Question About size Exhaust

[il_d0tt0re]

I guys!!!
I'm From Portugal and owner of a Polo Van Gt with a G40 engine made by me. and i learn a lot with you on this forum because you do a lot of good question and response for that questions, so...
I have a question you'd like to ask you, and it's about the exhaust size...
Why Pitstop and others Companys use a 63mm on exhaust???why not 55mm or 60mm??? do you understand??? my question it's understanding why they choose 63mm... Can explain me??? i'm just curious.
Many greetings from Portugal
Rodrigo

P.S.: sorry about my english i'm not so good 

[PeteG40]

my guess is that 63mm is approximately 2.5" so easy to get pipes in that size. Whilst this might be a little big for relatively unmodified g40s, cars with eaton and turbos will need to shift lots of air!

[il_d0tt0re]

yah pete i know that. but let's talk about G40's with a few things  like camshaft 1.4 or 1.5l with big valve head full blend g-lader you know 

[breadman]

Hi and welcome Rodrigo.
You certainly don't need to apologise for your English, it appears to be far better than some of the people who regularly post on Club Polo!
The reason such a relatively large exhaust pipe is used is because in a supercharged or turbo'd engine the exhaust gas temperatures are that much higher then in a normally aspirated engine. Hence the use of sodium filled exhaust valves on the G40.
As the exhaust gases leave the engine they expand, therefore for high powered modified engines you need an exhaust with a larger internal volume (bigger diameter). The higher the exhaust gas temperature, the bigger the exhaust needs to be - within reason.
PSD etc used 63mm because it was just about as big as they could comfortably get under the Polo floorpan.
Hope this helps.
Cheers,
Richard.

[il_d0tt0re]

tks breadman your reply was helpful and i know the point that you said "in a supercharged or turbo'd engine the exhaust gas temperatures are that much higher then in a normally aspirated engine" and "As the exhaust gases leave the engine they expand, therefore for high powered modified engines you need an exhaust with a larger internal volume (bigger diameter). The higher the exhaust gas temperature, the bigger the exhaust needs to be - within reason" that's a fact but we are talking about a 1.4 or 1.5L. i will tell you why of this question...
Here in Portugal you had Seat Ibiza (only a example) diesel engine 1.9l with a large modification, talking about 240hp or 280hp or a little more and the use 60mm of exhaust...i asked to a mechanical guys the reason why they used 63mm on a 1.3l or 1.4l, 1.5l engine but they can't answer me right. 

[breadman]

Remember, a diesel engine doesn't rev anywhere near as much as a petrol engine. So although the high EGT still exist within the turbo diesel, the actual volume/exhaust gas speed will be lower or similar to a 1341 G40/turbo operating at 7k rpm. I think!

[il_d0tt0re]

yes you're right but talking one petrol engine like an evo or sti the exhaust size that you usually bought it's 70mm you understand the comparation of 70mm for a 2.0l engine and a 63mm for a 1.3l engine?  i think with 63mm you will get a powerfull accelaration but a lower top speed comparing a 60mm exhaust or a 55mm!?!?!?!? what you think breadman?
Remember we are just talking about a question of what's a ideal exhaust size...because we know that G40 is good on acelaration and not for high speed 

[Dan]

It is not good to compare exhaust pipe diameters of normally aspirated and forced induction engines. There can be a lot of debate about this topic, but it all boils down to back pressure at the end of the day. There is a magic number for back pressure, but since it's rarely measured you can assume a certain diameter will work well from past experience.

I would use the following diamaters:

G40 with standard or slightly modified engine. 50.8mm or 2"
G40 with many modifications including charger, 65mm pulley, cylinder head, camshaft and increased capacity. 57mm or 2.25"
G40 or Polo engine with Ko3 turbo. 63mm or 2.5"

The reason why you use a larger bore on a turbo is there needs to be a greater pressure difference across the tubine. Hence reducing the back pressure as much as possible with a 2.5" downpipe and exhaust system.

[il_d0tt0re]

tks dan...
i like your forum because every one gaves rights opinions without fear of being wrong 

[breadman]

Remember we are just talking about a question of what's a ideal exhaust size...because we know that G40 is good on acelaration and not for high speed 

G40's are good for acceleration because they have exeptionally good power to weight ratio. Top speed is only limited by gearing and aerodynamics. I know of at least one heavily modded G40 that hit 7700rpm in fifth gear - 153mph or something silly! So I'd hardly say they aren't capable of high top speeds, it's just bloody frightening going that fast in what is essentially a shopping car!
I can see what you are saying regarding exhaust size on an Evo etc. However, a 70mm system that is also longer than the short Polo system will have an much larger effective pipe area than you'd think.

[djtez]

i was going to go for a 60mm exhaust?

mods are
gas flowed cylinder head
piper fast road cam
gas flowed gt inlet
4-2-1 manifold
toothed belts 65mm
57i kit with samco's
sns stage 5 chip
g60 green injectors.

would i be right on sayin thats enough mods to handle 60mm or am i pushin it and should pipe down to 2.25" ??

[il_d0tt0re]

Djtez for Pitstop use will need de 63mm or 2.5...But another question for you...What mainfold do you prefer 4-2-1 or 4-1???tks

Opinions needed lol
PeteG40
Breadman
Dan 

And who wants to...

[djtez]

4-2-1 for me coz it goes step by step down.
rather than rushin 4 cylinders compresion right into the 1 single exhaust pipe.

just my veiw
im sure the two will have advantages over each other.
i cant remember but i think the 4-1 has better top end (higher rev) power
and 4-2-1 is from low up through the mid-range.

i hope i got this right ^

[Dan]

djtez, I am not sure how readily avaialble 60 mm tube is availble for fabricating an exhaust. Most places would only have 2.25 or 2.5" in stock and have the bending mandrels to suit.

2.25" is more than adequate for what your spec is there. I would think about what diameter the downpipe is first. The bigger bore may also make it a bit boomier especially on motorway journeys.

4 into 1 isn't really useful on the road with camshafts we use, 4-2-1 is my choice (plus they fit easier ).

[djtez]

well i have access to any bore size really, i have trade access to piping.

i am supplyin the pipe myself to performance exhaust garage. jut i have to be sure to myself what is the best bore for my spec of engine, preferably to the mm as possible lol?