cat mpg opinions and advice

Morph;
One of the first things I would do, is check to see if your engine can be turned up. Some time back I remember reading that many of the 435 hp C-15s are almost maxed out. They are equipped with the high compression piston paks, and short of replacing them with low compression paks, you are kinda limited on options. It might be possible to post the serial # of your engine here, and one of the gurus could check that for you. Hopefully they'll also correct me if I am wrong about any info. If I am correct about the compression thing, this could affect how much you would benefit switching to a 6NZ ECM. If you get around a dealer who can hook up to your truck, be sure to get them to print out your engine configurations, this could be VERY helpful in your switch!
I THINK you can switch to a 6NZ no problem, if you have the low compression paks. DDS, Jas, Mr. Haney, and some others are waaay more knowledgeable than I about this stuff. DDS fixed me up with a 6NZ ECM, and it works great! I do know that with a 6NZ ECM, you would have to bypass that boost pressure thing on your engine (in front of the ECM, near secondary fuel filter, just follow the waste gate line from the turbo, back of engine, to driver side). But that is simple stuff.

As Martin stated earlier, I would first LOSE the catalytic style mufflers. This should be done, regardless, which brings us to another potential problem Big Brother. Removing them is illegal, and if ever they were to check.......
I did pull them off my truck, and replaced them with two of the older style mufflers, and also did it carefully, saving them, as the local Kenworth dealer quoted me $3000 for a new pair.
If it will be a while before switching to a 6NZ ECM, changing the injector codes would be my second step, (if it can be done on your engine). This was a big improvement.
My truck came with 24.5 tall rubber, and I almost need to stay with them for fifth wheel height. (chemical specs). If I had my choice, I would have 22.5 tall (smaller diameter= less rolling resistance= better mpg) that is a PROVEN fact. Lo pros would be even better, but I am of the opinion that more rubber sidewall = better ride, and my ride needs all the help it can get.

 

I was told that the 475s, and 550s mostly use the same turbo, yours may be too small, that is another consideration.
Don't know how much hp you are going after, but the Borg Warner I put on is supposedly good for up to 600hp, and was cheaper to buy a brand new B/W, than a reman Cat. No core required either.
I have also heard the PDI units are pretty good, and priced close to a Cat turbo. If I were to buy another, I'd look real hard at theirs.

 

All MBN motors have a higher compression ratio of 16.6 to 1 compared to the earlier C-15 of 16.0 to 1. You can still do the ECM upgrade without any problems. The mufflers need to come off, when you install the replacements just turn the part number where it can't be read without taking them off or the fairing off.

 

I agree, the first thing I'd do is mufflers, but be careful. Didn't you say you run Ca? You know how they are about that ####. As far as tires, listen to droy, 22.5LP is the best on mileage it's proven. I think you're rpms are close to where you want them if that engine has the same powerband as older C-15's. I know you're not going to want to here this, but I bet the best thing you can do right now for economy is slow down. I run 60 at just a hair under 1400 and my 6nz likes it. I think it likes 1350, but I'm compromising at 60. I wouldn't drop your rpms more than 100 if it pulls the same as mine. If you do slow down your rpms will probably be low for the hills so you might want to take it back up to 65 at the bottom to pull the hill better. You just have to feel that out for yourself. If you're runs will allow it try running 60 on the flat, I bet you'll notice a difference. BTW, I've found that running slower is FAR less stressful...as long as you're not in a hurry and fretting about it.



It is possible. (I fueled in wildwood,fl and reset the mpg, and clocked this about ft myers with a light load cruising)

My dash computer is normally pretty close to my actual too. I have the spreadsheet to back it up. I averaged about 8+ running light into FL and empty back with low idle time. If fact I pulled 34k into FL this week and was getting about 7mpg. I run 60 mph at ~1390rpm. Truck is an 01 Columbia with 500hp 6NZ.

 

I don't know much about the muffler legality issue, best to listen to the other guys. Sounds like you might can hide it if they appear the same on the outside. I don't and won't run Ca anyways, I don't think they have good enough paying loads to make me.

I know, slowing down seems so hard. I started out running 70-75 pulling containers getting 5.5mpg. The fuel prices going through the roof forced me to slow down. You get used to it, and I'm telling ya it's less stressful, set the cruise and watch the fuel guzzlers go by. I tell ya what, drop just 2-3 mph on your next run or two and compare the mileage just to see. (with your rpm 60 might be too low anyway) It's only going to take an extra 10-15min over like 500 miles. That's one pit stop, keep the left door shut.

P.S. I forgot, I definitely recommend getting the centramatics. I have them on all 6 positions. Don't know how much it'll help mileage but get some equalizers as well (crossfire,catseye) to keep the dual pressure balanced. It'll save tires for sure.

 

Good Job!! but beware, some late night with little traffic and spare time on your hands you will be tempted to slow down and try running at 1200 rpm, then you have to take another shot of the dash.

Ken

 

On a side note while it may be "proven" that 22.5LP get bet better fuel mileage. (not sure what the actual improvement would be, mpg wise) they also make about 31 more revolutions per mile than a full size 24.5 which means they will wear out sooner. So without doing any real experiments or hard math I'd say if you take any fuel mileage improvements and subtract the tire wear it may kill any advantage. BUT 22.5's are cheaper BUT they have a lower weight rating if you haul heavy, I blew 2 and gave up on them. It definitely wouldn't be worth buying new wheels and tires just for a fraction of MPG improvement.