Exhaust Backpressure Faq

[neutral]

The general rule of thumb for a N/A engine is 1" per litre of displacement.

So a 5 litre V8 about 5" ?

Yup. Had 2.75" dual exhaust pipes on my 5.9L Dodge Ram250 Van and that motor liked it very much.

[bigtime]

of course.
that's only the tips though..
practically useless if the rest of the exhaust is about half that diameter..

[tatsu]

What the previous posts are referring to is a true dual-exhaust system, not just twin-tips.

Basically, you have a separate exhaust system for each bank of cylinders - I.E. the header for one bank empties into one side and the header for the other bank empties into the other. We won't get into the complexities of crossover pipes, etc. that dual exhausts can also have - just think of it as basically dividing the motor in half, and giving each half its own exhaust system.

A 5.0L V8 effectively is treated as two, 2.5L 4-cylinders, and would thus need a 2.5" exhaust system for each side, therefore a dual-2.5" exhaust.

With the 5.9L, that would be 2 x 2.95", so the dual-2.75" system would be just about right, but a little under-sized according to the 1" per litre "rule of thumb".

Bear in mind that, as noted a few posts back, any number of variables can affect the optimum exhaust pipe diameter for your motor. 1" per litre is just a very general guideline.

[happyclown]

sorry to bring this back but i got a kinda tough question that i cant figure out...

I friend and I were thinkin about redoin the exaust on a car of mine to help open it up a bit... its a 3.8 v6... so i told my friend about that 1inch per 1 liter and was like "so we can go with a huge 4 or dual 2"... and he was thinkin about it for a minute and said well, you'll get more flow outa the 4. i was like wah? and so we looked it up and a 4" has more than twice the area as a 2"...

so that kinda confuses me... which would be better? a single 4 with bigger area for a single 3.8, or two 2" with less area for two 1.9 banks?


thanks!

[tatsu]

That is a dilemma. The surface area of a 4" circle is approximately 12.6 square inches, while dual-2" circles give you a combined area of precisely half that. The fact that this guideline comes up with such a widely varying range is further proof that the 1" per litre is JUST a guideline, and a very rough one at that. If I am not mistaken, this rule of thumb was devised by the domestic crowd, and usually dealt with dual-exhausts, whereby they simply divided by two for the diameter of each.

With a 4" exhaust, you will lose a lot of exhaust gas velocity at lower rpm, but it will flow a TON at high rpm. Besides that, the difficulty of routing a 4" exhaust system under the average car makes it somewhat impractical.

With a dual-2" exhaust, you should have great low-to-mid range torque, but it may be somewhat restricted on the top end. So, what to do?

Well, given that a 2-1/4" exhaust seems to be ideal for our stock 1.8L displacement despite being 25% larger than the 1" per litre guideline would suggest, and your displacement is a little more than double that at 3.8L, I would say that a dual-2-1/4" system would be your best bet.

It is worth mentioning that, back in my days as a water-cooled VW enthusiast, the "optimum" diameter for a road-going, 1.8L, multi-valve, naturally-aspirated, 4-cylinder VW motor was also 2-1/4" according to all of the major VW tuners. 2-1/2" exhausts would give you more top end at the expense of some low-to-mid range torque. Bearing in mind that many of the characteristics of those motors are quite different from ours, being that they are designed for low-to-mid range grunt rather than top-end power, I have always used a variant of that ratio for calculating the optimum exhaust diameter for a street-driven sport compact.

I look at per-cylinder displacement, because, in my mind, that will determine the volume of the exhaust pulse.

For a torquey street exhaust, I take the per-cylinder displacement in litres and divide it by 0.45 (which is that VW 1.8L divided by the number of cylinders), then I multiply the result by 3.98 (the approximate surface area in inches of a 2-1/4" circle). The result of that is then divided by pi, and then I take the square root of that and multiply by 2 to give me the diameter in inches, which I round up to the closest 1/4". That would give me a 2" diameter for a 1.8L six-cylinder, or 2-1/4" for a 2.5L six-cylinder.

For a better-flowing street exhaust, I use the same formula, but replace the 3.98 with 4.91 (the surface area of a 2-1/2" circle). This results in a 2-1/4" diameter for a 1.8L six-cylinder, or 2-1/2" for a 2.5L six-cylinder. For newer, multi-valve designs (E.G. most Japanese motors built in the last fifteen years or so), I tend to go with this calculation to allow for their better breathing on the top end.

For your 3.8L 6-cylinder, this would net you a 3" single exhaust. If you take that same surface area and divide it in two for a dual exhaust, you would get dual-2-1/4" exhausts.

Well, there's my 1.5 cents after taxes... flame away!

[tatsu]

Three other things to consider:

• 1. If a motor has a higher specific output (I.E. it makes more power from the same displacement), each exhaust pulse will have more “punch” (I.E. it will have a higher velocity and pressure than a lower-powered pulse), and would therefore require a larger diameter exhaust to accommodate the increased flow. Because high specific outputs in naturally aspirated motors generally mean a peakier power-curve, however, you need to maintain exhaust gas velocity at lower rpms to alleviate that, and you do that by going with a smaller-diameter exhaust.
  
  2. If a motor is a large displacement, high-torque motor, it can afford to sacrifice more torque down low for power higher up, so you would generally go with a larger diameter exhaust to accommodate this.
  
  3. Supercharged and turbo motors have both higher specific output, and more mid-range torque, so they would generally require a larger diameter exhaust.

All of this just proves – the only way to REALLY determine what is the best diameter exhaust for your setup is try different diameter setups and dyno each of them…

[Mr.mx-3]

Three other things to consider:

• 1. If a motor has a higher specific output (I.E. it makes more power from the same displacement), each exhaust pulse will have more “punch” (I.E. it will have a higher velocity and pressure than a lower-powered pulse), and would therefore require a larger diameter exhaust to accommodate the increased flow. Because high specific outputs in naturally aspirated motors generally mean a peakier power-curve, however, you need to maintain exhaust gas velocity at lower rpms to alleviate that, and you do that by going with a smaller-diameter exhaust.
  
  2. If a motor is a large displacement, high-torque motor, it can afford to sacrifice more torque down low for power higher up, so you would generally go with a larger diameter exhaust to accommodate this.
  
  3. Supercharged and turbo motors have both higher specific output, and more mid-range torque, so they would generally require a larger diameter exhaust.

All of this just proves – the only way to REALLY determine what is the best diameter exhaust for your setup is try different diameter setups and dyno each of them…

So basicly what you're saying that the velocity of the exaust is the speed and sound that the exaust makes as it leaves the pipes (ex) think back to the older day cars and when you were listening to them run the rate at witch you could hear the speed and depth of the exhaust break or in my words pop is the veloicty and also notice the diameter of the pipes.like for instance with a bigger or a oversized pipe the tone would be deeper and have a slower rate of POP in the exhaust than a stock exaust system would be more higher toned and would have a much more higher POP rate.

[tatsu]

The tone of the exhaust system can be influenced by a number of factors, including the exhaust gas velocity, the length of the exhaust system, the construction of the silencers, the size of the exhaust pulse, the speed (rpm) of the motor, and the number of cylinders among other things.

Generally, yes, though - a larger diameter pipe will have a slightly lower tone.

[Mr.mx-3]

The tone of the exhaust system can be influenced by a number of factors, including the exhaust gas velocity, the length of the exhaust system, the construction of the silencers, the size of the exhaust pulse, the speed (rpm) of the motor, and the number of cylinders among other things.

Generally, yes, though - a larger diameter pipe will have a slightly lower tone.

so a 2.5in. would be a fairly happy meduim for sound and performance like both the hole shot and top end??

[fowljesse]

It's also possible to have too much velocity. The [backpressure] pulse helps keep the new gas mixture in the chamber, since the valves are briefly open @ the same time. Someone built a system that created a slight vacuum, and he lost power because it sucked some of the new gas out before the compression stroke. A perfect system would give a slight backpressure pulse just for that split second. That's how and why Flowmaster mufflers were developed, and can (in the right position) have more HP than straight pipes.

[hgallegos915]

sprry to bump b ut aww man you mean i hurt performance by going to 2.5 instead of 2 1/4? i did have 1.6 tho..

[Tempus]

sprry to bump b ut aww man you mean i hurt performance by going to 2.5 instead of 2 1/4? i did have 1.6 tho..

go back and read the thread again.. Pay attention to the fact that there is no one magic number that gives the best performance in all situations. You are CHOOSING where to have the best performance.

For each engine there is going to be a range of pipe sizes that 'work' and above and below that sizes that don't 'work' due to too much back pressure, or too little velocity. Within that range you are choosing to either have your 'sweet spot' at lower rpm's (smaller end of the rnage) or the higher rpm's (higher end of the range).

2.5" is right in the range for a 2.5L engine.. Without crunching any numbers, and going just off one of the posts above, the 2.5" size is given as 'more freeflowing' (meaning you are choosing higher rpm power, vs lower rpm power)

OTOH the KLZE is going to be relatively speaking a higher 'output' engine than the normal domestic KL engine.. so if anything you want to err on the side of slightly bigger than slightly smaller..

I'm No expert, but based on what I read above, I'd think that a 2.5 pipe would work very well with a KLZE, unless you are all about 'hole shots' and want your performance located more in the lower RPM band.. In that case you might want a 2.25.. but I'd sure do a lot of research and number crunching before I ripped out the system and made any changes at this point.

frankly if it was me, I would not sweat it.. that should be 'in the zone' which is a hell of a lot better than being TOO small or TOO large and actually hurting your performance.

[MX34M3]

I have a klze ........and from the headers back (stock.....so I guess exhaust mani's....whatever).......I have 2.5" all the way back.....with no cat. converter.......a long flow through resonator....and no muffler.....pretty much as straight piped as it comes....and I notice some nice mid-high end torque with not real loss in the lower range......

engine runs/idles/responses perfectly

[ho bag]

Long story short.

Back preasure bad, Veloicty good. But dont sacrafice one for the other. you have to find the balance of least amount of back preasure with highest veloicty.

[mr1in6billion]

Long story short.

Back preasure bad, Veloicty good. But dont sacrafice one for the other. you have to find the balance of least amount of back preasure with highest veloicty.

Were we shooting for the "useless bump post award" or did I miss something?