Does a decat make a noise difference? Like a turbo whistle etc that's what im after

Does a decat make a noise difference? Like a turbo whistle etc that's what im after

Yes you will definitely get turbo whistle.

Decatted dervs sound atrocious!

The only person who thinks it sounds good is the owner!

They are a joke and the owl noise is pathetic

Have to agree but today many think go faster stripes and noise equals power

Does a decat make a noise difference? Like a turbo whistle etc that's what im after

 

It's a bit of a gamble really. I've got no silencers and no cat and mine barely makes any whistling sound. It sounds quite tinny almost at idle, my Dad keeps moaning at me as "your exhaust is blowing" but it's not! haha

 

Sounds great when really going for it though.

 

http://www.youtube.com/watch?v=LykXFEEyfVI&noredirect=1

Decatted dervs sound atrocious!

The only person who thinks it sounds good is the owner!

They are a joke and the owl noise is pathetic

 

false (my bold). I'm not bothered either way about the owl noise although I'm not aware of mine having ever made it and I don't feel I've  missed out at all

I've got to agree with jase they just sound unrefined.

Turbo/Subaru or high revving vtec engines sound amazing.

Mine sounds like a spitfire

From what I've read it seems that a decat can help the turbo spool quicker, so personally would decat then map it to take that into account.

As Dan says the standard exhaust is good for 200bhp

 

 

Mine whistles like a jet engine on idle but sounds good on boost, loads of people always look if I give it some because it's quite loud and growls. If there's no cat and no back box there is nothing restricting the exhaust gases so that's how it spools up quicker, makes it more free revving in my opinion for the simple reason there's no restrictions in the exhaust

 

Both of you are applying petrol engined turbo theory to a turbodiesel, so I'll walk you through it:

 

A petrol engined turbo car uses a FGT (Fixed Geometry Turbo) because they can cope with the much higher EGT (Exhaust Gas Temperature) of a Petrol engine, the larger the exducer or the higher the exhaust back pressure, the slower the spool, this is why large FGT's spool slowly and exhaust back pressure is a bad thing because it just makes the problem worse.

 

A diesel engined turbo car can use a VGT (Variable Geometry Turbo) because peak EGT is much lower, this means that the turbo can start boosting at lower engine speeds, it does this by using rotating vanes to restrict the exhaust gas flow forcing it to accelerate and drive the turbine faster, the rate of exhaust gas flow increases as the engine accelerates so the vanes gradually rotate until they are are wide open, at this point the VGT becomes the same as an FGT, only once this point is reached does exhaust back pressure become meaningful, this is why doing stuff to your exhaust CANNOT make any difference to spool-up.

 

FGT cars are insensitive to anything about the exhaust system other than back pressure so you can tune the turbo response using back pressure alone, if you want the turbo to spool earlier you INCREASE the back pressure, this is useful in LPT (Light Pressure Turbo) applications where you want to increase the boost at lower RPM, conversely if you want the turbo to spool quicker then you reduce the back pressure as much as possible so that you reach peak boost faster.

 

VGT cars have the best of both worlds, they shut the vanes to increase back pressure and boost at lower RPM and open them to allow peak boost, it is these vanes that control the spool, not the exhaust system. The consensus about the standard VRS exhaust system on here is that it's good for about 220 BHP before back pressure starts to limit the boost but ONLY once the vanes are wide open, it still won't affect turbo spool at lower engine speeds.

 

Petrol turbos, diesel turbos and naturally aspirated engines all work TOTALLY differently and also respond completely differently to tuning mods, this is why stuff that helps one type will almost certainly not help another type, in fact many mods will actually reduce power when applied to the wrong engine type.

 

There, I hope that helps, it's as concise a summary as I could manage and I've also greatly simplified things so thanks for sticking with me if you've read it all.

Thanks for the write up. As I say it was only what I read about the decat/ large bore down pipe from Miltek's website. Would have thought they know something

Wha?? Variable geometry is there to restrict the free running speed of the turbo because it doesn't have a wastgate. It's wide open at low rpm and gradually closes as boost rises.

Thanks for the write up. As I say it was only what I read about the decat/ large bore down pipe from Miltek's website. Would have thought they know something

 

The difference is that I'm not selling anything, you can check the information I've just freely given you for yourself if you like, at least you now know what to look for.

 

Remember that a turbo is a pump, the more oxygen you can get it to pump into the engine the more torque you can make, you can increase oxygen content by increasing the volume of air pumped, also by increasing the density of the air by lowering the temperature, the dominant factors in any turbo engine are therefore the turbo itself and the intercooler, as long as you can add more fuel to burn with that oxygen, not a lot else really makes any difference.

Wha?? Variable geometry is there to restrict the free running speed of the turbo because it doesn't have a wastgate. It's wide open at low rpm and gradually closes as boost rises.

 

http://www.autozine.org/technical_school/engine/tech_engine_3.htm#VTG

If it doesn't add power, how come the seat leon cupra makes an extra 7bhp and 10ft/lb when the silencers are removed on that video. Literally the only evidence I have of it making power though.

Both of you are applying petrol engined turbo theory to a turbodiesel, so I'll walk you through it:

A petrol engined turbo car uses a FGT (Fixed Geometry Turbo) because they can cope with the much higher EGT (Exhaust Gas Temperature) of a Petrol engine, the larger the exducer or the higher the exhaust back pressure, the slower the spool, this is why large FGT's spool slowly and exhaust back pressure is a bad thing because it just makes the problem worse.

A diesel engined turbo car can use a VGT (Variable Geometry Turbo) because peak EGT is much lower, this means that the turbo can start boosting at lower engine speeds, it does this by using rotating vanes to restrict the exhaust gas flow forcing it to accelerate and drive the turbine faster, the rate of exhaust gas flow increases as the engine accelerates so the vanes gradually rotate until they are are wide open, at this point the VGT becomes the same as an FGT, only once this point is reached does exhaust back pressure become meaningful, this is why doing stuff to your exhaust CANNOT make any difference to spool-up.

FGT cars are insensitive to anything about the exhaust system other than back pressure so you can tune the turbo response using back pressure alone, if you want the turbo to spool earlier you INCREASE the back pressure, this is useful in LPT (Light Pressure Turbo) applications where you want to increase the boost at lower RPM, conversely if you want the turbo to spool quicker then you reduce the back pressure as much as possible so that you reach peak boost faster.

VGT cars have the best of both worlds, they shut the vanes to increase back pressure and boost at lower RPM and open them to allow peak boost, it is these vanes that control the spool, not the exhaust system. The consensus about the standard VRS exhaust system on here is that it's good for about 220 BHP before back pressure starts to limit the boost but ONLY once the vanes are wide open, it still won't affect turbo spool at lower engine speeds.

Petrol turbos, diesel turbos and naturally aspirated engines all work TOTALLY differently and also respond completely differently to tuning mods, this is why stuff that helps one type will almost certainly not help another type, in fact many mods will actually reduce power when applied to the wrong engine type.

There, I hope that helps, it's as concise a summary as I could manage and I've also greatly simplified things so thanks for sticking with me if you've read it all.

You forgot one critical item with spool (in petrol cars) , manipulation of spool with an interupt etc 2 or 3 port boost solenoid to manipulate spool or make it worse in some wrongly configured pid tables

I have a milltec turbo back with 2 silencers and a decat very quiet when u drive sensible roars when on boost looking to swap for a blue flame full system as I like the letterbox backbox if anyones intrested and I got an m.o.t centre to swap them if need be.

If it doesn't add power, how come the seat leon cupra makes an extra 7bhp and 10ft/lb when the silencers are removed on that video. Literally the only evidence I have of it making power though.

 

Like I said, it's only when we consider an FGT or a VGT with the vanes wide open that exhaust back pressure makes itself known, I can only assume that the SEAT exhaust is not as well designed as the Fabias.

I'm not going to argue with what you said as that's abit too technical to me I've just always worked on the basis of get as much air and fuel in and get it out the exhaust as fast as you can which changes is why every car I have has some sort of intake upgrade, a remap and usually a decat exhaust system. Admitidly this is my first diesel but I'd of thought those modifications must make a slight difference

http://www.autozine.org/technical_school/engine/tech_engine_3.htm#VTG

good article that, I just admit I didn't word what I said very well, low rpm while it's spooling ALL off the exhaust gas is directed towards the turbine wheel ( closed in the classical turbocharger sense) and it gradually eases the gas away from the turbine as the speed increases, this reduction is gas in turn reduces the speed of the turbine, and the speed of the turbine and the boost pressure are directly related, so it limits the boost pressure.

the point in fact here is that there's nothing you can do to reduce lag, except by placing a restricter in the exhaust turbine to increase the gas speed, which would be harmful to flow at full chat.

The most important thing to note is how exhaust system back pressure plays NO PART AT ALL in how a VGT performs until those vanes are wide open because all the restriction to exhaust gasflow is governed by the vanes, what back pressure there is will limit the speed of the exducer and reduce full boost accordingly, however because your map limits boost by measuring MAP (Manifold Absolute Pressure) and adjusting the vanes accordingly your ECU will simply feather the vanes slightly rather than allow the boost to rise any further, so reducing back pressure will actually achieve nothing at all unless you have a hybrid turbo and more boost mapped i.e. above about 220 BHP. Even then any restriction due to back pressure will be very small although it will escalate quite quickly, so realistically above 230 BHP it's probably worth considering changing the exhaust system.

Just got the cat taken out of mine and I've got the back box out, it's loud when i put my foot down, quite on motorways and on idle it just sounds like a loud blowing sound ha. No noticeable power gains but it does defiantely spool up faster I don't care what anyone says. Mapping on Friday so will see what the outcome is.

I'm not going to argue with what you said as that's abit too technical to me I've just always worked on the basis of get as much air and fuel in and get it out the exhaust as fast as you can which changes is why every car I have has some sort of intake upgrade, a remap and usually a decat exhaust system. Admitidly this is my first diesel but I'd of thought those modifications must make a slight difference.

 

I'm sorry, it just doesn't work the way you think it does, so those mods at best will give you a noisier engine and at worst can actually lose power.

A remap is always worth doing as long as it's a good one.

 

I saw a comedy question on here a few days ago about tuning 'stages', so here's my take:

 

Stage 1 = Remap

Stage 2 = Stage 1 + better turbo

Stage 3 = Stage 2 + better intercooler

Stage 4 = Stage 3 + better injectors + higher flow filtration + performance exhaust

Stage 5 = Custom engine development at a proper race tuning firm, very expensive but power output only limited by the depth of your wallet.

 

Tuning 'stages' are simply convenient packages of mods for tuning firms to sell so they can make a profit, get paid and try and stay in business so they usually chuck a load of crap in there that you don't need so they can increase their margin.

Just got the cat taken out of mine and I've got the back box out, it's loud when i put my foot down, quite on motorways and on idle it just sounds like a loud blowing sound ha. No noticeable power gains but it does defiantely spool up faster I don't care what anyone says. Mapping on Friday so will see what the outcome is.

 

Good for you, as long as you're having fun then you're definitely doing it right.

I'm sorry, it just doesn't work the way you think it does, so those mods at best will give you a noisier engine and at worst can actually lose power.

A remap is always worth doing as long as it's a good one.

I saw a comedy question on here a few days ago about tuning 'stages', so here's my take:

Stage 1 = Remap

Stage 2 = Stage 1 + better turbo

Stage 3 = Stage 2 + better intercooler

Stage 4 = Stage 3 + better injectors + higher flow filtration + performance exhaust

Stage 5 = Custom engine development at a proper race tuning firm, very expensive but power output only limited by the depth of your wallet.

Tuning 'stages' are simply convenient packages of mods for tuning firms to sell so they can make a profit, get paid and try and stay in business so they usually chuck a load of crap in there that you don't need so they can increase their margin.

couldnt agree more with you on the stage ****, although on a petrol the stages do actually make sense. But with a diesel a remap is as far as you can go without changing the turbo, people say to me what stage is your car, I'm just like wtf are you on about? Is been mapped and that's it, no stage just a map, helmet.

Good for you, as long as you're having fun then you're definitely doing it right.

hopw that wasn't sarcastic after I've just agreed with your statement completely ha.

Bro', that was genuine, I know I can be a bit 'sharp' sometimes, but I meant that.

Was considering a bigger turbo plus decat at one point (several years ago). Was at that point I read/realised that cat converters do a reasonable job on a diesel. It's not all bolox. So if doing it now, would try to engineer a sportscat into it somehow.

 

Back on topic:

 

Improving the pressure differential across the turbine will have some effect, definitely. I've always thought anything to reduce back-pressure would be a good thing if fitting/mapping a bigger turbo. It's not quite that simple though. As peak back pressure won't be present in the exhaust system at boost threshold or spool from cruise (if that even applies in a turbo diesel).