Questions regarding Doug Thorley headers?
 

Currently have a CEL for low catalyst efficiency. I figured I'd go ahead and replace my cracked manifolds while I'm doing exhaust work, so I started looking back into a pair of Doug Thorley headers, which are something I've had my eye on for a long time.

I want to keep catalytic converters on both sides -- I really really don't want my truck to smell. I'm also a little unfamiliar with the exhaust system layout for my model 4Runner and can't find a helpful diagram. Will the long tube headers bolt straight to the cats (assuming I get direct-fit cats from Magnaflow or somewhere else), then the cats to the y-pipe? If not, how can I fit 2 cats into my exhaust system? Should I go for the short headers and weld universal cats to the headers?

I suppose I can run an O2 Simulator if I need to, but I won't run without any cats.

I love my mother earth very much. But your plan sounds redundant. Headers and cats?? It's not worth even putting on headers if you're running the cats. To answer your question the cats on mine were welded. If you want the same configuration weld flanges to the headers and get the cats with flanges. Simply bolt together. Your truck won't smell bad without cats after a proper air fuel ratio tune which you will have to get done afterwards. You won't need a dummy downstream sensor if you're replacing the cats correctly. My cats just started going bad. I have downstream dummy's for now so my truck runs properly. I'm going to straight pipe the cat sections get a ton of precious metal cash and use it on a free air fuel tune. Maybe have extra free $ to throw at some headers. After tuned right your talking substantial horsepower and torque gains. Probably better gas mileage. For free. I'm more worried about our planet being destroyed for profit than a catalytic converter.

I don't agree with your analysis on headers with cats = no performance gains.
Also, I don't think you can eliminate cats with headers and meet the necessary emissions regulations, smell or not, if I'm wrong on this then OEM's could save some $$ and more thoroughly tune engines and meet emissions.

Didn't say there wouldn't be a performance gain. For the price of headers the gain wouldn't be worth the investment. Why increase exhaust flow with multiple chokers down the line. The smell comes from your air fuel ratio being completely screwed up due to improper o2 readings. No I won't pass emissions and I don't have to in my area. The only reason someone would even look for them would be because they're thieves. Gasoline should be illegal too if no cats is. I burn more gas in my mower, weedeater, chainsaws, and welder. Where are those cats. Just information. I can't wait to do mine!

Since no one answered your question...

Right now there is no way to get stainless headers with cats built in. The long tube headers are for getting rid of the cats. In order to keep the cats, you have to get shorty headers, and then weld cats to them. What makes this particularly egregious is you end up having to put the shorty header in, mount it, tack weld the cat in place, then remove it, and finish welding the cat in.

This is what has kept me from doing headers :(

I wonder why they would want to delete the cats 🤭. Weld a flange to your headers that matches the flange on your new chokers. Bolt together the same way Toyota does on other areas of the exhaust. Now you can do headers.

Some 'meh' info here, OP.

Here's what I've done on my truck and I have no codes whatsoever and I didn't have to install anything additional to keep the codes from showing up.

DT Long Tubes, DT Y-pipe, and GSport Gesi Cats in the y-pipe.

I replaced the resonators in the DT y-pipe with some high-end GSport cats, links in my signature. All this required was an O2 bung plug, and a pair of new O2 bungs to weld into the y-pipe downstream of the cats.

These cats are extremely good and I have no rotten smell issues in my current setup.

The only downfall to moving the cats to the y-pipe is it may take a few extra minutes for them to get up to operating temp compared to if they were in the OEM location.

Here's a pic showing what this looks like on the passenger side. https://i.imgur.com/x54vA7t.png

Take the OEM manifolds off the truck, build a jig out of some 2x4's, remove OEM manifold from jig, put shorty's into the jig, fab cats to fit the jig... Profit... Several folks here have done it this way with great success.

Sorry for the meh info. Your setup actually looks like exactly what I was looking for. How does it sound with the magnaflow catback? I'm currently running a magnaflow catback also; I don't like obnoxious but I do like aggressive.

I'm sure I'd be rolling in free horsepower and torque gains but I can't afford to run without cats. My 4Runner is my primary vehicle and I need it to be able to pass emissions tests. I'd like to run headers because my manifolds are cracked to high hell and are annoying me so they need to be replaced, and DT headers are a better option for replacement than stock manifolds which will crack again.

Sorry, I didn't mean 'meh' from you... 'Meh' info in some of the replies from others.

I initially went to Shorties from OEM (long story, but it was due to fabrication issues that I got rid of the shorties and went to LTs), and noticed it was actually quieter than the OEM manifolds.

But, I've also got the Vibrant Ultra Quiet resonator to help with the drone.

Here's a quick cold start video from right after I got it all back together: 09 4Runner V8 Long Tubes + Magnaflow Catback.

It's a tad louder/more droney in the cabin than before due to deleting the resonators in the DT y-pipe, but it's tolerable. The worst part is the drone. Nice low rumble at idle, mellow tone while driving around normally, but really opens up when you get on it.

Slip em a gram of rhodium ✌️ id be interested in your results after the a/f tune!

Now this is an interesting idea. I could see that working well. I wonder why DT doesn't sell that as an option on their shortys.

I totally get why people delete the cats. I a) don't want to pollute more than it already does and b) live in a city where I have emissions checks. Honestly, there is a pretty decent chance even a shorty wouldn't pass our emissions check depending on where you go. Its obviously not "OEM or replacement."

Not sure. They could charge an extra stack for such a simple option. There's always a way.

That forest looks so fun! Pick up trash if you see any. That's how I contribute to the problem. That will have a much greater affect on nature than a straight pipe!