5.9 Mag stock LB or rebuilt SB

[JTinAZ]

I need you alls opinions. I’m looking for a 5.9 magnum for my 69 Dart build. I’ve found a complete stock LB with supposedly 96k miles on it. I’ve also found a rebuilt .020 over short block with new stock cast pistons, new lifters, cam (not one that I would use), oil pump, etc. fully assembled and never installed. Prices are close enough that it’s not a decision factor and I’m going to replace the heads regardless so the long block vs short block isn’t a big deal either.
My question is which would you prefer to start with? I’d like to get to 400 hp and this will be a street only car, no racing. Do I need to replace the stock pistons to get the CR higher to get to that HP or can I get there with stock pistons, aftermarket heads, intake, cam etc? I’m leaning towards the short block, but I don’t know the quality of the build. If I go with the long block I was hoping to get by with just new bearings and a hone.
I appreciate any feedback you all have.

 

[junkyardhero]

i'd probably start with the rebuilt short block, you can easily get to 400hp on stock compression & head with just a cam swap, intake and headers. aftermarket heads would make it a walk in the park.
if you go for the long block, magnums are easy on parts (except cam bearings evidently) and you should be able to get away with a soft parts fluff and fold. stab in a new cam and timing chain and you're set on the short block, just add your go fast goodies and you'll be ready to party.

 

[TrailBeast]

I need you alls opinions. I’m looking for a 5.9 magnum for my 69 Dart build. I’ve found a complete stock LB with supposedly 96k miles on it. I’ve also found a rebuilt .020 over short block with new stock cast pistons, new lifters, cam (not one that I would use), oil pump, etc. fully assembled and never installed. Prices are close enough that it’s not a decision factor and I’m going to replace the heads regardless so the long block vs short block isn’t a big deal either.
My question is which would you prefer to start with? I’d like to get to 400 hp and this will be a street only car, no racing. Do I need to replace the stock pistons to get the CR higher to get to that HP or can I get there with stock pistons, aftermarket heads, intake, cam etc? I’m leaning towards the short block, but I don’t know the quality of the build. If I go with the long block I was hoping to get by with just new bearings and a hone.
I appreciate any feedback you all have.

I picked up a 96k long block locally for my Dart.
Still obvious cross hatching in the bores, and the crank and rod bearings looked near new.
The heads were garbage with cracks and horrific valve seats, which I expected.
I put a set of LA drilled New Zealand EQ heads on with Hughes springs, retainers and locks from a V6 that allows up to about .525 lift.
Oregon Cam Grinders.512 lift and 214/224 duration and 110 LSA (torque with a little sound to it)

You might find the same used condition on the long block, but assume you will need heads, cam and timing set at the least.
On the rebuilt short block I wouldn’t be comfortable without being able to verify.
Too much BS in the world.

 

[AJ/FormS]

What cam is in the rebuilt?

? I’d like to get to 400 hp and this will be a street only car, no racing.

Yes you can get to 400 with a 9/1 Magnum but the power will be a good 400rpm higher than with a 10.5 or 11/1, and that means a cam one or two sizes bigger, which will move the torque up off the bottom end, requiring a higher stall convertor, and a rear end swap to at least the next size bigger. Add up the costs of the convertor and gears, the extra cost of fuel from the loss of fuel mileage, and your gonna get to the cost of the piston swap pretty quick.
and I gotta tell ya, once you drive a car with 190 psi cylinder pressure, you'll never go back to 155
In fact, if or when, actually, you do that, you won't need a big cam and you'll forget all about that 400 hp target. 300 is more than enough for that Dart. and such an engine will pull gears down to 2.94s, so you could be looking at sipping gas at hiway speeds. Couple that with an overdrive, the right cam and tune, and your 5.9/360 could be knocking on high 20s mpgs.
So, that said, I say build your own hi-compression engine with a small cam, a 2800ish TC, an overdrive, whatever gears will get you to about 60 mph at the top of second gear, probably 3.91s, and don't forget the posi and slapper bars; and who cares about how much power she's making, you'll be smiling no grinning like a kid after his first sexual encounter, every time the secondaries open. Oh, yeah, for full effect, put a spreadbore carb on it with tiny primaries and sewer-pipe secondaries.
Ok so; knowing what I now know, that's what I would do. and
BTW, to run 190 psi, yur gonna need to run alloy heads and a zero-deck, so there is that to think about.
Yes I'm spending your money, but, my way, you'll only spend it once.

 

[273]

Here's some 5.9l builds

400hp 5.9l 380 crate

https://www.motortrend.com/articles/mopp-0303-360-crate-engine-buildup/

Mopar Performance 360 cid 380 hp rated

RPM	TQ	HP
2500	380.6	181
3000	402.9	230
3500	415.1	277
4000	437.8	333
4100	439.5	343
4500	437.1	375
5000	416.3	396
5400	398.6	409
5500	388.6	407
6000	343.9	395

400 hp 5.9l EQ heads

https://www.motortrend.com/articles/59l-magnum-engine-make-mine-a-5-9/

On The Dyno5.9LMagnum
RPM:	TQ:	HP:
3,000	407	233
3,100	406	240
3,200	406	248
3,300	409	257
3,400	411	266
3,500	413	275
3,600	414	284
3,700	415	293
3,800	416	301
3,900	416	309
4,000	417	318
4,100	419	327
4,200	420	336
4,300	419	343
4,400	419	351
4,500	419	359
4,600	418	366
4,700	416	373
4,800	413	378
4,900	409	382
5,000	405	386
5,100	401	390
5,200	397	393
5,300	392	396
5,400	387	398
5,500	381	399
5,600	375	400
5,700	367	399
5,800	361	399
5,900	354	397
6,000	347	396

455 hp 5.9l

https://www.motortrend.com/articles/build-455hp-pump-gas-magnum-v8-around-4000/

5.9L Magnum Small-Block

RPM:	TQ:	HP:	TQ:	HP:
4,000	453.9	345.7	449.6	342.4
4,100	458.1	357.6	458.2	357.7
4,200	460.2	368	461.8	369.3
4,300	460.8	377.3	462.6	378.8
4,400	461.3	386.5	465.8	390.2
4,500	461.7	395.6	465.2	398.6
4,600	460.3	403.2	464	406.4
4,700	459.4	411.2	460.4	412
4,800	456.6	417.3	458	418.6
4,900	451.6	421.3	451.5	421.2
5,000	447.7	426.2	455	433.1
5,100	444.8	431.9	450.3	437.3
5,200	440.6	436.2	445.1	440.7
5,300	435.2	439.1	431.3	435.3
5,400	426.5	438.5	432.3	444.5
5,500	421.1	441	426.4	446.5
5,600	411.9	439.2	420.9	448.8
5,700	413.6	448.9	415.4	450.8
5,800	411.7	454.7	410.1	452.9
5,900	402.3	451.9	404.6	454.5
6,000	397.9	454.5	398.6	455.4
Average:	439.9	416.5	442.2	418.8

448 hp 5.9l, 300 hp crate vs cam and head upgrade

https://www.motortrend.com/articles/hrdp-0804-small-block-mopar-engine/

RPM	Base TQ	Mod TQ	Base HP	Mod HP
3,000	398	37.31	228	212
3,200	410	373	250	227
3,400	419	390	271	253
3,600	424	402	290	276
3,800	420	408	304	295
4,000	412	412	314	314
4,200	396	420	316	336
4,400	381	431	319	361
4,600	362	432	317	378
4,800	346	434	317	397
5,000	328	434	312	413
5,200	298	433	295	429
5,400	274	426	282	438
5,600	-	417	-	444
5,800	-	406	-	448
6,000	-	361	-	412

 

[TT5.9mag]

Junkyard magnums typically have good bores, but the upper bearing shells in the rods take a beating when the plenum gasket goes and the engine eats oil. And they all did at some point. Most guys only check the lower shell and they look great. Also, the cam bearings on higher mileage junkyard magnums are typically garbage as @junkyardhero said.

 

[JTinAZ]

Keep the opinions coming. I’ve been reading up on many different magnum builds throughout the forums, but I’m specifically looking at what you guys would prefer to start with.
To clarify, I will be swapping in a four speed in place of the 3 on the tree and of course a new rear end which I haven’t decided on yet. 8.25, 8.75, 8.8 are all on the table still. I’m leaning towards 3.73 or 3.91’s.
If I went short block I suppose I could order a set of forged pistons at .020 over. The guy said it had a cam for torque because it was going back in his truck but he got hurt and project no longer viable for him.
I haven’t decided on heads yet either. I wanted to look into IMM’s heads, but if I decide to get spendy then I’m going trick flow. I know some have had good results with these Chinese heads but I’d prefer to stay away from them.

 

[273]

I got a 68 Valiant Signet, with a 380 hp crate (400 hp) with 727 & 2800 rpm stall and 2.xx:1 gears, works pretty good still fun in the street it's a little hard to get up into the power band cause of the highway gears, it's still got plenty of power at lower rpms but if I really want to get up in the power band, I need to get it up to 30-40 mph in 1st then drop the hammer.

I drive it like a 2 speed most of the time in town 1st and 2nd (acts like 4.xx:1) and use 3rd like an overdrive. Obviously deeper gears and a higher stall would dramatically improve performance but it's fine for now even with it's 2800-4700 rpm power and 9:1 cr with a 750 single plane headers and something like 288/292 230/23? on 108 lsa .512" lift.

Works for me, maybe not for others.

 

[JTinAZ]

@AJ/FormS
I like what you’re saying and I do like the thought of the high CR. Can I do that on 91 octane? 1200ft elevation, not much humidity to speak of and hot as hell. At least right now. Not that it would be driven in the summer too much.
Would you start with the rebuilt SB or the 96k mile long block.

 

[sireland67]

I would buy a bore scope, and look at the bores of the 96K engine.
If it looks good, buy it and rebuild.

 

[AJ/FormS]

@AJ/FormS
I like what you’re saying and I do like the thought of the high CR. Can I do that on 91 octane? 1200ft elevation, not much humidity to speak of and hot as hell. At least right now. Not that it would be driven in the summer too much.
Would you start with the rebuilt SB or the 96k mile long block.

I am at 930 ft. My alloy-headed 367LA has had cylinder pressure as high as 195psi, and the Scr has been as high as 11.3, yielding a Dcr of about 9.2.
Currently she is sitting at 11Scr and 185ish psi, with a bigger cam.
She has always run on 87E10, and has never required an octane booster.
This engine was disassembled and inspected for detonation/etcetera, 5 times, in it's first 5 years, from 1999 to 2004, with none found. She now has over 100,000 miles on it. and The tune was easy.
Your results may vary, it depends on your skills.

As for which engine to start with; IDK anything about Magnums, but
since hi-compression pistons are a pre requisite to these numbers, as are alloy heads; all yur gonna need from a core is the block/crank/rods/and covers. At this point, the only good reason to use a Magnum, is for the roller lifters.
Do you need 195psi? No.
but, once you've driven something like that, you'll never go back to 155...... which at 1200ft, with a 400hp capable cam, is not gonna bring you any smiles. Rather 185 or more is nearly as much fun.
If the short, has dished pistons, those will have to be swapped out.
If the long, has iron heads, those will have to go; you cannot run much over 160/165psi with iron heads.
Iron heads, costing you 30psi, will knock your V/P down the same amount, taking your low-rpm torque from feeling like a bigblock, down to feeling like a stout 5.2M.
With a hi-stall convertor, this is not that big a deal; and iron heads at a Dcr of say 8.0>8.3, would still be fun, but those dished pistons still gotta go, cuz 8.3 Dcr may require an Scr of over 10/1, depending on the Ica of the chosen cam.
BTW
your 1200 ft elevation is costing you about 6 or 7 psi over sealevel, which translates to the same amount of lost V/P, which translates to about a half a point of Scr. What that means is that 11.3@1200ft runs about the same as 10.8 at sealevel. That's like 10 or more horsepower at say 3200rpm. almost a full cam-size over the nose., also probably worth 200 rpm stall. I'm guessing.
The point is this, when it comes to making low-rpm power;
the number one rule is that there is no replacement for displacement; but
the number two rule is that once the displacement is fixed, there is no better alternative than cylinder pressure.
If you have a clutch, pressure is paramount because the vast majority of time will be spent locked into low-rpm by the roadspeed.


Read about V/P here; V/P Index Calculation