Jeep 4.0 Myth Busting (What Mods Really Equal Power Gains?)

Chris

Administrator
Staff Member
Sep 28, 2015
35,380
Salem, Oregon
This article was originally found on fourwheeler.com. I'm reposting it because it's a helpful and informative article regarding the long-lived 4.0 engine found in many Jeeps, including the Wrangler.

Over the years, Jeep's 4.0L engine underwent a few changes here and there, but nothing drastic. Despite the similarities, many still think of the '91-'95 XJs as the fastest six-cylinder-powered Jeeps in stock trim. For some reason, the '97-'01 XJs just don't feel as fast. Did they change the engine? Did OBD2 kill the 4.0L? Probably not, because if you drive a '91-'95 YJ or a '97-'06 TJ, there's no appreciable difference. The older XJs probably just feel faster because they have less sound deadening material and some other heavy components. At least we think that's the case. And with that, let the myth-spinning begin.

We've heard dozens of fish stories about how to make power with the venerable 4.0L Jeep engine, which parts are better, and which ones are wastes. Well, we've got around a half-million miles under our tires thanks to the trusty 4.0L Jeep inline-six. We've also got well over 100 dyno runs on these engines and have a good idea of what makes power, what improves drivability, and what wastes your money.

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Cold Air Intake

Myth: A cold air intake with an open dry or oil-impregnated gauze filter will increase power and mileage.
Our Thoughts: While not all aftermarket filter media are as efficient at trapping dirt as the factory-spec paper plate filters, there's no denying you'll see improved mileage and feel a definite seat-of-the-pants improvement.

Claimed Gains: 10-20hp and 1-3mpg
Actual Gains: 5-10hp

Notes: A good cold air intake will wake up any '91-up MPI HO 4.0L with snappier off-idle acceleration, improved top end pulling power, and a definite 1mpg increase. On some models, the major benefit comes from replacing the kinked, convoluted factory ducting.

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Reflash Computer

Myth: "I'll just have my factory computer reflashed or a new program installed to deal with my automatic-to-manual transmission swap, stroker engine, supercharger, turbocharger, or whatever."

Our Thoughts: You can't reburn a Mopar computer. At least, we haven't found anybody who can do this. You can run aftermarket piggyback programmers like a Unichip or JET chip, or run a unit that will slightly overwrite certain performance parameters of the factory computer, like a Superchips or Hypertech. These can change the fuel maps or alter timing to some degree. However, we've found these have limited results on later '96-up OBDII computers.

Claimed Gains: That it's possible.
Actual Gains: Can't do it.

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Throttle Body Spacer

Myth: Adding a throttle body spacer will increase the intake plenum volume for more air and a higher velocity intake charge. Also, rifling an aluminum spacer helps air flow into cylinder head with less turbulence for more power.

Our Thoughts: Depending on the application, a spacer works wonders on a carbureted or a TBI-injected engine in which the air/fuel mixture atomizes and flows through a wet manifold, but results are less drastic on a MPI engine in which only air flows through the intake.

Claimed Gains: 10-15hp
Actual Gains: 0-3hp

Notes: With our Red and Mileage Master projects, the largest power increase we've seen on the dyno was 1hp.

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62mm Throttle Body

Myth: The factory 60mm throttle body is a choking point in the stock engine. Boring the factory throttle body or installing a larger-bore aftermarket throttle body will allow the engine to breathe and make more power.

Our Thoughts: On most stock or slightly modified engines the factory 60mm throttle body isn't the cork, and is capable of supplying the engine with enough air to get the job done. Only on a vehicle with airflow modifications like a cold air intake and free-flow exhaust will you notice a difference.

Claimed Gains: 10-20hp
Actual Gains: 5hp

Notes: We did see a 5hp difference with a 62mm throttle body on a 4.0L with a cold air intake and exhaust modifications and believe the increase would be larger with other airflow enhancements, like a larger camshaft or high-flow cylinder head. It's a complementary component best used in concert with other products.

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Head Games

Myth: The '91-'95 7120 casting number are hands-down the best power producers, followed by the '96-'98 0630 and then the '99-up 0331s. The early heads have far superior flow numbers and resist cracking better than the later heads and will make way more power.

Our Thoughts: We don't argue the flow numbers of the '91-'95s are the best, but the later-model HO head flow numbers are generally within 10cfm at the crucial mid-lift areas. Given the conservative factory camshafts, that 10cfm isn't going to do much. All 4.0L heads feature only moderate flow numbers for performance, but offer high-velocity, which is good for low-mid rpm torque. And any '91-up HO head will absolutely kill any '87-'90 non-HO head.

Claimed Gains: 30hp
Actual Gains: 5-10hp

Notes: With the relatively small duration, low-lift factory camshafts there's not much need for cylinder heads with huge flow numbers. The factory heads are well matched to the factory components. It's only when increasing camshaft duration and lift profiles that aftermarket aluminum or ported 7120 heads show their true advantage.

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Ignition Hop-Ups

Myth: A hotter spark will allow you to run a bigger plug gap and burn more fuel more completely.
Our Thoughts: It's sound theory, but impractical in practice for most of the 4.0L engines running around out there. Not only is the 4.0L calibrated lean from the factory to aid in mileage and emissions, but most HO engines come with a pretty good ignition that will allow 0.045-inch plug gap with no trouble. It's only when forced induction or larger injectors and different camshaft profiles come into play that hotter aftermarket spark components prove their worth.

Claimed Gains: 15hp, 20 percent better mileage
Actual Gains: 0-3hp

Notes: You may see some big numbers from 4.0L ignition modifications, but it'll only happen if there was something wrong with the existing ignition system, in which cases even fresh factory replacement parts may garner the same results.

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After-Cat Exhaust

Myth: Removing the restrictive muffler and exhaust pipe behind the catalytic converter will greatly increase drivability, performance, and mileage.

Our Thoughts: Car manufacturers often design exhaust systems to cancel out resonance and unwanted noise at some sacrifice to mileage and power. They almost always leave a little power and efficiency on the table as a result.

Claimed Gains: Up to 25hp and a 10-15-percent bump in mileage
Actual Gains: 5-15hp and 10-percent fuel mileage

Notes: We've realized very noticeable seat-of-the-pants improvements with after-cat exhaust systems, as well as bonefide dyno verification.

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Header/Manifold Crack Issues

Myth: All 4.0Ls will crack the factory tubular exhaust manifold at the collector. The extra air getting to the O2 sensor will make the Jeep run rich, foul cat, and run like crap. The fix is to add an aftermarket header for more power, durability, and longevity.

Our Thoughts: We agree with the above. The exhaust system of a 4.0L Jeep has to deal with very high temperatures due to the 4.0L's lean air/fuel calibration that, when coupled with a long and inflexible intermediate tube that leverages the tubular header, is a perfect recipe for cracking. In fact, even some aftermarket steel and stainless steel headers may not prove immune.

Claimed Gains: 5-10hp and long-life
Actual Gains: 5-10hp and long-life

Notes: Despite its fairly efficient design, you will notice a seat-of-the-pants improvement with an aftermarket header when used in conjunction with an after-cat exhaust and a cold air intake. Just buy the header with the thickest gauge tubing you can.

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Forced Induction Fueling and Mileage

Myth: Adding a supercharger or turbocharger will actually improve your mileage because the engine will be making more power and will require less throttle input to maintain the same speed.

Our Thoughs: Anytime you add a supercharger or turbocharger you'll need to up the factory 19lb-hr injector size up to 21-24 lb-hr injectors. Even though you're using less throttle per given road speed, the forced induction is still putting more than one atmosphere's worth of air/fuel mixture down the intake. The level of boost increases the brake-specific fuel consumption. Regardless of the techno-babble, we've always seen a drop in mileage with a 4.0L forced induction system in everyday mixed driving.

Claimed Gains: 0-3mpg
Actual Gains: -2 to -5mpg

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'99-Up Horseshoe Intake

Myth: According to that great wealth of information that is never wrong, the Internet says you'll get a 5-30hp gain thanks to the '99-up 4.0L's swooped, equal length runners and larger plenum volume.

Our Thoughts: While a free-flow intake manifold can deliver more power, it's only going to allow as much air to flow as the cylinder head and camshaft will call for. Upping the intake runner volume too much will cause the intake charge to lose velocity, killing low- and mid-speed power and torque. On a '98-earlier engine running a stock camshaft and unported cylinder head, it's wasted effort.

Claimed Gains: 5-30hp
Actual Gains: 5hp loss on Trasborg's '98 XJ (Project Mileage Master)
 
OP
Chris

Chris

Administrator
Staff Member
Sep 28, 2015
35,380
Salem, Oregon
Also, let me say that I'm VERY skeptical of any gains provided by a 'cold air intake'. However, I'll take that one with a grain of salt ;)
 

KhakiKraken

Member
Oct 29, 2015
55
If you run a cold air intake on the Dyno with the hood off so it can ACTUALLY pull in cold air then yes there are big gains. Us wrangler owners on the other hand know that we are pulling in that nice mega hot air from under the hood and that doesn't help. Talking with guys who have louvers installed say that it is more apparent. I notice a HUGE difference in the mornings here in PA when its cold, the jeep seems just to have more pull going up hills.
 
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OP
Chris

Chris

Administrator
Staff Member
Sep 28, 2015
35,380
Salem, Oregon
If you run a cold air intake on the Dyno with the hood off so it can ACTUALLY pull in cold air then yes there are big gains. Us wrangler owners on the other hand know that we are pulling in that nice mega hot air from under the hood and that doesn't help. Talking with guys who have louvers installed say that it is more apparent. I notice a HUGE difference in the mornings here in PA when its cold, the jeep seems just to have more pull going up hills.
That makes perfect sense. I think the issue I have is that most people assume that they'll make big power with a cold air intake, when in reality you don't really notice it at all. My Jeep came with a cold air intake but I would never pay big bucks like some of these companies want for one.

Actually, it's time to sell my cold air intake since I have a supercharger on the way.
 

pbuttiglieri

New Member
Jan 8, 2019
7
ma
considering you have a dyno and i do not, my comment clearly must be taken with a grain of salt. My 2002 4.0 absolutely saw gains with the K&N cold air and hypertech programming. when programmed to run on high test it will also lift the front wheels off the ground - only a little. I also have 35's with 4.10s, a banks cat back and throttle body spacer, which i dont think does anything outside of making a whistle. but the first 2 were absolute power jumps to the engine.
 

pbuttiglieri

New Member
Jan 8, 2019
7
ma
So for the fun of sharing. i decided to temporarily fix my 2002 4.0 and postpone the 6.0 ls swap till I have the 4000 readily available for the needed parts and to do the engine right. But since i had a cracked piston skirt (compression was still between 105-120 lbs on all cylinders) it required a chunk of disassembly. Parts being replaced are: piston rod bearings, piston rings (obviously the bad piston), Fuel injector filters and seals. Now the fun..... I port matched the gasket to the intake manifold, Factory 2 piece exhaust manifolds and head. I also ported and polished the head. Next i cut off the flat sections of aluminum on the intake as they are essentially heat catchers and we want a denser cooler charge of air into the cylinders. I wrapped the exhaust manifold also to control the heat against the intake and got a heat sleeve to go over the fuel rail to prevent the vapor lock i have heard about (was an early recall on the 2002 that i never experienced) as i am planning on not adding the recall heat shield that went between the intake ports. I am roughly under $400 including the machine work for the original valves. In a week or 2 i should have it all reassembled and able to give a yay or nay to improved usability.
Expectations are for a snappier engine throughout the rpm range as the exhaust seems pretty restricted, but may still be limited to the size of the valve/cam. I was good for 16 mpg before (with 4.10s and 35's, hypertech program, K&N cold air, Banks cat back, and possibly useless throttle body spacer). Stock i was good for 20-21 mpg.
 
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pbuttiglieri

New Member
Jan 8, 2019
7
ma
i actually started it Saturday and it fired up after twisting the cam sensor - which i removed to prime the engine. so now i need to find its sweet spot as this tells it when to fire the injectors! Gar! right now i cant say i found much for gains. i will add that i felt it was a bit snappier before i pulled it apart, so porting and polishing the head may have backfired on me. still need to fine tune and hopefully will have better results.
 

pbuttiglieri

New Member
Jan 8, 2019
7
ma
engine went back together and after a little re calibrating (lining up) of the cam sensor on top of the oil pump shaft (where the distributor would go) sounds great and feels great. some more umph. not gobs of it, but certainly better. Still off the road so i will find out how its fuel economy is in a month or so.
 
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Hook_62

Member
Mar 7, 2019
30
PA
engine went back together and after a little re calibrating (lining up) of the cam sensor on top of the oil pump shaft (where the distributor would go) sounds great and feels great. some more umph. not gobs of it, but certainly better. Still off the road so i will find out how its fuel economy is in a month or so.
The work you have done is all good, with one exception, I didn't see that you replaced the cam. I am currently doing a 4.0 rebuild myself and have done gobs of research into it. The stock head on a 4.0 has small exhaust ports which creates velocity, which is the part of the reason why the engine has good torque numbers especially at low rpm. When you port match and polish your head you almost have to replace the cam with a stage one performance cam to maintain the changes that you have made to the head. Good cam here, https://www.summitracing.com/parts/hrs-320111-12

This cam does not require any computer changes. Works in model years 1996 - 2006.
Basically what happens, you have increased the flow characteristics of the head (CFM) but, by not changing the cam you are not giving the head valve lift to actually increase the CFM the head can now handle.
 

pbuttiglieri

New Member
Jan 8, 2019
7
ma
I couldn't agree more. Part of me did the head polish because i could time wise and to play :D I was lightly on the fence of a cam (mostly because this was a quick fix until i drop in the 6.0) but i was having a tough time with a cam swap as they aren't roller cams and it would be an extra cost that wouldn't be long lived. I would be curious to how much gain i could get with a cam too. possibly the cure to holding 80 on the highway without shifting
 

Hook_62

Member
Mar 7, 2019
30
PA
I couldn't agree more. Part of me did the head polish because i could time wise and to play :D I was lightly on the fence of a cam (mostly because this was a quick fix until i drop in the 6.0) but i was having a tough time with a cam swap as they aren't roller cams and it would be an extra cost that wouldn't be long lived. I would be curious to how much gain i could get with a cam too. possibly the cure to holding 80 on the highway without shifting
Where did you find that hood, I love the louvers on the side?
 

Wildman

Over Analizer Extraordinaire...............
Supporting Member
Dec 12, 2015
313
In the hills of WA
Reactions: Hook_62

pbuttiglieri

New Member
Jan 8, 2019
7
ma
you got it! Daystar was also really cool as one of them cracked when i tightened it and they sent one out right away. I did also modify my install as those hood vents that come with 3 grommets per vent and the instructions are to drill 3 holes per vent so heat escapes and water doesnt fall in. I changed mine so one of them feeds my K&N cold air intake thats blocked off from the engine heat with home AC foam. i also made a piece of aluminum that hooks on the nose brace bar and prevents water from contacting the air filter. from a hose or something as it is protected with a plastic edging around the hole cut in the hood.
82284
 
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