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[WRC_Fiesta]

right, im really thinkin of keeping the fez now IF i can get a few niggling problems sorted out. This one is about my engine, which, annoying keeps chugging and stalling when i stop at lights etc. Ive had a stainless centre exhaust on now for about 6 months and tyhe problem seems to be getting worse. ive been told its due to a lack of back pressure in the exhaust cos of there being no middle box and the bore size is 1 3/4 inches My car has been regularly serviced every 6000 miles and is in good nick with only 71000 on the clock. Should fitting a standard exhaust back on help this problem????

[1600power]

Yes an engine requires back pressure otherwise it looses compression and this loss of compression, at low revs such as being stopped will result in there being alot less pressure than there should be in the engine, I think the diameter of your actual exhust piping is just to wide for the engine to handle . Alot of people say that the standard exhust pipes with a better backbox actually make a better combination than a full change, obviously this is based on the fact that yours is a 1.1 and not a FRST or Cosworth etc where the change to an aftermarket pipe is probebly a good idea!

[MadcatCraig]

your exhaust bore is too big !

[festajon]

My mates 1.1 was doing this, turned out the carb wasnt bolted down very well and kept coming loose, in turn causing a HUGE vacum leak and it stalling. In theory u can run a car with no exhaust. Yeah it will sound poo and run rough but it will run and not die all the time. I know this cos the same mates zorst snapped at the downpipe and fell off! we had to drive it through town like this and sounded like a tank!

[Project]

1.75" sounds ok to me. Engine should idle a little rough but not stall - most engines will idle without any exhaust at all after the downpipe, albeit very noisily.

Have you had it checked at a garage, set up and the carb cleaned out? Sounds more like a gummed carb.

(Sits back and waits to be told it *was* the exhaust after all )

[Sunil_FiestaRS_16V]

i went to rally the other day and to improve back pressure for the track some matey just shoved a coke with a hole in it up the exhaust. maybe try this for a day or somein and see what happens. then u'll know whether its ur exhaust or not

[WRC_Fiesta]

hmmm now this is something i havent heard before, about the carb, and it could be true cos i had it off to fit a k&n, which i later took off. i'll get on to it tomorrow and let u all know!!!!

ps. i clean the carb and all the pipes goin in and out of the airbox all the time and this seems to help a bit

[grahamb]

Sorry chaps, but thats horse s**t!

Compression in the engine is down to the pistons and the valves, now't to do with the size of your exhaust system.

Back pressure is a no-no. Performance is down to the ability of the engine to draw in air, mix it with fuel, burn it, and expel the exhaust gasses.

The easier it is for the engine to get rid of the gasses, the less likely that there are still some left in the cylinders when the piston starts on the inlet stroke, and therefore the better the air-fuel concentration when the piston comes up on the compression stroke.

What you are after however, is a high velocity exhaust system. ie once the exhaust gasses are flowing along the system, with the right bore you can expect the exhaust to help draw exhaust gasses out of the cylinders, which is what most people misunderstand, and commonly mistake for "back pressure".

Back pressure is when you have a restriction in the system, which prevents gasses from exiting at high velocity. Cheap and poorly designed systems exhibit back pressure, because of un-necessary bends and restrictions in the system, and cheap silencers that don't offer any gains in performance.

Bigger is better with an exhaust, but you must make sure that the engine is matched to the system. ie a 3" system on a 1100cc engine is likely to sap power, because the exhaust gasses never build up enough velocity. Much better to go for something like a 1.5" system on something that size.