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Exhaust system bore???
garage19 - 9/4/07 at 07:24 AM

After the for branch on a 150 bhp 2ltr zetec what diameter woud you make the system?

The choice is between 2" (52mm) and 2.5 " (63mm).

Thanks,

Doug

turbodisplay - 9/4/07 at 08:02 AM

I think 2" is okay for up to 200hp (but dont quote me!
I`m using a 2.5" for my primary turbo, secondary is a wastegate with 2" pipe.

The exhaust helps extract gases, so an optimal bore will exist for any given setup.

Mr Whippy - 9/4/07 at 08:10 AM

A bullyboy I'm sure would recommend at least a 5 inch bore, but 2 inches is really more than enough for 2ltrs.

SeaBass - 9/4/07 at 09:27 AM

quote:
Originally posted by Mr Whippy
A bullyboy I'm sure would recommend at least a 5 inch bore, but 2 inches is really more than enough for 2ltrs.



Do you mean a barryboy?? lol

garage19 - 9/4/07 at 03:03 PM

quote:
Originally posted by Scoobylav
On a normally aspirated car 2" will be better as it will provide slightly more back pressure. When utilising a turbo, the bigger more free flowing you can get the better. So 5" would be perfect. . . . . .


You should never create back pressure through a restriction. Back pressure is not good in any setup wether it be turbo or n/a.

The only reason you would run a smaller exhaust is to maintain gas velocity.

The small amount of back pressure (wave) used for pluse tuning is created by converging the primaries into a larger bore and the drop in gas speed/pressure.

[Edited on 9/4/07 by garage19]

Jasper - 9/4/07 at 03:15 PM

2 inch is fine up to 250 ish bhp, then 2.5 inch is good up to 450bhp - that's what Powerspeed told me when I was ordering my (2.5 inch ) silencer

Memphis Twin - 9/4/07 at 05:07 PM

My understanding is that by having a freeflowing exhaust on a n/a engine you will actually acheive less power output from the engine. I wasn't trying to say you should restrict exhausts to gain back pressure - however by derestricting you will adversly affect the performance.



Who told you that?!
What utter nonsense!

DIY Si - 9/4/07 at 05:19 PM

I think he's on about the pulse tuning build in to some systems and the lack of gas velocity leading to less scavaging (?) Bigger is not always better with this one.

Big Daz - 11/4/07 at 08:47 AM

quote:
Originally posted by Scoobylav
quote:
Originally posted by garage19

You should never create back pressure through a restriction. Back pressure is not good in any setup wether it be turbo or n/a.

The only reason you would run a smaller exhaust is to maintain gas velocity.

The small amount of back pressure (wave) used for pluse tuning is created by converging the primaries into a larger bore and the drop in gas speed/pressure.

[Edited on 9/4/07 by garage19]



My understanding is that by having a freeflowing exhaust on a n/a engine you will actually acheive less power output from the engine. I wasn't trying to say you should restrict exhausts to gain back pressure - however by derestricting you will adversly affect the performance.



A larger bore can reduce the exit speed of the gas, which as a result may end in a drop in power. This has been proven many times with the Max Power brigade running large bore systems on small capacity emgines.

Memphis Twin - 11/4/07 at 10:06 AM

quote:
Originally posted by Scoobylav
quote:
Originally posted by Memphis Twin

Who told you that?!
What utter nonsense!


Thanks for the replies Si and Daz I thought maybe I was going senile. Obviously I am not alone.


You were talking about maximum power.

A large bore exhaust can shift the rpm where the engine produces maximum torque higher up the rev range, which looks like a loss in midrange power, but in fact is just moving it further up the rev range. The larger bore system will allow the engine to make more top end power (which may or may be not what you're looking for).

David Vizard reckons that the best exhaust (for power)is not to have any restriction at all.

Just playing Devil's advocate...

bimbleuk - 11/4/07 at 10:21 AM

I've got a 4>1 exhaut manifold on my 1.6 4AGE with 150 BHP. The secondary and silencer pipes are 2" OD which works well. I've installed a supercharger and will expect 200-225BHP so I'm changing the secondary and silencer pipes to 2.25" OD pipes if it was turbo'd I would have considered 2.5" OD pipes as this can help to reduce lag.

May not be perfect for my installations but these are sizes I've commonly seen used on similar setups. I've also read a few reference books just to check I'm not miles away.