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Old 01-13-2015, 06:43 PM   #1
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Exhaust Backpressure for 5.4 V8 required?

I have a 2003 E350 EB with the 5.4 V8. Low mileage, runs perfectly, tuned "spot on", etc. Yet I'm getting only 14.3 mpg [regular gas] driving 55 - 60 mph via cruise control, with no "jack rabbit" starts, etc.

Everyone else seems to be getting 16 - 17 mpg. The only difference in my rig is that I have a "flow though" straight muffler instead of the stock Ford "baffle" muffler.

Could it be that this engine requires some "back pressure" to run efficiently?

Trent
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Old 01-13-2015, 09:27 PM   #2
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Re: Exhaust Backpressure for 5.4 V8 required?

Yes, that can have an effect, but unless you did full headers, you probably didn't reduce back pressure at all.

Do you have stock tires? What is your tire pressure? Do you have a roof rack?

How long are the drives where you're measuring your fuel economy?

We have a 5.4L E350, and it does 17 MPG consistently on the highway, fully loaded, at 68 MPH.
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Old 01-14-2015, 09:25 AM   #3
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Re: Exhaust Backpressure for 5.4 V8 required?

Quote:
Originally Posted by carringb
Yes, that can have an effect, but unless you did full headers, you probably didn't reduce back pressure at all.

....cut.......
I think he's asking if higher back pressure is a good thing that improves MPGs -- if I understood him right. Is that what you meant to answer?

If so, how does higher exhaust back pressure make engine more efficient? Does it affect controls, or is it that products of combustion that remain in cylinders allows engine to run at greater throttle opening at partial load and somehow reduces pumping loses? Or is it something else?

At full throttle less back pressure makes more power, but there isn't a lot of data on partial load like when cruising slowly.

Either way I agree the effect of a muffler or even full system is unlikely to register on the MPG radar. Our on-the-road testing accuracy isn't that good.
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Old 01-14-2015, 09:39 AM   #4
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Re: Exhaust Backpressure for 5.4 V8 required?

Quote:
Originally Posted by Trent4444
I have a 2003 E350 EB with the 5.4 V8. Low mileage, runs perfectly, tuned "spot on", etc. Yet I'm getting only 14.3 mpg [regular gas] driving 55 - 60 mph via cruise control, with no "jack rabbit" starts, etc.

.....cut......

Trent
I'm getting in the same range with 6.8 V10 at around 70 MPH. Although I rarely use cruise because I do better without. And that's strictly highway. Any city driving kills my MPGs. Anyway, it seems something must be off a little if yours is a stock van.

Can you share more information on your van, like gearing, 2WD or 4WD, whether lifted, tires, inflation, approximate weight, roof type, whether roof air, awning, etc....? Your 14.3 may be excellent already.

Sorry if you've already shared this information elsewhere.
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Old 01-14-2015, 09:39 AM   #5
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Re: Exhaust Backpressure for 5.4 V8 required?

No headers, no roof rack, stock size tires, pressure's about 60 lbs, rig weighs 7,600 lbs, round trip of nearly 400 miles with the "up hills" equaling the "down hills".

The reason I'm thinking "backpressure" is that I once had a Harley Sportster,,, changed the stock exhaust system to a "open" exhaust and lost about 40% of my horsepower. Found out that the engine required a certain amount of backpressure to run right [I changed back to the stock exhaust].

Sun Valley Express [Sun Valley, Idaho] runs an E350 EB shuttle from Sun Valley to the Boise Airport every day, 365 miles round trip, and they get 17.5 mpg running at 75 - 80 MPH [speed limit on the I-84 is 80 mph]. So something's wrong with my mileage, and I'm driving WAY slower than they do. ?????
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Old 01-14-2015, 09:46 AM   #6
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Re: Exhaust Backpressure for 5.4 V8 required?

I researched why back pressure was important, at least for the Harley.

Because of the cam profile it turns out that with reduced back pressure, the incoming fuel/air charge is “swept” right on through the cylinder and out the exhaust port before it can be compressed and “fired”. By adding back pressure more of the fuel/air charge is trapped in the cylinder which results in more efficient combustion.

My rig is 2 wheel drive.
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Old 01-14-2015, 10:12 AM   #7
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Re: Exhaust Backpressure for 5.4 V8 required?

I would personally not start dumping money into a 5.4 in search of better mileage. I think your numbers are respectable. I'm not discounting what CarringB says since I've heard others report 17ish out of a 5.4, but only a couple of people and I don't see how. My previous van, a homebuild 5.4, even before I added any weight or a top, got about 14 when I babied it. On average I got 11 or 12. I was usually loaded and had a CCV top on the van, but most people I know, especially running bigger tires or towing something, get about 12 with a 5.4l. The one thing I did to improve my van was a 5-Star tune, but that didn't live up to the 1 mpg increase I was told, however it did pep up the van quite a bit and was worth the money. I think any change in exhaust is only going to get you more noise. YMMV.

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Old 01-14-2015, 11:28 AM   #8
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Re: Exhaust Backpressure for 5.4 V8 required?

As another data point, my previous, late 90's 2WD SMB RB with 5.4, and pretty much stock everything (nothing but penthouse on roof) and a GVWR of around 7,600# would get a reliable 15-16 on the highway, with speeds in 65-70 range. I saw 17-18 on occasion but not a regular thing. Uphills included as long as there were equivalent downs (by which I mean this was not just flat Florida type driving). Maybe 14 with a "wrong" wind if I insisted on keeping my speed up.

Towing 5,000#, it was down to 12.5 mpg. City with a bit of highway thrown in (mix) was 12 mpg.

Subsequent owner reports about the same on the highway, although down closer to 11 for city (but that's with pure city for a whole tank, which I never did).
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Old 01-14-2015, 11:44 AM   #9
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Re: Exhaust Backpressure for 5.4 V8 required?

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Originally Posted by 86Scotty
I think any change in exhaust is only going to get you more noise. YMMV.

...and a lean condition at full throttle. I say that because I had no issues with my 5.4 until I had a flowmaster installed and the very first time I buried the pedal in the floor the check engine light lit up. It threw 2 codes (I don't remember the exact numbers) left bank lean and right bank lean. I cleared the codes and drove around like an old lady for a few days with no CEL and then intentionally tried to set it off by going full throttle... it turned on almost immediately (2-3 seconds.) I repeated this experiment a few more times and it was like clockwork.

In the world of motorcycles (which I have much more experience with engine modification, especially single cylinders) changing the muffler can be the difference between running acceptably (stock) and not running at all. I always try to change 3 things at the same time 1) more air in (be it an open airbox or a bigger cam) 2) more fuel (jets or ECU programming) 3) more air out (cam/ exhaust system.) Depending on the stock systems restrictions, sometimes you can get away with changing only 2 of the 3. Doing only one of the 3 usually results in a LOSS of power or poor running condition/ poor fuel economy.

That being said, I don't think the 5.4 needed more exhaust flow, I just didn't like it sounding like a leaf blower when I got on it! Adding more fuel (5 star programmer) will no doubt take care of the lean condition.
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Old 01-14-2015, 02:02 PM   #10
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Re: Exhaust Backpressure for 5.4 V8 required?

Do you have stock gearing? Likely 3.55 or 3.73 or maybe 4.10 (not sure if 4.10 was option).

Also, does your van typically hold overdrive on up hills? That's one reason I don't use cruise on rolling hills. The cruise overreacts and forces a downshift which I can often avoid when controlling throttle myself.
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