Yes. An overly lean engine doesn't give you better gas mileage. You want the biggest exhaust pipe you can afford. You will also gain performance by splitting the exhaust manifold. You can use headers, Dutras or something else. Each of these reduce back pressure and improve gas mileage.

The only time you need "back pressure" is when you have a situation where the carburation/efi system the can't supply the needed fuel to support the extra air flow.

Corrosives in the exhaust may eat the surrounding steel. It needs to turn down.

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'72 Duster 198 stock cam, 3:23's Hookers on jack stands for 8 years in the driveway
'79 Maxivan 360 Offy Qjet Comp RV cam/rusting in the driveway.
93 D350 160HP Cummins Auto Dually Clubcab needs a injector pump
2005 Golden Couch Buick

Very Nice Set up. ! A +++ THE 40 HAS A DRONE AT 2200-2280 MABY 3000. THEN SOUNDS GREAT. putting all the parts together to build the mtr.will stay posted.

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1964 Savoy post 426 Wedge 4 speed
1972 Challenger Rally 340
1973 Challenger Rally 440
AND SOME GM Stuff
DART 170 Manual

Thanks for the compliment!
I'll have to learn more about setting exhaust systems up in the future...I didn't realize that richening the AFR could compensate for lack of back pressure from extra air flow. That's one thing i didn't try before adding back pressure

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1984' Dodge D150 Utiline Bed
Slant 6 225, Holley 1920 Carb
A833 4 Speed O/D Transmission

Does that explain the exhaust restrictor plates they sell for 351C 4V ford motors? You can put a pretty big carb on an intake. I always thought they made the ports on thst motor too big.

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I didn' t buy it, I built it.....

Thank you, thank you, kesteb! I kept running into statements on this forum and others that said back pressure was needed. Then I thought about race cars, which are set up for as close to zero back pressure as possible. Dragster "zoomies" for example.

So, today someone finally explains that low back pressure causes the engine to run lean unless you deliver enough fuel to make it happy! Further, an engine running lean is not economical and too lean can damage the engine.

Back to the dragster "zoomies" - as Carroll Shelby said: (fuel economy?) "we don't give a sh*t."

BC

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'64 Dart GT convertible, 64 Dart 170 2-door post sedan in faded blue and rust (the Az Dart) & a sixty THREE Dart 170 2-door post sedan in faded blue and rust. (future project)
Early Dart Disorder (EDD) is real, and I've got it!

low back pressure means you are getting more of a scavenging from your exhaust stroke and you are filling the cylinder more because there is less "remnants" in the cylinder. So you run lean unless you fatten the carb up to compensate for your 'larger' cylinders now. Now you do want some velocity in your exhaust too to help low RPM scavenging. trick is to balance velocity with flow. BMW runs 2 exhaust paths, the second one opens when RPM;s get over a certain level, the best of both worlds. You can adjust back pressure with a supertrapp muffler with its discs, the more discs the lower the pressure. 1inch mercury backpressure = 1 hp loss in a published article.

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I didn' t buy it, I built it.....

[/quote]
Does that explain the exhaust restrictor plates they sell for 351C 4V ford motors? You can put a pretty big carb on an intake. I always thought they made the ports on thst motor too big.[/quote]

Never heard of such a thing. BTW if you think the 4V ports were big? Ever look at a set of Boss 351 heads?

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'72 Duster 198 stock cam, 3:23's Hookers on jack stands for 8 years in the driveway
'79 Maxivan 360 Offy Qjet Comp RV cam/rusting in the driveway.
93 D350 160HP Cummins Auto Dually Clubcab needs a injector pump
2005 Golden Couch Buick

Does that explain the exhaust restrictor plates they sell for 351C 4V ford motors? You can put a pretty big carb on an intake. I always thought they made the ports on thst motor too big.[/quote]

Never heard of such a thing. BTW if you think the 4V ports were big? Ever look at a set of Boss 351 heads?[/quote]

Yes, same head, 4v was 2 years older. Huge ports that stalled at anything under about 5000 rpm so they sell a plate for street motors. Crazy heads for a production street car.

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I didn' t buy it, I built it.....

Does that explain the exhaust restrictor plates they sell for 351C 4V ford motors? You can put a pretty big carb on an intake. I always thought they made the ports on thst motor too big.[/quote]

Never heard of such a thing. BTW if you think the 4V ports were big? Ever look at a set of Boss 351 heads?[/quote]

Ive seen a set my buddy has in his garage and yea, holy crap those are some healthy ports on those heads. From the good old days when engines were real power houses lol...

A certain level of backpressure is needed, no-where near as much as a stock system causes. Too large of an exhaust will lose the scavenging effect you want from an exhaust (helps pull the burned exhaust out of the engine), thats why its not good to go too big or too short of an exhaust. You'll lose quite a bit of power in the upper rpm range on a street engine, but may pick up a couple ponies down low in the rpm band.

I ran an open exhaust on my old 280z for about a year, off the engine and back to the diff and dumped it there. Was way loud, the drone inside was miserable for a daily driver and wouldnt rev past about 5500 rpm. Same car, same length exhaust, just added a cheap turbo muffler and it made a night/day difference. Drone was cut to less than half, and would easily pull to about 6800rpm with no sweat...

It all depends on car, engine, intake, head(s), cam, etc... You need the all around balance of things to make power and efficiency.