Motor has no balls past 50mph

Got a 97 3/4ton 4x4 with the 5.9 magnum bored .090 over and the light duty NV4500 5speed. Truck is sitting on 285/75/r16s with alloy wheels and has 4.10 gear ratio. The odometer reads 354,000 miles on it. I bought from a buddy I trust very well in HS a couple of years ago. He said the guy he bought it had the motor and trans rebuilt.
Well they both run great sort of. The trans is in perfect shape because I have taken it off after I blew the intermediate shaft and replaced that. All of the syncros and gears are in great shape. The problem is the motor has no balls past 50mph. There is no proof anybody has that the motor has been rebuilt. But the oil pressure is solid even after long drives and it idles and drives great. The truck is a regular cab long bed and has a grille guard (not front end replacement) and a Ranchhand rear bumper. I can burn the tires in 1st through 3rd gear without any problem if I want to. But once I shift into about 4th and get up to highway speeds I can stand on the pedal and the truck is awfully slow. I know the heads and larger tires have something to do with this but they offset each other and the truck doesn't weight much, only about 5400lbs.
I've been lurking on here and seeing that maybe the plenum has spewed some oil and it has maybe clogged up the cat convertor. I don't think that's the problem because I haven't noticed any symptoms of the leak.
I'm about to get the basic beginner mods like a CAI, programmer, long tube headers and an exhuast. I have some questions about pepping the truck up and the exhaust setup.

First off, I want the best performance exhaust I can get and I don't care about any legalities either. This obviously means no kitties and no muffler, but do I need short headers or long tube? And I think I need x-pipe setup somewhere around 2.5" but I'm not sure.

Second question is, once I get the exhaust, CAI, and programmer on how much of a difference is it going to make in the powerband? I'm assuming I need the SCT custom tuning, but I just want the best balls to the wall power I can get from that.

If the plenum is bad I'll go ahead and do that and just get a replacemnt intake. Also how much am I looking at to get a decent set of heads and a low powered cam for my truck? I can do all the work by myself except for exhaust.

I also want to throw on a hotter distributor, spark plug wires, and performance spark plugs? How much will these help?

FYI truck has had tune up and I am know it very well. FWIW the best fuel mileage I have ever gotten was 7.7 and that driving like a grandpa but not letting it get lugged down

I'd say hold off on the mods, check if your plenum is steel or aluminum (magnet trick), and if its steel do the replacement weather its showing other symptoms or not. While youre doing that, do your ignition stuff because its a hell of a lot easier to get to the cap whilst the intake is off. And route those wires so they dont cross.

Also think about doing an intake upgrade (mopar M1 or Hughes) instead of the plenum upgrade (intake upgrade gets rid of plenum pan alltogether if I'm not mistaken).

And welcome to RF!!!

What do you mean by a "hotter distributor" I don't really think that a distributor has anything to do with ignition power. If you want to do some ignition work first you can replace cap and rotor, plug wires and plugs. Then a better ignition coil like an MSD unit. Generally speaking also, performance plugs don't do all too much for a stock engine, NGK stock replacements will be best.

As for the plenum I'd pull it and check the gasket. If your gonna do it then I'd just replace the gasket. I think there is a good kit for it on ebay but I don't know what it's listed as Dodgedude94 might know and I'm sure he'll chime in soon.
Long tube headers are the best for high horsepower applications and if your going to install those then you could just run a true dual exhaust with no cats. 2.5 inch tubing would be the best for true dual and 3" for a single exhaust.
Hope any of this helps.

Check the cat by starting the truck and attempting to rev the hell out of it. If it revs easy, the cat is good. Simple.

Can rev it to 5500 no prob. If the intakes aren't too pricey I'll do that. How much do they help performance wise? X-pipe exhaust okay or is that overkill?
Thanks for all the info guys, I know a lot about diesels and I prefer them but I've had this truck a while and now I'm stuck with it.

Sorry for all of the questions, but is there a sticky or thread for engine performance? I've looked some but haven't found one yet. I am going to do all of the previous mods, but will I get more performance out of a cam, heads, or intake with a basically stock truck.

Sounds like a plugged catalytic converter

It's the stock cat and it has 354,000 miles on it

id suggest getting the programmer. you can change shift points around as well as control your powerband and such. intake and exhaust will also help, i would suggest a straight pipe with a side dump. also just take it to a shop who builds motors and see what they suggest for getting the most power out of it for the least amount of money. those big tires dont help any, but those gears should help out a lot. best of luck. my truck being a reg cab short bed sport has plenty of power with just intake and exhaust. plus i only have 120,000 miles and no huge heavy bumpers

Intake upgrade (NOT just a CAI), exhaust and programmer would certainly get you some power back. But I'd look for the cause of the symptoms and fix that before you start trying to throw more power at her... although, fixing a blown plenum gasket with the intake upgrade would kill two birds with one stone. I was going to do that but don't have the cashflow.

The motor has not been bored .090 over .060 max. Next do a compression test on the motor see what you get for numbers.

Max size pipe for a 360 mag is 2.25 for duals. As for headers don't cheap out is all I'll say, expect to spend more than a few hun on a set.

How many miles are on this motor?

Timing chain comes to mind, plugged cat con.

97 trucks and under don't get the big bang from the tuner like the 98+ trucks do unless they have the death-flash done to them. If you get into the mods you are planning then yes one is needed.

CAI save your money and just get a drop in K&N filter, unless you get a Volant or AFE CAI they are the only ones I would buy for these trucks.

Both the Mopar M1 and the Hughes F1 intake get rid of the plenum pan.

Don't be one of those....... run a muffler or two.

Merc225hp said:

Don't be one of those....... run a muffler or two.

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+1 for awesomeness.

If you really want to go dbag and wake up the neighbors, at least do it with a cutout so that you arent constantly ******* everyone off. Great thing about cutouts is you can sneak up on a prius and then WHAAAAAAHPPBLAPBLAPBLAPWAAAAAAAAAHHHHH!

Right now I'm looking at the CAI, Hughes Intake, long tube headers, 2.5" true duals no cats no mufflers, SCT tune, and a hotter ignition. That should wake it up a little. And guesses on how much extra horse that is? And are there any good threads regarding engine performance on the 360's?

Somewhere between 35,000 and 50,000miles. Has a small rear main seal leak. Lose about 1/2 qt every 3,000 miles. About 2.5" in diameter whenever I park. Don't think this has anything to do with performance but thought I'd throw it in.

MillenniumFalcon said:

Right now I'm looking at the CAI, Hughes Intake, long tube headers, 2.5" true duals no cats no mufflers, SCT tune, and a hotter ignition. That should wake it up a little. And guesses on how much extra horse that is? And are there any good threads regarding engine performance on the 360's?

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I'd seriously consider getting a high flow muffler, cutouts are cheap and quite entertaining. Also replace your IAC and TPS, often a bad plenum will foul both.

Not much and you will lose some bottom end going with the Hughes intake. I would install a smaller cam and or a set of 1.7 rr's before the M1 or the F1 intake was bolted on.

Merc225hp said:

Not much and you will lose some bottom end going with the Hughes intake. I would install a smaller cam and or a set of 1.7 rr's before the M1 or the F1 intake was bolted on.

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Mark is spot on like usual.


And I'm with ADudeOnSkis, install a high flow muffler like a SpinTech or something.

dodge dude94 said:

Mark is spot on like usual.


And I'm with ADudeOnSkis, install a high flow muffler like a SpinTech or something.

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Rasta.

ADudeOnSkis said:

Rasta.

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Que?


lol