Mild 400 build on a budget

Engine, Trans & Driveline

  1. 400Magnum

    400Magnum Well-Known Member

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    I’m putting together a mild 400 for a 78 Magnum and would like to get feedback on my plan. The goal of this rebuild is to stay on a very limited budget and have an engine with decent performance and be reasonably affordable to drive. This will be a cruiser, go to the occasional local car show, drive to work on nice days, and once in a great while take it for a long road trip. It will never be on the track.

    The engine has been rebuilt before, but needed going thru again for various reasons. The pistons were sitting very low in the bore when I disassembled the engine, on average they measured 0.152” below the deck. I don’t have the budget to take the block to a shop to get decked, so I’m just going to have to live with that.

    My 452 heads are shot, all the valve guides are worn and the exhaust seats are eroded. I’ve called several shops in the area and they’re all quoting me close to $1000 just to get new guides and seats. If I have to spend that much money, then I’d rather get Edelbrock or Stealth heads for about the same price with 75cc chambers, build my compression up a little and save some weight. With the right thin head gaskets it gets me into the 8.1 range versus 7 for the 452 heads.

    I bought my cam years ago when I was making decent wages. It’s the smallest hydraulic roller retrofit cam the Crane makes, .429”/.452” at 204/214. This cam builds pretty decent cylinder pressure, so I’m hoping the low static compression ratio won’t be as much of a problem as it normally would be. Crane says the static compression should be in the range of 8-9.5, so I’d just be getting there with the aluminum heads.

    The rest of the build is pretty much stock. Stock replacement pistons and rings, stock rods, etc. I will be using an Edelbrock Performer intake and a Street Demon 625 with the composite body too.

    So I’m just wondering how well you all think the combination of low compression pistons, cam, aluminum heads and intake, and carb will work together for my goals?
     
    Last edited: Feb 22, 2018
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    • 91r/t

      91r/t FBBO Gold Member FBBO Gold Member

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      How are the cylinder walls for size/roundness? Are you having any machine work done to the block what so ever?
       
    • 400Magnum

      400Magnum Well-Known Member

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      I haven’t done any machine work to the block, but it was bored 30 over when it was rebuilt when the previous owner had it, and I’m not planning on doing anything to it if at all avoidable.

      I’ve not done any real measurements yet on the cylinders to know what they’re like for roundness or taper. That will be the next thing on the to do list as soon as it’s warm enough in the garage.
       
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      • 91r/t

        91r/t FBBO Gold Member FBBO Gold Member

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        Yea I would go from that. I mean if you had to clean up the cylinders, you'd be nuts not to deck the block for some more compression but if you don't need to, I would think the combo would run a hell of a lot better than the stock configuration.
         
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        • 400Magnum

          400Magnum Well-Known Member

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          Yes, ideally that’d be what I’d like to do, but unless something is seriously wrong, I won’t be doing that. I don’t think anything is really bad, since the car was running when I parked it there all those years ago. As it is right now, I’ll just be running a hone thru the cylinders to remove some rust and break the glaze to get the new rings to seat.

          Yes, I’m thinking it should run better than stock too. If nothing else, just getting the compression up a little and taking some weight off the front end will be nice.
           
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          • 69 GTX

            69 GTX FBBO Gold Member FBBO Gold Member

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            If you went with the aluminum heads you could take some off them to help the compression. Sounds like a good plan otherwise. On the up side you can run the cheap gas.
             
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            • 400Magnum

              400Magnum Well-Known Member

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              Yes, I thought about that too, but the price from the shop was going to be about $300 by the time they did everything, and I’m just not sure the minor gain in compression is worth the expense. And I really like the idea of being able to run cheap pump gas.
               
              Last edited: Feb 22, 2018
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              • 91r/t

                91r/t FBBO Gold Member FBBO Gold Member

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                Plus you'll have new heads with valves that seat completely and flow alot better. The engine should be a torquey engine with that small cam and a little more compression.
                 
                Last edited: Feb 22, 2018
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                • 400Magnum

                  400Magnum Well-Known Member

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                  Right. That’s what I was thinking/hoping, it’s just nice to hear someone else think it too. :)
                   
                • 91r/t

                  91r/t FBBO Gold Member FBBO Gold Member

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                  Did you say you are buying pistons or using the originals from the previous rebuild? Reason I ask is that would be the best money spent in this build, even if they were the cheap cast units. That would get you a choice and get your compression way up. If you scrounge up a few hundred extra, that's where I would put my money in this build. If not, not a biggie, I am sure it will still run pretty damn good.
                   
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                  • 400Magnum

                    400Magnum Well-Known Member

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                    Another question I had regards which piston rings to go with. My initial goals were to run moly rings. But after talking to a Tech at Keith Black, he said that since I’m honing the cylinders at home I’ll never be able to get them round and perfect enough for moly rings to seat properly, so I should just use plain iron rings. Is he right? I was thinking that if I take my time and measure carefully, and use the correct grit obviously, that I should be able to use the moly rings. Any advice?
                     
                  • 400Magnum

                    400Magnum Well-Known Member

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                    I am getting new pistons, but they are just stock cast replacements with the same compression distance. I’ve looked at Keith Black, ICON, Ross, and SRP, Sealed Power and SpeedPro, but can’t find a replacement piston that maintains the weight of the original, so I don’t have to get the crank rebalanced, with a higher compression distance. Do you know where I might find some?
                     
                  • 91r/t

                    91r/t FBBO Gold Member FBBO Gold Member

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                    Pulled this quote from another forum and it kinda goes with what I have heard about the moly rings from alot of others as well.

                    "It is highly recommended, unless the cylinders are rebored, to use a "power hone" or "rigid hone" for moly rings. Molys have a hard time "seating" in bores that aren't "round". ANY taper or out-of-round will inhibit that. Iron rings are recommended if not reboring."
                     
                  • 91r/t

                    91r/t FBBO Gold Member FBBO Gold Member

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                    What are the weights on the original pistons? Is the crank on the 400 you are using cast or forged? Externally or internally balanced engine?
                     
                  • IQ52

                    IQ52 Well-Known Member

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                    What do I think? Hmm.....this is what I know.

                    https://www.forbbodiesonly.com/moparforum/threads/the-slug-400-on-the-dyno.80843/

                    The aluminum heads will flow much better than the iron heads we used. Read that as even better power from your 400.
                     
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                    • 400Magnum

                      400Magnum Well-Known Member

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                      The pistons that I just took out weighed, on average, 790 grams and the pins were 205 grams.

                      The crank is an externally balanced cast crank.

                      Yes, I saw this thread, it’s what gave me encouragement to think my combo might not be too bad. But I still wanted to ask.
                       
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                      • XCELLR8

                        XCELLR8 Well-Known Member

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                      • 400Magnum

                        400Magnum Well-Known Member

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                        Yes, I talked to the Tech at UEM about those pistons and a few others they had. They’d be perfect if they were the right weight, but they’re about 100 grams too light. So then I’d have to get pins that are about 100 grams heavier to balance the weight. And that makes things a lot more expensive for very little gain. It just isn’t worth the expense to me for what I’m wanting out of the car and considering all the other stuff that I still need to get.
                         
                      • Garys1969RR

                        Garys1969RR Well-Known Member

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                        I think you can get a lower end balanced for around $100. I would try to get the piston up near the deck, no more than .050 below deck. Most of those low compression 400s just don't have much power. A few do, if built right, as has been shown, with good flowing heads. But even those will still produce more torque and power with a compression increase.
                         
                      • oldbee

                        oldbee FBBO Gold Member FBBO Gold Member

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                        I measured carefully & used one of those “ball”(?) hones, 500-600 grit(?) & have had no problems.
                         
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