Hypothetical 360 magnum build

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volaredon

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I might be buying a replacement for my either my 01 Durango or my 96 Dakota tomorrow.
Found an 03 Durango RT that supposedly needs a motor, I rented a tow dolly and am going to look at it tomorrow early it's cheap and supposed to have a very clean body and interior. If it isn't as advertised I won't be getting it.
Being an RT it's guaranteed to be a 360 and not a 4.7.

I've asked this elsewhere and discussion has gone sideways.
No "find another" response.
No " what happened to" either engine
What I have is what I have.
Yes I know that the engine in this thing might really be alright, it's 90 miles away I haven't seen it myself yet.
But the idea is to use what I already have here if possible.
I have a 00 short block here that was a doomed rebuild. Less than 3 hours on it and between a set of "ready out of the box" EQ heads that weren't (valve guide clearances were way too tight) and a stuck wide open injector or 2, it ended up with the break in oil coming out a silvery slurry.
That and a melted, glowing red magnaflow cat that was put in brand new with the rebuild.
Plans are to re rebuild this 30 over short block to be able to make lemonade out of lemons so to speak.
It has a set of speed pro stock replacement coated hypereutectic pistons in it now. At a minimum it will need rings and bearings again. Don't know yet what else.
First question is HOW different is the external balance mismatch between a LA and a magnum?
And where? Pistons rods or crank?
Reason I ask is I have a set of speed pro or sealed power 30 over flat tops for a LA 360 with only valve relief eyebrows in the tops.
For starters I'm wanting to use the LA pistons in the Magnum block. Possible? Feasible?
goal of engine is hopefully "some" mpg improvement vs the stock 360 that's in the Durango now. Improved torque for towing the camper. Being an RT means 3.91 gears. Has a 46re behind the engine.
also I have an old but brand new MP "RT+10" cam that was "current" in the OBD1 days.

on the heads I'm kicking myself for not going after the nos factory new magnum heads I saw on CL earlier this year. For $400 that was a deal in my opinion. I have 1 known good factory magnum head here. I have 1 more candidate here that needs checked.
the engine in the possibly new to us Durango has its heads. But supposedly the reason it needs an engine according to the seller is it overheated. So doesn't bode well for those being any good.
again I haven't seen it for myself, I don't know yet.

failing that I'm going to need some heads for it. I want to run no more than 89 in it, will be a daily driver. Not looking to race, probably won't see over 3500 rpm. Do these speedmaster ones that everyone is talking about come in a magnum version? I cringe on those because of the Chinesium content. How bout Edelbrock? So they?
just spit balling at present/ main question I need answers for have to do with those LA pistons in magnum block, mostly having to do with engine balance. They are currently still attached to LA rods. I have yet another set of magnum rods here that I could switch them over to.
 
as a caveat, i know just enough to be dangerous. but, the pistons will physically fit.

the pin height is probably going to be different and the ring pack/location is for sure different. with the difference in deck height of the magnum this may put the piston where you don't want it-- too far in the hole or possibly above deck. i *think* i've heard of using them as a lower cost alternative when supercharging.

if i were in your situation, i'd probably try and sell a few parts off and secure the correct pistons and rings just to make my life easier on the build and not have to potentially mess with expensive proposition of balancing the assembly.

if it's a DD and more tow mater than rickey racer, i'd go with as stock of heads as i could find. if they happened to be RT heads, awesome. if not, then whatever was cabeza du jour.
 
> any cam with a later-closing intake valve will lose cylinder pressure.
> any pistons with a shorter pin-height will reduce cylinder pressure.
>Giving up cylinder pressure is never a good thing, especially for fuel-economy, in a heavy vehicle.
> those 3.91s x .69od= 2.70 final drive, just about right for the factory cam. The tall tires will make that feel yet smaller.
> the octane requirement changes as a function of effective cylinder-pressure, which changes with load/rpm/and chamber temperature. You don't get to decide what octane to run in an unknown combination of parts, unless: 1) you have empirical supporting evidence from a prior similar build, or 2) the combination is EFI with a knock-sensor feedback, or 3) the Scr is low enough, that it doesn't matter, or 4) the engine is never required to work, and/or operates as a stationary engine.
>If the computer, in response to detonation,
1) pulls timing, then fuel economy will take a hit, or
2) doesn't pull timing, then the engine may overheat and destroy itself in the long term, or just rattle the skirts off and destroy bearings, in the short term.
> Since you are on a restricted-budget, I'd recommend one of two things
1) spread the project over time and do it right, or
2) just stay home.

BTW, I love the First-generation Durangos. If they weren't so heavy, and so hard on gas in my application; I mighta bought one years ago.
 
Yes you could use those pistons in a magnum, not knowing exactly what they are I guess you would have to measure and weigh them. Magnum has a little shorter deck height, magnum pistons and pin are lighter by quite a bit. May have to juggle head gasket thickness to make it work.
Afew post here about the newer eq heads, not as good as the old ones but not much else for choices except for stock probably cracked ones.
Back to your piston question, if you ran this rotating assembly already, just use the same rods, pistons, balancer, flex plate. The cranks are all the same, la- magnum.
 
You will have to balance it. Magnum parts are lighter so the bob weight on the crank will not be correct for what ever rod/piston combination you use.
 
Yes bob weight is different, that is why the balancer and flex plate need to match the assembly, la to la , mag to mag. Not knowing what exact pistons he has it is a crapshoot, I was assuming this assembly was factory balanced or balanced before assembly.
 
I figured that the pistons out of the LA should have the same pin height, as the magnum and LA were same bore and stroke. I don't think there were any difference there. The LA pistons and the original LA rods are in my son's garage, he's not around at the moment. I can mock one up and see where it lands compared to TDC and go from there I guess.
I just didn't figure on having to be the one to science it out.
I know for sure that you can interchange LA and magnum pistons on a 318.so i figured you could on a 360 as well. Crap. I have the receipt here from the pistons in the doomed rebuild, which would get me a PN. If I can get a PN off the LA ones I guess I can go look them up and see what the weights were.

Nobody yet has answered as to why they castrated every 360 built, with dished pistons from the factory to begin with. Makes zero sense to me.
From what I can find a stock, as built 360 LA vs magnum there's a whole 0.010 (yeah, only 10 thousand) difference in deck height between them
Not enough to make a difference in whether the pistons pop up, as everyone I have ever had apart were in the hole at max TDC.

If money weren't an object, (it kind of is, that's why I'm looking for best "bang for buck" ) or if I had access to a machine shop and knew how to run it (so I coud do my own work and not have to pay for sublet) id like to build a neutral balance 360.

Another thing, most come on here wanting to modify their engines "build them up" where Ive been stuck in a "rebuild them stock" rut, now I come on looking to possibly build something up and everyone seems to be telling me to stay stock.... another thing that makes no sense.
 
volaredon said:
Another thing, most come on here wanting to modify their engines "build them up" where Ive been stuck in a "rebuild them stock" rut, now I come on looking to possibly build something up and everyone seems to be telling me to stay stock.... another thing that makes no sense.
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building something up? sure, but that requires money.

wanna spend some dough? build it up. don't wanna drop that coin? keep it stock.

honestly going with stock pistons & rings is probably the most economical route here. stock head gaskets v whatever you wind up needing? balancing the rotating assembly? etc. etc.
 
Guess I’m missing the boat, thought it was for a 3500 or less rpm Durango, could put some icon 742’s in it if you want and stay with pump gas. Not knowing what pistons you have I am just guessing. Putting a scat light rod and a light piston will save you for balancing but that cost money also, nothing wrong with external balancing on lower rpm motors like this.
 
Some pistons can sit out of the deck on a magnum (no factory ones) kb 107 can be close, not a big issue but if you are running a close chamber head (magnum) you have a little work to do. Might have to taylor your head gaskets to the application.
 
JunkyJimmy said:
Guess I’m missing the boat, thought it was for a 3500 or less rpm Durango, could put some icon 742’s in it if you want and stay with pump gas. Not knowing what pistons you have I am just guessing. Putting a scat light rod and a light piston will save you for balancing but that cost money also, nothing wrong with external balancing on lower rpm motors like this.
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But I don't have those. I was hoping to use up what I have here. Don't know the CR difference between what I have and the CR107. I guess I'll have to get the number and look up the specs on them..
 
I built one a few years ago, KB107, balanced bottom end.
Eldebrock heads, and a comp roller 262 cam, M1 duel plane.
It made a good tow engine in a 78 powerwagon.
 
If you are putting this in a Durango and using the factory EFI it's not going to make any top end power anyway. It will be done at about 3500 RPM. That manifold is made for bottom end. So just put it together with what you have.
 
Well I don't / won't race it, probably don't need more than 3500 rpm anyway/ but I want to make "idle to 4500" better and if I can do so with what I have collecting dust, all the better.
If I can't, or if there's some reason it absolutely won't work out, !parts are gonna "crash" or blow out the side of the block, etc. I get that. I also had the joy of building an engine years ago that was "lots of noise but less power than stock. Don't want that either. I'm not saying I don't want to or can't spend "any" money on it but what's wrong with using what I have if I can? I don't think I would be able to get pennies on the dollar of money already spent, trying to make lemonade out of lemons. But I want it to be an improvement over stock, at the same time
I'm looking at having to buy a set of heads most likely at any rate, and with all the talk of aluminum heads I'm considering that. But I want to work with what I have at the moment and that's short block parts.
 
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volaredon said:
Well I don't / won't race it, probably don't need more than 3500 rpm anyway/ but I want to make "idle to 4500" better and if I can do so with what I have collecting dust, all the better.
If I can't, or if there's some reason it absolutely won't work out, !parts are gonna "crash" or blow out the side of the block, etc. I get that. I also had the joy of building an engine years ago that was "lots of noise but less power than stock. Don't want that either. I'm not saying I don't want to or can't spend "any" money on it but what's wrong with using what I have if I can? I don't think I would be able to get pennies on the dollar of money already spent, trying to make lemonade out of lemons. But I want it to be an improvement over stock, at the same time
I'm looking at having to buy a set of heads most likely at any rate, and with all the talk of aluminum heads I'm considering that. But I want to work with what I have at the moment and that's short block parts.
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Let’s start over.
What is this engine going into? The Durango?

Is it going to be running factory fuel injection manifold and serpentine drive from a Magnum 360?

Can the EQ heads that were previously damaged be fixed?

Edit: This site glitched and I lost half of what I typed. We’ll just start with those questions.
 
Yeah a Durango.
Yeah the factory fuel injection. Hopefully, maybe with a slight tune.
Yeah the factory serpentine setup but I don't know why that's important at the moment.
The EQ heads that got damaged were sent thru a machine shop and are on another engine.
 
If you are putting it a Durango and using the factory fuel injection just put it together. Send the PCM out and get it flashed to the specs you want it to run. You can put all the stuff inside you want but if the ECU isn't tuned to the parts it will run like ****. This isn't magic. You keep asking questions but then change your mind. If it's EFI you WILL have to change the programing to fit what you change in the engine. Keep in mind the PCM controls the transmission shifts as well. Now you have to deal with that. Unless you put a programmer on it or program in the PCM you are making a monster. I can guarantee your first program will be close but it will not be the best. Plus it's about $350 to $400 just to have it done. It can be done but it will not be cheep. A programmer might be hard to find for it at this point. It's old school now. Now on top of that if it has a Security or SKIM in the PCM you CAN NOT GET RID OF IT!!! The BCM will have to see the correct PIN from the PCM of it will not run. So you will not be able to just use any PCM out of a different vehicle. The VIN and PIN must match.
 
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More food for thought... keep the compression close to factory. Those do not run have knock sensors so detonation will be a problem. So you will have to factor that into your tune as well.
 
volaredon said:
Yeah a Durango.
Yeah the factory fuel injection. Hopefully, maybe with a slight tune.
Yeah the factory serpentine setup but I don't know why that's important at the moment.
The EQ heads that got damaged were sent thru a machine shop and are on another engine.
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It's all important because you mentioned the possibility of aftermarket aluminum heads. The Edelbrock LA heads and the Chinese copies like Speedmaster do not have enough material on the ends to bolt on the factory Magnum serpentine setup. And there is no material where a couple of the bolts would go, so can't even drill them to fit. You will definitely want to know that before considering new heads.
 
I haven't opened that link yet but to address the post 2 above the one with the link
NO I'm not changing my mind.
The whole idea all along was to see how feasible it would be to build an engine starting with what I have on hand. And I haven't gained or lost anything since I started this thread.
I know about the computer on these, I had my fits a few years ago with the computer on the Durango I currently have. I sent it's original in for repairs, it didn't work out, I went to a boneyard and got one off an identical Durango, and it would start/quit like a carbureted car with a bad ballast resistor.
So I sent the junkyard computer in and the computer repair place reprogrammed that one to be plug n play. I won't forget that one. Everything is as built, stock on my current Durango, except for the fact that I have unmodified EQ heads on a stone stock short block, and 1.7 roller rockers.
Hellrats said:
https://www.hughesengines.com/TechArticles/4buildingupmagnumtruckengines19932003.php

Read this. It may help you out on your project before you get too deep in to it.
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I see either posts have. Been put in since I started this reply so I quoted THAT to as a place holder to show what I meant "2 above WHAT post".

But I came asking before I started swinging wrenches, because I figured I'd run into somebody who had at least attempted what I am thinking about.
The particular Durango that I was looking at to put this prospective build into ,wasn't as advertised So I didn't get it /but I'm still looking for another one to replace our current one.
Our current Durango is over 300 k so I'm scoping out it's replacement while things are still good with this one, I've talked about current one before
Maybe that's where the confusion comes in, where you're thinking i'm "changing my mind".
I know that one well as we've had it for 10+ years, I'm wanting to improve on the next one based on how this one acts.
I'm just surprised how many replies om getting to "just build it stock".
 
Bobzilla said:
It's all important because you mentioned the possibility of aftermarket aluminum heads. The Edelbrock LA heads and the Chinese copies like Speedmaster do not have enough material on the ends to bolt on the factory Magnum serpentine setup. And there is no material where a couple of the bolts would go, so can't even drill them to fit. You will definitely want to know that before considering new heads.
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I was wondering about that. I thought that Edelbrock and speedmaster had a magnum based aluminum head on the market.besides the LA ones.
I know that the MP R/T heads are as common as hens teeth.
I'm actually still hoping that the set of NOS Mopar magnum heads that I saw a few months ago, show up on CL again. But exploring all options.
I know that if I can use those replacement LA flat top pistons that the compression would be higher than stock, and was "considering" a set of aluminum ones as I know that aluminum heads can support more CR without detonation than cast iron ones can.
 
For your original question, the Magnum pistons and connecting rods are both lighter than the LA parts. You can run an entire LA rotating assembly, including the flex plate and harmonic balance, in a Magnum block while balancing it as an LA. Once you start mixing parts, like lighter pistons on either rotating assembly, you will have to get it all balanced unless you can match the weights.
 
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