How will an exhaust make you lose POWER?

[claytarget_god]

I am looking at getting a custom exhaust on my vRS.

I am looking for some info on this, was looking at getting the Milltek cat and back system. cheapest Ive seen them is about £680.

have seen few bits on these and they apparently are a tight fit where it bends near the back. plus I think the tail is a bit small for the cut-out in the bumper.

so i decided to have a word with Longlife who make there own from the same stainless as Milltek and they are cheeper, plus you can choose the tailpipe you want and how you want it to look.

I am going for their downpipe with a french made sports cat and the cat back system.

now I have been warned that the Milltek is made for the car so there will be no probs with it but the one from Longlife might do something to lose power?

can anyone tell me what would cause this to happen?

plus any other tips would be helpful.

cheers

[martziniuk]

Do you know how many cel the cat is?
Also is it a 2.5" system?

[Fluffmeister]

Some engines engines can make less power with a full system because of a lack of back pressure.

I can't really comment on cars because I don't know enough, but have experienced it with bikes. A forced induction motor may need a "little" back pressure to keep the turbo operating optimally.

[nbr_uk]

I would have thought a company who make exhausts for specific models of car on an ongoing basis like Milltek would spend more time on testing and refining the design than a company who make it as a one off like longlife.

[Angry Dog]

Have you considered Blueflame?

I arent a fan of these "custom" exhausts. Hayward and Scott are pretty much the only company I rate, but it comes at a price!

[bengie]

an exhaust isn't just a pipe that directs gases away from the engine, plenty of science in them!

[claytarget_god]

have seen the blueflame and I dont like the upswept tail.

I am not sure what cell the cat is, Ill call them on monday and find out. plus find out what the pipe diam is.

cheers for the info lads n lasses. most helpful.

the manager showed me her pc while she put in all the details of my car and it came up with a detailed pic of the system so dont know wether they know things like pressure.

surely there will be back pressure so long as there is a cat there?

has anyone had there Octy De-cat??

cheers

[Lady Elanore]

There might be standing waves of pressure at certain revs and this could work against the gas flow leaving the cylinder. Tuning…..geddit

We used to spend a fortune to get proper tuned parts when I was young, inlet and output parts were all 'tuned' for this purpose and as far as i know it is why we call it 'tuning'

[redbaron]

Originally Posted by bengie

an exhaust isn't just a pipe that directs gases away from the engine, plenty of science in them!

agree... but in general on a turbo the lower the back pressure the easier the turbo spins....

[Beretta]

Originally Posted by claytarget_god

have seen the blueflame and I dont like the upswept tail.

I am not sure what cell the cat is, Ill call them on monday and find out. plus find out what the pipe diam is.

cheers for the info lads n lasses. most helpful.

cheers

You dont have to have the upswept tail on a Blueflame exhaust, just have a word with Nigel at Blueflame as there are a few different ones to choose from.

I have the turbo back Blueflame with the 100 cet sports cat and cannot fault it.

A few have run de-catted, but you will have the cel light on all the time.

[claytarget_god]

rgr that.

redbaron, that is what I thought, the easier it is for the gas to get out the better the turbo runs,

so long as there isnt a sudden pressure increase id of thought it would be fine.

[Lummox]

Originally Posted by redbaron

agree... but in general on a turbo the lower the back pressure the easier the turbo spins....

Yeah, unlike a n/a engine a turbo motor would be quite happy with no exhaust at all, just be a bit noisy....lol.

[claytarget_god]

cheers lummox, so long as that is right. jus that they are very close and cheaper that one form milltek. plus they fit it and there is a guarantee on it for the time I own the car so sounds good, plus it is custom so i get to say roughly what it looks n sounds like and they build it.

ie the back box i want to bee tucked up so you cant see it, jus like the original, they said that was fine.

gonna book it in for the 02 may ish, that is the onlw weekend i can spare the car.

Ill put up some pics of them fittin it and what it looks like on if they dont mind

cheers

[bengie]

Originally Posted by Lummox

Yeah, unlike a n/a engine a turbo motor would be quite happy with no exhaust at all, just be a bit noisy....lol.

i can confirm this after a weld failed on the exhaust fitted to the mazda, sounded effing awesome at full chat in 2nd at 6000rpm though a right rally slag sound!!

[Lummox]

Just thinking of running a pipe off the turbo and out the front wing on the Golf

[bengie]

it'd be a tad loud i think.

evie wouldn't like it.

[speednut]

I removed a Jetex cat back system and replaced it with a custom built one by Exhausts UK in Sheffield. A far better quality system than the Jetex and I had the restrictive centre section removed straight through to the cat. All in cost me £300 and you can choose tailpipe designs and tailor the overall sound to your own taste. Sounds like the Longlife setup is similar, but I would check out there work first before you commit money.

I have had 3 systems fitted by Exhausts UK and rate them very highly, all quality materials and the work has a lifetime guarentee. For me custom built systems are better than stock off the shelf items and you can stamp your own identity on your motor making it that bit more unique.

Steve

[JRHartley]

From what I can remember from my you'ff , the turbo will spin more freely with less back pressure from the larger bore exhaust system but you can get into issues with the cam profiling. Some petrol engines use cams with valve overlap profiles to give a scavenge effect. This used to be the case on A-series engines whereby the inlet valve begins to open whilst the exhaust valve is still open at the end of the exhaust stroke. The port velocity of the escaping exhaust gases going down the exhaust system used to help draw in air/fuel into the cylinder as a way of optimising the intake stroke. If the exhaust system was not matched to the cam profile and head flow eg by simpling fitting a larger bore exhaust system on its own with no changes to fueling or induction then the the exhaust port velocity could be reduced due to the larger volume of the exhaust system. This slowing down could lead to a reduction in scavenge and produce less power rather than more power.

With regard to turbo diesels IIRC the main problem lies with exhaust gas temperatures. A large bore exhaust system will have a larger 'expansive' volume which may allow gases to cool down quickly but still remain in the system due to the lower velocity of the escaping gases . The colder gas will be of higher density and this can in effect increase back pressure rather than decrease it.

The oposite could also happen with a a large bore - straight through system. If the gases escape too fast they will not take heat away from the turbo quick enough and the cool air in the intake side of the compressor can get warmed up too much for the intercooler to cope with the knock on effect of less power.

Again all this depends entirely on the engine design so ideally the best exhaust system will be the one tuned to compliment all the other[ if any] changes made to the engine. Big is not always best [ well thats what I tell the missus :-) ]

[martziniuk]

There was a guy on the vxr forum who did the decat on his vectra vxr and lost quite a bit of power.Before and after RR results taken.

[claytarget_god]

JR

that was interestin mate, cheers for that. had to read it carefully.

if noone else has lost power from a 3" system hopefully I wont have any trouble. my car has CAI and a remap so I am hoping that the remap can take into affect the way the exhaust gasses escape with this system.

cheers