Someone here was asking about these, and I found this old document on them. It raised a general question for me. Is ONLY swapping a cam (and in this case intake from dual plane to single or vice versa) acceptable to change idle characteristics of an engine? I was always under the impression that cams were match to other parts of an engine, and therefore should not be swapped without additional changes such as valves. Am I not necessarily right, and is it more about converter and gears? The chart below didn't align properly but on left is 380hp and right 300hp, but the only change from the 360/300hp and 360/380hp was single plane intake and the cam. I had the 300hp version and it was very impressive and smooth idle with stock converter.


Mopar Performance Magnum V-8 Crate Motors
Feature 360/380 360/300
Assembly part number P5249499 P5249498
Compression ratio 9.5:1 (advertised), 8.7:1 (JTE nominal)
Cylinder heads Cast-iron, 18-degree valve angle high-swirl intake ports, combustion chambers
Intake valves 1.925-inch diameter, 5/16-inch stems
Exhaust valves 1.625-inch diameter, 5/16-inch stems
Rocker arms Ball-stud pivot, 1.6:1
Camshaft Hydraulic roller tappet
part number P4876348, without lifters P5249663, with lifters
application MP replacement '89 OE 360
duration 288/292 degrees 274/264 degrees
lift 0.501/0.513 inch 0.385/0.401 inch
Valvesprings High-performance, 1.42-inch diameter
Valve seals Banded Viton rubber, static
Retainers Chrome-moly steel, 7.5-degree plunge angle
Oil pan Center sump, 5-quart capacity (with filter)
Intake manifold Single plane Dual plane
M1 aluminum, square-bore carb pattern with provisions for
multiport injection
Ignition system Mopar Performance electronic kit vacuum-advance distributor
Valve covers Cast-aluminum, black-wrinkle
Fuel Premium only Best available grade

Changing just the cam and intake will definitely change idle quality.

The roller cam in the 360/300 magnum crate motors was the same one used in the LA 360 TBI motors. However, the magnum rocker ratio amplified the lift. I thought the 360/380 engines used a different grind, although it's been a long time since I looked into these. I know at the time I had searched the pn for the 360/380 cam and came up empty.

I know changing those will change idle quality, especially cam, but just wondering if this is "acceptable" or "proper" engine building or if all components are considered to be tied together? Something I've always been curious about in the engine building world. Is just swapping a cam for example okay?

Just swapping a cam, assuming you are reasonable in your choice, is ok. As I recall the difference between the 300 and 380hp version was cam and intake.

Now if you were to slap a .600" lift cam into a stock 318LA you'd be asking for trouble there, interference aside.

My favorite low buck 318 LA hop up back in the day was the 340 cam and a stock 4bbl intake with a ~600 cfm carb.

Though to be honest I'd probably not do that today.

Thanks for info sniper. Yes, those crate 5.9's were identical other than cam and intake. I would think the 380hp might be a bit lopey on the idle. The 300hp had stock idle. I was thinking rocker ratio might need a change too but I guess 1.6 is okay for either cam. Always looked at a build as a recipe.

I posted the exact numbers from MP above. It's not aligned very well, but you can see the specs on each. Just the cam is different and single plane versus dual plane M1 intake.

Here's the link. Scroll down to bottom just above the ads: https://www.hotrod.com/articles/hrdp-9809-magnum-and-non-magnum-parts-exchange/

No, one version came with the upgraded R/T heads till they ran out.

never saw that documented

Was in the one of the MP catalogs, I know I was researching the different packages when I was building a 5.9 for my Duster.

Went with a stock short block with windage tray, Enginequest heads from Hughes, Comp Cams XR265HR-14 roller cam, Performer RPM intake with 750 CFM AFB, Wide Band O2 for tuning, Doug's headers, Harlan Sharp Roller Rockers.

Thing just hauls ass and reliable.

I think the 300 hp had the RT cam the 380 has the .501. I have both. I put a 360 together one with the ‘kits’ 1.7 roller rockers duel springs-retainers guide plates push rods. Ported the heads with the factory port templates M1 intake and those finned Magnum valve covers. Did this 15 years ago and engine is still on the stand yet. Also bought used 380 pistons (.020 over) to stick in another block, still in the box. Much different piston than stock (coated, higher compression). Almost done with the E/Stock Duster, might stick it in first to see how well it runs while I finish up the 340

I wish I had bought another one back when they were available for my D150. Years ago I traded a complete race ready 904 to a friend for his 5.9 / 360 crate short block. it was drilled for early model head oiling, we put in a 590 lift cam , put on my W2 heads and it ran a best of 10.70 making around 525 hp, after spinning a bearing we put a steel crank to replace the cast one, better rods and pistons with the same cam and heads made 549 hp and went a best of 10.40 in my Dart. After many years and over 1,000 runs I took it out in favor of my present 408 , it still ran great and is being redone as we speak for lower compression and pump gas should be around 5-520 hp. I haven't decided yet if I am going to keep it as a spare or sell it.

From my 2002 Mopar Performance catalog..
Looks like the only difference is the cam and intake.
Cams
300hp 250/264D 335/401"L
380hp 288/292D 501/513"L
My understanding is that R/T heads never came on the crate motors.

That's what I posted originally with all the specs

I bought one in early 2000's. The 300hp version and it was only $3000. That was a bargain. It had every bit of advertised torque of 375! Tire shredder and pulled hard all the way through the gears.

They stopped selling them when the heads ran out.

A friend on mine was partner for KRC Performance, they were actually buying the crate engines and stripping the R/T heads off.

I saw the advertisement myself several years ago.

Edit: Just off the phone with Bill from KRC and he confirmed it.

I thought they stopped when the 360 stopped production. They were pulling blocks off assembly line at truck plant but when they stopped the 5.9 in truck production, no more blocks.

Its right there in the building a Magnum V8 book by Larry Shepard under crate engines.

Yes intake and cam but would think they also matched the spring's to match cam. IE would not use the same ones.

That's what I thought but THEY used same springs, "Valvesprings High-performance, 1.42-inch diameter" same pn. Both were 1.6 rocker ratio. Maybe that's why?

Neither the 300 or 380 hp crate motors used the R/T head according to your information. Only the 402 crate motor used it and no one was talking about that engine.