h-pipe x-pipe or neither?

In your case I think you're best off just keeping them separate. You won't see any mpg difference, and when kept separate the motor sounds a little more mean.

Now I just have to hope the cheap turbo mufflers sound good

If you want good MPG consider the flowmaster pulse tech header collectors and phase each bank 180 degrees for a 4 to 2 to 1, You Pair 7-1 and 3-5 on the Drivers and 8-2 and 4-6 on the Pass side....sounds pretty good too For mpg you want to keep the restrictions down but the VELOCITY up, you when cruising the motro is most responsive in that 35-75 MPH range and wherever your powerband falls for that speed. For max MPG I'd run generally the exaust system 1 trade size smaller than you would for max power. you may lose some peak power (maybe but doubtful) but you'll gain some effectively 'free' part throttle torque/MPG through the middle. This is generally how Mopar approached it from the factory BTW and in part why so many people were so happy with Chrysler's "passing power".

FWIw the factory mopar dual exhausts of the 60's and early 70's were exceptionally well engineered for what they were. so don't reinvent the wheel....except for a better flowing muffler system.

I rode in HotrodDave's mileage master 318 in his Cuda had TTI's straight pipes with an H pipe all the way out the back. It was a 273 2bbl cam. That car sounded sweet, and scared the Mustang boys.

The H pipe quieted my slant down.

Use whatever fits. The MOPAR engineers were very good at low dollar improvements, the H-pipe is one of them. The X pipe setup is better but harder to get under the car.

I like Dynomax Hemi Super Turbos. Not too loud but you know they're there. Listen to Streetwize as his advice is gained over years of experience.

R.

At part throttle daily driving the pressure in your exhaust pipe is so low its effect on fuel economy is aleady low.

You are only going to see a hit on fuel economy if the pressure goes over 2 psi.

Find a 0 to 10 psi gauge and plumb it with steel brake line to the exhaust just after the manifold. You need the metal line to dissipate some heat to protect the typical pressure gauge.

If you add special long tube headers (36 to 46 inch primaries)
and a short duration camshaft
you can get more low rpm torque.
With this additional torque you could use a numerically lower differential ratio and still climb mild hills.
These three mods working together would improve highway MPG but have slight effect on stop and go city driving.

360view. I believe the H-pipe works the same way as crossover circle track headers in that it connects the two sides of the engine's exhaust together.

It has, in my opinion, very little to do with backpressure. Another example of this is the old header vs manifold argument. Headers do their best work if they're long tube. This is again due to the tuning effect, separating the exhaust pulses for the correct amount of time. Many many dyno tests have shown long tube headers to increase the torque curve below the torque peak. Shorty headers do not have as much of an advantage because they aren't long enough. They're more like manifolds.

R.

The H pipe as i understand it, helps create a scavenging effect in the pipes, which helps with flow at higher RPM.

Since the car in question will be spending 90% or more of its time at 65 MPH +, then I would say it would be a benefit.

My 71 Newport with a factory 383HP motor, came with the high flow manifolds, and an H in the exhaust. It was only a 2-1/4" system, but im guessing it was there for a reason.

What MPG do you get now, or think you'll get?

If you're buying a new system, X will give you best economy by a slim margin, given that +HP = more efficient combustion = best fuel econ. Proper tune will factor much more into MPG.

But the money you'd save on gas might not equal the cost difference between a cheap stock system and the added cost of X or H.

ASSUME you might improve fuel econ about 5% - you would save 1 gallon for every 20gallons you buy. So what's the diff in cost for a new X exhaust system vs stock? $400? At $4/gal you'd break even after buying 2000gals of gas ($8,000 x 5% = $400). That's between 30-and40,000 miles of driving.
Best,
-Art

Remember the Headman Hustler 4-2-1's?

H pipe needs to be as far forward as possible.

Too big diameter pipes? Crap fuel mileage!

Ill have less than $300 in the whole system with the x or h pipe. The dual kit is $225 and an h or x for a cheaper one is $45. The car needs exists anyway vast the po had somebody delete the cat and did a horrible job cause it hangs about 2-3" off the ground.
I will also be installing headers. The car already gets upper teens stock and not running 100%

I put an H-pipe in my '68 chevelle back in the day, cost about $5.00 for the pipe and something like $20 to have someone put it in. That was back in my chevy days. Who knew that, even way back then, MOPAR engineers were putting them in their higher performance cars as OEM?

R.

If that 5th Avenue has the stock 2.21 or 2.45 gears in the rear axle, the engine is running quite slow even on the highway. With the engine running so slow, it shouldn't take a very large exhaust system to virtually eliminate back pressure at cruising speed. A large, freeflowing single exhaust (2 1/2 or 3 inch) may be nearly as good for mileage as a dual exhaust system in that particular application.

A vehicle with say 3.23 or 3.55 gears will force the engine to run considerably faster at highway speeds which means more exhaust will be produced. A dual exhaust would be more important for MPG with the numerically higher gears.

5th Avenue owners who want better performance or improved exhaust sound should definitely get dual exhaust. Dual exhaust could offer a slight mileage improvement over a freeflowing single exhaust. But I suspect it would take some time for the improved mileage to pay for the dual exhaust.

I know it wasn't asked, but if it was my car, it would get dual exhaust, a good 4 bbl carb., 2.94 or 3.21 gears, and an an overdrive A500 transmission from a Dakota. Installing a cop car suspension under it would be another great improvement. That would make a NICE cruising car!

I haven't checked the ratio yet. The cost of the dual kit alone is cheaper than the exhaust shop running a single setup. Its getting a 4bbl and a carbon off a 360 from an 85 Powerwagon. I also plan to run 215s instead of stock 205s. It currently has 195s. Running cop suspension will lug it down slightly do to added weight. I also plan to go with an electric fan and possibly underdrive pulleys