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possible cause of loss of power

574 views 6 replies 4 participants last post by  adamSRi 
#1 ·
whilst having new backbox fitted yesterday, the mechanic bloke told me, that because the bore is 2.5", the only thing in the system which is causing backpressure will be the cat. he says ideally the bore for my engine should be a 2" bore, and could cause loss of performance, especially if i was to rid of the cat at the mo cause then there would be no back pressure being built up.

not really sure if what he says is true etc so am putting it to you. would having a 2.5" bore, which is normally used for turbo engines on a 2.0 8v result in loss of performance?
 
#4 ·
there was a big thread about back pressure a while ago. people associate back pressure with restrictions to flow, i.e silencers and cats and small bore tubing. but emphasis should be on gas speed, and not on the "sticking your finger over the end of a hose pipe" meaning.

i think the physics behind it for an appropriate exhaust was keeping gas moving at a good speed, with as least drops in temperature as possible, in relation to the gas being produced i.e engine size and state of tune.

so, a huge exhaust on an engine not shifting much exhaust gas, like a 1.1 fiesta say, the gas speed will slow down, will have time to cool a fair bit at some points before it exits, the exhaust is no longer working like an extractor unit, helping draw gas from the cylinder head. as mentioned before, for this to be effective the gas needs to keep moving.
 
#6 · (Edited)
0ddball said:
there was a big thread about back pressure a while ago. people associate back pressure with restrictions to flow, i.e silencers and cats and small bore tubing. but emphasis should be on gas speed, and not on the "sticking your finger over the end of a hose pipe" meaning.
Indeed, "exhaust vacuum" would be a better term than back pressure really, as although its not actually a vacuum that gets across the principle better.
As its a drop in pressure thats important, ie the high gas speeds create a drop in pressure.



i think the physics behind it for an appropriate exhaust was keeping gas moving at a good speed, with as least drops in temperature as possible, in relation to the gas being produced i.e engine size and state of tune.

so, a huge exhaust on an engine not shifting much exhaust gas, like a 1.1 fiesta say, the gas speed will slow down, will have time to cool a fair bit at some points before it exits, the exhaust is no longer working like an extractor unit, helping draw gas from the cylinder head. as mentioned before, for this to be effective the gas needs to keep moving.

Thats pretty much bang on, the only other thing you arent mentioning which is vitally important is that the rev range used has a huge effect on what exhaust diameter you require (as much so as the size of the engine), so if adam had a 320 degree cam and throttle bodies etc, and hence was permanently at 9Krpm+ a 2.5" bore exhaust would probably be ideal as he wouldnt care anymore about low RPM gas speeds and the flow characterstics would be more important.


so am putting it to you. would having a 2.5" bore, which is normally used for turbo engines on a 2.0 8v result in loss of performance?
Yes at low rpm and in the mid range, probably not at the top end, this is basically what i was trying to explain to some of the guys at the SW meet if you can remember that far back, when exhaust sizes were mentioned.


Chip
 
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