My 2.5" system is pretty good only 1 muffler and a cat. Although I think in a heavily worked motor there would be some advantage but anything with a stock head would be worthless going bigger. My setup would lose more then it would gain.

You can have a giant pipe with an equally giant blockage/restriction so that it flows equal to a 2.5" pipe, but the internal flow gets weird.
Imagine a sewer pipe bolted up to the cat, with the other end off the pipe with a cap with a hole in it that restricts the flow equal to a 2.5" pipe at whatever length.

There's a lot of things going on in an exhaust system that can affect it's performance. Some more than others, but effects none the less.

For example, as the exhaust gases move along the pipe, they cool and subsequently take up less space. I don't know how much it cools but I'd be guessing a couple of hundred degrees from the cat to the exhaust tip easily.
Going by the gas laws, the exhaust gases are effectively seeing a pipe that is gradually increasing in diameter.
If the gases maintained the same temperature for the full length of the pipe we would see different results. (I'm not saying the results would be good or bad, just that they would be different.)

I guess this is where megaphone collectors have come in to fruition. Over their short length, the gases don't cool, and the megaphone allows the gas to expand gently into the rest of the exhaust system. In this type of arrangement I would expect to see very little change with or without an exhaust system attached.

I wish I had the ability, money, and resources so I could play around with countless combinations to see how the theoretical corresponds to real life. The engineer in me wants to see data, gauges, graphs and all that geeky stuff.
It's one thing to read about it, but it's another to actually do the stuff yourself.

So many thoughts running around in my head.

Last edited by MAD on Tue Jul 09, 2013 12:58 pm, edited 1 time in total.

"I wish I had the ability, money, and resources so I could play around with countless combinations to see how the theoretical corresponds to real life."

Right on! - that's why when someone like TheKrazyD comes along talking about trying something a little different I'm keen to see how it goes - not rain on his parade...

Its a BA twin cammer right? Actually ive seen catastrophic engine failures on those with people mucking about with the exhaust.

There are a few good programs you can buy. (and plenty of free online ones that are s**t)
Pipemax is an awesome bit gear for under $100.
Put the right data in and it will tell you a lot of very useful info.
Pipemax is based on both theoretical and real life results.

Engine Analyzer Pro is also awesome, but you'll pay almost $500 for it.
You can get very close the first time by using the above properly.




IMO the part of the system that he is doing different isn't really much to get excited about.
At that point the only job it has is to get rid of the exhaust gas and deal with noise.

The power making part of the system is in front of the cat/s and the position of the cat on these cars does a good job of ruining it for you. There is nothing worse then a compromised header (every off the shelf header falls under that title) with a cat bolted right at the collector.

If you look at the lengths needed, especially if you are trying to build on low rpm torque, the cat will end up behind the gearbox.
From the cat back your only concern should be, do the cat and muffler flow enough? By the time you get a cat and muffler that flow enough, the size of the pipe will be decided for you and will be plenty big enough.

I know that facts shouldn't be brought into arguements on here anymore, but do people not understand the reasoning behing most Falcon 6's running 2.5" exhausts? Yes, The minimising back pressure is a necessary thing, but going too big ruins the car. You want to achieve the least backressure, whilst maintaining Exhaust Gas Velocity.

Im sure that this is a stupid question. But isnt optimum exhuast 76% of intake on a naturally aspirated engine. Regards Daz.

This only rings true because the header section is compromised in the first place.
The reasoning behind it is package constraints, and cost to the customer.
All the necessary 'gas velocity' is done in the header down to the collector at which point anything after does 3/5 of stuff all, unless grossly undersized. (when designed properly)

All headers are a comprimised design. You have to build a header for one rev range, then make them work over the entire spread of the engine. There is an optimal pipe length and diameter for each individual engine, but you need to maintain exhaust gas velocity right up until the point it exits the tailpipe.

That used to be the rule for intake port flow vs exhaust port flow.
But that is out dated and proved wrong.

Yes there is an absolute optimal, but the difference between them is not as great as you might think, until you get too far out of the optimal region. As I said before if the header is designed properly up to, and including, the collector then the pipe work after it is fairly irrelevant unless undersized.
The compromised designs that we have available are not a compromise by way of a chosen rev range. They are a compromise because of packing and cost constraints and that has made them even worse.
For the everyday punter, I'm sure there is a design that is far far better than what we currently have on offer.

Check out www.burnsstainless.com and read some of there stuff on merge collectors, they've got some great stuff there, and will design a header for you if you supply them with certain engine and cam details.

Something that surprised me was through all the searching I did for BMW I6 header designs I found only one person that hand made their own set with the same primary pairing as the pacemaker 'competition' headers.
I found a few discussions on that type of pairing, but I couldn't find any that are made and sold other than what we have here.
I also found HM and Genie also make a 6>3>1 header but their pairing is 1-2, 3-4, 5-6. Quite different to pacemakers pairing of 1-6, 2-5, 3-4.

Absolute crap.

You need to maintain velocity until the gas exits the correct length collector. The rest of the exhaust system serves only to limit noise and provide a way to route the gas out from under the car.

1-6 2-5 3-4 is the correct way of doing it. Each pair is perfectly separated.
With this pairing each pair has pules----gap----pules----gap----pules----gap----pules----gap----

1-2 3-4 5-6 is the lazy easy to build way of doing it.
1-2 and 5-6 have each pule close together,
like this Pules-gap-pules-----gap-----pules-gap-pules-----gap-----
1-2 and 5-6 end up with pules fighting for area in the secondary, which means you either need a larger secondary or it will choke sooner.

When sorting through the list that is the off the shelf header, any header that pairs up 1-2 3-4 5-6 should be the first one cross off the list.
The for under 150rwkw you really should be using a 1.5inch primary pipe. It will make the power easy and will have a better torque spread. I s**t me that pacemaker no longer do the 1.5inch primary pipe.

As far as the best of a bad bunch go, the old style pacemaker comp with the 1.5 inch primary pipe was it.

I'm sure that explains the reason many people said that Genies had a very distinctive sound.

This is basically what I want to try and fit under the EL, with an X-pipe after the cats, and continued as dual to the rear.
The cat placement will probably be different than this picture, but the same basic dual layout.


I'm thinking dual 2" to try and keep the cat size down, cost down, and for easier pipe routing at the rear, but we'll see when I get the chance to get it on a hoist and play with some bits and placements.

As this is going to primarily be an exercise for sound, I'll probably just cut the collector off a set of pacemaker 4499's and go from there with the dual.

Dual exhausts is something I have wanted to do for a long time on a Ford I6.