I have an 03 dodge ram hemi 4x4 with 35's, stock gearing (3.91?), with a superchip, TBS, intake, and cat back, and was thinking about installing some long tube headers, and maybe deleting the cats. i was wondering how much low end i would lose, as most of my driving in highway and i occasionally haul a boat. i am looking for some gas mileage improvements obviously, but will i see any, and if i do will it be worth any loss of low end? gears are a little out of my price range right now with school starting up unless i only put in the rears for now. any thoughts would be appreciated

 

Hammer should be chiming in here soon, I know he has long tubes and had the cats off, but put em back on because of the low end torque loss.

 

removing the cats, you'll loose a good bit of low end power. your best bet is to throw some hi-flow cats from magnaflow in. your also going to have to move the cats backa few feet because of the long tubes. your also going to have to extend the wires on the o2 sensors (not a big deal). as far as gearing goes, its somewhat a necessity. you'll see a huge difference when you change them out. and you CANT just do the rear. you gotta do both front and rear at the same time. you cant be driving around with different ratios. most of our truck come with the 3.92 from factory, some have 3.55. with 35s you want to opt for 4.56 gears for the more power. but since its mostly on the highway you may want 4.11's. some one will know better on that though.

oh and you can ditch the paper weight (TBS). it does just about nothing for you!

 

Ok, I'm sure my critics will chime in, but this is what plain old experience has taught me:

First of all, the "I'm looking for gas mileage improvements" went out the window when the 35" tires went on the rims. Single worst thing you could have done for fuel economy except maybe put a big old parachute behind the truck.
LT headers DID get me about 1-1.5 mpg after I installed them, provided I could drive the same way I did before them (which ain't happened after I got over my mpg tests).

The gearing IS super important for performance after installing the tires, because, and I know there are many who will disagree, but the Hemi has a sweet spot, for maximum performance and optimum mpg's. Don't let it get to that spot and both will suffer. The taller tires are not letting it get there or stay there.

You get the engine breathing a lot better and you will lose low end torque, plain and simple, because it's back pressure that is supplying you with the torque at low rpm's.
Back pressure is inefficiency, plain & simple, therefore you are not going to have low end torque and maximum fuel economy. The only way I know of to have both is to maximize air flow and change to a cam with a grind specific for this.

Since I am still running with the stock cam, and it looks like I will be for a little while anyway, the only advantage I get from the CAI, exhaust & LT headers (besides really awesome sound) is that my mid/high end HP is up quite a bit from stock and I get slightly better mileage when cruising (which is pretty much negated with the 4.56 gears - great for mpg in city driving, but hurt you on the interstate).

The deletion of the cats was felt right away after I installed the headers, not a huge loss of low end, but noticeable, and since I have always had the plan to INCREASE low end torque, to me this was unacceptable. So the cats went back on (about 18" farther back to allow for the header length and I stayed with the fairly restrictive (but great sounding) Flowmaster 50HD muffler.
When I do the cam I want (and have more low end then I'll know what to do with) I'll go to high flow cats and a less restrictive muffler (Cherry Bomb I'm sure). For now, I'm relying on inefficiency for my low end torque.
So if I were you, I'd make up my mind how important fuel economy is. If it's the most important thing, ditch the 35's, go back to at max 33" tire, keep the stock gearing and you can get the headers and you'll see better mpgs on the highway.
If it's performance and you want to keep the big meats, then ditch the header idea for now and save up enough to re-gear...

 

the only online shaft dyno test
of 'long tube' headers on a 5.7 Hemi
that I would consider trustworthy
is Steve Dulcich's test prior to his work on 5.7 camshafts:


Caption:Baseline testing the crate engine in totally stock form netted peak power numbers of 376 hp at 5,700 rpm, and 372 lb-ft of torque at 4,600 rpm, at STP correction factor. Not bad! We figured since we were going to run some hot cams through the engine, we'd first address the exhaust situation by ditching the factory log manifolds.



Caption:TTI makes these beautiful 1 3/4-inch full-length headers, designed to fit a new Hemi into a conventional A, B, or E body chassis. The headers were worth big dividends in power and torque, raising the bar to 393 hp at 5,700 rpm, and 390 lb-ft at 4,700 rpm.

Note that the above tests are without mufflers in both cases
and that any muffler will reduce the percent gains found

the torque gain found here was about 5%
(390-372/372)

typical gain with mufflers and catalytic converters is more like 3%

Depending on primary tube length
headers improve torque in some rpm range
and unfortunately,
take torque away from other rpm ranges

if you are serious about wanting improved torque in the
'daily driving and towing'
1500 to 2500 rpm range
you need primary pipe length in the 42 to 48 inch range
and I don't know of any headers for sale for the 5.7
that have those lengths