Common Misconceptions about backpressure...

[SConnors]

I've read about an engine "requiring backpressure" quite a few times in this forum. I beleive this is a misconception. Engines do not require backpressure. They do, however require exhaust scavenging.

Headers have been a popular modification for engines for decades. Higher flow tubular headers, substituted for stock cast manifolds, has resulted in increased performance for enthusiasts for years. A common "bigger is better" attitude resulted in oversized header tubing, which resulted in poor exhaust scavenging, possible transfer of the intake charge straight to the exhaust, and/or less power. This is where the school of thought that an engine "requires backpressure" was born.

I've read that people are scared to go with a free flowing exhaust because it will "limit torque". I would love to see evidence of this, but have not been able to discover any.

Any engine with similar exhaust scavenging (ie. same headers) SHOULD respond favorably to less overall backpressure. Fluid dynamics, however, still applies. Gradual transitions from increases in diameter are required to reduce the restriction of the transition (a pipe that starts as a hole in a wall will flow better than an unsupported pipe due to venturi effect.)

The above asumption requires that the air fuel ratio remains the same throuought the RPM range and ignition timing remains optimal...

Anyway, prove me wrong... I love to learn new things, but this new concept runs contrary to my personal experience and observations.

Keep it civil, please

:bash:

 

[Even Steven]

Any mechanic worth his tools knows that a lack of back pressure can lead to burned valves, but that's much more of a problem on older vehicles that were not computer controlled. Today's vehicles obviously have the capability to adjust air/fuel mixtures, so that's not really even a problem anymore, as long as you're talking about MODERN vehicles. Older vehicles can still suffer ill effects from a lack of back pressure.

BTW, it's not the lack of back pressure that burns valves, it's the fact that a freer flowing exhaust system tends to draw more air into the intake, causing a leaner than optimal air/fuel ratio. This leaner mixture is what burns valves. And of course newer vehicles have the capability to adjust to any increase in the volume or density of intake air, so it's not an issue.

BTW, even the older speed density systems that were more popular in the 80's were able to compensate and adjust mixtures on the fly. That's why the mid and late '80's Mustangs responded so well to intake and exhaust mods.

 

[drmorbius]

Back in the day when I used to race Chevys it was necessary to learn about exhaust systems. First I agree with you that headers can increase performance, however they work best when completely uncorked. I agree that large tubing is not where it's at. The benefit comes from a tuned length of tubing. Also these benefits reach optimum at higher RPMs and are usually in concert with a tuned intake runner and flowed heads. I think the idea of needing back pressure may have come from the fact that if you are running uncorked and shut off the engine you need to stuff rags in the collector box to keep cool air from reaching the hot exhaust valves which will cause warped valves. Headers on the street can be a problem as the fasteners used to mount them to the head can (and do) vibrate loose over time causing exhaust leaks. I think that headers are meant for racing situations and that the benefits for a street driven vehicle are minimal at best.

 

[SConnors]

The concept introduced apply to all naturally aspirated internal combustion engines regardless of when they were designed. The question at hand is the misconception that backpressure reduction leads to a loss of torque or power.

We are assuming AFR and timing remains optimal.

It is not lack of back pressure that leads to valve problems, rather thermal shock from cool air entering short pipes as drmorbius mentioned. Ask yourself if placing the exhaust under vaccum would lead to valve problems. Remember, we're discussing a reduction in backpressure only.

Burned valves are not relevant to the discussion, as they are generally caused by an insufficient margin on the valve grind, reduced valve guide surface area, inappropriate material used on the valves or guides, or incorrect AFR. All these issued are assumed to be kept constant.

That said, my Jeep Cherokee saw 15 percent increase in torque when I switched to Doug Thorley Tri Y headers (from improved exhaust scavenging). The benefit for steet driving was signifigant. I used a Walker Super Turbo exhaust system to keep the exhaust volume (sound) comparable to the stock exhaust.

The only reason I have the stock exhaust on my runner is the lack of options and hard data with the VVTI V8. Placing the cats in the header section increases the heat of the exhaust entering the first stage cats, but it severely limits aftermarket exhaust options.

 

[Even Steven]

Burned valves are not relevant to the discussion, as they are generally caused by an insufficient margin on the valve grind, reduced valve guide surface area, inappropriate material used on the valves or guides, or incorrect AFR.

The title of your thread is "Common Misconceptions about backpressure", of which one of the biggest is burned valves. That's why I mentioned it in the first place. And like I said, the primary cause of burned valves in a lean air/fuel ratio, which can be caused by decreasing the backpressure on an older engine.

If you want to talk about running big, open headers, just look at the cars racing in the NHRA. That should pretty much answer all of your questions without the need for a debate.

 

[SConnors]

What I was getting at is the misconception that a reduction in backpressure causes a reduction in torque. This is simply not true, but I have seen it written and accepted a few times on this forum.

NHRA and their supercharged engines don't apply here.

 

[Even Steven]

I see... I didn't realize there was such a big misconception when it comes to backpressure and torque.

BTW, depending upon the class, NHRA doesn't always run forced induction engines. Some are naturally aspirated and still use a huge, open headers.

 

[Even Steven]

That said, my Jeep Cherokee saw 15 percent increase in torque when I switched to Doug Thorley Tri Y headers (from improved exhaust scavenging).

That's probably considered an exceptional result, but I think it speaks more to the inefficiency of the OEM design. In other words, if the stock system is horribly inefficient, then I can see how big gains can be made by improving it with aftermarket compenents.

I'm not so sure how much this applies to the 4Runner though, because the stock system may not be so inefficient in the first place. Another thing to consider is the possibility that the V8 and V6 exhaust system is identical, at least from the end of the exhaust manifold to the tailpipe. I'm not sure if that's the case, but I wouldn't be surpised...

 

[RonMAIDEN]

I've seen plenty of Civics with youngsters throwing big 3" exhaust on 'em and lose both torque and horspower on a dyno;too much flow is bad on a naturally asirated engine.

 

[drmorbius]

The only way to really know how much gain you get by adding headers or any other aftermarket bolt ons is to dyno the vehicle before changes are made to establish a baseline. Then dyno again for each change. Doug Thorley makes some of the best headers in the business, and has been at it since the days of **** Landys Dodge if anyone still remembers that name. Many people think that louder is faster when it usually means that you have detuned the factory setup, but hey whatever makes people happy, right? I'll bet the 4runners are pretty well optimized from the factory. Also, making changes to the exhaust systems, at least in California, is illegal and you will not pass the smog test even though the emissions may be in the right range. Headers won't limit torque but they may shift the peak to a higher rpm IE not what the factory designed the vehicle for.

 

[SConnors]

A sudden change fron a 1.25" tube to a 3" tube can result in signifigant reduction in flow. It's the similar to the restriction resulting fron the resonator on our 4runners. It causes a reduction in exhaust scavenging. Like a slowdown in traffic on the freeway affects the overall speed of the route...

The slow down comes from the following equation:

Rate of flow (V dot) = Velocity * Area(cross sectional)

That said, the backpressure created that far from the resonator on our Runners' exhaust is probably not that noticable. And I'm fairly sure it's not affecting the Civics to any huge degree.

Ron, if there was a power reduction in the case you mention it was probably due (in small part) to the above effect or an incorrect AFR/timing setup. That said, I'd love to see the dyno runs of before and after. Post 'em up!

I think the whole thing is kind of like an urban myth... maybe we should send this idea to Mythbusters (I love that show!)