Based on 2 recent 750 mile round trips (i.e. 1500 miles total), driving my 12k# motorhome with its "new" 1st Gen Cummins, i'm getting 12-13 mpg. Is this a great, good, fair, poor, or horrible mileage?

The trip was from Phoenix to San Diego, through rather unusual 50-mile climbs (sea level to 4200 feet)and 6% grade drops on the I-8. The way to San Diego was a LOT harder on fuel than the way back; It really worked the engine out to 2400 rpm at times going around 1200 degrees EGT for straight 15 minutes at a time, with boosts reaching 28 psi. Other than that, the drivetrain works fine; I never floored it nor had the need for more rpm although I still had 300 left to use if I wanted to. On the flat portions I went 70-80 mph, 1800-2100 rpm, 8-15 psi boost with TC locked most of the time.

 

sound's Darn good to me good Job Man

 

Doesn't sound bad to me with all the work you were putting it through. Get that on the flats and I bet you'd see 15+ MPG. How heavy is your set-up?

 

12,000 lbs. and almost 11 feet tall.

 

Sounds good to me considering you have a huge amount of aero drag, and were going pretty fast. I bet you could see 15+ if you were willing to drive 55mph

 

Try keeping it under 2000 RPM. Over 2000 RPM the mileage takes a pretty sharp drop. Your vehicle frontal area is about the size of a sail used on a small sailboat. That + high revving the engine and the overall weight while getting 15 was pretty good.

 

I drove 55 on the steep climbs and still passing some vehicles. If I drove 55 on the flats everyone would be passing and staring at this wierd motorhome. :-) Most of those diesel semis go 75mph!

My first couple thousand easy miles with the new engine I thought my MPG was 16-20, until i took these tough trips, and the mpg was not that high as I thought/wished :-( Also I found out my speedo/odo is 6%off so that explains that high reading of the mpg. The 12-13 is the adjusted mpg at this time.

 

and what were you getting with the gasser? at least you doubled your fuel range, you can still toy with your air, running 1200*, and im assuming your pyro is at the manifold split, is still a bit high for a 15 minute duration, id feel alot better with even 100* drop, hx35's are cheap, and bolt right in. in accelerating, are you seeing any smoke? and what size is your exhaust tube?

 

I was getting 8mpg with the V8 gas engine BUT only up to 65mph (no overdrive) at 3100rpm on the freeways. Yes the egts are a bit high even at lower rpms but well below dangerous levels, even at full throttle and full load (@2700rpm).

The stock downpipe is 2.5 inch I believe (edit: it's 3-inch) and it is terminated by a 3-inch glasspack with a 3.5inch turndown tip under the vehicle. Do you think I'll lower egts 100 degrees if i replace the downpipe with a full 3-inch pipe?

What year does the HX35 come with? I had intended to get an HE351CW but now I'm satisfied with the stock performance. Besides, putting an HE351 would open for more upgrades (air, fuel, studs, valves, cams, etc.) which I feel at this time i don't need. My rig already looks and sounds "cool" enough as it is I think :-) After working on it a lot, I think I'm at the point of "getting sick" of tweaking it and just want to drive/enjoy it :-) but I'm still open to simple/easy mods.

I wish I could use a 3-inch intake intercooler tubes, but i don't have room in there, having a stock intake horn facing forward.

No discernible smoke when accelerating, which I like.

PICTURES HERE: http://www.pirate4x4.com/forum/showt...1#post12676786

 

I would go full 4''.

 

x2 on the the full 4" i saw a 250* drop when i put mine on.

 

Billtit and Bludodge, thanks. I think I can fab a 4-inch one easily. Or is there one out there I could buy, ready to bolt in? That would even be better.

 

if you can use a 2nd gen down pipe you can probably get a full 4" system with a straight through muffler for ~$300 on ebay.

 

the diamond eye 4" system is about $280 + shipping, and it comes with a decent muffler. That would probably be just about as easy as anything

Also throwing something like an he351 on there would make it a nice towing rig too.

 

Hi Rick, do you know if the second gen downpipe clamps/bolts into the exhaust housing of the 1st gen turbo? While I'm there I'd like to replace the S-shaped downpipe with a straighter one.