I know this is more of a truck question (F150) but the exhaust and the 4.9 have to work together. What is the diameter of the exhaust head as it connects to the catalytic converter? I just bought a 1992 F150 4x4 with the 4.9l - 130k miles on it. It is in really good condition but is missing most of the exhaust system after the cat. Will it do any harm to have a 3" exahaust pipe after the cat? My goal is improved mpg via more hp and torque. The big picture: I have been told that the stock exhaust headers are about as good as you can get. People have suggested mods to the cam and intake manifold but only if there are other mods after that. If I can get enough Ben Franklins, I plan to pursue this route, hoping to get higher hp and torque, and better mpg as a result. What would you suggest for an exhaust system for this engine? I prefer to keep it sounding on the conservative side.

 

3" is plenty. 2 1/2 would probably be fine.

 

You have to watch the mods you want to do as you have EFI and unlike modern cars & trucks you CAN NOT chip it.
So if you start adding say a bigger cam the computer will freak out and run like crap so you cant go much bigger on the cam.

If you cant go bigger on the cam do you really need to go bigger on the exh pipes?
Do you have smog check where you are at so you have to run a cat?
I believe the the EFI 300 motors have the largest pipes of any of the F trucks that had a 300.

I know for a fact that the 1994 F150 system has larger pipes than my 1981 F100 truck and why I went with the 94 system.
My truck is also running EFI manifold, replacement Y pipe, no cat, Thrush welded muffler and 94 tail pipe and I like the sound.
The stock 81 smaller pipes and factory muffler was to quite.

Do you plan to replace the Y pipe with 2 pipes off the EFI manifolds, I seen some factory that was that way. and a Y just before the cat or muffler.
The Y pipe is I think 2.25" as are the inlet / outlet of the muffler.

Are you going to have a shop make up the larger system or find a F truck that has larger pipes from the factory and try and make it fit?
BTW even the diesel did not use larger pipes than a gas motor till they started using turbos and all the gas motors 300 on up all used the same size from what I looked up.
Now will a diesel turbo that has larger pipes, think was 3" but not sure, able to fit the older F trucks is the question?
I have not looked but I would think the under sides have not changed much so the diesel pipes and muffler may work and you would need to work out how to hook up the Y pipe or cat to it.

See I believe in trying to find a system that may work this way
1- you dont have to pay a shop to make it
2- if you need to replace it down the road you just order for the year truck you get the system from.
Both is why I went with the 94 system.
Good luck
Dave ----

 

You know I dont even know if you can run the Chevy 1.7 rockers and not freak out the EFI computer?
Any one know?
Dave ----

 

I had a 1990 F150 4.9 six (bought new, special ordered- loved the truck!!!!) and we cut the exhaust system off from behind the cats and ran 2" diameter dual tailpipes straight out the back. Ran great, nice deep tone- like a SBF but a little quiet. no drone, passed smog in SoCal regularly with no issue....the ECM will automatically adjust the fuel mix to maintain A/F ratio as designed. No gain or loss of HP/TQ. Had the truck for 17 years with that!