here you go. It was a link in a thread where people were arguing about 2.75" DP vs 3" DP and Zeus posted this. https://www.evolutionm.net/forums/sh...5&postcount=57

 

nice find. learn something new everyday!

 

a few subtle points. bigger is better, that does not mean open exhaust is best. why do individually throttled cars have velocity stacks on them? well... because it increases the intake gas velocity. how? by forming a uniform fluid flow front by drawing in air at the same speed from all angles. it FORCES the head to injest a laminar flow, barring (by geometry) flows that would disrupt.

same thing for exhaust. as jay says in the article, the best way to transition sizes in piping is actually to use a hyperbolic transition, this will keep things laminar as the expanding gasses will just slide off the pipe, with no chances of reverse flow turbulence, becuase the hyperbolic shape is holding the air.

this means that you benefit most not from an open header but a header with a velocity stacks coming off it. but you never see this... why? firstly because it's difficult and awkward and a bit ugly to do. secondly cuz many assume as some have in this thread that there is no such thing as a restriction if you're blowing into nothing. so what is the general purpose solution? remove the flow from the source as far as possible, until this phenomenon is MINIMIZED in it's flow disruption effects. that means having a longer exhaust.

you can even test this yourself, if you have a way of measuring horsepower or afr on a lawnmower engine try it. take the exhaust pipe out and see how the motor responds. it will lose hp. then put an overly long pipe on, it will gain some hp, but not much. then successively cut off pieces of the pipe. until at some point your hp is maximized (this will not be at 0" of pipe).

so what does this have to do with anything? why do we 'need' a 3" dp or a 3" o2?

look at the design of the turbo. the turbine outlet is not 3" by any means, it's not actually that big at all. but what else is there? the wastegate port. WHY IS IT OK FOR GT STYLE TURBOS TO RUN 2.75? because by and large most of them are wasting externally, which gives 2.75" of LAMINAR FLOW. this is significant, the o2 is matched to the size of the turbine outlet, either directly or close (no such story in an internally wasted turbo) in an internally wasted turbo, if you can seperate out the waste flow and recombine it when the disruption affects are minimal, then you've won. example is many garrett setups, buschur external wg setup on stock turbo, jspeed downpipe for evos etc.

now this is not the final reasoning for anything... i'm not saying that you must have 3" of pipe for garret turbos that waste internally. what am i saying? i'm saying that it matters MORE for such setups. why? if you read jay's article he mentions that abrupt expansions are bad, hyperbolic is best, but most likely you'll find a linear expansion (just like you do in the stock mitsu turbine). but none of that really matters if you have a disturbance in flow that is just another source entirely (the wastegate flow). so you see he addresses waste gate flow, how it should be seperated, we have no such luxery if we wanna stay stock looking and be quiet, so what happens? we need to make the overall size of the o2 housing larger because the disturbances in flow are larger and if you increase the size where the disturbances take place you alleviate at least SOME of the pressure. i hope that makes sense, making a high pressure zone larger reduces the pressure...

 

Has anyone measured the back pressure in their exhaust (post turbine) before and after doing any exhaust work?

I am curious to learn if there is some general rule of thumb or ratio for backpressure/power.

On a non-EVO installation I have measured 9psi in the DP with a homemade device; a copper tube mounted in a O2 bung plug to a rubber hose and spare boost gauge. 9psi sounds like a lot considering that was only at 15psi boost. Do we have any info to calculate what power would be at say half that back pressure?

-Brian

 

mmm there is a misunderstanding here. 0 pressure behind the turbine would actually imply 0 flow. see the flow work term in the fluid equations contributes a pressure factor, so if you have a fluid flowing then you have a pressure associated with that flow. the idea is to minimize the pressure LOSS associated with the flow going out, eliminate the back pressure and to expedite the flow so that the overall energy is maximized going to the turbine.

how would this be measured? uhm... i think measuring a pressure difference would be most important , cuz if you think about it, a higher initial flow in will definitely have a higher final flow out across the turbine. so what fullrace was referring to with the temp losses is the enthalpy term in the fluid equations, that's where the work is derived. if the mass flow is conserved then have the lowest temperature post turbine would give you the lowest pressure and the the most work to the turbine.

i just point this subtle stuff out cuz it's not so simple as pressure 1 means power 1 means most efficient.

 

on aircraft they measure this as EPR..engine pressure ratio. thats how they determine the efficiency and power of jet turbines..... I think this is what you are trying to point out triny...

 

Thanks very much! It is nice to learn something new that will benefit all of us. Great find...

 

I ran this question briefly by my brother who has a masters in thermo-dynamics and what i got from his reply was that a linear expansion of exhaust gasses through the exhaust system is going to be more efficient than any abrupt expansion such as going from a small turbo outlet and immediately expanding it to a larger diamter as found in many of these "coke can approved" systems. Linear expansion creates a much more uniform and fluid flow of hot expanding gasses and eliminates turbulence, thus creating a more efficient exhaust system.

His explanation made more sense to me than my re-hashing of what he said, so i hope this makes sense to you guys.

It does back up Buschur's response to this flame-bait thread though. While I have no allegience to any Evo tuner since i'm new to these cars, I will say that similar views are repeated by all the Audi and VW tuners i've been involved with over the years. I don't see why the concepts would somehow abruptly change because this is an Evo.

 

so any of you running the RS*R 80mm DP?? seems like there's a few people running 80mm exhausts so i don't see why they aren't using a matching 80mm DP??

and in case you need an update on your conversion of metric to standard 80mm=3.1496in

 

welp couple things.

yeah linear expansion is a compromise with production complexity and maximum performance. mitsu wants to have a little of both.

that said, we're mainly discussing the o2 ourlet diameter (being 3") and that is because i said with the highly turbulent and high pressure streams coming together need the room more than an externally wasted setup.

additionally, i think you might not have given your brother the whole picture and i apologize also for not disclosing all the facts in one post. but here's another bit to think of.

the inlets of the o2s are big, they're bigger than stock even the ebay ones. this is bad as your brother pointed out. the abrupt expansion is definitely not conducive to maximum flow laminarity. BUT if we limited the o2 inlet size to the exact diameter of the turbine outlet (exducer) diameter then we'd encounter a maximum flow (volume) problem.

back to my lawnmower example. the exhaust pipe i stated would make a certain maximum horsepower, but at what rpm? this is where i apologize for not giving the whole story. because it IS rpm dependant, until you can have a variable size exhaust outlet you can't maximize the hp (or more correctly, TORQUE) with one exhaust. so you optimize the motor to run in some rpm band. mower example is really simple because there's no gearing and the flow is always the same at so much load and rpm.

now our cars have a few conditions that far supercede the simplicities of the mower. firstly what works best for torque doesn't work best for hp. the torque maximizing, exhaust gas velocity maximizing 2.75" dp setup (which buschur himself has proven to make less power than 3") will likely not make the same peak power, the differences are probably slight for lower hp number but CAN become quite pronounced at higher hp levels. for a lil qualification, torque comes not from back pressure but from exhaust gas velocity, this is coming striaght from jay's article agian.

so combining all these subtleties together you might notice that keeping a non expanding o2 housing is great for flow velocity and flow laminarity but in the end you'll be limited to the mass flow that a certain cross sectional area can produce and since we have many rpms of use we must account for, we must then make sure we're maximizing the most of everywhere and losing out on the least areas.

and in such a way we sacrafice a little laminarity for some maximum volume flow.

also keep in mind that jay says, LEAST pressure out back, is the RULE, while introducing rapid expansion turbulence DOES introduce a rise in pressure this is outweighed by the higher rpm, higher mass flow rates that the larger housing is capable of.

 

I forgot to answer, yes, the machined fit on the 02 housings to our downpipe flange is all that is there, no gasket required. The extended lip on the 02 housing fits into our flange rather tight and then extends to the end of the flange.

I happen to have the turbo off my RS right now and was looking at the turbine housing to 02 housing fitment. We gasket match our turbine housings/manifolds/02 housings. The SS 02 housing on my car is a perfect match to the turbine housing, there is no abrupt change in size in the flanging.

 

the abrupt change i'm referring to is where the wg flow merges with the exducer flow. the exducer flow is not pouring directly into a pipe and that said it's not pouring directly into a pipe smoothly either.

if nothing else the exducer flow is expanded into the wg flow at least, but in any case the o2 housing is larger on the inside and is the expansion that i was referring to. you need that extra volume so you don't choke off or build pressure cuz pressure IS built there, the shear fact of the ebay making more power than stock directly attributes to that fact.

the wider o2 exit is the other expansion that alleviates merging collector flow pressure.

 

I completely understood everything you just explained. Thanks.

Weather or not I'll remember it again in 24hs is another story...

 

Let me tell you something man... ITS ALL ABOUT THE MONEY. They tell you "yea we put customer service first"... LIES. It's all about the money with these guys. They rant and rave..."yea I tuned this car and it made sooooo much power". Man you dont know how much power that car really made. Dyno graphs look too smoooth. It's almost too good to be true. Once again - ITS ALL ABOUT THE MONEY with these vendors.

 

dude get a clue. if it wasn't about the money EVERYBODY would be working for mcdonalds or mowing lawns. the point of a business is to sell a product, to make money to pay all of your workers, as well as fund other projects like new product desings, project cars, trips to track events, sema, also cover utilities and the other bills that come along with owning a shop then in the end of course they are hoping to make a bit of a profit. which most of the time in the car industry end up going back into the shops since there can always be slow months. hell if i had a business i'd be doing the same damn thing, and i'm sure you would to.