zeep99xj

So your saying i will see no gain if i upgrade to a 2.5" system from the headers back? Im really looking for better MPG and highway performance on my DD XJ, but i dont want to lose too much torque either. Will it balance out the low end torque/ high end HP if i were to run a 2.5" system with some better headers?

TheJerm

Its all about back pressure, To make low end torque you wan short intake runners and short exhaust runners that is rather restrictive to have back pressure. If you open the system up with fake CIA (like most ppl to), larger TB, 99+intake on older HO's, Header, hi flow cat and larger exhaust pipes then you going to lose toque but make the engine breath better and make more power at higher RPM's

You cant have both good low end torque and great HP in higher RPM's, it just doenst work that way.

If you want better MPG then keep it stock with smaller tires and regular maintenance. If you make it breath better then its going to use more fuel to even it out and thus mileage will go down.

CAEMI

[QUOTE=TheJerm;577642]Its all about back pressure, To make low end torque you wan short intake runners and short exhaust runners that is rather restrictive to have back pressure. If you open the system up with fake CIA (like most ppl to), larger TB, 99+intake on older HO's, Header, hi flow cat and larger exhaust pipes then you going to lose toque but make the engine breath better and make more power at higher RPM's

You cant have both good low end torque and great HP in higher RPM's, it just doenst work that way.



I agree. Torque and HP are in conflict. You can't have both unless you increase displacement, all other things being equal. I'm not so sure about the gas mileage effect. In my experience, larger tires mean lower RPM, mean higher mileage. Also, an engine that makes more power (within limits) using the same displacement usually gets slightly better mileage.

Personally, I don't think an upgrade to 2.5" from the cat back will add power. Good headers will add a wee bit of power but, frankly, you'll be hard pressed even to notice. It usually takes significant changes to notice significant changes. I know that sounds, well, smart-alecky but that's not how I intend it.

I found that if you change the intake manifold on a hotrod engine, you won't notice a material change. If you just change from cast iron exhuast manifolds to headers you might notice a wee bit - maybe the effect of 3 to 5% maximum. On a 375 hp engine, that won't hardly change your dragstrip ET as much as the correct tire pressure will even though you pick up 20 hp.

But, if you change the intake and headers, you might get 10%. That you will notice. If you change to better flowing heads you'll get another 5%. If you change your camshaft to give more lift and duration, you'll get 5%.

Add them all together you might pick up 40 to 80 hp. Forty hp on 375 will change your ETs. 80 hp will make you smile...a lot. Big smile.

So, in my experience - and that is pretty limited when you consider all the other stupid mistakes I have made with cars just in the last two weeks - it takes several changes working together to make a difference that makes you feel like you got your money's worth...especially if you dont' have money to burn.

My 94 came with a Borla header because the DPO said the stock exhaust cracked. Apparently, it is common. So, if you've got a bad exhaust manifold, might as well put a header on it. That said, headers burn out and cast iron doesn't. It's a coin toss in my mind.

Don't mean to blabber but my thinking is change several things if you really want more power. Just one thing out of four won't give you the big smile.

That said...I love changing stuff. I change things just because I can. My kids got me a new creeper for Father's Day. I am geeked. I've been using the same wooden one my Mother bought me for my birthday thirty-five years ago. Old style with steel wheels that hardly rolls on concrete anymore. The new one has PADDING. And a headrest. And ball bearing wheels.

I'm gonna go change the trans fluid or something.

zeep99xj

Well stock and smaller tires just arent an option

TheJerm

^ larger tires means lower rpm's and better mileage? Everyone running larger tires get terrible mileage because the motor has to work harder to push those around. I get 20 mpg with a mix of city/mountian/highway driving everyday and 23 with all highway. I have stock height and 235-75R15's. 99 intake, APN header, high flow cat, muffler mech fan and a/c delete. I lost some low end torque but whatever.

I'm sure you running larger tires can only dream of getting better than 16ish

CAEMI

I agree - large tires - 31" or bigger - is going to cost you mileage. Like you, I run 235-75R15's but I think stock was 215's. I think a little larger tire will help, IMO, but way larger - like 31" - will hurt.

I'm hoping that 235's and the Borla header will give me a wee bit more mileage. I've got 4.11's so any improvement I get will be a gift. I could probably turn bigger tires yet but the 235's are about I think will clear in stock config.

TheJerm

the only difference between 235's and 215 is the width of the tire. The second #, 75 in our case, is height based on the % of the width. You could get 215-95R15 and be the same diameter as 235-75R15. Just an FYI.

This thread got way off topic lol

Stock exhaust manifolds crack all the time and upgrades are always better than replacing the stock manifold a couple times. I think the only type header to keep low end torque would be one with small tubes coming from the ports. Correct?

edit: if you want better mileage you might want to look into getting stock 3.55 or even 3.07's from manuals if your going to run lifts and bigger tires in the future

CAEMI

Yeah, it did wander off topic, didn't it? But it's not that bad...it started out a thread on header size. This is complicated stuff if you mean to get your money's worth. Nobody likes to spend money and not get the results they expect.

BTW, my DPO put a Detroit Locker in back and I plan to leave it. The 4.11's use a different carrier...so to go back to 3.55 I have way too much hardware to change.

And I forgot that the 215/235 thing was width only. Duh. Guess I suffer from bigger number syndrome. 235 is bigger than 215...must mean BIGGER.

As for header tubes, I think the manufacturer (Borla, others) know. I really only understand headers from a hotrodder's POV and even then only with BBC motors. The principles are all the same, of course, but I really understand BB chevy motors.

I do know that headers have to be matched to the rest of the respiratory system or they can do more harm than good. Somebody earlier noted that an engine is an air pump and you can only pump out what comes in. On hotrod motors the idea is velocity of flow - pump alot an pump it fast (RPM). That is why the cam profile is critically related to RPM. Too open and the combustion chamber empties before all the fuel/air mixture is burned and sucks your power potential out. Too restricted and you leave burned mixture in the cylinder...which means fresh mixture can't get it. (This is kind of why the cam is sooo tied to RPM. In a drag car, you want to hit maximum RPM at the timing light. If you build a 6800 RPM motor, you choose gears to hit 6800 at the finish line and choose a cam/heads/header to make max HP power at 6k on the dot.

This is all deeply related to when, exactly, the valves open and close, how long they are open, the cam lobe "ramp" (how fast they open and close), and many other variables. This is why roller cams work so well - you can have much more radical lift and duration without damaging lifters and that gives you the option of changing ramp slope (very nifty).

All this intel is way too deep for the average guy to make use of. Frankly, some of it is voodoo - the guys that REALLY know what works won't tell you. Drag racers are friendly guys but they play to win.

BTW, that's why Eldebrock sells "kits" that include heads, intake, and camshaft setups. This way you can be confident the stuff will work together - somebody bolted it on and set up on a dyno and found out the truth - no "butt dyno". Comp cams does the same - you call them up and they'll tell you, given your heads, headers, intake, carb/EFI, which cam will do what you want (torque or HP). It's all fun to learn about but man can you burn money and time exploring it on your own.

These days...stock works for me. All that detail gives me brain fever anymore. I just want the thing to stop making clank and bang noises while I'm still in the driveway...they upset my wife!!

96_xj

Alright let me chime in on something. I run a stock rewelded(it cracked =[ so i fixed it) header, down to the stock down tube (the restriction/needed back pressure). I then run a full 2.5" high flow system after that. I run a TRUE cold air intake(it is sealed off from the engine bay completely and about to be heat wrapped). stock TB, (i do have upgraded ignition and injectors tho..) And i can smoke my 33" swampers with 3.07's in the axles. I have not lost power in low RPM's and on the highway i can cruise. i get about 15 average with the way i drive (mind you if i re gear i will get extremely close to 20 mpg)

TheJerm

How can you stand driving with 33" tires and 3.07's? I hated the 3.07 in my heap when it was 2wd and stock tires, given it was in the mountains of NC.

96_xj

I drive mountains often. And it is not to bad. the only problem i have is on long hills i have to drop into 4th. I want to re gear down to 4.10's. but it all takes money. But like i said i can smoke my 33's. I can get up to 50 in 2nd in no time at all (4400 rpm -personal redline) and i have 175,ooo on the clock.

i dont race it regularly but i do drive it fast every now and again. (there is a difference in reckless and driving fast, i drive fast not reckless )

TheJerm

20 mpg with 6" lift is a big dream you have. I doubt even with regearing your going to get better than 15, it will probably fall actually.

Its not very hard to break the tires loose with a manual and i love having a locker to do powerslide around turns and what not. 4400rpms is right when the 4.0 starts making great power. Ive bounced mine off the rev limiter a couple times and i have 202K

96_xj

I think 20 mpg is about right/ Stock i was getting 23, It is not the lift, our cherokees are boxes from the start. It is the strain you put on the motor. when you regear to a tire/gear ratio that is close to stock the only strain on the motor is the extra weight of the tires. (or like have 2 adults with some luggage in the back of a stock cherokee).

What causes peoples mpg loss is what they add when they lift. They add roof racks, lots and lots of weigh in armor and it puts more strain on the motor.

So calculating that i have a big *** roof rack(altho it is pretty airy and dont provide a whole lot of resistance when empty) and a heavy 3/8" steel bumper are the only things playing into wind resistance 4.10's will put me in a stock ratio a 3mpg hit seems to be about right.

my engine does not have any tick, knock, or taps. i pulled the valve cover off, and the oil pan cover and they looked nearly new and shiny-no build up at all. and i am very strict on how well i keep my motor maintained inside and out i dont expect more than 3mpg loss over stock in that sense either.

only actually doing the regear will tell tho.

TheJerm

Once you lift and put larger tires on it, you will never get mileage like a stock XJ does. Its plain and simple. Ive never heard of any lifted jeep getting mileage close to a stoker cause its not possible with a lift and larger tires.

zedpapa

my 89 with a 3" lift and 31x10.50 BFG's still gets 18-19 commuting and very close to 25 on trips. the 4.0 really responds to opening up the exhaust. i have done enough to know. if you want to make a difference in torque, go 2.5 from the manifold to the cat, 2.5 from the cat to muffler and use 2.25 for the tailpipe. i prefer to use a flowmaster with center in/offset out. there is not a lot of room under the cherokee so its limited to the 50-series as far as size goes. i have a 40-series on mine and sounds good.

zedpapa