Hi,

 

Unfortunately, my car recently developed a head gasket failure so I've taken it off-the-road for the winter. Guess it's as good a time to start the various projects I've had planned for a while.

 

Apart for the obvious engine rebuild I am also looking into the exhaust side of things. My car is a MY1990 JDM Twin Turbo and has the original catalytic converters still in place (but with custome cat-back free flow exhaust). I don't particularly want to go down the route of decat pipes (would make the car far too loud) but was wondering if anyone had fitted so-called "sport" cats (also referred to as "hi-flow" cats)? Does anyone have a good known source for these?

 

As always, any and all information is greatly appreciated! :)

 

Dan

Danny, Leon at Alunox supplies these, think he's had good results with them. He's in austrailia working for 6 weeks at the minute though.

De-cat doesn't make much difference to the noise when you have turbos.

Also you can a small backbox in the test pipe, to help with noise if you want?

Thanks for the info guys.

 

John - how's it going chap? Long time no hear/see! When abouts will Leon be back? I'm not in a hurry as this is the last aspect of the "project" - must first get the engine and suspension organised.

 

Andy - The car is pretty loud as it is at the moment - especially between 2500~3500 rpm (heavy drone). As it's an AUTO it spends a lot of time in that rpm range. Hence wanting to quiet it down a little

 

Mark - I've thought of doing that and will ask Leon about it as well when I speak to him. I see Luke (ZCentre) does these types of test pipes as well.

 

Cheerio guys!

 

Dan

I agree with AndyP Decat makes sod all noise really.

 

A turbo is a baffle think about it :)

 

Lloyd

I agree with AndyP Decat makes sod all noise really.

 

A turbo is a baffle think about it :)

 

Lloyd

 

Fair enough. I am still concerned about (a) turbo seals prematurely perishing (despite them being brand new) and (b) emissions at MOT time.

 

However, I may consider a straight through system as well because I can always have some silencers added at some point in the future.

Depending on the age of your zed, it is immune from decat restrictions because it is not on the DVLA's database!

Depending on the age of your zed, it is immune from decat restrictions because it is not on the DVLA's database!

 

 

That's true and it is a pre-1992 car so should be ok come MOT time.

 

One "Sports Cat"

 

One "Sports Cat"

 

:wack: LOL!

 

Reminds me of a joke I saw written down a long time ago.

 

An airplane's log book shows the pilot heard "mouse-like" squeeks coming from behind the instrument panels. The following day, after the technicians had a look at it, there was the following entry "cat installed" ... :D

 

I'm not very good at retelling jokes, as you can see, but it was hilarious when I first read it!

bwhahahaha i remember that one, there was loads of those... im sure ive got en e-mail with them in somewhere

you get away with de-catt on cars up to 1995 if they are not in the data base (which the zed isn't) but after 1995 your car is tested as having catts if its in the data base or not

I have heard that Mini Cooper S cats flow well.

Fair enough. I am still concerned about (a) turbo seals prematurely perishing (despite them being brand new) and (b) emissions at MOT time.

 

However, I may consider a straight through system as well because I can always have some silencers added at some point in the future.

I don't think you will need to worry about those issues as all the engine parts will be new, just go for a straight through system. This should help it spool up quicker too. :dance:

I don't think you will need to worry about those issues as all the engine parts will be new, just go for a straight through system. This should help it spool up quicker too. :dance:

 

Yes - agreed.

 

I have been compiling a (wish) list which includes MS Manifolds and Greg Dupree divorced wastegate pipes (5-bolt). So if that lot flows into a set of unrestricted decat pipes it should all flow pretty well. I'm just concious not to make the final system too free flowing as that will have the opposite effect (ie slowing the gasses down and therefore reducing power).

 

Thanks again for all your feedback!

 

Cheerio!

 

Dan

I'm just concious not to make the final system too free flowing as that will have the opposite effect (ie slowing the gasses down and therefore reducing power).

 

Just the opposite the more free flowing you can make a turbo system the better as you will avoiding backpressure the enemy of a turbo system!

 

With an NA system you are right!

I have been compiling a (wish) list which includes MS Manifolds and Greg Dupree divorced wastegate pipes (5-bolt). So if that lot flows into a set of unrestricted decat pipes it should all flow pretty well. I'm just concious not to make the final system too free flowing as that will have the opposite effect (ie slowing the gasses down and therefore reducing power).

Dan

I know this always causes a debate on here! :headvswal

Using my vague understanding of turbo charged engines, I thought the more free flowing the better?!? :confused:

I know this always causes a debate on here! :headvswal

Using my vague understanding of turbo charged engines, I thought the more free flowing the better?!? :confused:

 

Beat ya to it by one minute :dance: :D

Agreed - get that filthy burnt mess to the outside world ASAP so there's room for more nice new stuff !

Just the opposite the more free flowing you can make a turbo system the better as you will avoiding backpressure the enemy of a turbo system!

 

With an NA system you are right!

 

Mark - I agree with that statement to a certain extend. To the best of my knowledge, a turbo charged system needs a certain amount of back pressure to operate correctly. Just for argument sake, lets say I connect a straight through system that goes from 3" at the turbos to 6" at the other end. That is going to slow the gas flow down. This is also the reason why it isn't adviseable to bolt on an exhaust system on a TT with pipes that are more than 3" in diameter. IIRC I have seen dyno graphs comparing a TT with 2.5" pipes and then with 3.5" pipes (believe it was in the US). The latter system produced almost 30bhp LESS than the 2.5" system. I can't remember where I saw this so can substantiate my claim but that is my understanding of exhaust systems. I obviously now wait to be corrected! LOL ;)

 

Cheers!

 

Dan

Mark - I agree with that statement to a certain extend. To the best of my knowledge, a turbo charged system needs a certain amount of back pressure to operate correctly. Just for argument sake, lets say I connect a straight through system that goes from 3" at the turbos to 6" at the other end. That is going to slow the gas flow down. This is also the reason why it isn't adviseable to bolt on an exhaust system on a TT with pipes that are more than 3" in diameter. IIRC I have seen dyno graphs comparing a TT with 2.5" pipes and then with 3.5" pipes (believe it was in the US). The latter system produced almost 30bhp LESS than the 2.5" system. I can't remember where I saw this so can substantiate my claim but that is my understanding of exhaust systems. I obviously now wait to be corrected! LOL ;)

 

Cheers!

 

Dan

 

That is really just a symptom of a badly designed exhaust. From memory we're talking about the Bennouli (spelling?) principle here, which basically says that if you increase the diameter you slow the gas velocity, if you decrease it then you increase the gas velocity. Obviously, if you take this to it's logical conclusion the you could say that a smaller exhaust is better - wrong. The reason being that the volume of the pipe is increased as well, therefore the same volume of gas moves except at a different rate. Where you run into problems is when the input doesn't match the capability of the output, this is what people like Leon (and hopefully most exhaust designers) understand.

 

The only reason to have long exhausts is to avoid burning the front of the car (and to make room for big silencers) ! Think about drag cars with those metre long tubes and flames coming out the end ...

Here is a little write up from a Garrett turbo designer alledgedly!

 

http://www.fiatforum.com/cinq-sei-technical/44817-turbo-exhaust-design-theory-garrett.html