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exhaust routing, What's the best exit location?

reamer

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Mar 20, 2008
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Going with a new header-back, 2-into-1 exhaust in 2 weeks, What is the better location for the tailpipe? Out behind the rear right tire, or straight out under the tailgate?
Tired of smelling fumes...
 

okie4570

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Behind passenger tire unless you you're going to be in the rocks, then in front of the passenger tire. Out the back is the worst for fumes.
 

jckkys

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Mar 15, 2012
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5,196
I've seen several including my own that backed into a rock, stump, or curb that bent the pipe against the tire putting a kink in the tube. I went with the '77 tail pipe pattern the exits just in front of the bumper pointed down and out.
 

bwalls

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Aug 1, 2021
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When I bought my bronco my duals were going straight out the back. It was AWFUL. So bad that I had some buyer's remorse. I reran them out the sides behind the rear tires hoping it would help a little. I can barely smell any exhaust now at all. It's a totally different driving experience!
 

Broncobowsher

Total hack
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Jun 4, 2002
Messages
34,835
Passenger side, behind the rear tire, side exit. Just like stock.
Consider adding cats to the exhaust to further clean it up.
Put a wide band O2 in front of the cats and make sure the "tune" is correct as well.

That will get it as clean as it can be.
 

jckkys

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The stock '77 exhaust was nothing like that. The rear stock hanger itself is behind the tire. On '77s the tube extends past the hanger to just in front of the rear bumper where it turns 90 degrees to exit to the side with a tip turned down. Mine is as close as I could get without a bender. When describing something as stock be cognoscente that stock is different for different years. The '77 routing does a better job of keeping exhaust gasses out of the passenger compartment than other yrs. It also gives far better ground clearance.
 

Slowleak

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Sep 12, 2013
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There were two different outlet pipes in ‘77, and maybe in other years. One is the standard and other was the “low noise” option which also had a different clamp.

The ‘77's I have owned had the pipe exiting out the side behind the tire like other years. I assume that was standard. I do not have “low noise” listed on my Marti. Perhaps the low noise pipe turns down at the tip.

595ef36f1124092cab04c37f58c84c88.jpg
 

Digger556

Sr. Member
Joined
Aug 8, 2013
Messages
793
I've used side exit behind the passenger rear tire:
45463333595_f6e3e83ecf_o.jpg



...and corner exit:
50580715446_fcd13e1166_o.jpg



Side exit has some exhaust leakage. Not terrible, but I could smell exhaust on my clothes, similar to when I rode my dual-sport bike. Corner exit has been great. No exhaust smell.
 
OP
OP
reamer

reamer

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Messages
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Passenger side, behind the rear tire, side exit. Just like stock.
Consider adding cats to the exhaust to further clean it up.
Put a wide band O2 in front of the cats and make sure the "tune" is correct as well.

That will get it as clean as it can be.

I have a '73. How would a Cat and O2 work with a 2-v carb?
 

Quick & Dirty

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Feb 15, 2004
Messages
826
I remember seeing a test done many years ago (early 70's), and the 45° corner exit was the best for both noise and fumes. As far back as possible, but out of the slipstream.

As for adding cats, you also need the air pump to make the catalyst work properly.
 

jckkys

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Mar 15, 2012
Messages
5,196
I find it hard to believe any above bought a '77 new, as I did. Another problem is the lack of specifics. Sure they all exit behind the tire, but how far? Most stock EB systems wrap the pipe around the tire and point down. These were vulnerable to bends when backing against an object. The as new '77s had the exit just ahead of the bumper and tucked up to just below the frame. They had far more ground clearance ie about 1 foot.
As to cats, mine only had one on the dr. side upstream from the Y union. It only effected 4 cylinders. The DSO was Detroit. So maybe that was reason for just one. No one at the dealership in Libertyville, IL knew why. The weird little coil spring hanger and heat shields were on mine tho.
 

Slowleak

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The as new '77s had the exit just ahead of the bumper and tucked up to just below the frame. They had far more ground clearance ie about 1 foot.

The ‘77 Ford manual lists two different outlet pipes with different part numbers. What were the differences between the standard outlet, and the low noise outlet?
 

Broncobowsher

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I have a '73. How would a Cat and O2 work with a 2-v carb?

A cat will burn off unburned fuel in the exhaust.
The O2 will tell you if rich or lean and where in the throttle that happens. The cat takes unburned oxygen in the exhaust and the unburned fuel and burns them. You can't just drive around with a rich fuel mix and have the cat clean it up. There needs to be lean enough conditions to get some oxygen into the cat.

Air injection works a couple different ways. One is blowing air directly into the exhaust ports. The exhaust leaving the port is still hot enough to burn if it only had some oxygen. The air injection adds that oxygen. Still used to this day on some cars as it helps clean up the cold start emissions.
The second part is injecting at the cat. This is used for a couple different reasons. Today it is done so the oxygen sensor does not have a polluted (with air added) reading and can tell what is really happening in the combustion chamber. The old school reason was the cat was a 3-way. The first part was to reduce NOx emissions. That needs a slightly rich (oxygen deprived) condition to break down the NOx. After that reaction it was to reduce the HC and CO, those need oxygen to burn and break down.

The typical exhaust smell people complain about is the HC (hydrocarbon) in the exhaust. Burn that off and the smell goes away. A byproduct is the poison CO goes away as well. NOx is typically ignored unless emissions testing says it is a problem. That is high combustion chamber temps where nitrogen is fused with oxygen, typically in a lean combustion on hot running engine.
 

jckkys

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5,196
I wish I knew what a low noise option would have been. My '77 was the last one they had in stock. The '78s were already out and I knew I didn't want a chopped short F150. I had to take what was there. The option list was short and it looked like it was tailored to plowing. I never considered plowing but liked the 4900lb. GVW and heavy duty cooling. An old photo I had showed me just today that the tailpipe was bent in an S behind the rear hanger and the exit was down and straight back. It was hard to recreate a picture in my mind of a tail pipe unseen in 40+yrs. So I got it partially wrong. Luckily I still have the original hangers. A low sound option would have been great as my '77 exhaust was nearly as noisy as my '68 and '73s were. The straight back exit was good and caused little or no fumes. Sitting in traffic with a tail wind did push some in. Now that I have hydraulic lift gate cylinders I'll have to monitor things more. My new CJPony system looks original and is quieter than the OE systems.
 

Slowleak

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I wish I knew what a low noise option would have been. My '77 was the last one they had in stock. The '78s were already out and I knew I didn't want a chopped short F150. I had to take what was there. The option list was short and it looked like it was tailored to plowing. I never considered plowing but liked the 4900lb. GVW and heavy duty cooling. An old photo I had showed me just today that the tailpipe was bent in an S behind the rear hanger and the exit was down and straight back. It was hard to recreate a picture in my mind of a tail pipe unseen in 40+yrs. So I got it partially wrong. Luckily I still have the original hangers. A low sound option would have been great as my '77 exhaust was nearly as noisy as my '68 and '73s were. The straight back exit was good and caused little or no fumes. Sitting in traffic with a tail wind did push some in. Now that I have hydraulic lift gate cylinders I'll have to monitor things more. My new CJPony system looks original and is quieter than the OE systems.


I still have the original hangers and clamps on mine as well. I did some investigating based on part numbers etc. The low noise versions were sold in “reduced noise areas” which I assume were based on local noise regulations. My first ‘77 which I bought in ‘80 had original exhaust and it was as pictured on the special decor below. The ‘75 Ranger below has the “s” shape. Both of these pictures are from the Ford archives. I have never seen an original “s” shaped one but there is no doubt they existed as you confirmed. Do you have a Marti report on yours?

076d3fd63d09a935b617c343aacc98f0.jpg



9f23bed5b362adc4fb42afaa77d61996.jpg


‘77 with the rear exit “s” pipe…
f079a32091c4a9addd9bf55690ba0474.jpg
 
Last edited:
OP
OP
reamer

reamer

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Mar 20, 2008
Messages
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A cat will burn off unburned fuel in the exhaust.
The O2 will tell you if rich or lean and where in the throttle that happens. The cat takes unburned oxygen in the exhaust and the unburned fuel and burns them. You can't just drive around with a rich fuel mix and have the cat clean it up. There needs to be lean enough conditions to get some oxygen into the cat.

Air injection works a couple different ways. One is blowing air directly into the exhaust ports. The exhaust leaving the port is still hot enough to burn if it only had some oxygen. The air injection adds that oxygen. Still used to this day on some cars as it helps clean up the cold start emissions.
The second part is injecting at the cat. This is used for a couple different reasons. Today it is done so the oxygen sensor does not have a polluted (with air added) reading and can tell what is really happening in the combustion chamber. The old school reason was the cat was a 3-way. The first part was to reduce NOx emissions. That needs a slightly rich (oxygen deprived) condition to break down the NOx. After that reaction it was to reduce the HC and CO, those need oxygen to burn and break down.

The typical exhaust smell people complain about is the HC (hydrocarbon) in the exhaust. Burn that off and the smell goes away. A byproduct is the poison CO goes away as well. NOx is typically ignored unless emissions testing says it is a problem. That is high combustion chamber temps where nitrogen is fused with oxygen, typically in a lean combustion on hot running engine.

Still not getting how an O2 sensor will "tell me anything" on a 1973 running points.....
 

jckkys

Bronco Guru
Joined
Mar 15, 2012
Messages
5,196
The O2 sensor and wide band A/F gauge are effective in identifying lean and rich mixtures in either full or part throttle. It tells you when the power valve opens and if the power valve channel restriction needs to be enlarged. Altitude vs. mixture can be quantified and correctly adjusted. So jets and accelerator pump nozzle size can be corrected too. The points vs. electronic ignition is irrelevant in these adjustments. The only exception would be very lean mixtures where the higher voltage of electronic ignition can ignite when points ignition can't.
 
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