Chris White

Very Fishey....

gt37vgt

so after years of testing you now have a huge turbo with a NOS fogger in the side of it ?

Willard Bridgham 3

All turbochargers require pressure to run or, like Chris, you get infinite spooling time.

Assuming constant turbine radius, small, quick-spool turbos with small a/r ratios require more pressure to run properly than turbos with large a/r ratios that spool more slowly. Constant volume from a positive displacement engine causes higher-velocity through the a (from a/r) and causes higher velocity and higher pressure drop; small turbo. Higher turbo operating pressures reduce engine VE and Hp.

From the turbine outlet, you want the largest pipe possible all the way to atmosphere.

Back-pressure is one of those catch-phrases that make what is fairly simple seem complicated....it's not.

V2Rocket

i believe he means, if the cold side pressure is higher than the hot side pressure, the turbine will not spin the right way due to the higher cold pressure on the compressor wheel side which it is connected to.

Mike Lindsey

We have 8 years studying exhaust pressure and it's effects. It's been interesting. That's why we
built and sell a exhaust pressure kit so the customer can see what the limits of their turbo is. You
can have reasonable or even 1 to 1 exhaust pressure, to a certian boost, then the exhaust
can sky rocket out of control. That's a head gasket killer. We have a 1 to 1 set-up on a 2.5 at
23 psi. Motor revs quicker once under boost, less torque drop off, and it's more reliable.

gt37vgt

so no giving away the recipe but how about just some of the secret sauce... what size exhaust AR?

toddk911

I had thought a small cold side with larger hot side would be a nice mix for quick spool, and more top end, but ST did a lot of testing and found the better result was a more closely matched cold/hot side.

How about a large cold side and smaller hot side????

Chris White

You have it backwards…the smaller the hot side the quicker the spool up – but you will limit the flow potential (top end). In reality you pick the hot side based on your performance desires and then match the cold side to the hot sides abilities.

Willard Bridgham 3

Probably the Engineer in me, but all I have to do to know that turbo input/output is not linear is to look at any turbo map. This stuff is proportional, but to the third or fourth power, ie, small rpm increases cause huge increases in pressure.

Laust Pedersen

It may be of interest to look at numbers I posted here some 6 years ago. The data and graphs can be found in post #21 by “tazman” who managed to post my spreadsheet.
The turbocharger was a standard K26-6 and exhaust system was standard as well except for the muffler. I later discovered that the cat was bad, partially explaining the high backpressure at high flow conditions.

Btw, the -6 and -8 turbocharger labels means that the minimum cross-section areas in turbines are 6 cm2 and 8 cm2 respectively, which corresponds to tubes of 1.09” ID and 1.25” ID, so imagine squeezing your exhaust through that.

Also, it may seem a little counterintuitive that (for lower flows) the backpressure is about the same as the boost pressure, which would imply near 100% efficiency. However the gas temperature has to be taken in to account, so the exhaust gas flows much faster and therefore provides the needed energy to the turbine to provide the boost (and the unavoidable air temperature rise).

Lastly, measuring pressure can be a little tricky, since it for a flowing gas is necessary to distinguish between static and dynamic pressures.
The boost pressures are static pressures when taken from the factory outlets on the intake manifold, which I am sure Porsche took care of during the manifold design.
However the exhaust pressures, when taken from the CO2 test tube, are not calibrated, but my guess is that they are adequate for the purpose used here. At least they are consistent between our cars with the same headers and X-over pipes.

Laust

TurboTommy

toddk911;
you have a LR turbo
What hotside is it? Stage 5 or p-trim?
You should get the backpressure test kit from them. It's like a 100 bucks and a fairly easy install.
You'll see your back pressure vs boost, and then you too will know where you stand; no more guessing how your system performs.
Report back!!

TurboTommy

What do you mean by: "when taken from the CO2 test tube are not calibrated"

It did cross my mind whether or not I'm getting a proper reading, but I would say it's static there, isn't it?

Chris White

Sooo....27 lbs of boost by 2500 rpm....interesting....

Willard Bridgham 3

The pressure measured is static pressure at factory outlets and static pressure + velocity pressure = total pressure. To measure velocity pressure requires a probe facing into the exhaust flow and, unless you have access to high-tech military stuff, a velocity pressure probe will depart quickly in the hot exhaust gasses and trash your turbine.. Given that the diameter of the header/crossover is the same on most of our cars and the displacement is 2.5L, static pressure is a relatively good indicator of total pressure.

Laust Pedersen

I just mean that the CO2 test tube to my knowledge was not designed to measure pressures, but it is also my guess, that it probably does a reasonably good job at measuring static pressures, since the inside hole in the X-over pipe is flush with the wall and therefore reasonably perpendicular to the flow, in spite of the 90 deg bend in front of it.

Yep, but true for a K26-6 with clamped wastegate, standard header and X-over pipe thermally coated inside and outside ... But it took forever to get to that boost level.