Exhaust size vs. power potential

[EspenW]

Good info guys!
Will dig up some parts and rig a backpressure tester shortly.
Nice to have, not only for tuning purposes, but also as a diagnostics tool for checking for clogged cats, etc.

I see now that most brakelines available over here are metric, typically 10x1 thread, and the smaller hose barbs for 6mm ID tubing have 1/4 thread, but those are typically brass, so very simple to drill and chase some 10x1 thread.
The OXS bung plugs are often stainless steel, and that can sometimes be a PITA to drill/thread, but I see that one made of mild steel is available on Ebay for £2.50 shipped from China, so will get one of those if the stainless one that I have is tricky to modify.

[Jamo]

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?

have a read of http://www.s2forum.com/forum/showthr...?t=5297&page=7

See the comments from Repo and Mihnea on their target back pressure figures of either 2:1 or 1.5:1 backpressure:manfold boost

[EspenW]

Yep, I found that classic thread while searching yesterday too. Good read.

Important to differentiate betwen backpressure in manifold and backpressure post turbo.
The 29psi that Dmitri measured on the car with 2.5'' exhaust was post turbo, and that is a massive amount.

[kinaputt]

This numbers must be different for differendt engines, compair a 2L turbo engine running 3BAR of boost and a 5L V8 running 0.5BAR of boost, example 600hp on both.
The V8 will have less backpressure with that boost than the small 2L running high boost even if it has more exhaust pressure than the 2L without any boost.
Or am i wrong? This is just what i am thinking in my head

[driftin jo]

I managed to talk to someone very knowledgeable last night that I have not spoken to for a while, he worked at Garrett when and I worked at earlson, which was next door, we made all of the bearings for them at the time.

I mentioned a few things that were posted and the first thing he said was that JP was doing really well if he had near 1/1 ratio of ex manifold pressure / inlet boost pressure and if on full chat you get more boost than exhaust man pressure you have broken in to the black art arena that 1500cc 1200hp F1 turbo cars of old did. He said they ran 5bar boost and had 3 bar exhaust manifold pressure.
This is how you make huge power as the cylinders are purged of hot gas so you can run much more timing. That is one of the limiting factors with a turbo the fact that some of the hot 900deg c gas is left as the piston comes up to TDC on the exhaust stroke, this is then heating your next charge up a lot and limiting the amount of timing you can run.

He said that stock OEM set-ups can be 2.5-3/1. He echoed what Jay Kavanaugh had said and that if you lower the pressure post turbine the hole thing works better, even a small drop post turbine makes a big difference to pre turbine pressure and thus efficiency.
I mentioned Corgy bells graph and he chuckled and said it was probably not a good ref for modern turbos with generally little lag compared to old school turbos.

As we already know a medium turbo running 30 psi boost may make say 500 HP but a big as turbo with a big A/R may make 700hp at 30 psi boost.

He commented on this and said as the big as laggy turbo has less overall restriction it actually does not need as big and exhaust to make 700hp as a modern GT for example. The reason is because the pre turbine pressure on the GT can be high and if you don't give it a large exhaust to lower this you will loose a lot of potential.
The affect of going to a bigger exhaust on the same power output with a much bigger turbo will make much less difference in HP as the turbine is now much less of a bottle neck.
If you then take this to Corky's graph it all makes sense, i.e. you CAN make 700hp on a 3 inch pipe but this will be with a big turbo where as with a smaller turbo that can make say 600 HP you would need to go bigger than 3” to make it happen.

Closing point really is, with a modern turbo, especially a street car, go as big as you can post turbine.


One other thing he mentioned is that a lot of people who measure back pressure don't get the right reading as the the way they have placed the pick up for the gauge is wrong and that the pressure is much higher than what they see on there gauge. That's another can of worms !

[Sonic]

Interesting reading chaps... and definitely explains a lot after changing to 3" rather than 2.5" on my setup!!

[andy10v]

One other thing he mentioned is that a lot of people who measure back pressure don't get the right reading as the the way they have placed the pick up for the gauge is wrong and that the pressure is much higher than what they see on there gauge. That's another can of worms !

How do you measure it properly?

[driftin jo]

How do you measure it properly?

I'm not sure, I think the problem is likely to do with placement location and Bernoulli's principle.

[andy10v]

I'm not sure, I think the problem is likely to do with placement location and Bernoulli's principle.

So possibly a high temp pressure sensor mounted directly to the exhaust, rather than a boost gauge on a long line?

[driftin jo]

So possibly a high temp pressure sensor mounted directly to the exhaust, rather than a boost gauge on a long line?

Probably yes.

you can get pressure transducers that monitor combustion chamber pressures, F1 cars etc use them, they are the ultimate tool for tuning as you can for example predict knock before it happens etc etc

[my2000apb]

great thread!

[EspenW]

I'm not sure, I think the problem is likely to do with placement location and Bernoulli's principle.

I see that point. Since we are measuring pressure in a dynamic environment,
the speed of the gases will create a pressure drop (port vacuum) depending on the direction of the opening of the port. However, if the port faces into the flow, we will have a ram effect and measure higher than real pressures.

The same applies for boost measurements, though, so not sure how much it affects readings.

[driftin jo]

My conclusions:

after my conversation that I wrote about and reading the Jay Kavanaugh stuff again,even though I have read it a least twice before already, my conclusions are.

1: Read it again

2: Especially this bit as it makes you realise that you need to measure exhaust manifold pressure
and the exhaust pressure after the turbo to calculate the total back pressure that the engine is seeing!

Quote from Jay Kavanaugh :

"Here's a worked example (simplified) of how larger exhausts help turbo cars:

Say you have a turbo operating at a turbine pressure ratio (aka expansion ratio) of 1.8:1. You have a small turboback exhaust that contributes, say, 10 psig backpressure at the turbine discharge at redline. The total backpressure seen by the engine (upstream of the turbine) in this case is:

(14.5 +10)*1.8 = 44.1 psia = 29.6 psig total backpressure

So here, the turbine contributed 19.6 psig of backpressure to the total.

Now you slap on a proper low-backpressure, big turboback exhaust. Same turbo, same boost, etc. You measure 3 psig backpressure at the turbine discharge. In this case the engine sees just 17 psig total backpressure! And the turbine's contribution to the total backpressure is reduced to 14 psig (note: this is 5.6 psig lower than its contribution in the "small turboback" case).

So in the end, the engine saw a reduction in backpressure of 12.6 psig when you swapped turbobacks in this example. This reduction in backpressure is where all the engine's VE gains come from.

This is why larger exhausts make such big gains on nearly all stock turbo cars-- the turbine compounds the downstream backpressure via its expansion ratio. This is also why bigger turbos make more power at a given boost level-- they improve engine VE by operating at lower turbine expansion ratios for a given boost level.

As you can see, the backpressure penalty of running a too-small exhaust (like 2.5" for 350 hp) will vary depending on the match. At a given power level, a smaller turbo will generally be operating at a higher turbine pressure ratio and so will actually make the engine more sensitive to the backpressure downstream of the turbine than a larger turbine/turbo would.

3: I think that anyone interested should read it again, all of it.

http://www.theturboforums.com/thread...-Jay-Kavanaugh