Thread: Turbo Over Speed From 3inch exhuast. on td5

The 2.5 inch exhaust has merit though. What they are saying is with the smaller diameter of the exhaust the gas flow is quicker than a 3 inch exhaust. This creates less back pressure. With a larger volume pipe the gases flow slower. So the bell auto and td5 alive guys think the 2.5 flows better, after all if 3 worked better wouldn't they just sell that as there performance pipe. Just putting it out there.

IMHO having worked on the 3 main types we're discussing here

a 3inch normally bent exhaust works about the same as the 2.5 in mandrel bent unit (the 3 inch being the cheaper of those 2) the 3 inch mandrel bent unit make a different noise and under the right conditions seem to me a little less crisp in picking the turbo back up from mid boost ranges, like when you back off a little then floor it again BUT from very low boost if you hold it pinned the boost seems to come up a little better.

Its not anything Id pin down as being absolute as other factors may have come in to play but thats how it felt, sounded and how the boost gauges were tweaking.

Originally Posted by lambrover

The 2.5 inch exhaust has merit though. What they are saying is with the smaller diameter of the exhaust the gas flow is quicker than a 3 inch exhaust. This creates less back pressure. With a larger volume pipe the gases flow slower. So the bell auto and td5 alive guys think the 2.5 flows better, after all if 3 worked better wouldn't they just sell that as there performance pipe. Just putting it out there.

Sorry, but this is a crock of bullplop, sprouted by idiots who don't understand maths or fluid mechanics. (not having a go at you lambrover)

Assuming two pipes (say 2.5" and 3" diameter) the same length, made of the same material, and both operating in the turbulent regime, then:

Pressure Drop = Velocity^2 / Diameter

So pressure drop (or back pressure) rapidly INCREASES as velocity increases due to the ^2 term. Increasing pipe diameter increases D and decreases V, both of which reduce pressure drop (back pressure).

So anyone who says smaller pipes give less back pressure has NFI what they are talking about.

oh come on IR Everyone knows that pressure is just the resistance to flow.....

if the gasses are moving faster they must be at lower pressure...

quick everyone go out and retro fit 1.25 inch series one exhaust systems and get those exhaust gasses screaming out your exhaust pressures will drop dramatically.
you might want to consider that this post might have been sarcastic and made with full awareness of just how much easier it is to get to other parts of the vehicle when you fit a spaghetti thin pipe

But isn't it true that different size pipes affect scavaging.

I am not saying fit the smallest pipe, but could there be a specific size that performs better.

I am not convinced either way but still wouldn't it behoove them to sell the 3 if it was better.

Really i don't know enough about exhausts but the theory of the 2.5 made sense.

There does seem to be a lot of guys running 3 though

Overspeed is overboost. They are the same thing.
You can fit the largest exhaust you want and as long as you control the boost, there will be no problem. If you fit a large exhaust and clamp the wastegate shut then overboost and overspeed is probably going to happen.

There is no scavenging on a turbo exhaust, it's simply a drain pipe (or drone pipe in some cases).

i see where your coming from lambrover and alot of people use similar theorys on petrol motors with extractor designing but as isuzurover has posted with the formula that proves it.
i look at it like putting your finger over a hose it boosts pressure up because its coming out of a smaller hole but there isnt any flow and upstream is being restricted compare that to no finger at all and there isnt much pressure but its getting rid of the water faster and hence its getting rid of it up stream which would relate to getting the gasses away from the turbine.

although if someone with a 2.5" exhaust and someone with a 3" exhaust were to both shut there wastegates off and whoever's turbo goes poof first wins