What Sort Of Exhaust System For A Gti-6 Conversion?

[cybernck 402 1 Cars]

What do people normally use?

 

Is standard 205 system with OE middle box and back box too restrictive for this engine?

[chipstick 382 1 Cars]

^{You want a fairly substantial bore. I've read people saying about the engine feeling throttled with a standard bore exhaust.}

 

^{Magnex units seem to be common.}

[stef205 92 1 Cars]

i went from a magnex 2" up to a custom 2.5" bore pipe much more power, worth the stretch in my opinion.

[dcc 855]

Bullcrap.

 

Standard exhaust flows fine for a standard 6 engine. There was a big write up on this somewhere, too big-of-a-bore reduced the performance iirc.

 

 

[stef205 92 1 Cars]

i had a rolling print out and the print out with the larger bore made more power and more torque using the same rollers

[chipstick 382 1 Cars]

Miles in particular spoke about the throttling feeling from a small bore IIRC

[Anthony 1,003]

Remember that standard (pattern) systems vary massively in terms of bore size and flow, and even to my comparitively untrained eye, it's clear that the bad ones are very bad indeed and little doubt that would prove restrictive.

 

I'm convinced that a decent pattern system is fine for a standardish '6 engine, and the fact that Maxi managed 241hp (IIRC) on a standard Bosal exhaust suggests to me that it is unlikely to be a particularly noteworthy limitation on something with 70hp less.

[sambaileyw 2]

To big and the gases slowwwwws due to not holding the heat, to small and it also slows, needs to be just right but your only talking small amounts of power..

[Batfink 201]

Remember that standard (pattern) systems vary massively in terms of bore size and flow, and even to my comparitively untrained eye, it's clear that the bad ones are very bad indeed and little doubt that would prove restrictive.

 

I'm convinced that a decent pattern system is fine for a standardish '6 engine, and the fact that Maxi managed 241hp (IIRC) on a standard Bosal exhaust suggests to me that it is unlikely to be a particularly noteworthy limitation on something with 70hp less.

 

Without a comparison with a larger bore its not that much use as a guide. Yes the car made 241bhp. Could it have made more with a larger bore? I'd expect it to!

[Anthony 1,003]

I'm not suggesting otherwise - I'm simply pointing out what a decent standard system has been shown to be capable of, and that if it is capable of that, it stands to reason that it isn't going to be proving a massive bottleneck on a standard engine.

[Miles 331 1 Cars]

From experience, the Phase 1 system is not suitable at all (3 piece)

The later 2 piece providing it's a 2" bore is OK on std power, but allot now are smaller than this so watch out

2 1/4" is ideal for upto around 225bhp

[Tesstuff 222]

Is that just with the 6 engine?

 

My v6 has a smaller bore than that.

[welshpug 1,661]

IIRC the system I removed from the 406 V6 donor for AdamB's 306 was 63mm at the cat and 50mm at the tail.

[cybernck 402 1 Cars]

I've just measured the entry of a GTI-6 OE middle box and it was something like 53-54 mm, going by my vague measurements.

 

Surely if the 205 system is 2" or thereabouts, it will be fine for a standard engine.

 

 

However, what about the silencers? The middle box seems to be a straight pipe and the back box is quite bigger than 205's.

 

Are 205 boxes going to be a somewhat limiting factor or just make it a little bit noisier?

 

Also, I assume people don't transfer CAT's over to 205's when performing an engine swap?

[Miles 331 1 Cars]

The v6 309 I did had a 2 1/4" system on it again, any smaller and it will restrict, even on the 406 the bore is large.

 

CAT's are rarely swapped over unless it's a post 1993 car

[petert 588]

Unless you've got 250hp, 2.5" is a total waste of time. It's just all noise. I use to put single 2.5" systems on 300hp 5L V8's.

[Cameron 16]

To big and the gases slowwwwws due to not holding the heat, to small and it also slows, needs to be just right but your only talking small amounts of power..

 

Not true at all. In a large pipe the gases flow slower for the same mass flow rate (i.e. same power output) but the restriction is FAR less so more power is possible. In a small pipe the gases have to flow faster for the same mass flow rate, so the restriction is far greater! Exhaust pipe diameter isn't nearly as important as primary / secondary diameter, since all your tuning is in the manifold the rest of the system is just regarded as a means to get the gases from the manifold to the point at which they exit the car.

 

Unless you've got 250hp, 2.5" is a total waste of time. It's just all noise. I use to put single 2.5" systems on 300hp 5L V8's.

 

As above, certainly not a waste of time, but maybe a waste of money and package space.

[chipstick 382 1 Cars]

Mine is a 2.5" system IIRC.

 

Fairly loud. 90dB @ 4500RPM and 96 @ 6000RPM. Made 172bhp on the rollers a couple of weeks ago practically standard, so i'l stick with the current setup.

[sambaileyw 2]

Not true at all. In a large pipe the gases flow slower for the same mass flow rate (i.e. same power output) but the restriction is FAR less so more power is possible. In a small pipe the gases have to flow faster for the same mass flow rate, so the restriction is far greater! Exhaust pipe diameter isn't nearly as important as primary / secondary diameter, since all your tuning is in the manifold the rest of the system is just regarded as a means to get the gases from the manifold to the point at which they exit the car.

 

Surely as the pipe diameter increases (substantially) the gases will slow due to heat loss as the expansion of the gas from heat will drive it to escape faster? there is obviously an optimum size?

[Cameron 16]

Mine was 2.5" as well, made >180bhp with Emerald and mild cams - not bad.

[petert 588]

It would have 180hp with a 2" exhaust.

[NNN 1]

How do mufflers affect this? Would bore need to be bigger to overcome mufflers restriction and retain the same gas speed / power output?

Just wondering if the loss of power could be compensated when (for any reason) going from a free exhaust to a std one by increasing bore diameter...

[Cameron 16]

Depends on the muffler design, I don't know what the 205's internals look like but generally broadband attenuation and restriction go hand-in-hand.

 

Yeah I probably could have made 175bhp with a 2" system, but I made mine with more horsepower in mind. 2.25" would have been ideal, but I had easy access to thin wall 2.5" so there was no cost or weight penalty.

[billybluesky 1]

The size of an exhaust should surely be an exact science, if i remember way back wern`t people creating "tuned length" systems, using optimum pipe diameters and lengths to provide extractor systems, which scavenged the exhaust gasses by providing a negative exhaust pulse at the valve just before it closed, obviously this isn`t applicable where a turbo is used , but the point is, like women say , size isn`t everything

[BlueBolt 156]

Well, after a run out last night, I get the feeling that the Magnex system I'm running on mine is somewhat too free flowing...

She just feels a bit too boggy and like she wants to give more but can't?? I'm pretty sure the baffles in the middle box have blown out at some point, so that may be just the answer I'm looking for but will see....