318 rebuild for turbocharging

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MopaR&D

MopaR&D

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So the original 318 short block out of my '70 Duster is sitting in my garage and for a while I had toyed with the idea of doing a turbo build. It seemed like a far-off dream until I finally got a steady income; now I think I can start putting together a basic plan and it won't just be a pipe-dream (hopefully).

What I'd like is 550-600 HP ideally from one of the kits a fellow FABO member has posted about making or something similar. The 318 needs a full rebuild (rings are gone, smoked like crazy) but I'd like to keep it on a budget, I've read the stock cranks are pretty strong but should I keep an eye out for an old forged one? I do intend to upgrade the connecting rods and put in some affordable forged pistons with rings gapped for boost of course, target 9:1 comp max (I'm at high altitude ~5000 ft.). I'm also aware the Mopar SB heads don't clamp as tight with 4 bolts per cylinder so I'll probably go with head studs and good head gaskets. For the top end I might take the stock Magnums off my 360 and have them gone through (VJ, guides etc.) and use those on the 318, at the same time I upgrade the heads for my 360. The biggest expense (after turbo setup of course) will be fueling though as I fully intend to go EFI, I need something that can adapt on the fly to changes in altitude.

I just want a strong reliable engine that is built to handle decent boost and last while doing it.
 
Well, my 360 was just built into a 408; however, prior to the rebuild it had a supercharger on it. It was completely stock, cast internals, and only had ported J heads and a Mopar M1 intake. I know at first the original owner of the engine was running 7-8 lbs of boost but swapped it down to 4-5 lbs. When the engine was taken apart, I believe one of the pistons had cracked. I definitely wouldn't cheap out on the bottom end if you want to add boost.
 
For sure, with a 300-350 HP (at sea level, if I'm lucky) 318 I'd need at least 10 lbs to hit 550, plus a couple on top of that to make up for the lower air density. I'm definitely going with forged pistons that's a no-brainer for me. Another aspect is the cam i'd like to run a roller for longevity but the cost to convert is too much for me.
 
For less than the cost of the roller conversion you could come up with a later model LA roller or magnum block. My blow through turbo 318 has Wiseco flat tops and 360 J heads, head studs, cometic MLS gaskets, steel crank, larger late model rods with ARP bolts, jeep timing chain tensioner and roller chain, 340 profile cam.... I also incorporated a crankcase evacuation system with separator that is powered by the turbo inlet via filter resistance. I would love to tell you how wonderful it runs but it is still on the stand.
 
I can tell you what I did with my 318 and I also live high altitude at 6280 ft. (Colo. Springs)

I ran a 318 magnum for multiple reasons. It was cheap to source a used block. The roller cam was important to me in the fact that I too, wanted to run EFI. I did not want to attempt to break in a cam and experiment with EFI at the same time. Otherwise I would have to break in the cam with a carb and since my setup (megasquirt) was going to control ignition and fuel, I would also have to slave in an ignition system, it was easier to just find a roller block. I ran EQ magnum style heads because they are stronger between the valve seats (wont crack like stock), flow a little better, and the deck surface was already true. I used ARP head bolts.

I accepted a goal of 400-450 hp and it seemed right for a 318 at my altitude. I used the stock rotating assembly which has been proven to hold up as long as it is rich enough. My pistons sat pretty low and my final compression ratio was about 9:1

The thing about "needing a little extra boost on top" because we live high altitude is untrue. 10psi at sea level is the same as 10psi at 10,000 ft. Air density is all the same under compression because the turbo is the compressor, not the atmosphere. It's just going to take a little more work to get there, if you even can. And it will take a more defined combination of parts including the turbo to get you there. For example, if you were to make 10psi max at altitude, you could go to sea level and make 12 psi max. This is where the power gain comes from. That extra 2psi worth of compression is available in the atmosphere.

For 600 Hp, that's really pushing the strength of the block some would say. And yes, likely the strength of the crank.
 
I hear you T56, sounds like a nice 318 build btw. My understanding is that a compressor takes in air at a certain pressure and increases it by a set amount (boost pressure). If you increase the inlet pressure the turbo will just compress it 10 psi more whether it's 5 or 20 psi going in. I'm pretty sure it does work like that or else compound turbo/supercharging wouldn't work...?? Not trying to knock you btw just discussing :)
 
Well that is a good argument, and in a sense, we are both correct. I did some extended research, It is just a matter of how we measure pressure. Because a manifold pressure of 14.7 (which is sea level on a fair day / and WOT on a NA engine) inside a 10000 ft engine (10 psi atmosphere) would be 4.7 lbs of boost. This is measuring absolute pressure, where we compare the measured pressure to absolute vacuum (0 psi). Gauges we use, however, use 14.7 psi as the starting point upon which all other measurements are gauged. But for us, we would start out at about 12 psi for being at altitude and then the turbo begins to compress that to our desired boost level. I suppose our gauges would be a misrepresentation of the boost level goal we are trying to achieve when we are at altitude?
 
Ah OK you are right then T56 my understanding of boost reference was wrong I thought they measured with respect to pressure outside the engine.

I thought some more about the cam too, I like the idea of a solid flat-tappet with the special lifters that direct oil to the lobe face. Is there any reason to go with hydraulic vs. solid on a boosted engine, aside from needing adjustable rockers etc?
 
The only reason I can think of is if you build it with minimum oil pressure and the feed to the turbo bleeds off too much....
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The main issue I'd see with a solid flat tappet is that you'd have to take off the valve cover periodically to adjust lash, and that can become a pain if there are parts in the way you constantly have to remove and reinstall. So it all depends on how you build it. Not sure if you'd really receive all the benefits that a solid cam offers. Those are great for high rpm stability but most turbo engines operate on a lower rpm scale, unless you have a high rpm/high boost objective
 

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I know this is old but I'm bringing it back because my 360 recently took a dump (broke a piston, cracked cylinder) and it looks like I'll be building this 318 sooner than later.

I am on a budget but I'm willing to spend extra money on necessary good parts. So how necessary are forged pistons, aftermarket rods and crank? The Magnums I pulled off the 360 look good, I will have them gone through by a machine shop and if they are still good I'm going to swap them over to the 318. I also have the Air-Gap manifold and carb I can swap over so I can run the engine N/A initially while I save up more for all the turbo parts.

I am a bit leary of running cast/hyper pistons again because those are what broke in my 360 however I was pinging the crap out of it on a regular basis; had quite high cylinder pressure (10.5:1 and 256/262 adv. duration cam) so I know if I re-do it things will be done differently. For starters I'm definitely building for 9-9.5:1 MAX static comp...
 
Detonation will waste a forged piston just like a hyper piston. I use the stock hyper pistons in my 318 turbo mag, as have lots of people. The key to doing this is the tune. You must have precision fuel and timing control.

You're saying you want to swap the 4" pistons into a 318? If you're going that far, you're likely going to need to rebalance, not to mention seriously over bore the 318. With that much work, just buy 318 replacement or aftermarket pistons to fit the smallest bore size you need to machine. Strength matters.

For what it's worth, an oem 318 mag I pulled apart had the pistons sitting down .047", with a 62cc chamber and .039 gasket I came to a compression ratio of around 9.15:1. This is ok for me at an altitude of 6500 ft, with a mild cam, 13 lbs boost, e85, and no intercooler. If I made my combo any more scary like run premium, or increase boost, or went to sealevel, I'd be a little worried.

You should be shooting for the lowest comp ratio possible while maintaining as much quench effect as possible. A step dish style piston set at zero deck would work well in this case.
 
T56MaxTorq said:
Detonation will waste a forged piston just like a hyper piston. I use the stock hyper pistons in my 318 turbo mag, as have lots of people. The key to doing this is the tune. You must have precision fuel and timing control.

You're saying you want to swap the 4" pistons into a 318? If you're going that far, you're likely going to need to rebalance, not to mention seriously over bore the 318. With that much work, just buy 318 replacement or aftermarket pistons to fit the smallest bore size you need to machine. Strength matters.

For what it's worth, an oem 318 mag I pulled apart had the pistons sitting down .047", with a 62cc chamber and .039 gasket I came to a compression ratio of around 9.15:1. This is ok for me at an altitude of 6500 ft, with a mild cam, 13 lbs boost, e85, and no intercooler. If I made my combo any more scary like run premium, or increase boost, or went to sealevel, I'd be a little worried.

You should be shooting for the lowest comp ratio possible while maintaining as much quench effect as possible. A step dish style piston set at zero deck would work well in this case.
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Not gonna reuse my 360 pistons those are actually 4.060" no way they'd fit, plus one is busted. What would you think of me putting one of those Scat 4" stroker kits in it? $1400 for crank (cast), rods, forged pistons, bearings and rings... that's a pretty smashing deal, you think it's a good idea? Or should I just get new pistons and resize the stock rods with ARP rod bolts I already have? I even have ARP head bolts I can reuse from the 360.
 
My mistake. I read you wanted to swap over the pistons. You meant heads. Which is totally acceptable.

With a 318 stroker, you're going to increase the compression beyond boost safe levels I'd imagine. Maybe they give you piston options to lower it, I don't know. I honestly don't see the point of stroking a 318 just to add boost later on. For example, I set a goal for my car to make 450 ish torque. I had the option of a 360 mag but it was a little harder to find than a 318. To make the same power goal in a 318 vs 360 I'd just have to add a little more boost. whatever the engine is, I'd figure I'd have to limit boost with the wastegate anyways. Some guys run an oem short block to 16+ psi. I'd never need to go that high. It's either 13 lbs with a 318 or 11 with a 360. My transmission, axle, and unibody strength determined my realistic power limit, not my engine.

If you go stroker, I wouldn't bother adding boost. But I'm not here to spend your money.
 
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Well the boost is the main priority, I'm only considering a stroker kit because of the value... that Scat kit comes with dished pistons, +23cc so the compression will still be ok. I'm not set on it though just thinking out loud.

Overall I want to push the power up to the 550-600 range (at the crank), basically the limit of the stock block or a tad under. Supposedly turbocharging is easier on the bottom end anyway as opposed to a built N/A engine of the same size and power... right?
 
Well, brainstorm on exactly what you want to accomplish with the vehicle and set the goal of how to obtain that. This post from a year ago said you wanted to make 550-600 horse. Is that still what you want? You're getting close to questionable block strength. To be reliable, I'd say you're going to need all forged internals, high strength fasteners, and some block filler. Not to mention you Need the cooling system, chassis, transmission, gear, and tire to handle it as well. It's all about how much you want to spend to achieve a certain goal.
 
If all you want is 550hp, either do a stroker kit and keep it na. Or use a stock motor with a turbo.

At 550hp it won't even be breaking a sweat. And is easily obtainable, and nowhere near block breaking hp.
 
1980volare said:
If all you want is 550hp, either do a stroker kit and keep it na. Or use a stock motor with a turbo.

At 550hp it won't even be breaking a sweat. And is easily obtainable, and nowhere near block breaking hp.
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That's certainly nice to hear... N/A stroker would definitely be harder to hit that HP mark living in CO, elevation starts at 5000' where I live and just goes up from there. That's really my motive for going turbo.

I already have a set of ARP bolts to put in the stock rods and have them recon'd, just need pistons and then have it all balanced and the block machined.
 
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