4.0L intake and exhaust upgrades

[cdrake261]

How much more torque and HP can one gain by changing the intake to a ramair intake and make the exhaust more free flowing? I'm not as worried about HP as I am about torque, and I know on smaller engines(honda's, etc.) require back pressure, but not sure how the 4.0L responds to that.

 

[Dr. Dyno]

No engine "requires" backpressure. The optimum amount of back pressure is as little as possible.
The important thing is to get the correct pipe size for the expected HP output. On a 4.0L Jeep that's 2.5". A free-flowing exhaust from the downpipe backwards should add ~15hp. Aftermarket headers do very little except mellow the exhaust note and make it more pleasing to the ear. Most people buy them as replacements for the crack-prone stock header.
A good cold air intake combined with a 62mm bored throttle body will add a further ~8hp and tremendously improve throttle response. Combining that with the exhaust upgrades will add torque across the rpm range and give you a slight improvement in gas mileage (provided you can avoid the temptation to step on the gas).
If you have a '91-'99 XJ with the earlier-style log type intake manifold, updating it to the later-style curved runner manifold will add a small amount of HP/TQ and improve gas mileage.

 

[cruiser54]

One noticeable improvement is to get rid of the headpipe with the factory "crush" in it. If you're lifted 3" or more, you can purchase them from the aftermarket and install it yourself. If lower, any good muffler shop can make and install one custom by routing it in two 45* angles for front driveshaft clearance.

 

[Dmondeac]

Who makes a new head pipe?

 

[cruiser54]

I don't know. You're gonna have to shop around the parts stores. Supposedly, some of the replacements don't have the "crush". I always have my muffler shop guy make and install one. Do me a favor. If you do this, post up a pic showing just how crushed that pipe is. I don't have any old ones anymore.

 

[cdrake261]

So would I gain torque if I went 2.5 free flowing exhaust?

 

[cruiser54]

So would I gain torque if I went 2.5 free flowing exhaust?

Sure. Get rid of the crush though, for sure.

 

[dachacho]

borla makes a very nice header and downpipe/crossover pipe. expensive but very good quality. my .02 is that i dont like 4.0l exhaust to be too free flowing. a high flow cat and flowmaster muffler is perfect and offers the best sound hands down. ive seen 4.0l's come alive with intake upgrades. a simple high flow cheap filter wont really do anything substantial. where it will shine is once you get a larger throttle body as well as a real cold air intake with smooth bend pipes. its all about ambient air temp, and how much air the engine can possibly pull in. an exhaust wont do much without intake work because you can only push air out as fast as you can bring it in.

 

[cdrake261]

borla makes a very nice header and downpipe/crossover pipe. expensive but very good quality. my .02 is that i dont like 4.0l exhaust to be too free flowing. a high flow cat and flowmaster muffler is perfect and offers the best sound hands down. ive seen 4.0l's come alive with intake upgrades. a simple high flow cheap filter wont really do anything substantial. where it will shine is once you get a larger throttle body as well as a real cold air intake with smooth bend pipes. its all about ambient air temp, and how much air the engine can possibly pull in. an exhaust wont do much without intake work because you can only push air out as fast as you can bring it in.

I'm a big fan of ram intakes, but I absolutely hate the popping sound some cars has...reminds me of my honda days....never again!

 

[i973eat1]

borla makes a very nice header and downpipe/crossover pipe. expensive but very good quality. my .02 is that i dont like 4.0l exhaust to be too free flowing. a high flow cat and flowmaster muffler is perfect and offers the best sound hands down. ive seen 4.0l's come alive with intake upgrades. a simple high flow cheap filter wont really do anything substantial. where it will shine is once you get a larger throttle body as well as a real cold air intake with smooth bend pipes. its all about ambient air temp, and how much air the engine can possibly pull in. an exhaust wont do much without intake work because you can only push air out as fast as you can bring it in.

I just made my own CAI, I used a 3" piece of PVC at a 45 degree bend and a spectre air filter, just using the rubber piece that goes to the stock airbox to put it onto the 45. It definitely gave it some more oomph. I also have the stock throttlebody with a 1" spacer on it, and stock manifold and exhaust. (180,xxx Miles on it) That is all going to be upgraded soon. I cant wait to feel the difference it will make!

 

[crimsondragon]

Make a cowl air intake. This ensures cold air and raises up the air intake by about 4 inches. It will be loud though since the cowl also provides air for the cabin.

 

[MoparManiac]

I'm not 100% sure if it fits a stock application or not, but the YJ downpipe I used when I had the 4.0 in my MJ did not have the factory dent in it.

I got it through Crown Automotive.

 

[Talyn]

I'm a big fan of ram intakes, but I absolutely hate the popping sound some cars has...reminds me of my honda days....never again!

Ram intakes? lol.

As for the pooping sound with a FI engine that shouldn't happen. I've heard some people actually retard their timing to make it happen.. but I haven't seen it first hand.