Engine Masters - 3" single vs 2.5" dual

[OutlawDrifter]

Not looking to stir the pot but depending on what you're doing a little back pressure is good.

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Agree to disagree. Back pressure is not good...period. An engine is an air pump. The butt/ear dyno has a tendency to lie. A properly sized exhaust system that is designed to scavenge and keep the exhaust temp up and moving will perform better than one with giant pipes that allows the exhaust to cool and not flow as quickly.

If back pressure was necessary, why do top fueler's run zoomies? Why do NASCARS run headers into boom tubes?

 

[Erik the Awful]

Scavenging is good, but the exhaust temperature is merely a byproduct and not the goal. You ever hear professionals talk about porting heads and recommend that you don't port match your heads and headers? That step up in size from the port to the primary kills your exhaust gas speed, but prevents reversion. Put a good X or H pipe in place and you'll get the draw you need to evacuate the stalled gases.

 

[Erik the Awful]

If you haven't noticed, I like stirring up a bit 'o trouble with exhaust discussion. Engine Masters just put up a new video, if you have Motor Trend access, on mandrel-bent versus crunch bent exhaust. They got 2 hp difference on a 540 hp LS3 with a 2.5" exhaust system. The crunch-bent tubing they had was a worst-case job. There was a whole psi of difference in backpressure, but the power was almost within a margin of error. They also tested straight-through mufflers versus Dynomax Super Turbo mufflers on the exhausts and got nearly no difference, but the Dynomaxes were quieter.

They do have a bit of discussion on backpressure, because below 3500 rpm the full exhaust picked up torque compared to the open headers with extensions, but they state they believe that's a result of the length of the pressure wave and not backpressure. I'd love to see a test on that.

I remember back in the '80s when mandrel-bent exhausts became "the new big thing". People were getting amazing horsepower gains by ditching the factory crunch-bent pipes and putting on big mandrel-bent tubes. We were all sold on mandrel-bent tubing, but the infamous "header bash" episode of Engine Masters made me do some re-thinking, and for a while now I've believed the problem was less the crunch bends, and more the tiny tubing the manufacturers were using in the '80s and '90s. My '99 Suburban had 1 7/8" crunch-bent tubing behind the manifolds. It would have been small on a 305 TBI with a factory cam. Putting any larger tubing in front of it was going to be a positive change.

The one positive thing about mandrel bends is that you can buy pieces ready-to-go fairly cheap. If I'd taken my truck to the exhaust shop and had them bend tubing for me, I would have been out more money than buying pre-bent tubes and welding it myself.

I am thinking about replacing my Cherry Bomb Salutes with some Dynomaxes now.

 

[L31MaxExpress]

If you haven't noticed, I like stirring up a bit 'o trouble with exhaust discussion. Engine Masters just put up a new video, if you have Motor Trend access, on mandrel-bent versus crunch bent exhaust. They got 2 hp difference on a 540 hp LS3 with a 2.5" exhaust system. The crunch-bent tubing they had was a worst-case job. There was a whole psi of difference in backpressure, but the power was almost within a margin of error. They also tested straight-through mufflers versus Dynomax Super Turbo mufflers on the exhausts and got nearly no difference, but the Dynomaxes were quieter.

They do have a bit of discussion on backpressure, because below 3500 rpm the full exhaust picked up torque compared to the open headers with extensions, but they state they believe that's a result of the length of the pressure wave and not backpressure. I'd love to see a test on that.

I remember back in the '80s when mandrel-bent exhausts became "the new big thing". People were getting amazing horsepower gains by ditching the factory crunch-bent pipes and putting on big mandrel-bent tubes. We were all sold on mandrel-bent tubing, but the infamous "header bash" episode of Engine Masters made me do some re-thinking, and for a while now I've believed the problem was less the crunch bends, and more the tiny tubing the manufacturers were using in the '80s and '90s. My '99 Suburban had 1 7/8" crunch-bent tubing behind the manifolds. It would have been small on a 305 TBI with a factory cam. Putting any larger tubing in front of it was going to be a positive change.

The one positive thing about mandrel bends is that you can buy pieces ready-to-go fairly cheap. If I'd taken my truck to the exhaust shop and had them bend tubing for me, I would have been out more money than buying pre-bent tubes and welding it myself.

I am thinking about replacing my Cherry Bomb Salutes with some Dynomaxes now.

I saw power gains from shorty headers and a muffler swap and then even more with tri-ys. I still have the stock 3" dual exhaust tubing to the muffler on my 97 Express van. Was not that puny 1-7/8" pipe my 99 Tahoe had. Backpressure does effect fuel economy at lower rpm. Running a well tuned Q-Jet on my 383 at a steady 2,000 rpm on an engine stand I got an extra 30 seconds of run time on 1 quart of gasoline by adding knock off flowmaster 40s to the header collectors. Cost 1 in/hg vacuum having the mufflers. 24 in/hg without mufflers and 23 in/hg with them.

 

[alpinecrick]

They do have a bit of discussion on backpressure, because below 3500 rpm the full exhaust picked up torque compared to the open headers with extensions, but they state they believe that's a result of the length of the pressure wave and not backpressure. I'd love to see a test on that.

A good friend of mine who is a pretty decent mechanic/hotrodder says backpressure is a bunch of baloney. Instead he makes the argument that headers and even factory exhaust manifolds need the exhaust pipe behind the headers/manifolds to complete the scavenge on anything less than a full house race motor.

My '99 Suburban had 1 7/8" crunch-bent tubing behind the manifolds. It would have been small on a 305 TBI with a factory cam. Putting any larger tubing in front of it was going to be a positive change.

The mechanic I use when I don't have the time or am stumped on some problem worked at a 4wd/hotrod shop in the 90's with a wheel dyno. They dyno'd a number of 5.7 and 7.4 Vortecs back in the day. He says the exhaust system on the Vortec 350's and 454's were plenty for the hp those motors were producing. The biggest choke point was always the factory mufflers. He also recommended modifying the factory muffler for more flow but not more noise by drilling holes in the exhaust tube wall where it passes through the third chamber in the factory style mufflers. That third chamber is a dead end and acts as a expansion chamber normally. I did this to my factory style Walker muffler. Can't really tell an increase in performance via seat-of-the pants dyno. It made the muffler slightly louder though....

 

[Erik the Awful]

A good friend of mine who is a pretty decent mechanic/hotrodder says backpressure is a bunch of baloney. Instead he makes the argument that headers and even factory exhaust manifolds need the exhaust pipe behind the headers/manifolds to complete the scavenge on anything less than a full house race motor.

Yeah, I totally agree with their assertion that having too short of an exhaust hurts scavenging, but I'd like to see them test that versus backpressure. That way when somebody pops off, "you need backrpessure" there's a body of scientific data to help refute it. Of course, we could all be wrong and there could be a kernel of truth there. We need good data.

 

[Schurkey]

An engine "tuned" to compensate for a ****** exhaust system may run worse when the exhaust system is improved.

This does not mean ****** exhaust systems are good for power anywhere in the RPM band. It means that it's possible to screw-up the tune to make up for a crappy exhaust, and when that crappy exhaust is improved, the tune is no longer correct.

Back pressure isn't beneficial...but it can be dealt with to a greater or lesser extent.

 

[PlayingWithTBI]

This does not mean ****** exhaust systems are good for power anywhere in the RPM band. It means that it's possible to screw-up the tune to make up for a crappy exhaust, and when that crappy exhaust is improved, the tune is no longer correct.

Well said! I'm a strong believer that it's hard to emphasize how important a tune is AFTER just about any modification.

 

[Erik the Awful]

An engine "tuned" to compensate for a ****** exhaust system may run worse when the exhaust system is improved.

That's what I like about Engine Masters, they tune the engines to pull the best numbers for that combination. When the engine only made an additional 2 horsepower, that was with Steve Bruhle tuning it.

 

[95 Truck]

PlayingwithTBI, in post #2 you say you had a chambered and then installed a Bolra muffler on your truck, do you have a video or audio off truck running with both on it? like before and after?
Also what high flow cat and what cam?