JDMCRX, Even thou this is only my second post here, I work at http://www.ssheaders.com/.
One of the headers we make are for the Lexus. Which also has a very restictive and short factory manifold.

If you go to the web-site, click on the link for "Header 101". It should help you with more than just the basics. Also click on the Online Store to see pics of the Lexus Headers.

After looking at pics of the 2.7 V-6 manifolds from the 4 to 6 engine swap thread here.
I will give this 30-second analysis.

#1. Don't bother making a classic 4-into-1 header type header bolted to the Cats.
The primary tubes will be so short that they won't help the engine breathe much untill about 10,000+ RPM!
4-to-1's are great for Racing but not for the Street.

Classic 3-to-1 headers for a V-6 Truck would need to have at least 32" inch long primary tubes to work. 36" would be even better. Too bad that's well past where even the Primary Cat is.
It is much easyer to make and fit a Tri-Y like our Lexus one.


#2 Don't take off the Cats.
Those first two cats are called Pre-Cats. When the engine is cold they electricly heat up and burn off hydrocarbons to help the truck run clean untill the Primary Cat gets hot enough to work.
By-Passing them with that O2 simulater is hit or miss. (mostly miss, now or later)

Better to just keep 'em.


#3  Yes, We are willing make them, IF....
You can scrounge up 25 others (Minimum) who want to do a Group-Buy, get a GV in our shop to work on and find someone to ram-rod the whole Group-Buy and send in the order.
This is how the Lexus headers got started.

Otherwise we will only make them as a one-off custom. ($$$$)
That's what I plan to do with my 4cly Tracker on my own time.

Good luck,
TR

02 simulators? I want in on this. what do they do and where do you get them? sounds like a good idea. would you have to tune your efi to get those to work?

man, I can't wait to see your headers!

http://www.google.com/search?hl=en&q=o2+simulators&btnG=Google+Search

They send fake O2 sensor signals to the ECU so you don't have to have working O2 sensors (ie. removed a catalytic converter).  They are illegal if you have to pass emissions.

screw the hippes and thier crappy emissions tests.. o2 sensor simulators are gnarly it seems. catalytic converters are really restrictive so those are a good idea

O2 sensors are hit and miss.  Some people have good luck with them and others have none.

Remember that they're illegal on a street driven vehicles.  Just like removing your cat if you're caught doing so it's a FEDERAL fine.  A not cheap fine either, a couple grand last I knew (late '90's).  I'm no tree hugger but I figure if the vehicle was designed to work a certain way, especially something like this that has to do with the computer controlled functions of your engine, better safe than sorry.

O2 sensors are hit and miss.  Some people have good luck with them and others have none.

Remember that they're illegal on a street driven vehicles.  Just like removing your cat if you're caught doing so it's a FEDERAL fine.  A not cheap fine either, a couple grand last I knew (late '90's).  I'm no tree hugger but I figure if the vehicle was designed to work a certain way, especially something like this that has to do with the computer controlled functions of your engine, better safe than sorry.

I think here in oregon there aren't very strict emissions laws. but I could be wrong. I agree with better safe than sorry with the o2 sensors because of all that computer stuff, that's the problem with efi.

I think AJMBLAZER is right, it's a federal fine.  So it doesn't matter if you have to pass emissions in your state, it's not street legal.

I know it's a federal fine.  Messing with emissions stuff, which your o2 sensor(s) should be considered part of because of the part they play in tuning your engine for best emissions, is a federal fine if you run across a cop or inspection shop that wants to report that sorta stuff.  Friend of mine got in some trouble back in '96 with his big, lifted Chevy K10 and the cops pulled out the rule book.  They slapped him with a federal fine for about $2500 that set him back A LOT after you include the towing, impound fee, and cost of a new exhaust system.

Keep your engine happy and safe.  Contrary to what some people might think you can in fact make performance and keep your vehicle emissions legal.  Your locality might not test and inspect vehicles but emissions is a FEDERAL law so it applies everywhere.

I know it's a federal fine.  Messing with emissions stuff, which your o2 sensor(s) should be considered part of because of the part they play in tuning your engine for best emissions, is a federal fine if you run across a cop or inspection shop that wants to report that sorta stuff.  Friend of mine got in some trouble back in '96 with his big, lifted Chevy K10 and the cops pulled out the rule book.  They slapped him with a federal fine for about $2500 that set him back A LOT after you include the towing, impound fee, and cost of a new exhaust system.

Keep your engine happy and safe.  Contrary to what some people might think you can in fact make performance and keep your vehicle emissions legal.  Your locality might not test and inspect vehicles but emissions is a FEDERAL law so it applies everywhere.

thanks, I'll keep that in mind and probably not hack catalytic converters off things like my friends do. Really appreciated to know it's sucha big fine... those greedy environmentalists

Car Craft did an artical testing Cats a few years back and found that hi-flow after market cats cost 0(!) HP compared to a straight pipe.
The only gain was in ditching the factory one. About 5 HP on 305ci Camaro IIRC.

so high flow cats dont' bother the power? nice. I am glad my secondary cat is high flow now

Hi-flow cats are fine and if you are concerned, you can always go one size bigger than the pipe you are running.

I've heard that's a good idea with mufflers. I like the idea for cats as well. I am not too worried about it but sounds like a good idea anyway. I am pretty sure the one size up for mufflers is only for open chambre.. doesn't make sense to do that for something like a magnaflow or borla

Car Craft did an artical testing Cats a few years back and found that hi-flow after market cats cost 0(!) HP compared to a straight pipe.
The only gain was in ditching the factory one. About 5 HP on 305ci Camaro IIRC.

Except in Kaliforniastan where you aren't allowed to replace the Cats unless it isn't working right  :'(

Most After market Cats MFGs only make few size biscuqets(sp?) and just change the size of the inlet and outlet pipes welded on.  Most are rated by CI of the engine, as long it can be used for a 200CI+ engine, you can't go wrong.