de cat pipe

[ben24]

has anyone had a decat pipe fitted on there 172 ??

if so how much differnce did u see from it ??


ben

 

[Turkish]

Don't expect noticeable power, just better throttle response, freer revving so better for blipping on downshift etc. I would say loads of noise too but then when i fitted my decat it was as part of a supersprint race exhaust so bound to be loud, lol.

 

[ben_p]

Oh, don't forget to add that there a b**ch to fit too

 

[ben24]

Oh, don't forget to add that there a b**ch to fit too

im quite handy , but they look quite easy , but i guess not

 

[Teleturbie]

I'm putting my cat back on, it's just gutless on torque lower down the rev range since the decat. Don't think it gained any power top end either.

(this is in combination with a Milltek cat-back)

 

[ben24]

I'm putting my cat back on, it's just gutless on torque lower down the rev range since the decat. Don't think it gained any power top end either.

(this is in combination with a Milltek cat-back)

if thats the case ill sack that idea then as my car seems to have no torque lower down the revs anyway

 

[Turkish]

I'm putting my cat back on, it's just gutless on torque lower down the rev range since the decat. Don't think it gained any power top end either.

(this is in combination with a Milltek cat-back)

In some cases that can be more a case of perception than fact, it will make the kick at 5k less noticeable so some people "feel" they've lost performance. As for fitting unless you have the hands of a 5 year old girl it is a pain in the ass on a 172.

 

[ben24]

In some cases that can be more a case of perception than fact, it will make the kick at 5k less noticeable so some people "feel" they've lost performance. As for fitting unless you have the hands of a 5 year old girl it is a pain in the ass on a 172.

ill get jeremy beadle to lend a hand then

 

[Teleturbie]

I'm putting my cat back on, it's just gutless on torque lower down the rev range since the decat. Don't think it gained any power top end either.

(this is in combination with a Milltek cat-back)

In some cases that can be more a case of perception than fact, it will make the kick at 5k less noticeable so some people "feel" they've lost performance. As for fitting unless you have the hands of a 5 year old girl it is a pain in the ass on a 172.

I was thinking about that either, but the perception is quite convincing tbh. Are there any before and after cat/decat RR graphs available (where all the other parameters stayed the same)?

 

[Turkish]

tbh a RR graph before and after purely a decat wont really tell you anything. RR settings, temperature, moisture content, tyres pressures etc all play their part. I wouldnt be overly convinced in being able to use one as a yardstick if it was the same RR, same operator and done on the same day literally before and after the decat was fitted.

 

[Teleturbie]

That's why I mentioned parameters staying the same, but I agree it's quite hard to achieve that.

Anyway; for the moment I trust my perception, to little backpressure is only going to give you losses imo. I'm convinced by this theory, because my exhaust broke off once and the car almost lost all power.

I've asked before if any of the techies could explain the theory of backpressure, but no result on that yet .

 

[mattjk]

Loss of backpressure will lose you bottom down power but top end power will improve. (or so i understand/heard)

 

[Fred@BTM]

there is a reason behind it, clios like free flowing exhausts with minimal back pressure

if i'm right its got something to do with the pulses generated inside the engine, on serious machinery the exhaust length is pulse tuned.

4 clyinder engine - each cylinder isn't continuosly expelling exhaust gas from the exhaust ports, each port expells gas every 4th rotation (in turn, so same order as the clyinder firing order of the engine, we'll call it 1-2-3-4 for example purposes) this is where the 4-1 and 4-2-1 manifolds come into play

so cylinder 1 has fired and the exhaust gas exits the engine into the primary pipe of the exhaust manifold, we'll say that volume of gas from the exhaust port to the point at which 2 (or all 4) primary pipes meet is 1L

now clyinder 2 has fired, the exhaust gas is the same volume, the exhuast gas from clyinder 1 is the same volume ahead of the gas from exhaust 2

same happens with clyinder 3 and 4

so the end of the gas from 1 links up with the start of the gas from 2 and a constant chain of gas is formed

its hard to explain! LOL

 

[SteveC047]

Fred

That was quite a good explenation considering the science behind it is extremely difficult to get your head around it.

 

[Fred@BTM]

i might be barking up the wrong tree completley but i think i'm correct, i'm sure someone will correct me! LOL

 

[fuel_slut]

there is a reason behind it, clios like free flowing exhausts with minimal back pressure

if i'm right its got something to do with the pulses generated inside the engine, on serious machinery the exhaust length is pulse tuned.

4 clyinder engine - each cylinder isn't continuosly expelling exhaust gas from the exhaust ports, each port expells gas every 4th rotation (in turn, so same order as the clyinder firing order of the engine, we'll call it 1-2-3-4 for example purposes) this is where the 4-1 and 4-2-1 manifolds come into play

so cylinder 1 has fired and the exhaust gas exits the engine into the primary pipe of the exhaust manifold, we'll say that volume of gas from the exhaust port to the point at which 2 (or all 4) primary pipes meet is 1L

now clyinder 2 has fired, the exhaust gas is the same volume, the exhuast gas from clyinder 1 is the same volume ahead of the gas from exhaust 2

same happens with clyinder 3 and 4

so the end of the gas from 1 links up with the start of the gas from 2 and a constant chain of gas is formed

its hard to explain! LOL

I understand but still doesn't really explain why a decat might lose or gain you power?

 

[Fred@BTM]

it will always gain you power IMO its not possible to loose power from a decat (well i've never ever seen it!)

a cat restricts the flow

try going to the loo with your undercrackers still on, there's a restriction so the performance isn't as good

 

[ja.jack]

there is a reason behind it, clios like free flowing exhausts with minimal back pressure

if i'm right its got something to do with the pulses generated inside the engine, on serious machinery the exhaust length is pulse tuned.

4 clyinder engine - each cylinder isn't continuosly expelling exhaust gas from the exhaust ports, each port expells gas every 4th rotation (in turn, so same order as the clyinder firing order of the engine, we'll call it 1-2-3-4 for example purposes) this is where the 4-1 and 4-2-1 manifolds come into play

so cylinder 1 has fired and the exhaust gas exits the engine into the primary pipe of the exhaust manifold, we'll say that volume of gas from the exhaust port to the point at which 2 (or all 4) primary pipes meet is 1L

now clyinder 2 has fired, the exhaust gas is the same volume, the exhuast gas from clyinder 1 is the same volume ahead of the gas from exhaust 2

same happens with clyinder 3 and 4

so the end of the gas from 1 links up with the start of the gas from 2 and a constant chain of gas is formed

its hard to explain! LOL[/quote

But you bored the tits off us trying. I'd agree with Teleturbie. I fitted my de-cat on standard exhaust and took it straight back off as the sound it made was horrid. Might have been able to live with it if it had felt any more powerful, but it did'nt.

 

[Lagerlout]

Mine made a difference to my 172. Definitely. Big difference in fact. More eager to rev, pulls harder low down. No placebo - definite gain.

Let us not mix the water here though. Ignore these 182 owners - The 182 was fitted with a more free flowing cat from the factory so it stands to reason that it seems to have less effect on 182's.

 

[ben24]

Mine made a difference to my 172. Definitely. Big difference in fact. More eager to rev, pulls harder low down. No placebo - definite gain.

Let us not mix the water here though. Ignore these 182 owners - The 182 was fitted with a more free flowing cat from the factory so it stands to reason that it seems to have less effect on 182's.

yea , good shout

 

[Graham Gooch]

For once I agree with Ja Jack.

To me all the decat did was make the car sound like a nova. This may of been due to the Blueflame crap exhaust, but I felt it was the same.

Sounds raspy as fook lol.

 

[Lagerlout]

I'm not really sure what my car sounds like. I only hear it from inside lol. Although it sounds f**king ace imho, it might sound like an old nova to others. Need someone in the know to have a listen. lol.

 

[Graham Gooch]

Oddly enough mine didnt sound anything like a decatted 172 cup? Probably the exhaust manifold perhaps.

 

[Lagerlout]

Coupled with the twin pipes I guess it's going to be completely different.

 

[Teleturbie]

there is a reason behind it, clios like free flowing exhausts with minimal back pressure

if i'm right its got something to do with the pulses generated inside the engine, on serious machinery the exhaust length is pulse tuned.

4 clyinder engine - each cylinder isn't continuosly expelling exhaust gas from the exhaust ports, each port expells gas every 4th rotation (in turn, so same order as the clyinder firing order of the engine, we'll call it 1-2-3-4 for example purposes) this is where the 4-1 and 4-2-1 manifolds come into play

so cylinder 1 has fired and the exhaust gas exits the engine into the primary pipe of the exhaust manifold, we'll say that volume of gas from the exhaust port to the point at which 2 (or all 4) primary pipes meet is 1L

now clyinder 2 has fired, the exhaust gas is the same volume, the exhuast gas from clyinder 1 is the same volume ahead of the gas from exhaust 2

same happens with clyinder 3 and 4

so the end of the gas from 1 links up with the start of the gas from 2 and a constant chain of gas is formed

its hard to explain! LOL

Thnx Fred, but if I follow your 'less backpressure = more bhp' explanation, why is it when you take off the complete exhaust, it loses so much power (unless that was a perception too:S)? doesn't make sense..

 

[ja.jack]

You could try the Oracle at Delphi, might make more sense than Fred.

 

[foxon]

Oh, don't forget to add that there a b**ch to fit too

piece of piss took me 10 mins!!

 

[foxon]

got 182 manifold, d-cat n 6*4 magnex oval, seems to rev alot quicker than standard 172 exhaust but it may just be me finkin its quicker if ya catch me drift.

 

[ja.jack]

Have you ever had it dynod, foxon?

 

[foxon]

not until GDI did it b4 they started the work, but won`t put down what it made cause haven`t got the printout yet (at GDI obviously) otherwise `sum` people on here will only start chatting s**t!!

 

[ChrisR]

Oh, don't forget to add that there a b**ch to fit too

piece of piss took me 10 mins!!

Depends on the model, a ph1 172 is a right arse to do.

 

[Fred@BTM]

if i'm right its got something to do with the pulses generated inside the engine, on serious machinery the exhaust length is pulse tuned.

4 clyinder engine - each cylinder isn't continuosly expelling exhaust gas from the exhaust ports, each port expells gas every 4th rotation (in turn, so same order as the clyinder firing order of the engine, we'll call it 1-2-3-4 for example purposes) this is where the 4-1 and 4-2-1 manifolds come into play

so cylinder 1 has fired and the exhaust gas exits the engine into the primary pipe of the exhaust manifold, we'll say that volume of gas from the exhaust port to the point at which 2 (or all 4) primary pipes meet is 1L

now clyinder 2 has fired, the exhaust gas is the same volume, the exhuast gas from clyinder 1 is the same volume ahead of the gas from exhaust 2

same happens with clyinder 3 and 4

so the end of the gas from 1 links up with the start of the gas from 2 and a constant chain of gas is formed

its hard to explain! LOL

Thnx Fred, but if I follow your 'less backpressure = more bhp' explanation, why is it when you take off the complete exhaust, it loses so much power (unless that was a perception too:S)? doesn't make sense..

because the explanation i give above won't happen, basically what i'm saying is the gas from each clyinder work together, ie one pulls the other one along (well thats what i understand the theory to be anyway)

i supposed you could also look at the exhaust system on a car as an instrument, say like a trumpet or something, if you make the pipework too big then it won't work, if its too small it'll be impossible to blow air through it

f**k knows, lets just call it magic!

 

[ja.jack]

^^^ A good example of how a little knowledge can be a dangerous thing.

 

[Fred@BTM]

very hard to explain jack, so leave off

 

[Gaz_]

i think james should give us an explanation

 

[Gaz_]

Oh, don't forget to add that there a b**ch to fit too

piece of piss took me 10 mins!!

No chance. On a 182 only. All 172's are pretty much a two man job.

 

[ja.jack]

i think james should give us an explanation

I do'nt presume to be a self styled expert on the subject, do'nt have that kind of ego problem.

If you hold on though I'll go and look through my old copys of Hot Car ect.

 

[Turkish]

piece of piss took me 10 mins!!

No chance. On a 182 only. All 172's are pretty much a two man job.

Exactly, if you have man's hands (I don't know what Ben's excuse is we all know he's 12 ) I found it the same! 172 it's behind the bloody subframe and a right wanker to get at but on the 182 it's well clear and takes less than 5 minutes to swap.

 

[phill172]

well if you guys want to hold on for a month, im going to be fitting my decat next month, and i will be doing it at a local garage with a rolling road, so i will do a test before and hopefully if its not a mission to fit, straight afterwards (yes i know the exhaust will be hot!), i have a cat back system by longlife on at the moment too. so will see what difference it does make

 

[Teleturbie]

http://www.superchevy.com/technical/engines_drivetrain/exhaust/0505phr_exh/

A long and complex bit of reading, but some very interesting issues:

- The so called 'scavenging effect'; this effect occurs when the intake and exhaust valves overlap, the vacuum at the end of eacht exhaustpulse creates suction of fresh air/fuel mixture into the cilinder, this generated suction is up to 6 times greater then generated by the piston moving to BDC, indicating how important this is for an optimal volumetric efficiency! These exhaustpulses are created with a carefully tuned cam/manifold/exhaust partnership.

- A bigger bore is not necessarily better; there is a balance, too big and it slows down the speed of escaping exhaust gasses, so less flow and less vacuum at the end of the pulses. Another issue with a bigger bore is that it cools down gasses, cooled down gasses weigh more per volume and therefore restricting the flow.

Anyway; this is what I understand from it all..

Would have thought some of the exhaust specialist on here would have knowledge of this subject or do they just weld up some tubes and baffles till it sounds 'nice' :quiet: .


@Jack: who's the oracle at Delphi?