Outer Limits 4x4 Board

Hi guys, I had the factory fitted muffler and tailpipe replaced on my '93 F300 Rocky yesterday, and noticed a significant power loss, even as I drove away from the muffler shop. I commute 25km's daily, mixed driving - lotsa hills, and as of yesterday, I now need select a lower gear, and am using more fuel. The muffler shop, today insist the muffler fitted, is suitable for up to 2.5 litre engines, and when I had them inspect the vehicle, they don't feel the exhaust system restrictive in any way. My enquires suggest that the new muffler has influenced how the engine produces torque...it now needs more revs to produce the same power output. This mightn't be of concern in a saloon, but it seems detrimental to a 4 wheel drive, especially a 1.6 litre.
I was achieving 9 to 10.5 km's per litre previously...but now that I need stay in lower gears and/or use a heavier right foot, I appear to have reduced this to 6 or 7. Has anyone else noticed such a radical change in fuel consumption/loss of power since changing their muffler? Thanks in advance for your responses.

.

they don't feel the exhaust system restrictive in any way

This is the problem.

It may be that the exhaust is too big... Your engine needs a certain amount of restriction in the exhaust to create torque.. any larger than this and you will lose effective torque, but have more power up high due to the more free flowing exhaust.

They said that the muffler is suitable for up to a 2.5L enigne.. It's only a 1.6L!!!
You need an exhaust suitable for YOUR car... not just any type will do

Go back and say you want one suitable for a 1.6L car! Not a 6cyl car.

I'll certainly suggest that to them. I'm surprised though that they have not
'so they claim' encountered this issue before though. I put it to them that the EFI and the exhaust system are surely designed for each other...
It can't be a case of 'pull the wool over a dumb blond', because I'm anything but, both figuratively and litterally. I've driven my Rocky for 15 montrhs now...I think I'd know when somthing odd happens, coincidentally. Cheers.

You've probably lost torque at lower revs, but most likely will have more power at higher revs? Does the engine rev out more freely now?

The first thing I noticed when changing my rear exhaust was the loss at low revs. In some ways I think the standard system was better for everyday driving.

The standard exhaust system is optimised for lower rpm torque and fuel efficiency.
Anything too much larger results in a loss of both of these.

But yeh, it will rev out more freely due to a lot less restriction.

I quizzed the muffler shop today. They reckon the muffler fitted is all they can supply re size, shape and tube size. I suggested they think of a way to restrict the exhaust flow, so they made up and fitted a restrictor into the end of the tail pipe. Everything is back to normal now, my Rocky has power/torque again. Thanks guys for confirming my thoughts.
Cheers.