Old 5.9 crate magnums

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.

I didn't say they (the 300 and 380 hp) did, I said there was a third option, four if you count the stroker.

I did confuse the aluminum head which I assumed was a variant of the R/T head but looking at the specs it really is by itself aside from being interchangeable.

We were specifically talking about the 300 and 380 hp versions and you claimed "No, one version came with the upgraded R/T heads till they ran out"

You did not say there was a third version, you stated I was wrong.

Worked there almost 9 years, still remember a few things.

Springs, locks and retainers were changed on the .501/.513 cam variants per BOM.
You absolutely do not want to run that kind of a cam with the wimpy stock valve springs.

At the start, The entire engine (not just the block) began as production pulled just for us,
The parts added/painted Orange at Cummins recon. These were generally good values but were not
At all “race engines”. Race engine builders would sometimes tear them down and complain the clearances were
the same as a 1999 truck engine. Go figure...that’s what they started out as.

When production stopped, we then ate up OE service long block stock. Slight cost increase.

When that was gone, we ate up OE service short block stock and added OE service heads.
It cost more, the price went up.

Then we ran out of OE heads and tooled up the magnum r/t iron heads.
They flowed better than stock. With the r/t head swap, The 380hp variant pulled easy low 400s in testing and was
Re-rated to 390hp. The magnum r/t heads were not without issues, most problems were
Due to the type of tooling used to make them. I Could go on for a long time on the topic but it was
Somewhat Hobbled from the get go with the wrong type of tooling. Still, they had impressive potential
“For what they were” as articles and testing by Dulcich back then proved.

The price went up on crate engines again due to the heads’ expense.

The aftermarket noticed there were no OE heads to be had, and EQ tooled up.

Then we ran out of short blocks. The solution came from reman.
Reman drove the price up a whole lot. The price was high, and some were understandably
Disappointed they were getting a bored out rebuild instead of brand new.
The reman versions had a slightly higher perceived quality piston,
A hypereutectic, but most all other specs were the same.

Then reman kept saying they weren’t satisfied with
Core quality, and that 1 block in 20 was good enough for performance crate engine
Use. Personally I didn’t buy it. But Whether true or made up, they wanted out.

The last nail in the coffin was a super expensive deal, based on a brand new MP aftermarket race block version built
By a pro race engine builder. These were very nice pieces but the high price tag was a deal killer for most folks.
I always assumed more of these were given to show car builders for advertising than were sold.

Around this time a blowhard magazine writer who will go unnamed basically said Magnum R/T heads were overpriced garbage and EQ heads rule so RT heads were killed off, replaced by repackaged EQ heads. I heard about this and promptly collected
A few RT heads for later, just in case.

Now, EQ has also got rid of the product but some surfing shows a made in China casting that looks
Suspiciously close to the EQ, and of course the price is next to nothing.

The 4” stroke iron head version was probably my favorite, as the output made a lot
Of folks happy.

Looking back: some catalogued products you’d think were widely used were not popular AT ALL....
Other items that seem like almost stock replacement (iron heads) were actually pretty darn killer/sleeper products.

Thank you- Your recollection matches mine, the packages were always changing due to parts running out.

Also if thinking about buying heads from China when Relentless Performance was doing business I looked at purchasing a pallet or two of new China cast 2.2 SOHC heads and they had a 50% failure rate during routine pressure testing.

Yeah, that's a mighty high failure rate!!

I'm not expecting much from them.

https://www.alibaba.com/product-det...list.normal_offer.d_image.37581517wtWUgr

I swear they were using aluminum from recycled cans cast in a backyard foundry.