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Shooting for 400HP, Need Bigger Exhaust?


Elite Explorer
December 6, 2007
Reaction score
City, State
Montrose, CO
Year, Model & Trim Level
2000 X-Edge
My antics! Jeez, I think I have been chasing power that the current exhaust set-up won't support? Duh:banghead: My butt dyno was not reading 400HP!

A recent post made me look at my exhaust again, rethink it, research it as I had not considered how much power the exhaust would take. In building this, I was doing the best with what was stock in getting it done as it's my daily. Calculating it, I think my exhaust is good to about 350 horses??? I want 400 and have the boost, but am not getting 400. I have detuned to 350 horse from 9.1lbs to 5.7lbs of boost till I get more exhaust work done.

What I am running: TMH's into the stock downs with all 4 cats. I cut-off the neck-downed pipe after the second cats and put a high flow chamber muffler on (two 2 1/2" in and 3" out) with 3" tail pipe.

I am going to start searching, might modify something larger for an F150 or something. It will likely take some custom work. I would keep the TMH's. I believe they are good to 400-425HP? I am thinking most of the restriction is in the stock downpipes and cats? I think 2 1/2" downpipes and two high flow cats would be large enough? Then comes the single 3" which is a little small, but I would try it first before I went to a bigger muffler and tailpipe. I think I can give a little power on the top end, with a 3" muffler and pipe restriction for those brief moments at high RPM. However, a mandrel bent piece would be nice.

The downpipes and cats should make for a big improvement and I think I will try that first. No hurry, spring weather is much nicer to work in at the earliest.

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I wish you success at hitting your power goals, that's my #1 intent.

Exhaust is a massive subject. There are many concepts, many ideas, and opinions, plus countless multiples of those in numbers of examples. There are way too many incorrect assumptions, most in those examples, and they spread and multiply throughout the world, people's opinions, and they end up in many stories, reports, and tests by magazines etc.

I have read tons, like you and most avid car enthusiasts. But it takes a lot of thought, patience, and rethinking concepts, to discern the real critical facts.

As short as i can state it, I have failed many times I know; The headers are critical, they are about 90% or more of the power potential of any combination. In race cars, they have zero exhaust, headers only, and no cross over. That is simply due to rpm power band, they never run low enough rpm to need any lower level torque, which would come with a proper cross over. People miss the purpose of a cross over, it's completely lost on 99.99% of everyone. The only correct place to put a cross over for exhaust pulses to improve flow at the collector, is at the tested for location(WOT run with fresh paint on the collectors(hottest burned off spot is where the collector should go)). The problem there is that the location is always right next to the transmission, not behind it. No one can place a cross over there, it needs to go as smoothly directly to the other side, which is not possible on anything except a race car(and they don't need it).

Thus all cross overs are placed too far back to gain anything from exhaust pulses pulling from the collectors, the merge point is the critical area of the subject. Without that close proximity, the cross over is simply an additional volume for the flow to spread into. It's a reduction of restriction, that's all, which is a tiny increase of airflow, which is power. So the point is the cross over is very minor, real testing always shows less than 5hp in normal combinations(not 750hp engines etc).

The main point I try to make is that the effect of everything beyond that one spot in the collectors where a cross over could help, does not help with exhaust pulses pulling upon the collector merge point. It's all simply a restriction to air flow, some varying amount of restriction. All of the pipe sizes, bends, lengths, cats, mufflers, resonators and tail pipes, they just reduce air flow(and sound). That is all that they do, they do not produce power, they lower it or reduce it in some amount.

The maximum power is made with zero exhaust, headers only and collectors cut to the ideal length, which is full on racing at the track. So anything else a person does will result in less power than that header only condition. If you can wisely see that, then working on the exhaust is easier, you stop stressing over where to locate cross overs and mufflers etc, and think just about restrictions.

The stock cat pipes are restrictive for sure, four OEM cats are terrible compared to a pair of high flow cats. The sharp 2.25" bends and pipes are minor issues for mild power, and a bigger deal when you get to and above 350-400hp. Two mufflers flow twice as much as one, same for tail pipes. The stock headers are horrendous, most people know that. But the 1.5" primaries of the TM headers are much better, yet small for a 400hp engine. You can only do so much with the Explorer chassis, the space. The headers are the worst issue, and the TM versions help of course. The cat pipes are low flowing as expected. The rear single exhaust is fine for a stock V6, but too small for any V8. The old 80's Mustang 302's had two mufflers and tail pipes, and those were lesser 302's than these mild GT40 302's.

So I'd call 2.5" cat pipes a nice upgrade, and the mufflers ought to be at least as large as the flow of two 2.25" mufflers and tail pipes. Best of luck,


Ole Red is just constipated from the headers back for 400HP.

That's why I think I will back off the 400HP mark. I hope to give her the benefit of custom 2 1/2" downpipes with high flow cats though.

I am trying to get an estimate of what it would handle, sure would be nice to get some real world testing and tuning based on how much boost I throw at her.

You are still at a mild boost level right, 14.7psi boost is a full extra displacement of air. So for a vague guess at the airflow pushed by that boost, multiply the NA power by the boost/14.7 figure. That also has the issue of net power versus gross, do you use 210hp(advertised) or something like 275hp-300hp(I've read that as the gross engine dyno figure for a stock Cobra 302)?

The guesses get harder, so many factors and even experts have a tough time predicting things like that. I'd say the power might be 1.6 times the original power, in net hp terms, so possibly 325-350rwhp. I'm being conservative, who knows what the IAT's do as they go up. It may make 375rwhp, but the exhaust should be a factor at those levels. Bigger cat pipes will help absolutely, and remember that it cools as it gets farther back, so the tail pipes don't necessarily need to be the same size as the pipes forward of them.

I'm planning to build from the rear to the front, welding is not a skill I have done much of. So making things easier first is better to begin with. I'll stick with 2.25" tail pipes, and change to 2.5" pipes in front of the mufflers. Working around the rear diff, and front diff, will be harder pipe work than the center section.


Supporting close to 400 hp now

X pipe added after 1st photo was taken


Don I am speaking to power at the flywheel.

I figure 210 stock which this build is, but for headers/some exhaust +25HP, electric Fans 5HP and boost (6lbs to 9lbs 90 -150HP)


Looks like the stock down pipes and cats? If so, I think you are about at the max power for the exhaust. Do you have a dyno of the current set-up with the larger blower?

I am thinking the exhaust does not need to be as large with a mild cam and not revving it much past 5,000rpm. I found a calculation which lowers the exhaust size if the peak torque curve is lower in the rpm range like with a stock or near stock cam. I get an idea that my current set-up is actually good for around 370HP. At that power level, it is hard to spend a bunch more on exhaust to get to 400HP and at 400HP the Torque starts creeping close to the tranny max rating of 500ftlbs.................................


Here is the Dyno sheet for my AWD after the Xpipe, exhaust and cats magic but before the blower rebuild.

Will have another Dyno sheet for after the rebuild and going back to the 2.86 pulley

Great curves! Or somewhat lack thereof as the torque comes on stronger and earlier with boost. Nice!

Additional efficiency in the blower, a bit more boost, and it has got to be close to 400!

Yup, thanks for the posts.

I see high flow cats and downpipes in my more immediate future. We got emissions testing so I need to see if I can remove cats. If not they might be able to be modified. I also think more of the heat will flow downstream and perhaps help with some engine compartment heat. Four cats have to be blocking heat in as well.

If that's not enough, a crossover where the second cats are now, if emissions possible, and 2 1/2" duals.

I think I will get enough power with better downpipes/cats and could pull back my horsepower goal a bit. I can rationalize by thinking 15-20 horses are not that noticeable.

I'll keep this thread up with what I find regarding emissions, tubing and cats.......

Emissions testing is a big deal, some states require OEM parts or part numbers, while others just look for the existence of emissions related parts. The OEM knows now that a second set of cats are not needed to meet emissions, so they are not on anything past that late 90's era. They finally did away with air pumps and injection into the cats, but some states like CA kept requiring them indefinitely. Some people learn faster than others.

Be sure to maintain all four O2 sensors, they are all needed for emissions.

In Colorado Emissions testing areas: "Please be advised that "gray market" vehicles and customer-built vehicles must either have all required components or present paperwork from a state-operated Emissions Technical Center documenting an exemption from this requirement."

You might first try to find a shop that has successfully built cars in CA that did pass testing. They might have a better path to getting that exemption, before you talk with them.

I called to see if I can get an exemption for two instead of four cats. They will not allow it and told me that Colorado has gone to California CARB requirements this year. Any parts added must be made for the vehicle. I am done posting here for what I am going to do or not do because of this Eco Nazi crap! May get to be time to leave CO if it keeps going downhill......................

Great curves!
I called to see if I can get an exemption for two instead of four cats. They will not allow it and told me that Colorado has gone to California CARB requirements this year. Any parts added must be made for the vehicle. I am done posting here for what I am going to do or not do because of this Eco Nazi crap! May get to be time to leave CO if it keeps going downhill......................
sniffer wise itll pass in CA (or so i hear one v8 i saw had hollowed rear cats) so i assume it will pass?

Honestly you could probably get to 400 with a cam swap and current exhaust with slight modifications

When I lived in Colorado I removed all the secondary cats
The referee allowed this for Colorado emissions but labeled me as a "gross polluter" for Federal Emissions??.
I did not really care since Colorado registration is all I was after. I know they have been getting more strict and adopting CA emissions laws.
The referee allowed me to do at least 3 v8 conversions this way, all primary cats in place all factory emissions components, only thing not on the rigs were the secondary cats

Anything after the downstream 02 sensor should be fair game IMO

The v8 stock cats trap alot of heat that is for sure

The 07 I am working on is the first exhaust for a v8 I have built with no cats and straight 2.25" tubes all the way back to the small mufflers. I put an X pipe right after the transmission, It sounds pretty nasty, IMO like a proper 5.0

Honestly you could probably get to 400 with a cam swap and current exhaust with slight modifications
I've heard this a few times. My shop has been recommending me to change my cam as one of the next steps in my build. I guess I need to start educating myself on the option.

Emission here suck and Colorado seems to be going to the ways of Commiefornia. We see a move in our future, at least out of the metro area and emissions testing but for other reason as well. Crime is up, they let dangerous drug addicted mental health people camp on the street here, and they want to build and are building section 8 housing into the suburbs. We worked all our life to get this house and away from scum.

I AM YELLING!!!! I do not get requiring CARB original OEM style parts when I could put on 2 hi flow, high efficiency cats and run clean. Emissions matter to me and that is what they test. If emissions parts are there and she is running clean they shouldn't care beyond that!

I could cam-up and get 400 but would loose the low end torque I like. It is fast off-the-line for a heavy brick. Not interested in camming-up and getting a high stall torque converter to get it moving. Then, for what it is, it would still have a constipated exhaust. The exhaust is not built for 400HP but I am going to get a bit more out of it. The 2 1/2" downs with larger cats will wait till we move.

I am sure a custom cam for supercharging would help, so would larger heads and I could keep most of the torque, but I would loose mileage. I actually get Explorer mileage if I keep it out of boost.

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I lost a little low end when I cammed my ranger. But the 3k+ power was well worth it. Plus it ran to 5600 which put me right back in the power band after shifting. I still had wheel spin issues off the line with 35s though.