I really couldn't be bothered with this topic anymore. It has been done to death over the years and it's obvious that no one pays any attention.

I had a 3" exhaust on my DOHC EB ghia. It was a slug dog off the mark and it was tuned. with 4.11s it ran a 2.3 60ft and did a 14.4 although the auto was slipping as well on changes. with a 3" exhaust it didnt start pulling till 3000 with stock cams.

My el is faster off the mark and to 100 and is 2.5" exhaust.

I agree thats for sure.

So, you've obviously tried multiple header designs with varying primaries, merge collectors, std collectors, 6 into 1 headers, different cylinder pairing, various length secondaries, duals, x-pipes, etc?
What was the best? Let me guess. Pacemakers with a 2.5" cat back, or 3" if supercharged/turbo'd.

I really couldn't be bothered with this topic anymore.
More so when people are going to be smart a***.

To be clear - this all kicks off with people asking about / mentioning what they want to do and then the thought police jump in... it's not like anyone ever kicks things off by saying "all you people with smaller than 3 inch are no hope clueless losers cause I'm gonna put twin 4 inch on MY Falcon and show yer what it's all about" - quite the contrary.

Something that also makes me quick to fire up over this particular topic is that I'm old enough to remember when EXACTLY this sort of "debate" was in full flight over fitting anything bigger than 2.5 single on even mildly worked 253 / 302 / 308 V8's...

I've never suggested BTW that smaller is bad either - just that under certain circumstances the negative effect of larger is overstated and the treatment of people wanting to try it is not fair - I'm well aware of BIG power being made by monster V8's in the states running 2 1/4 pipes...

VERY interesting info edfairmont4.0!! Did you change that 3 inch setup to 2.5 and recover the performance? (I mean that as serious interested question - not a smart a*** comment).

- all I can say is that mine certainly isn't a slug off the line - and even before the J3 chip I noticed a slight but definite improvement right off idle as well as thru to top end when I went from std 2 1/4 factory to this 3 inch cat back - which surprised the heck out of me - because except for feeling choked in top end it was already going bloody good compared to the old ED with 2.5 single system.

One smart a*** comment deserves another.
It just seems that the topic of exhaust discussion is locked around the fact that no-one is using anything other than standard, off-the-shelf, retail headers. The biggest mod I've seen to a set of 4480's, used on the street, is to increase the collector to 3" to match up with a 3" cat, then reduce to 2.5" afterwards.
In the saloon car series they add about 300mm to the secondaries of the 4480's and gains are achieved across the board.

On a serious note I actually would like to know your experience with header designs for these engines.
As I mentioned earlier, the multitude of designs that are available for BMW and Chev I6's is staggering, and most of the BMW look like masterpieces.

Do you have any dyno sheets of when you went from 2.5" to 3" or 2 1/4" to 3"? Have you taken your car down the strip?

No - the changes have been obvious enough it hasn't been necessary - and no I don't drag the car. Sorry - need to quit doing things on the move and actually think (for the benefit of anyone who saw my original comment during the short time it was up).

As a matter of interest, because altho the note is fine I am finding this 3 inch system just too loud so I'm about to fit the Berklee 2.5 cat back system - will get to see the direct difference between the 3 inch and 2.5. Obviously I'm not expecting to loose so much in the changeover that it bothers me

Last edited by Greenmachine on Fri Sep 21, 2012 7:42 pm, edited 1 time in total.

I wasn't being a smart a***, Being over it was the truth. I am sick to death of this same debate going on and on and on, when the reality of it is that for the everyday street car it doesn't matter two knobs of goat s**t what you use.

If you want to get into header design for a particular application, no one has even asked the first question yet. How much money do you have? and second, What cam shaft and how is it timed?
then we'll get into expected hp and RPM.
After that the numbers need to go into Pipemax and then I would verify the outcome of that by using Engine Analyzer Pro. (but remember Garbage in garbage out) then I would think about building the header. But in any case no one is ever repaired to pay the price.
When a pair 3 into 1 merge collectors cost more then an off the shelf set of pacemakers it becomes a pointless debate on a forum as focused on price as this one is.


You say 'these engines' as if there is something special or different or magical about them that means they are different to any other engine. It's s**t, an engine is an engine is an engine.
If ou want to 'design' headers, you do it the same as any engine, you work out what lengths and diameters you need to compliment the package and intended purpose, then IF you are so inclined you can take the extra steps like true merge collectors.

If you really want some real discussion, come around for a few JDs and give it the time it needs to be a meaningful conversion instead of being a pointless forum 'debate'.

Acutually no it didnt go down to 2.5 on the BF motor, before that i had a Worked AU motor in the very same car. it had a 2.5" system 4480's with 2.5" collector, I then got the collector changed to 3" along with the rest of the exhaust - It immediately felt CRAP off the mark but maybe revved a little nicer about 5k for the loss in torque It was not worth it. I know the BF motor would have been better with 2.5" system. Lord knows why i kept 3" on the BF motor that had modified 4495s i think... I think I loved the sound sitting on 6000

Yeah I agree totally. It really is a case of chasing the last few percent, but some people are keen to do it. I just hate to see the topic shut down all the time.

I know cost is extremely prohibitive, but if you're handy you could make some stuff yourself, and it shouldn't be a reason to shut down the discussions.

When I say 'these engines' I mean that they are not some super high revving race engine. They are loaded with torque, that's why I said think long primaries would do well to enhance that.
Even in standard form, without going into cam timing, desired operating peak, etc. I reckon you'd see a pretty decent improvement.

If I wasn't at the other end of Australia, I would take you up on that offer.

Oh - bummer!. Well all I can say is that on the face of it I've experienced pretty much the opposite - which might well mean my factory exhaust really was crap - but at that point this car with the cam + std manifold, std cat and std pressed 2 1/4 exhaust was going pretty much as well (except in the top end) as my old ED used to with the same cam + complete 2.5 system and 4499's. To be frank when I first put a 2.5 system on my ED waaaay back when, I never did notice really ANY difference (if anything my feeling was that it maybe lost a tiny bit - that was with std cam) - I never started getting any noticeable improvement with that car until I started messing with the cam - but when I put this 3 inch on the EF I have gotten a noticeable improvement. Being clear here, in all this we're talking a SMALL improvement - small but definitely noticeable - and across the whole range.

I'm not and never have said bigger than 2.5 systems will necessarily be better - tho the overhead for top end is obvious - but simply that I don't believe they're gonna automatically be worse (they evidently CAN BE worse - but I reckon not automatically in all cases) - ie. that there's at least sometimes more than one way to skin the cat...

with my 3" system I gained 10rwkw from 2000rpm all the way up to 5900rpm.
I went through 3 mufflers with my 2.5" system because it was so restricted.
I had to put a 2/14 standard system on for a road worthy and the car wouldn't rev over 3000rpm because of the restriction.

A 3" system is needed if you have are chasing more than the usual 150-160rwkw.

When I really think about it, my interest in larger than single 2.5 really comes from wondering about the difference in real world vs theoretical ideals - ie. sure a theoretical length of 2.0 straight pipe might flow enough for 300kw - but what if with real world things like bends, welds, mufflers it actually takes that same length of 3 inch to be able to really deliver that flow...

GeZza200's experience there really points in that direction too.