87 300D Turbo Exhaust Mods

[retmil46]

I've been contemplating some exhaust mods to my 87 300D - basically just to open it up for better flow and possibly better fuel economy. In the past, I've heard that when new with the original trap oxidzer, with the trap ox hollowed out and the shell reinstalled, these cars could see in the high 30's on the highway.

When I bought mine over 2 years ago, the trap ox recall had already been done and it has a conventional cat mid way under the vehicle.

From searching thru the "Exhaust" related threads here and on other forums, it seems the conventional wisdom is, at a minimum, to remove/hollow out the cat and remove the resonator.

The stock muffler is still a question mark with me. If I understand some posts correctly, the particular muffler on the '87 is already a straight-thru affair to begin with - one gent mentioning if you cut off the inlet and outlet pipes, you could look straight thru it. If that's indeed the case, I see no need to replace it - still in excellent shape with no rust. Can anyone confirm/deny this?

Stock 2 1/4" exhaust piping is also in excellent shape - so I don't plan on replacing it for doing a budget mod - unless someone has data that says going to a larger diameter along with removing cat/resonator has some benefits.

2 years ago I had the stock POS Chrysler 4 banger gasser muffler removed from the Jeep CRD diesel and replaced with an Aeroturbine straight-thru muffler - made a big difference in that vehicle. At the time, the gent doing the work asked me if I wanted the cat cut out as well - he stated that NC didn't require a cat on a diesel. Being that it was a brand new '05, still financed and under warranty (which did cover the exhaust back to and including the cat), I passed on that option. But he was apparently right - first time I took that '05 Jeep diesel to get it inspected, as soon as they found out it was a diesel all they did was a basic safety inspection - NC gave an automatic pass to all diesels, no emissions inspection or hookup to computer.

If they don't care about an '05 OBDII diesel, that old '87 probably won't even get a second glance sans cat.

Any ideas and opinions are welcome.

[sixto]

An 87 300D will not return high 30s mpg under any rational circumstance.

I wouldn't just hollow out the cat because you might introduce turbulence that will impede rather than improve exhaust flow. Leave it as is or replace it with plain pipe.

As an experiment, cut the downpipe before the cat, get some earplugs and burn a couple of tanks to see if mpg improves. If so, attach a cherry bomb with side exit (no tight bends) and enjoy the better mpg. Unless you build a 3" exhaust yourself, the payback period might be years.

IMHO, and I have no experience to corroborate this, this car will cruise at high speed with a full load without blowing out the exhaust system. That tells me it has sufficient capacity to cruise at 65 mph with one person aboard. If you're out for maximum mpg, use no more than a third of the pedal travel and keep it to 65 mph. The 2-1/4" exhaust system will be more than you need.

You were misinformed. NC requires a cat on a Diesel; they just don't check that you keep it in place

Sixto
87 300D

[babymog]

If you're looking for more top-end power, your case with exhaust flow enhancement might have some merit. However, using around 25hp to cruise on the highway, I don't think that the flow is significantly restricted.

Further, unless you're finding something amazing aerodynamically, you're not going to go from 30mpg to 38 for example, you might be able to get a couple of MPG out of some basic engine and airflow mods.

The '87 300D from most reports here can get low-mid 30s driven gently at 60mph with good fuel an lubes on properly inflated high-quality tires. With cheap tires, cheap oil, and winter fuel and 75mph, you'll be lucky to break 30 on the highway.

Be sure your belly panels are in place, your tires properly inflated, your wheels aligned, and your foot light, it'll save a lot of tubing and noise.

[C Sean Watts]

Originally Posted by retmil46

I've been contemplating some exhaust mods to my 87 300D - basically just to open it up for better flow and possibly better fuel economy. In the past, I've heard that when new with the original trap oxidzer, with the trap ox hollowed out and the shell reinstalled, these cars could see in the high 30's on the highway.

When I bought mine over 2 years ago, the trap ox recall had already been done and it has a conventional cat mid way under the vehicle.

From searching thru the "Exhaust" related threads here and on other forums, it seems the conventional wisdom is, at a minimum, to remove/hollow out the cat and remove the resonator.

The stock muffler is still a question mark with me. If I understand some posts correctly, the particular muffler on the '87 is already a straight-thru affair to begin with - one gent mentioning if you cut off the inlet and outlet pipes, you could look straight thru it. If that's indeed the case, I see no need to replace it - still in excellent shape with no rust. Can anyone confirm/deny this?

Stock 2 1/4" exhaust piping is also in excellent shape - so I don't plan on replacing it for doing a budget mod - unless someone has data that says going to a larger diameter along with removing cat/resonator has some benefits.

2 years ago I had the stock POS Chrysler 4 banger gasser muffler removed from the Jeep CRD diesel and replaced with an Aeroturbine straight-thru muffler - made a big difference in that vehicle. At the time, the gent doing the work asked me if I wanted the cat cut out as well - he stated that NC didn't require a cat on a diesel. Being that it was a brand new '05, still financed and under warranty (which did cover the exhaust back to and including the cat), I passed on that option. But he was apparently right - first time I took that '05 Jeep diesel to get it inspected, as soon as they found out it was a diesel all they did was a basic safety inspection - NC gave an automatic pass to all diesels, no emissions inspection or hookup to computer.

If they don't care about an '05 OBDII diesel, that old '87 probably won't even get a second glance sans cat.

Any ideas and opinions are welcome.

NC inspections are "safety only" for diesels. BUT...all vehicles "must have emission control equipment in place and not be defeated, bypassed or tampered." Diesels do NOT get an emission test regardless of OBD category. The shell (as least) of your trap ox and/or cat must be there, the rest is up to you . *HINT* they never say what should be inside it. I cut mine open like a turtle shell - for test purposes only - and cleared out a LOT of sooty obstruction in one piece, leaving a smooth, clean inside, and welded it back, to great improvement. Pictures to come soon.

[Hit Man X]

Just hollow the cat out and be done with it