Author Topic: Question About size Exhaust  (Read 8349 times)

Offline breadman

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Re: Question About size Exhaust
« Reply #15 on: December 16, 2008, 08:13:06 pm »
In most cases, a 4-2-1 manifold is better for torque without being too restrictive at high rpm - you could say the best compromise on the majority of engines.
However, on a naturally aspirated full race "screamer" engine, a 4-1 manifold with long primary pipes is normally used. Torque isn't of as much consequence when the powerband is very narrow and at very high rpm.
For any supercharged Eaton/G40, regardless of spec I'd choose a 4-2-1 manifold everytime. TBH, I've never seen a 4-1 option for the G40?

@djtez, I'd think that 60mm would be fine if that's what you want to make it from. Make sure first that the company making the exhaust have facilities to bend 60mm, it isn't a standard size.

Offline djtez

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Re: Question About size Exhaust
« Reply #16 on: December 16, 2008, 08:18:25 pm »
ok doky, its 52mm stainless on it at the moment...
company called zaust who made it up .
this and a jetex single round tail pipe.

how much does the back box alter the back pressure of the full system?
surely having a 1" pee shooter on a 63mm (2.5") center pipe would almost be same as
 having a 3.5" box on a 2.25" (57mm) ?
or am i way off track

Offline breadman

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Re: Question About size Exhaust
« Reply #17 on: December 16, 2008, 11:33:02 pm »
Well, back pressure is always present due to the exhaust pipe itself being a restrictor. That is why a decent size system and silencer(s) are important in keeping the back pressure at a minimum.
For this reason you'd be better off fitting a large bore silencer (not one of the many that just  have a small inlet pipe and a bloody great big tail pipe!) at the end of a system as opposed to a pee shooter which would effectively strangle the large bore system.
It's like everything when you modify, you have to find a good compromise. You need an exhaust that's big and free flowing enough to keep the exhaust gases from being too restricted, whilst at the same time not being too big and or short to keep the gas speed up to aid extraction and noise levels reasonable. 

Offline polo classic

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Re: Question About size Exhaust
« Reply #18 on: December 17, 2008, 09:52:44 am »
Jabba tested 60 and 63,5mm bore exhaust on their old demo and found small differences. The 60mm lost 1 hp to the bigger one, however it had 3 nm more torque. Personally I'm sure a 55mm exhaust would net even better results over the 2,5" one. Just remember when Steve put his old G40 on the rollers at Jabba when he and Mark Hunter helped me get the ported head on back in 2001 or 2002. They thought there where something wrong with the rollers, at it produced so much power for the spec, 168 bhp with 68mm pulley, full blend charger, chip and a  Sebring 55mm exhaust

Offline Yoof

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Re: Question About size Exhaust
« Reply #19 on: December 17, 2008, 05:43:35 pm »
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.

In a well set-up turbo system your EGT won't be anymore than an n/a car. Out of a V8 twin turbo at work we'll see around 950'C on full chat, I've seen 920'C+ with my old 3f engine.... EGT has no relevance to bore size, unless there is a flow restriction and the gas is further heated. THe actual reason a larger exhaust size is used is to shift the extra air flow a G40 has over a GT, at a rough guess (without doing any maths at all) an extra 50% gas is exhausted after combustion between g40 and gt.

Personally I'd not get too hung up on bore size on a supercharged system, the differance between 60mm and 63mm is minimal, you'd be better spending your time calculating primary, secondary lengths and collector volumes.



Offline il_d0tt0re

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Re: Question About size Exhaust
« Reply #20 on: December 17, 2008, 05:54:34 pm »
so...Polo Classic your opinion for a guy who have port and polish head with big valves, full blend 1.4l engine block, injectors, 4-2-1 mainfold exhaust and GT intake for example can get better results with a 55mm exhaust, then a 60mm one or 63mm??? in reality it's my opinion to but i will do the test  ;D

Offline polo classic

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Re: Question About size Exhaust
« Reply #21 on: December 18, 2008, 12:43:10 pm »
As you said, it needs to be tested. Test seems to indicate that up to at least 170 hp, a 55mm exhaust is better. But it is not that easy to compare, as there are more than one way to build a silencer  ??? and most manufacturers use either 51mm (2 inch) or 63,5mm (2,5")