I dumped about $4000 into my truck over the time that I have owned it and after going through some head aches I hope the following post will make someones life easier.
After using Doug Thorley headers for about a half a year I would like to post my experience and suggestions on what to do when installing them.
First of all, if you are planning to install these, make sure you are happy with what they do to your power band. The regular exhaust manifolds seemed to have a slightly different power band but not that much different from the headers. So you might want to check with someone that already owns these to see if you want to put these on your truck.
Are your studs broken?
The factory exhaust manifold studs are small, only 8mm in diameter so if you have the original studs, they are prone to cracking or breaking. If your current studs are broken, get the 300ZX turbo studs. They are initially golden in appearance but turn white as they will get hot. If your studs aren't broken, they are probably the revised grade 8 studs, which are probably the same as the 300ZX turbo studs. I suggest using studs over bolts as they are much easier to work with and are designed to work properly with the expansion/contraction rates. Go to your nearest Nissan dealer and buy all the proper fasteners. It will be around $120 but it is worth the expense.
Now what is that all about extending the EGR tube?
There are two different sets of headers made by DT. One is the short tube variant other is the long tube variant. The short tube ones are for the VG30E application and have the EGR port in a different location than the VG30i. The EGR tube on the VG30E engine does not need to be modified, it will fit right in. However, if you want to use the short tube headers on the VG30i engine then you need to extend the EGR tube to match. I don't know where the fitting goes on the long tubes and if its made for VG30E or VG30i.
Long tubes vs Short tubes?
I would say go with short, they are easy to install and have pretty much identical flow of the long tube. I don't think the long tubes are in production anymore.
Nickel Chrome vs Ceramic?
If you don't care about the looks, go with the chrome coated ones. They are cheaper and are just as tough as the counterpart. What will happen is the primary tubes will turn dark blue but because there is a stream of air by the crossover pipe, that part will stay shiny for a while. The ceramic coating will give you a few extra HP and will have a more even color tone over time but for a premium.
What about the Oil Filter Relocation Kit?
You do not need to install it, the oil filter can be changed easily without it. However, if you really do hate the filter in that location then you can get a kit and mount it happily next to the transmission somewhere. WARNING! Any damage to the hose or oil filter will leave you disabled and without an engine.
How do I connect the stupid slip fitting to make it leak proof?
So if you put these things together without anything, you will have an exhaust leak at the slip fitting. What you need to do is put some high temp silicone (preferably copper based) into the slip fitting before you slide the two pieces together. If you haven't done that then you can still wedge the silicone in on around the side. Then you put a 2.00" lap joint band clamp on the slip fitting. If you have a MIG welder and want more stability then put a small tack weld where it will be easy to cut it off in the future should you choose to take the system apart (disconnect the battery if you are welding). You can remove the black sealer from the band clamp as you won't need it. Torque that band clamp to specification which is around 50 to 60 ft lbs. Do not use a regular exhaust clamp as it just will not work (it will break). You can weld the slip joint around but then you will have to take the torsion bars out to remove the crossover pipe.
Why does the DT install manual says to use high temp silicone on the gaskets?
I was told by a professional mechanic (on the job for 15 years) that putting silicone on new gaskets is stupid so you don't have to do it.
You should check the torque on all the nuts after an initial drive because they sometimes tend to back off.
What the hell is that rattle?
When you put your headers together, you may hear a clunking sound from the front end. That is the exhaust pipe hitting the frame, but not where you might think. Yes the headers do run close to the frame but they will not touch. The part that actually does touch is that U-shaped bend thing around the left side of the transmission crossmember. Take a grinder and grind off about a half inch off the crossmember and the rattle will be gone. You should get rid of that rattle because it will trigger your knock sensor.
If you don't hear a knock but your truck still hesitates, it is most likely that the O2 sensor has failed from your modding. Go get a new one ($140 from dealer) and swap it in then reset the ECU by pulling out the negative battery terminal for a few minutes.
What to do next?
The Doug Thorley headers are designed to exit into a 2.5" pipe (or 2.25" for the long tube version). If you want to go smaller then use a reducer.
What to do for Muffler/Cat?
Anything. Make sure your muffler isn't too loud because you will hate the highway drone. The Flowmaster 50 seems to be perfect for me. While you are there, be sure to check on your catalytic converter and make sure it isn't plugged.
When installing the muffler, make sure it fits or make it fit. If you are re-using the old midpipe and notice the crimp that was made from the old exhaust clamp and muffler, don't worry about it, it will work as a flow enhancer. If you are installing a tough metal performance muffler like a Flowmaster, do not use the regular clamp. In fact, when doing a performance exhaust system, forget about regular clamps. You can use a band clamp and silicone combo identical to the header crossover pipe slip fitting or you can weld or both.
If you hear banging by the gas tank, the tailpipe is probably not bent correctly. In my case, my Magnaflow catalytic converter expanded so much that it forced my tailpipe against the gas tank. After bending the tail pipe a little bit, that problem went away.
For a finishing touch:
Install a tip on the exhaust with an outlet diameter between 2.50" and 3.00". Not only does it look nice, it sounds good too. Get a stainless steel one because the chrome ones look like crap after a few weeks. Clamped or welded is up to you but I prefer a clamp one because they are easy to swap and clean.
So there you have it, I hope this helps someone with their sport exhaust system.
FYI:
If you do not wish to install headers, fix your exhaust manifold studs and throw out those heat shields, they are useless. Also, if you don't see any black spots on the engine from the exhaust manifolds, they are fine but dump those heat shields anyway as they might hurt the studs and cause an annoying rattle when they rust. Instructions on how to change studs can be found throughout the forum and here: http://www.nissanpathfinders.net/forum/index.php?showtopic=16196
Another FYI:
If you do mod the exhaust system, it is a good idea to install a transmission cooler as it will keep the transmission happy with your mods. Look elsewhere on this forum for instructions on that.
Any ECU problems?
The ECU does not have to be modified to run the headers however if you have a higher flowing set up, your truck will hesitate for the first few minutes when started from a cold state. This happens due to the timing being retarded for a quicker warm up. If you can hack the ECU, you can set the proper timing during warm up to prevent this from happening. I don't know if the California model ECU units respond in the same way. It is only a small problem and will go away as soon as the O2 sensor kicks in. Any other issues may be either MAF or O2 sensor related.
