Back Pressure Myth?

Alright, guys.

I've been hearing this ever since I bridged the gap between true DIY and the tribal knowledge of the world by hopping on my first automotive forum. Why is it that every time exhaust gets brought up, someone mentions this inexplicable need for "back pressure?" I never understood it. It goes against everything anyone would think about performance. Today, I was validated.

I just watched Engine Masters run a comparison test between 2.5" and 3" exhaust. Max HP on the 2.5" was 601, and the max torque was 554. The 3" put out 614 HP and 569 torque. The 3" was shockingly just below the numbers for open headers.

Office chair experts will rest on their laurels, citing all kinds of things like physics laws, thermodynamics, and years of experience, but I just saw the numbers and that's what matters in the end. It's going to take some real mental gymnastics for me to believe an engine needs back pressure. Go home, internet. You're drunk.

 

I'd say it will depend on the engine, but you'll just call me drunk. So wtf ever.

 

Haha. In jest. I can't imagine a fundamental difference between how the motors work to make this true.

 

If that 3 inch was a 5 inch then what would happen? Power loss!

 

N/A. Open headers (no pipes) performed only slightly better than the 3", but better still.

 

But even header provide back pressure. I knew a guy that ran an open block, don't ask me why but he was an idiot with a fohawk but said he noticed a power difference

 

Any engine will run at peak wide open, no back pressure. However, over the long duration you will suffer. In addition, you gain low end bit lose top end. I only post when I'm drunk as well.

Also, anything you watch on TV is slanted to market or sell a product regardless of the show or sport. Win on Sunday, sell on Monday. Butt dynomometers work best...

 

But you've pretty much just discredited your source. Hahaha.

 

Barring the idea that the show is trying to sell steel, they did use identical mufflers from the same company. How do you think it will suffer in the long run? And I feel ya on the butt dyno, but the numbers are there.

 

I feel like it depends on the engine. Yes, they "work" the same, but they're designed differently with different tasks to perform. My 2.7 is designed to produce torque down low, compared to a NASCAR engine which is design to run balls the the wall. So yeah the rate in which it exhausts well, exhaust, I feel could help or hurt "performance". Rather it be for pure HP or torque.

 

If you want to pay dyno time, I have a big block sitting at the shop right now. I'm sure he would be happy to test different pipe size and style headers.

 

Mind my spelling of the words

Look up
Quescent And laminar flow it's all the same principal about smooth flow

Oh and I didn't read any of the post except for the first one

 

By the way. I hope I don't sound like a dick. I just mean this as a fun debate over something I saw that looked pretty damn convincing.

 

No offense taken were all dicks

 

I believe (sitting here with my armchair physics degree hanging on the wall) that it has less to do with tubing diameter and more to do with how the system is designed as a whole. Having a 5" pipe doesn't necessarily mean that the exhaust gases will leave the engine any faster. As someone mentioned earlier, laminar flow is the smoothest and most efficient means of a liquid or gas moving through any container. If you have, for example, a 2" manifold exit diameter and immediately transition to a 3" exhaust tube, the gap on either side will introduce ripples and swirls in the current that will actually slow down the flow of the gas.

 

First off, I like the sig. Second, the ripples and things make sense to me, but seem to be more applicable to a flow with considerably less pressure that that of an exhaust stroke. Even if it IS still applicable to this, I can't imagine the effects of a ripple such as these to be even measurable when compared to something like diameter of exit.

 

What TV dyno showed you was peak numbers. What you need to look at is the power band/curve, and translate that into common street use. Dyno queens usually struggle to be street practical. Driveablitiy is a characteristic of low RPM torque, and back pressure /scavenging helps with that.

And we're not including the scavenging effect of the system here. For example, in tuning to run the 1/4 mile, before dynos were available to the common man, we hung a length of collector pipe on the header flange. Changing the length of the collector (with a hacksaw) changed the performance of the vehicle. Not the overall power made, but where in the RPM band max torque occurred. Kinda like advancing ignition timing moves torque down low but reduces it up high.

When you give everything to coming off the line, you can fall on your face and not finish strong. If your power is all made at high RPM, you can't launch hard enough.

Complicate that with ignition timing, gearing, tire bite and diameter (more gearing) and carb jetting, and it takes a lot of 'test and tune' to get it right.

So, as a crew member on an NHRA E/S ET record holder (even if it was less than 2 weeks) I can say we were pretty proud of our tinkering.

But it was the exhaust collector/scavenging that put us over the top for a while.