I have a 01 Screw with the 4.6 motor. I wanted to improve towing performance and MPG so I've installed a Magnaflow 3" SI/SO and the Iceman intake system. To my surprise the towing MPG went down from 10 to 9 MPG. There was some performance improvement above 3,000 RPM but you had to really put your foot in it. Shuould I have used the 2.5" Magnaflow or is the motor just to under powered for heavy towing (boat and trailer = 5,000 lbs). Any ideas?

 

My guess is that the performance mods didn't hurt your MPG. The engine is the problem. I ran into an old friend and he said that he got a Sierra about 6 months ago and has the 4.8L in it. I asked what his MPG was and he told me 18-20.

 

We have two 4.6L's, and it depends on the driving. If I'm on the freeway, I can get 18-20 MPG. If I'm on the city streets, I can get 11 MPG. Modifications shouldn't be the cause of your problems, nor should the engine. Check into your driving and most of all maintenance habits. Run a bottle of a good Fuel system cleaner (Red Line SI-1) change the oil and the Fuel filter. Tranny fluid is a must, too. When was the last time you changed your plugs and air filter.

My point is, you shouldn't suspect the modifications as the problem without ruling out other issues.

 

The maximum size exhaust piping should be no larger than 2.5" on a 5.4L (and you have a 4.6) or you will get a "scavenging" effect in the engine exhaust-to-intake phase. I lost about one mpg when I put on the Gibson 3"SISO with a new Y-pipe. Lost low-end power too. I put a reducer in the tail pipe and gained back some of the low-end power that was lost.

 

If you have a SI/SO exhaust system the piping can be as large as 3". If you are running SI/DO or DI/DO you should go with the 2.5" pipe. A Magnaflow system (15609) comes with a 3" pipe and all the dynos that I've seen come back with positive HP and TQ numbers. Maybe it depends on the system.

 

Thanks for the replys. My engine has about 25K miles on it so I think the tune up stuff is OK. When I first installed the performance goodies I only really noticed power improvement above 3k RPM so maybe that's just the best it will do. For everyday driving the truck runs fine just was wanting better towing performance, oh well $*&# happens .

 

I agree with the 3" piping not being a problem, heck ythey recommend that you can use 2.5 on the ranger! The problem of losing power with the truck may be an issue with the Y-pipe. True driving habits cause the biggest difference in gas mileage but my Father(58, turning into an old fart driver) took my truck to Arizona last X-mas and he got a best of 15 MPG highway. I haev heard of many preople getting better MPG with lower rear ends and since I am goint to be going to 4.56 very soon than I will let you all know when I get results.