Replacing the H-Pipe
Paradise Garage

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© 1999 Brian F. Schreurs
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It should come as little suprise that the stock intermediate pipe, or H-pipe, on a SN95 Mustang is wholly inadequate. We say this because everyone is aware of the performance gains to be had with a new set of headers, or a shiny new catback exhaust. Yet not as many people think about the H-pipe, which connects those polished shorty headers to the 2.5" Flowmasters. Perhaps this is because the H-pipe costs as much as the rest of the exhaust combined, thanks to the presence of catalytic converters.

Though they may be expensive to replace, the benefits are worth it. With both ends of the exhaust opened up, the stock H-pipe acts like a straw, restricting your exhaust flow and nullifying most of the gains from the rest of the system. The 1995 Ford Mustang GT, a frequent visitor to Paradise Garage, already has a set of Jet-Hot coated shorty Mac headers and a twin-chamber Flowmaster American Thunder catback (see Installing Cat-Back Exhaust). It was time to replace the straw with a sewer pipe.

The Bassanni X-pipe is a very popular piece, but in a recent comparison test it was outperformed at all RPM by a newcomer, Tri-D Industries. Tri-D has been making catalytic converters for years, but the Mustang is their first application in the aftermarket exhaust business. Impressed with the numbers, we bought their H-pipe (#60181). It doesn't come with instructions or spare parts, but even so we only needed a bit of hose and a couple of clamps from Home Depot Racer's Supply. Other than that it's ready to go.

Lift the front of the car as high as you can safely go. No matter what you do, things are going to be cramped near the back; ideally it'd be nice if you can at least get enough clearance that you can fit under the pipes. Also set jackstands underneath the mufflers to support their weight once the H-pipe is out.

Take a look at the situation. The H-pipe is held in place at the headers, the tailpipes, a bracket in the middle, a hanger between the catalytic converters on the passenger side, and the AIR tube (Air Injection Reaction, in case you were wondering).

This is the oxygen sensor and harness, from the H-pipe to its connector.
Before continuing, unplug the oxygen sensors. If you follow the wiring harness a few inches, you'll find they have a coupler. This saves you from trying to unscrew them from under the car.

Start with the back of the H-pipe. Unbolt the flanges to the catback. The nuts are 9/16" and the studs, when you realize you'll need to grab them, are 1/2". The studs ultimately have to come off so if you can get them now, it'll be easier to remove the H-pipe. If they're particularly stubborn (as a couple of ours were), you'll be able to wiggle the H-pipe out with the studs still attached. They can be removed once you're not flat on your back.

Once the bolts are removed from the back, proceed to the headers. The nuts holding the headers on are 11/16". You'll probably need a 6" extension for your ratchet, unless you're built like Stretcho from the Fantastic Four. If one of the studs decides to come out instead of the nut, that's okay. There's no harm in it.

The old and new H-pipes compared.
Next, undo the hanger on the passenger-side pipe, connected with a 5/16" bolt. Then the bracket on the crossbrace. The nuts on this bracket are 1/2". Finally, the AIR tube has a clamp held together by a 10mm bolt. Yep -- metric.

You'd think the H-pipe would just drop on you at this point but it won't. It's going to take some serious wiggling to get it away from the four flanges, and once those are clear, there's still the little matter of the AIR tube. Twist it up and down while pushing rearward. Eventually it will pop loose, and yes, whack you on the noggin if you're not careful.

If you thought the replacement H-pipe was heavy, wait'll you take the stock one in the knee...

Using your good leg, kick the bugger out from under the car. Drag it next to the replacement and marvel at the differences in design. The replacement has only two catalytic converters instead of four and the bends are a little smoother. On the other hand, Tri-D is a little sloppy with the welder (but who cares, no one will see it under there, so long as it works). And the crossbrace is designed differently. Fortunately, the bracket under the car will accommodate the change.

Oh yeah... and the AIR tube... that's not an optical illusion, it really is totally, utterly, and irreparably wrong. We don't know what is going on at Tri-D but this tube isn't even close. It's not even close to not being even close. About the only thing right about it is that it faces toward the front of the car. Normally, with a manufacturing defect this glaringly bad, we'd send the part back and make them send us another. Alas, the visiting Mustang is a daily driver and the show must go on.

The outside diameter of the AIR tube is 3/4". Go to an auto parts store and find some high- temperature hose with an inside diameter of 3/4". If that can't be found, Home Depot Racer's Supply has some hose that will do the trick; just probably not as well. Don't forget two hose clamps.

Here's one way to remove the oxygen sensor plugs.
We'll get back to the AIR tube in a minute. For now, find the oxygen sensor bungs on the Tri-D pipe. They are plugged, but the plug simply screws out. Ahhhh... but what size? You'll need a 5/16" hex key. Oh, don't have one that big? You can also use a Torx T-50 screwdriver. Ohhhhh... don't have one of those either? Okay, you can also use a Craftsman 5/32 punch: put the butt end of the punch into the keyway and turn it with an adjustable wrench.

No punch either? Well then we can't help you; you're screwed.

The oxygen sensors themselves go on and off with a 7/8" wrench. In addition to swapping the sensors over, you'll also need the studs from the catback-side of the H-pipe. PB Blaster is your friend.

Now drag that Tri-D H-pipe under the car for an alignment check. What you're looking for is to determine how much of the stock AIR tube needs to be cut. You'll want to minimize the gap between the tube on the car and the tube on the pipe, while still leaving enough wiggle room to get the new piece of hose attached. We wound up with about a 1" gap. Get a good idea of where you're going to cut, and cut it.

Time to lift the pipe into position. The hardest part will be getting the crossmember bracket attached. It helps to have a friend under there as well -- in fact it may be required. Whilst you attempt to align the center bracket, the front end will be getting stuck on the engine, transmission, and headers; the rear end will be snagging the catback. What fun.

With some cutting and splicing, we finally got the two ends of the AIR tube to connect. Hopefully Tri-D will fix this little detail.
But it does eventually line up. Be particularly mindful of the flange on the driver-side header, as it can slip and get pinned below the tranny bellhousing. Then you'll have to pull the H-pipe and start over.

With the bracket bolted down, start tightening the four flanges. Do the rears first as they're not too hard. Then move to the front ones which are no fun at all. Be patient and tighten the flanges evenly. Tighten one side too far too fast, and the other side won't line up right.

While you're there, reconnect the oxygen sensors. Then move back to the AIR tube. The car-side of the tube is pretty floppy so it shouldn't be too hard to jam the hose on and line it up with the pipe side. Don't forget to slide the clamps onto the hose before connecting it to the pipe.

And that's it! Get out from under your car, get it back onto terra firma, get cleaned up, and get ready for a test-drive!

So what can you expect from this? Well, there should be a horsepower gain, especially if you've already replaced the rest of the exhaust like we have. But this is best proven at the track and on a dyno, not by idle seat-of-the-pants speculation or "horsepower calculators." We'll give you hard, real numbers just as soon as we have some.

As for sound, though, we can report on that. The tone of the Flowmasters has not been lost, but rather emphasised. The exhaust is now deeper, more menacing. And slightly louder. Instead of sounding like it has a good catback, the Mustang sounds like it's supposed to be growling. A subtle distinction, which you'll understand when you hear it for yourself.

The fit and finish of the Tri-D piece was nice with the exception of the AIR tube fiasco. Would we recommend this pipe? Yes... with the caveat that you'd better be ready to cut your car for the AIR tube. Hopefully they'll discover their mistake and start shipping pipes with the correct tube fitted. We'll be more forgiving of the AIR tube incident if the dyno numbers look nice.

Update: You want hard numbers? How about two-tenths on a dragstrip. Check out our day at the MIR Test-n-Tune.