum heads should allow me to get between 300 and 350hp pretty easily with a similar setup.  

hey cleeve ive already done this swap magnum heads and all i ran the comp cam 280h with keith black heads and a zero deck block im dynoing at 400 hp i built my engine based on the article in mopar muscle magazine heres the link very informitive http://www.moparmusclemagazine.com/techarticles/mopp_0409_318_engine_build/index.html and check out my cardomain  http://cardomain.com/ride/3125670  

if you got it apart you should look into the keith black 167 pistons well worth the money and will increase your compression  

mr_horsepower:
if you got it apart you should look into the keith black 167 pistons well worth the money and will increase your compression

Well, the short block is still in the car.

I'm putting the magnum heads on the block, and if a compression test doesn't turn out particularly well I will abandon my low-cost effort and get the short block redone ground-up with new Keith Black pistons.

If compression is OK though, I'll be throwing in a Cam, roller chain, magnum 4bbl intake and carb and I'll call it a day!