1970 318 duster exhaust question???

bighammer said
I'm callin BS! Don't get me wrong here alpha13, your argument is well worded and all, but it doesn't make sense to me. I'm no expert on the topic, lol, but this goes against common sense. BTW, yes, there IS something as back pressure. And to illustrate my point, try running your manifolds thru some 1 1/2" exhaust. I think the effects of back pressure may be noticeable.

Yes, velocity will be greater with smaller diameter pipe. This is true, but does not equate to better performance. It's actually robbing potential horsepower.

Thirdly, again, not my own experience here, but I have always heard that the larger diameter pipes will make your exhaust quieter, not louder.

Yes, the smaller pipe is going to be easier to work with! That may be the only advantage with it though!

If I'm completely wrong with everything I've said, and you are correct, my apologies! Either way, no offense intended!

See this is where people are wrong BigHammer. There is no such thing as back pressure. I have asthma so it's pretty easy for me to understand. Your analogy with the manifolds and small pipe size simulate restriction, not backpressure. And yes you are right, if you use too small of a pipe you will restrict your engine, but velocity and volume go hand in hand as well. If you take that same 1 1/2" pipe and put it on a small 3 cylinder smart car it will have a sufficient amount of pipe size to flow it's particular volume. Now performance exhaust is typically bigger so you can flow more volume, but to achieve a reasonable exhaust velocity you must create more volume. When you rev higher you create more volume thus allowing the velocity to remain at a decent rate.
I'll give you an example. Take a stock 318, put a dual 2 inch exhaust on it. (I'm guessing on this next part for comparison.) Exhaust gas velocity will peak at around 2500 rpm. Now take the same 318 and then put a 2 1/2" dual exhaust on it exhaust gas velocity will now peak at a higher 3800 rpm.
Why you ask? As engine speed increases, volume increases, and power increases. But limit(restrict) the amount of air exiting the engine and it will hit it's peak velocity at a lower rpm. As I've stated before, obviously if you restrict the engine too much it will hurt it's amount of airflow thus decreasing power. Alot of engines come from the factory with insufficient flowing manifolds and exhaust for performance use. That's why putting a larger exhaust pipe,within reason, on a stock engine will make more power. But it will also make more power in a higher rpm. How much power is debatable. Thats why when running a higher rear gear(numerically lower) you need alot more low speed torque A.K.A velocity because you spend more of your time in lower rpm. Now when you move your torque curve higher because your velocity peak is higher due to your particular engine combo you need a lower gear (numerically higher)as a crutch to make up for the loss of low speed torque. That's a whole nother aspect though.lastly, a 1 inch pipe and a 3 inch pipe can achieve the same velocity,but they would also require two different volumes to achieve it. The 1 inch pipe having a lower volume and the 3 inch having a higher volume. I could keep going on and on about this, but if you don't understand by now there really isn't much more I can do for you.:violent2: of course in no way am I trying to come off rude.