backpressure simplified?

[baglock1]

One of you budding physicist types care to explain to me how a lack of backpressure can reduce torque output? I understand how it affects horsepower in the upper range, and I can visualize a motor falling on it's face (in regards to power) at the lower end from an exhaust that flows too much, but I can't explain it. What did I miss while I was ditching my physics class?

[wish]

Scavenging and the venturi effect ... when you use a bigger pipe the pulse through the exhaust gets weaker (moves slower), thus it doesn't draw the next pulse out as well. The more it draws, the better it pulls the spent gasses out of the combustion chamber. The better you pull those out, the more lively the next load of fuel as it's not diluted as much by the gasses left from the previous cycle.

That's the short of it anyway ...

[Msquared]

Originally posted by wish
Scavenging and the venturi effect ... when you use a bigger pipe the pulse through the exhaust gets weaker (moves slower), thus it doesn't draw the next pulse out as well.

That's a different issue: pipe diameters and exhaust gas speeds, to be exact. Backpressure in and of itself does not under any circumstances increase power or "torque" at any rpm on any engine, and is always bad. A smaller-diameter pipe boosts low-end power by increasing low-end exhaust speed and scavenging, not by adding backpressure. The fact that the same small pipe adds restriction at higher rpms and flow rates is an unfortunate side effect that builders would love to eliminate. A variable-diameter pipe would be a good solution if it were feasible. A good example of this in the numerous variable or dual-stage intakes on production cars these days - same laws of physics apply - in which the low-rpm tubes are not only longer but of smaller diameter as well. So baglock, the reason you can't explain it is because it doesn't work that way (go with your instaincts, Grasshopper)!

[wish]

I'm wondering how we're defining back pressure ?

[baglock1]

Perhaps I was using the term “back pressure” improperly. Instead of saying that, I should have said a conduit of smaller diameter. But if I had said that, I would have made the mental connection from “decreased conduit diameter” to “increased flow rate.” From there, I would have answered my own question. Thanks for the moment of clarity guys.

[carrew]

This is a subject every ricer should read.

[RACER7088]

When the exhaust system is sized too small or is restrictive at some point then the residual pressure in the pipe may be higher than atmosheric and this is termed backpressure. If you are opening the exhaust valve way too soon then a restrictive exhaust system may help make more power but on a normal system that has correctly timed events all back pressure does is increase pumping losses and increase intake reversion and dilution which are both detrimental to power. At low rpm though even a "restrictive" exhaust system may not be restrictive. It's at some rpm that the system will start topping out in flow capability and retaining positive pressure and then usually it's bad.

Conversely when you have bad exhaust ports and/or a higher backpressure system you may need to run more lobe seperation at the same intake centerline to open the exhaust valve sooner and/or longer. Of course like Matt and others said there are also resonant tuning phenomenon at work due to header diameter length and there's overal header pipe cross section.

[PaulL]

Thought I'd resurrect this thread because I'm about to make an exhaust change.

I currently have the stock H pipe on my 01 Cobra, with MagnaFlow catback. I want to remove some weight and hopefully pick up some power (low end torque would be even better), so I'm installing a Bassani X Pipe. I chose this brand because they had good tech on the phone prior to order, I could find out how much it weighed, good HP claims (yeah, I know) and it installs in sections so that the cats that come with it can be removed and test pipes installed (the test pipes come with the kit.) Bassani said they've dyno'ed the kit both ways and the pipe actually makes more horsepower and torque with the cats on.

Anyway, I hope to dyno the car before and after and I'll let you know how it goes. I've heard that 4 valve engines are sensitive to changes that could hurt their low end torque, so that's where my concerns are. However, since the pipe will remain the same size, the exhaust velocity should remain the same, and all I will do is hopefully reduce backpressure without reducing velocity. It will remain to be seen how well the X chamber scavenges the exhaust gasses and how/if it improves it over stock.

Any experience with this change?
Any thoughts, or suggestions on things to check out when I test?
I'll check the stock H pipe weight .vs. the X pipe, and I'll try to do the dyno stuff.

[Msquared]

Paul, fwiw my 97 Cobra seemed to like the Bassani X-pipe just fine. I also ran it with cats. I think X-pipes have generally shown a slight improvement in power over H-pipes by more effectively evening the scavenging effect between the banks of 90-degree V8s (which is all production V8s except Ferraris and the Lotus Esprit, I think). A flat crank engine would not benefit from this, and neither would a 90-degree crank engine with 180-degree headers (too complicated for anything but a full-race car).

I bought my Cobra with the exhaust already installed, so I don't have a before/after comparison. But just based on when the exhaust "came on song" with that X-pipe wail, the scavenging probably only works near peak-power rpms. I think that increasing the low/midrange power with exhaust requires lengthening header primaries (i.e., long-tubes) and/or increasing collector volume (which the H-pipe may do, depending on where it's placed, but the X-pipe probably does not do). All the mods on my Cobra consisted of ProM MAF, underdrive pulley, the Bassani cat-back with hi-flow cats (but stock mufflers), and a very conservative chip. It made 291rwhp, about 20hp more than a typical stock 97. Most likely, about half that was due to exhaust mods.

I hope this helps. If nothing else, the X-pipe will indeed make your Cobra sound very cool at WOT!

[PaulL]

Thanks Matt. My car last made 310 rwhp and 310 rwtq. I've made some changes since then, though, and hope to get another baseline maybe this weekend.

[fordfnatic]

In the current issue of Popular Hot Rodding there's an article by David Vizard about making torque. One of the subjects touched on is header and collector lengths as well as exhaust diameter.

And, in the last issue of Mustang Enthusiast, there was a big article comparing X-pipes from different manufacturers.

Hope this helps.

Bruce

[PaulL]

OK ... baseline dynoed the car Fri night, and got 313 rwhp, and 316 rwtq. I got the X-pipe installed this morning, and the "how-far-does-it-burn-out-when-shifting-to-second-gear" test showed an improvement. ;)

When I get it re-dynoed, I'll post a chart showing before and after.

Sounds pretty cool. too.

[Auto-X Fil]

Did you ever re-dyno?

[PaulL]

Oh yeah, I did. I forgot to post the results.
Got 326 rwtq, 326 rwhp. Not too shabby! I don't recall doing anything else to it since the 316 results, so I suppose the increase IS due to the pipe!

http://corner-carvers.com/forums/attachment.php?attachmentid=9705&stc=1