So previously I have been running Downpipe - Precat (soon to be cut out) - CAT - Centre silencer (straight through with rifle barrel style holes on inside) to straight pipe to tailpipes.

Car sounded pretty good except for epic drone at 70mph. Was getting around 48mpg average on a run as high as 52 average sometimes.

today I replaced the straight pipe with the standard backbox, car is now completely silent which is boring but practical.

But wierdly I can't get over 32mpg now. Even driving like a granny I get 31.9MPG.

How can a simple backbox have so much effect on MPG, almost 20 mpg down AVERAGE?

It is not even the computer but the actual dial drops rapidly the car guzzles fuel at this rate whereas before it was fine.

My only explanation is the backbox must be so packed full of baffles and what not that the exhaust gas hits a wall when it enters, so it must take more fuel to push the exhaust gas out?

Performance/lag etc don't seem changed.

I am wondering about getting a straight through silencer made and welded into the straight pipe, I measured the straight part and I only have about 10 inches available. Is this enough for a quiet but not-mpg/performance affecting exhaust?

Or could I open the backbox up somehow and take out some of the baffles or wadding to make it more freerer flowing? I wouldn't mind SOME noise but I don't want any drone.

HELP!

 

Back pressure.

Most exhausts from the factory work on a small amount fo back pressure to control some emissions (helps keep some exhaust in cylinder after combustion to prevent new fuel/air mix from getting pushed past exhaust valve). Works great in N/A engines.

On turbo cars you want a very little amount of back pressure to force all that exhaust into that wonderful snail to put some nice fresh air in your combustion chamber. Better the flow the more efficient the turbo and engine generally resulting in better fuel economy. Happened to the new ford F-150 EcoBoost engines, too much restriction was causing Check Engine lights for too much back pressure on cylinders. Ford put on betterm higher flowing exhausts and the problem was fixed and fuel milages returned to their usual claim of 9.8L/100KM, 29MPG in Queen's English, in a full sized truck.

I would check some companies like Flowmaster as they have a low amount of sound until you really hit the higher RPM's. I run one of my XR and it's great not too loud until you really get into it. It's a full 3" inlet outlet exhaust and I have only a downpipe, muffler(silencer), and full 3" mandrel bent pipe out the back nice simple and fairly quiet. No 70MPH hum.

Hope this helps some, I'm sure there are some Technicians on here that may have some products that may help.

Happy Motoring!
TMitch

 

Or you might have just had a coincidental failure of a sensor which could explain your poor mpg

 

Cool explanation Tmitch.

Dont worry about mpg when driving performance cars. Just enjoy the car for what it Is.

 

I know my motor is a diesel but the best thing I done was have the exhaust changed and the CAT removed. The turbo comes in much sooner which makes the car far easier to drive as you don't have to worry about the revs so much!
I have a straight through silencer and it's quiet unless I put my foot down, then it's loud.

 

I agree with you Glenn!

Once the nice weather hits you should only worry about your Smiles per gallon.

 

Can anyone recommend a straight through silencer? I only have about 10 inches to play with (that sounded dirty (and i wish!)) and it needs to fit 2.5" pipe. Unless I bolt one in further down the car near the CAT where I have about 2 feet of spare straight pipe. Magnaflow do all sorts of suitable fitments but no one in the UK sells them.

I have been running the car with the completely standard exhaust for longer now and I have seen 35mpg (once) driving like a complete granny.

I have also been looking at these Hemholtz chambers but you have to find out what the EGT is where you place it; normal egt's work fine but EGT's in my car are around 900 degree C so finding the formula could be difficult.

 

Found my solution.


Will cost about Ł400 to get it fitted including 3" to "2.5" adaptors. I am going to purchase the 3" one even though they do a 2.5" one - so if I ever go for a bigger turbo I don't have to buy a brand new one if I need a larger diameter exhaust!

 

What car are you getting 48mpg out of?

 

Bump.

Looking for some kind of 2.5" inlet/outlet middle box to reduce exhaust drone.

 

I'd worry about the 20mpg loss, theres something not right with the engine if thats happened!

 

http://www.motorsport-developments.c...AF251.tech.pdf