2bbl EFI Exhaust System for High Elevation Performance

[1975 DSE]

I have owned a 1975 for over 20 years. It started getting tired so I began modifying it. I started with a 2bbl Sniper on a 302 good to 350 hp. I had my engine rebuilt to a 347 and I am putting the Sniper 2300 2bbl back on. I live at 7,000ft, and I drive roads in Colorado up to 13,000ft. I am installing shorty headers and dual exhaust. Here is my question: Will I see any difference with a mandrel bent exhaust over pinched bent exhaust with a 2bbl at higher elevation?

 

[DirtDonk]

I don’t have a definitive answer for you. But I do have my “educated opinion for what it’s worth” answer for you.
Is there a measurable difference between the two? Yes.
Will you feel the difference? No.

The difference is there, but it’s relatively slight for what we’re talking about. Doubtful that there’s much difference below 3000 RPM, but at higher RPMs and full throttle openings, such as on a dynamometer, there’s a difference.
There’s probably a bigger difference between 2 1/4 inch, 2 1/2 inch, and 3 inch tubing.
The 3 inch is too big for what you need in dual form I’d say, but would be just on the high side of acceptable as a single exhaust.
A single 2 1/2 would be my choice, but a dual 2 1/2 wouldn’t be the end of the world, and might show some benefit for the conditions you’re talking about. Higher altitude mainly. Higher performance engine secondarily.
Even a dual 2.25” with the right mufflers is a pretty good breathing set up for a relatively stock engine and two barrel TBI.

Here’s where we get into your specific engine more than anything else. You say you’re building a 347, but what camshaft what other modifications besides the headers? And what sound are you looking for?
And since you’re taking it on long trips, and I assume driving it, are you willing to put up with drone from the exhaust?
I wouldn’t be. I lost that desire 50 years ago!
For a semi-performance 347, I would think dual two and a quarters run through straight through mufflers like Dynomax Ultra Flows, or dual two and a halfs with Dynomax Super Turbos would be a pretty trick set up.

And here’s the crazy part of my dual exhaust recommendation. Run both of them over to the passenger side and out the rear corner at an angle.
There just isn’t enough real estate under Broncos for really clean dual exhaust routing down both sides.
And shorty headers lets you cross the driver side over to the passenger side fairly conveniently. Running the two together can also let you use an X pipe or crossover chamber ahead of the mufflers.
That way you get the benefit of both single and dual exhaust. Fine tuning and scavenging at lower RPM, and overall breathing at higher RPM.

So there is a whole slew of my opinions. Take them for what they’re worth, and put it together with what others say.
If you decide to go discrete duals down both sides, you might benefit from some type of glass pack due to their convenient shape.

Have fun!

 

[Broncobowsher]

It depends on how bad the old exhaust is and how matched the new system will be.
But my general feeling is, no. You can have enough flow without going all out.
Stuff like this you won't know unless you do both and test back to back. And there is almost as much of a chance of it making less power than more.

 

[AzBroncoGuy]

Here's my 2cents: Unless you are going to be running the motor above 4,000 RPM headers are a waste of money. Headers are designed to "draw" the exhaust out when running at higher rpms by creating lower pressures in the collector. The result of headers in most broncos w/302 v8 is more noise and heat and less torque. I ditched my headers, went to dual 2 1/4" exhaust. Better low end performance and now my spark plug wires last more than 1 year.
I used a kit from James Duff. Fit like a glove and very happy with it. Dirt Donk was correct in the tight fit on the left side. It did fit and sounds nice with the Magna Flow mufflers but not so loud to make you crazy on a long drive,

 

[Broncobowsher]

I could theorize a little on the high elevation...
Lower ambient air pressure will help pull exhaust, or at least not impede the flow as much
Lower air density will mean less cylinder filling, thus less exhaust mass going through the exhaust.
So a good flowing exhaust should be less critical at higher altitude than at lower altitude.
Less mass going through the pipes would care less about bends and crimped pipes and have less resistance to getting out.

Not knowing what heads or cam you have, another mystery. Not a huge fan of shorty headers, but they should flow better than stock manifolds. I never had issues with burned plug wires on Bronco headers. Other headers, yes. I'm also the person who took off a good set of headers and put on manifolds, on an engine with a set of trickflow heads. But I also spent hours porting the manifolds and they were K-code manifolds that are known for better flow.

 

[DirtDonk]

Are those K-codes still available through Dorman?

 

[DirtDonk]

Being the fickle decision-maker that I am, I’ve been thinking about trying those. Even though I have three different sets of headers that I could choose from!

 

[73azbronco]

So, with an EB, at any elevation, I think you want torque. meaning you want an exhaust giving you torque. I use WH (maybe BC) coated headers, 2 plus inch pipes to 2 to 1 exhaust..

 

[Broncobowsher]

Are those K-codes still available through Dorman?

I got them from a mustang shop 20 years ago. They worked on my 351 automatic combo. Passenger side was tight to the frame. Might be an issue with mechanical clutch linkage from memory, but memory doesn't remember if that was 351 or 302 deck height.