bigger exhaust or not

[sagman82]

I've get psycho when I think about aftermarket parts and upgrades. So I've been obviously learning about perfect exhausts. Bottomline, for my V8, getting a bigger 3" tailpipe won't really help much because the engine is steady around 2-3k rpm. For a sports cars, a bigger tailpipe can be benefical because it's engine would go for higher rpms. During my enlightenment of exhausts, it's the velocity of gases expelled by the exhaust that's important, not the amount and smaller pipes create a faster delivery of gases. I'm by far NOT a guru, just explaining what I've recently learned. In summary, why go aftermarket?
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Is Bigger Better or is Faster Best?

When contemplating a modified exhaust system there are those who want the biggest diameter pipe that can be had. Their idea must be that fatter pipes are more effective at venting than narrower pipes. This sounds reasonable but it is not quite correct. Sure wider pipes have greater volume and higher flow capacity, but that is just half of the story. Capacity is one consideration but gas velocity is the other factor.
An experienced exhaust designer knows that the best exhaust is one that balances flow capacity with velocity. A given volume/time of gasses will travel faster through a 2" pipe than the same volume of gas passing through a 3" pipe. So when taken to its extremes we can see that a too narrow pipe will create backpressure (restrictions to positive flow) problems and a too wide pipe will cause a very slow flow with no backpressure.
The optimum is where the fastest velocity is achieved with the least constriction possible.

This situation will arise when the pipe is wide enough so that there is the least level of positive backpressure possible whilst achieving the highest exhaust gas velocity.
The faster the exhaust gas pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The scavenge effect can be visualised by imagining the high-pressure pulse with a trailing low-pressure area behind. The faster the high-pressure pulse moves the stronger the draw on the low-pressure gasses and the gasses behind that. The scavenge action is like (but not exactly) suction on the gasses behind.

The greater the clearance burned fuel from the combustion chamber the less diluted the incoming air/fuel mix is. Scavenging can also aid intake on overlapping valves (where the exhaust and inlet valves are open at the same time) by drawing in the intake. These are good things to happen.

So instead of going for the widest pipe possible we should be looking for the combination of the narrowest pipe that produces the least backpressure possible. In this scenario we achieve the least restriction on positive flow and the highest gas travel speed.

Exhaust pipe diameters are best suited to a particular RPM range. If we used a constant RPM engine this would be easy to specify. But a variable RPM engine will mean that not one size suits all. It is possible to vary the size of exhaust volumes according to rpm but it is very expensive (Ferrari has done it). The optimum gas flows (volume and speed) are required at the RPM range that you want your power band to be located. For a given engine configuration a small pipe diameter will produce higher exhaust velocities at a low RPM (good) but create unacceptably high amounts (bad) of backpressure at high rpm. If you had a car with a low RPM power band (2,000-3,000 RPM) you would want a narrower pipe than if your power band is located at 5,000-7,000 RPM.

 

[sagman82]

any comments?

 

[Turdle]

I think that with the vehicles we are talking about, being basically trucks, we want to maximize our torque benefits, both to get the thing moving, and to increase low range scavenging for fuel economy. BUT at the dragstrip, where high end is important also, I believe a progressively larger pipe would be beneficial, starting with 1 3/4 headers, 2.5 at mufflers, 3inch after that.

I am running stock header pipes, and my mounty launches good enough for me, and on the highway I get 20-22 mpg at 68 mph if I keep my foot out of it. ( hard to do with flowmaster sound)

 

[sagman82]

Yeah I agree if you want to drag your truck, a big pipe is benefical to achieve higher rpm torque. But I'm not gonna drag... so the topic stands, why go for a big aftermarket pipe when the smaller pipes are deemed to give better velocity of gases for our lower rpm engines.

 

[snocross1985]

interesting...

 

[Turdle]

Yeah I agree if you want to drag your truck, a big pipe is benefical to achieve higher rpm torque. But I'm not gonna drag... so the topic stands, why go for a big aftermarket pipe when the smaller pipes are deemed to give better velocity of gases for our lower rpm engines.

For the noise man! A musical instrument ( horn) is just a tube with valves, and the tube gets progressively larger, therefore LOUDER
I'm kind of proud of my noise, at times, at others I'd like to shut it off. You be the judge, can you live with it?
yes i'm posting noise link again

http://media.putfile.com/exhuast-1

 

[unclemeat]

Exhaust system designs are somewhat complicated. Two things are happening 1. exhaust gasses are flowing through a pipe and 2. gasses are cooling.

For the flow of gasses through a pipe the velocity of the gas is very important. In fluid dynamics, there are two conditions of flow that must be addressed. Laminar flow is when flow streamlines are straight and flow is most efficient. Laminar flow in low flow conditions and is what most people equate to low RPM torque. Adequate backpressure is necessary to slow the pulse of the gas from the cylinder. The perfect low RPM flow rate optimizes torque by providing the most flow volume with the least resistence in the exhaust system.

Condition 2 is turbulant flow. In turbulant flow streamlines are random and flow is much less efficient. Which means that more fuel is required to push the exhaust out.

The third condition is the cooling of the exhaust gas. When gasses cool they compress. When gasses cool their density and viscosity change, so you are dealing with a differential condition.

When designing an exhaust system all three conditions must be considered. Of course there is a simpler solution, go to a good exhaust shop, tell them what modifications you want and how you drive and listen to their recommendations.