4inch exhaust for the AU

[Stav]

Originally Posted by stockstandard

Your maths skills could use a little work.

Id LOVE to see the look on the faces at the exhaust shop when you ask them to fit the 4" pipe.

Ill take a photo. :Reverend:

[Lukeyson]

go the 4", if your putting a 460ci in it

[Stav]

No probs.it wasnt long ago where 3 inch exhausts were rubbished until Stiddy came along with his standard xr6hp manual and runs flat 15 second 1/4 miles.

Too many people take what they are given and jump on new ideas

[EDXR8]

Originally Posted by useless

No probs.it wasnt long ago where 3 inch exhausts were rubbished until Stiddy came along with his standard xr6hp manual and runs flat 15 second 1/4 miles.

So how did that run compare to a run with a mandrel bent 2.5" system?

[DanXR6T]

I really think Useless needs some help in the maths department.

area = 3.14 x (radius^2)

therefore 2.5" = 4.9 square inches
2 x 2.5" = 9.8 square inches

4" exhaust therefore has an area of 12.56"

applying your "4L/5L" theory the optimum area for a 5L is then 15.7 square inches or twin 3.1"

[Stav]

Originally Posted by Axeman82

I really think Useless needs some help in the maths department.

area = 3.14 x (radius^2)

therefore 2.5" = 4.9 square inches
2 x 2.5" = 9.8 square inches

4" exhaust therefore has an area of 12.56"

applying your "4L/5L" theory the optimum area for a 5L is then 15.7 square inches or twin 3.1"

My maths does suck.But I like the sound of a 4inch xorst sounds exotic.

[stiddy]

when i had a 2.5in cat back system my pb was a 15.2 only ran it once, most times it was low 15.3's
with the 3in cat back system ive run a 15.009 and now running consistant 15.0's and 15.1's

im interested to see what a set of headers will do... but i'd say 3in is the MAX for an NA I6......

[stiddy]

oh and i'll add that its a Mandrel bent 3in system with only one small 14in straight through muffler...
its incredibly loud, really not for street.. no need for it, but i love it..
scared people, just ask HLC about our highway incident LOL..
but i think going twin 2.5's on an I6, would just be too much..

[stockstandard]

Are you going to modify the extractors to have a 4" output?

I would be doing a bit more research before you go ahead. When people are jumping on new ideas it is usually for a good reason.

[OED666]

this is interesting... i am researching exhaust size, and thinking of going 3"...

when doing my engine conversion, i noticed the extractors have a 3" flange, then going into a 2.5" system.

my only concern is whether 3" will be too big, as with the cam i have i have already lost a fair bit of bottom end power, but then i just have to launch harder and slip the clutch more...

[stiddy]

go the 3inch..... so what if you have to launch/slip harder at the track :P
i let mine go from 3500 - 4000rpm...

[stockstandard]

Originally Posted by stiddy

go the 3inch..... so what if you have to launch/slip harder at the track :P
i let mine go from 3500 - 4000rpm...

It really depends on how much you lose down low to how much you gain up high. I really struggle believing that a good 2.5" exhaust is any sort of restriction on anything other than a highly modified 6. Im putting out 146kw and making 95mph passes with 2.5" press bent exhaust.

If I was going to be giving up any more of my low end power I would make damm sure I was getting way more back at the top end. That is unless I gave up using my car as a daily driver.

[stiddy]

whats done to your car?

i wouldnt say you loose alot of bottom end power.. but you can notice a gain up the top.. sort of feels like they have moved the power band up 500rpm.. or cut 500rpm power from down low and stuck it uptop..
i certainly dont regret getting a 3in system..

[Falcon Coupe]

Four inch on a 6 cyclinder is not only overkill, it would be horrible noise pollution.
And not all cars gain power with pipes dropped, mine makes 14rwkw more with the first section of exhaust on, but loses 30rwkw with the tailpipes on.

[EvilChief]

i used to run a very interesting setup ...

two sets of extractors (a 3 cyl) ending in a 2" flange, connected to two 2.5" cat, then 2.5" piping for another meter before cellecting into a 3" pipe that connected to the 3" muffler, from there 3" over the diff to tip.

bottom end was house even before i put that exhaust due to the cam i was running. however it allowed to rev quite freely and quickly in the top end. performance gains were only minimal, however did show small increases in the overall power. Overall noice was actually a tad quiter in the bottom end, but definatly quite angry in the top end. Great little experiment ... took two weekends and lots of swearing

BTW best results i gained from no exhaust :p suprise suprise

the new engine will receive a twin 3" exhaust with twin 4" cats ...

[Stav]

Originally Posted by EvilChief

i used to run a very interesting setup ...

two sets of extractors (a 3 cyl) ending in a 2" flange, connected to two 2.5" cat, then 2.5" piping for another meter before cellecting into a 3" pipe that connected to the 3" muffler, from there 3" over the diff to tip.

bottom end was house even before i put that exhaust due to the cam i was running. however it allowed to rev quite freely and quickly in the top end. performance gains were only minimal, however did show small increases in the overall power. Overall noice was actually a tad quiter in the bottom end, but definatly quite angry in the top end. Great little experiment ... took two weekends and lots of swearing

BTW best results i gained from no exhaust :p suprise suprise

the new engine will receive a twin 3" exhaust with twin 4" cats ...

Actually evil chief this is exactly the setup we used in the 2502v xa six but had it dual exhaust and we couldnt reach the radio buttons when accelerating from all the torque.From each 3 cylinders a sepereate exhaust system.I want to do this but want to check insurance.It is either 4 inch or dual exhaust.
How did you run the ego sensor?

[EvilChief]

Originally Posted by useless

Actually evil chief this is exactly the setup we used in the 2502v xa six but had it dual exhaust and we couldnt reach the radio buttons when accelerating from all the torque.From each 3 cylinders a sepereate exhaust system.I want to do this but want to check insurance.It is either 4 inch or dual exhaust.
How did you run the ego sensor?

i assume u r asking about the oxy sensor?
the ox sensor was fitted to the extractors of the rear 3 cylinders. we tested which one (the front 3 or the rear 3) were running leaner and found that the rear ones were just very slightly leaner. That way we could be sure that the leanest reading was used as point of refrence. Engine ended up running just a tad richer, but it was acceptable, as reading were a max of .1 off.

[jonbays]

Originally Posted by EvilChief

i used to run a very interesting setup ...

two sets of extractors (a 3 cyl) ending in a 2" flange, connected to two 2.5" cat, then 2.5" piping for another meter before cellecting into a 3" pipe that connected to the 3" muffler, from there 3" over the diff to tip.

bottom end was house even before i put that exhaust due to the cam i was running. however it allowed to rev quite freely and quickly in the top end. performance gains were only minimal, however did show small increases in the overall power. Overall noice was actually a tad quiter in the bottom end, but definatly quite angry in the top end. Great little experiment ... took two weekends and lots of swearing

BTW best results i gained from no exhaust :p suprise suprise

the new engine will receive a twin 3" exhaust with twin 4" cats ...

This would be worth trying I think too. I had an XT with a 221 and cam with a similar setup with balance pipes between them after the transmision and it sounded beautiful and reved cleanly to 6000rpm. The cam helped here but pre exhaust it wouldn't get past 5500 easily. i used to get asked if it was an 8 all the time

[EFFalcon]

Bwahahahahahhahahah
anything i'd have to say would be too mean.
lol!

[JC]

If you really want the maximum exhaust flow from your engine, you need to match the cross sectional surface area of the exhaust pipes to the surface area of the exhaust manifold (for most people this would be extractors). The best system would actually be one pipe per cylinder, but running 6 cats and 6 hego sensors is a bit expensive, fiddly and you will probably get incorrect readings from that many O2 sensors.

I have PH4499s, so I think they are a 1 5/8" primary. 4480s are 1 1/2" primaries - I'll use them as they are pretty close to each other anyway. So if you want to keep the same surface area as 6 x 1.5, work out the surface area as Pi*R^2 for one, and multiply by 6.

Surface area for 1 primary = 3.14159 * 2.25 = 7.07 square inches.
6 of these is 42.42.
To find the best single pipe, now divide 42.42 by Pi. The answer is 13.5.
Get the square root of 13.5 and that's the ideal diameter for a single system = 3.67" pipe.

For 1 5/8 primary, the answer is 3.98" - close enough to 4" really. So useless may be onto something, if it weren't for things like back pressure, torque etc. In an ideal world, 4" would be ideal

[Walkinshaw]

Originally Posted by JC-XRWgn

If you really want the maximum exhaust flow from your engine, you need to match the cross sectional surface area of the exhaust pipes to the surface area of the exhaust manifold (for most people this would be extractors). The best system would actually be one pipe per cylinder, but running 6 cats and 6 hego sensors is a bit expensive, fiddly and you will probably get incorrect readings from that many O2 sensors.

I have PH4499s, so I think they are a 1 5/8" primary. 4480s are 1 1/2" primaries - I'll use them as they are pretty close to each other anyway. So if you want to keep the same surface area as 6 x 1.5, work out the surface area as Pi*R^2 for one, and multiply by 6.

Surface area for 1 primary = 3.14159 * 2.25 = 7.07 square inches.
6 of these is 42.42.
To find the best single pipe, now divide 42.42 by Pi. The answer is 13.5.
Get the square root of 13.5 and that's the ideal diameter for a single system = 3.67" pipe.

For 1 5/8 primary, the answer is 3.98" - close enough to 4" really. So useless may be onto something, if it weren't for things like back pressure, torque etc. In an ideal world, 4" would be ideal

Do all your cylinders fire at the same time?? I don't think mine do.

[JC]

Originally Posted by Walkinshaw

Do all your cylinders fire at the same time?? I don't think mine do.

True, but at 3600 rpm, that's 60 revs per second, so while the cylinders fire at different times, at those revs it's almost at the same time. Plus there's some overlap - that's how extractors work, the exhaust process from one cycle helps scavenge the exhaust from the next by providing vacuum (also known as back pressure). That's one fo the reasons a 4" system wouldn't work, but that's real world stuff, not theory.

[stockstandard]

Originally Posted by JC-XRWgn

True, but at 3600 rpm, that's 60 revs per second, so while the cylinders fire at different times, at those revs it's almost at the same time.

Yes, but because of the increase in piston speed at higher RPM, the exhaust gas is moving at much higher velocity and therefore clears the extractors/exhaust MUCH faster. Bottom line - 4" exhaust on a na 6 is just crazy overkill from a performance point of view. If you want it for brag factor fine, but not for performance.

[outback_ute]

Originally Posted by JC-XRWgn

That's one fo the reasons a 4" system wouldn't work, but that's real world stuff, not theory.

lol, since when did the real world matter... :

[EFFalcon]

might as well just run straight out of the head then

[JC]

Originally Posted by EFFalcon

might as well just run straight out of the head then

Don't mention that. Someone may try it! LOL.

[EFFalcon]

Originally Posted by JC-XRWgn

Don't mention that. Someone may try it! LOL.

We have tried it... its good

[INJECTED_250]

Originally Posted by EFFalcon

We have tried it... its good

Yup me to.. nice 10 inch long blue flames from the head

[Walkinshaw]

everybody loves a melted radiator headder bottle

[EvilChief]

fact is big diameter exhaust are good for high rpm applications ie racing (drag or strip) where the engine is always within optimum rpm range is never left and where so called back pressure is not that important

smaller diameter exhaust are good for street application, where engine is mainly used in low rpm application to provide increased torque and driveability and of course less noise ...

id like to see a race car with undersized exhaust for the low end torque jk