Exhaust size vs. power potential

[EspenW]

Some good discussion of this over at Sonics project thread, but instead of derailing that thread, better to discuss it here.

Corky Bells ''Maximum Boost'' ( http://www.amazon.com/Maximum-Boost-.../dp/0837601606 ) has been a turbo charging bible for many, but among the things that I wondered about in that book was his chart/graph for exhaust diameter vs. power potential.

Have a look at this:


I have a 3'' Milltek catback, but stock RS2 2.75'' downpipe (and stock, dual RS2 cats), so I did some research and googling after reading Corkys chart. To my surprise, I didn't find a whole lot of comments on the chart, and real world testing of size vs. power in general, so Dmitris real world test of 2.5'' vs. 3'' is very interesting.
Did find some info, though, for example:

http://www.supraforums.com/forum/showthread.php?33491-Downpipes-3-quot-or-3.5-quot-Which-one&

http://www.koracing.net/viewarticle.php?article=7

Discuss

[Error404]

There are three different aspects in my opinion:

Firstly, when do you notice the benefit of the larger diameter pipe?
Secondly when does it become a restriction?
Thirdly when do you hit the wall?

You notice a 2.75" or 3" downpipe over a 2.5" at as little as 250hp in my experience. If you notice the difference, then it is already affecting performance, maybe not ultimate power but certainly spool and boost response. At some point you can't make any more power. Max power is only one indicator of performance

[Thuppu]

I know of a 930hp UrS4 with a 3" back end of the exhaust(backbox). I would personally use bigger pipe at that power level...

[Jamo]

I take issue with the chart being measured against bhp alone ( as with most things ), if it was measured with typical back pressure measurement it would be more usual for me.

infact max power indications in general are selling points, the more over time i've learnt the more the tuning industry is garnered to be enticing to people based on max power outputs.

[EspenW]

Agreed on max power, Jamo.
Peak hp is a pretty useless number by itself, unless you have a CVT tranny and can stay right at that RPM or DSG/PDK dual clutch type gearbox and can stay very close to it.
I'm aiming for max average power put down in the speed range that I'm interested in, and that is 50/60-100mph, so 3rd gear territory. A huge turbo might pull awesome numbers on the dyno in 4th or 5th gear, but might not see enough load in the lower gears on the road.
Analyzing the in-gear acceleration data from a (close to) 700hp 2.2L Audi was very revealing to me. It is significantly (0.25s) faster than mine from 140-160kmh in 3rd (approx 6300-7300rpm), but from 100-160km, it is only 0.18s faster, and if the pull was 80-160km, I would be 0.5s ahead at 160 (and 0.75s ahead at 140km), despite the slighly lower overall gearing and weight of the other car.
Peak hp only tells part of the story.

[elaw]

This is just a single data point so take it for what it's worth...

When I had my 4KQ with an MC-1 engine, I hooked a pressure sensor to the CO test pipe to measure backpressure from the exhaust. The car had an UrQ downpipe, no cat, UrQ front muffler, and Magnaflow rear muffler. All interconnecting pipe was 2-1/2". The engine itself had stock internals and the stock K26 turbocharger, but was Megasquirted. I used the Megasquirt to datalog the output from the pressure sensor.

At 18 PSIG boost (so what, maybe 225 HP?), the backpressure never exceeded about 1.4 PSI.

[driftin jo]

This chart is one I made years ago and was taken from SAE testing on an engine dyno not theory.

Restriction was defined in this test as the the back pressure exceeding 2 psig


Pipe bore size. BHP obtainable without
inch / mm. pipe causing restriction.

2 / 50.8 ------- 185

2.25 /57.15 ----- 235

2.5 / 63.5 ------ 285

2.75 / 69.85 -----350

3 / 76.2 ------ 415

3.25 / 82.55 -----490

3.5 / 88.9 ------565

3.75 / 95.25-----650

The above figures are based on petrol engines running a full length exhaust system 4 metres / 13 feet in length with free flowing silencers . Much more HP is obviously attainable from each size of pipe but beyond these figures the exhaust is increasingly restrictive .

After looking at lots of real world figures I would say you can go 20 % in power above the figures here before you start to see noticeable restriction in power due to excessive back pressure as was the experience with prj between the 2.5” @430hp and 3.0” @ 510hp.

IMHO 2 psig is not going to make any difference and I remember reading that if you are below 8 psig you are not loosing much, compared to 28 psig back pressure were you are loosing 80 HP !

I think it would be useful for people to fit back pressure gauge and post the results for a given dia and HP output.

With a turbo the lower the the back pressure the better as you have a better pressure ratio across the turbine so quicker spool etc.
if you compare the chart to the graph by corky bells you will see they are at opposite ends of the scale, the graph being what is max potential and my chart showing were the back pressure would start to exceed 2psig when beyond the figures.

Please read this for what it is, I have posted this before and all I got was lots of “ ive made 50% more power than that on a 2.5” etc etc . No where in this does it say you can't make twice the power but simply that beyond this the pipe dia will start to cause a restriction (back pressure exceeding 2psig.)

This is also a very good read and fits with the above and IMO much more useful than Corky's graph.
http://www.theturboforums.com/thread...-Jay-Kavanaugh

[EspenW]

Good read, and good link there, driftin jo.
Anone have a nice 3'', 3.25'' or 3.5'' downpipe for sale that is bolt on (on the RS2)

[EspenW]

Got to think of something: One would think that E85 means more exhaust gas volume, since the charge cooling and increased amount of fuel (stoich is approx 9.6:1) results in more molecules of air and fuel burnt for a given RPM. However, EGT is drastically lower on alcohol, so that would mean less thermal expansion and lower backpressure vs. petrol. Not sure if one cancels the other or if the net effect is higher or lower backpressure.

[caterpillar]

http://www.exhaustvideos.com/faq/how...-pipe-diameter ;some nice info.... les backpressure, more power, also earlier rpm...

[EspenW]

Tip from Corky himself (from the Miata forum) on how to measure backpressure:

''Take a small copper tube (3/16, 1/8), with an end fitting of 1/8NPT. Drill/tap the manifold or turbo. A magnet can lift out most of the shavings if you choose a spot that can reach where the bebris will fall. If you miss a few, they will blow through the turbine before it even begins to turn. A bit of an oily rag on the end a wire will catch most everything.
Coil the tube around something to use up about two feet of length. This will shed enough heat to permit the existing boost gage and signal line to be used to measure the pressure.''



EDIT: Doing some reading, it would appear that making use of the existing OXS sensor bung would make more sense. Find a bolt or fitting with the same thread and attach some copper pipe to that.
That would mean measuring post turbo, though.

A few links:

http://www.tuneruniversity.com/blog/2012/08/how-to-measure-exhaust-backpressure-for-smarter-exhaust-upgrades/

http://www.autospeed.com/cms/article.html?&title=Franks-Exhaust-Part-1&A=108185

This one is pretty nifty, but shipping cost to Europe is ridiculous:
http://www.ebay.com/itm/Backpressure...6d6d5d&vxp=mtr

Should be easy enough to make with some parts from a local auto parts store. Might have to drill and make some threads but that is not exactly hard.

[EspenW]

Easy, peasy:

Buy/find a section (50cm or so) of hard brakeline with fittings installed at each end. Approx 5 quid



Buy/find female thread to barb type hose connector (same thread as brakeline fittings) Approx 5 quid

Buy/find O2 sensor bung/plug:


Drill and tap the O2 bung plug for the proper thread size of the brakeline fitting.
Bolt together, hook up some vacuum tubing and a pressure gauge.

[JP]

I'm measuring backpressure on my car at the momeny in the exh manifold and most the time cruising it's negative however as boost increases and rpm so does back pressure sometimes to the extend of exh pressure equalling intake pressures with no more than 5000rpm and a bar of boost.
Now I can adjust back pressure via cam timing, but I've also decided to upgrade to a 3.5in system first so will pull some ecu logs before and after the exhaust change for comparison purposes. As I still wonder whether the exh manifold and hotside are the ultimate restriction over the exhaust.

This will hopefully be done over the next two weeks with no changes other than the exh and I'll post the results here once done.

[quattro84]

Corky Bell scaled actual 4-stroke Piston engine parameters (flow / backpressure / temperature) down to Power in order to make this graph. It's a good estimate but it is not exact.

You are right about thinking the actual pressure dropping when swapping to E85. Temperature has the greatest effect whilst the actual increase in mass number due to the higher fuel content i reckon is neglicable.

The source of it all it to what components are actually limiting the total flow (head /mani VE / turbine housing IC / bends) and for that this is a good reference.

[Error404]

I'm measuring backpressure on my car at the momeny in the exh manifold and most the time cruising it's negative however as boost increases and rpm so does back pressure sometimes to the extend of exh pressure equalling intake pressures with no more than 5000rpm and a bar of boost.
Now I can adjust back pressure via cam timing, but I've also decided to upgrade to a 3.5in system first so will pull some ecu logs before and after the exhaust change for comparison purposes. As I still wonder whether the exh manifold and hotside are the ultimate restriction over the exhaust.

This will hopefully be done over the next two weeks with no changes other than the exh and I'll post the results here once done.

Won't the back pressure pre-turbine always be roughly similar to charge pressure, since the exhaust pressure pre-turbine provides the energy to boost the intake charge? i.e. if the turbo and intake / exhaust manifold paths were 100% efficient, would pre-turbine back-pressure exactly equal charge-pressure?