ross255

I am looking for any figures that are out there comparing

#6 hotside and #8 hotside at 15psi

measuring the pressure in the crossover pipe. Using the little co2 pipe that comes up at the back of the engine is an ideal take off point.

The maximum ratio should be 1:2 e.g if running 15 psi, the max you want to see in the crossover is 30 psi.
This being the case, for people with say, a K27 are they going over this ratio with a #6 and would be better with a #8.

Also with bigger coldsides (eg k27), the turbo has to spin at lower rpms to get the correct psi. Once up to the right psi, the wastegate opens, which releases pressure. So does this mean you can get away with a #6 hotside provided you have a coldside that is of sufficient size to provide the right psi at low rpms.

GPF

Hi Ross,

Are you sure 1:2 is OK? I thought it should be a lot lower than that?

Having a #6 hotside I thought my backpressure may be too high so I tried to measure it recently.

I don't have the sniffer pipe you you refer to, so I installed a pipe on to my crossover using the test port, ran it into the car and stuck a gauge on it.

I found that the pressure in the crossover is not constant with boost but increases with engine revs/sustained load - almost like there's never going to be a maximum? Not sure if that's the norm.

On a straight drag at WOT in 3rd on the flat with 17 psi boost I got around 20psi at 3000 rpm but got to around 30psi by 6000 rpm. I plan to do some more testing over the next week or so to get some more accurate figures.

Short of going to a #8, any good ways to reduce the backpressure?

Or is 1:2 OK after all?

Regards

Graham

fast951

Thank you GPF, no you are not seeing incorrect backpressure results. Now lower the boost to 14psi and note the results.. Now go to 18psi, and see the backpressure skyrocket..

Ideally you want a 1.2:1 ratio (Exh/boost). You can manage it up to 1.8:1 .. When you hit 2:1 it's out of control, detonation city!! The exhaust gases will flow back into the cyl. head..

You want to test under max boost/rpm/load. This is where things get interesting...

As I always said, the K27/6 produces lots of backpressure. There are few things you can do to reduce the backpressure, but why even bother, it's a bandaid solution.

OriginalSterm

Out of curiousity, what are some of the "bandaids" you can do to reduce back pressure?

fast951

Perhaps you should ask this question to the guy taking the money to build the K27/6??

ross255

GPF, no 2:1 is not OK, it is the max you wan to see, and as fast951 said, even that is not good at all!
It will probably explain a few blown headgaskets using a 26/6 to try and get 18psi.

Your data does show that even with a bigger coldside, so you would be on boost at lower turbo rpms, even with the wastegate slightly open to control boost, you still get to high a backpressure figure of 30 psi with a #6 hotside.

Now if only someone has some data on a #8 hotside for us to compare this to.

ross255

Fast 951
Can you say what the highest ratio you would get on one of your turbos,

say if i run a stage 2 at 15 psi at around 6k rms

OriginalSterm

John-
I guess I was asking a more general question, how can one reduce back pressure with the turbo one is currently using?

fast951

The Stage 3 flows much more than Stage 2, however the hot sides are "fairly" identical. On a Stage 3, running 24psi (much higher than the 15psi you asked about). ST reported 30psi backpressure as the highest he recorded. 30/24 = 1.25:1 E/B.

The stage 2 has a smaller compressor, so I expect the same the results.

Interesting, someone decided to modify stage 3 turbo. They used the "same" wheels, and housings. But did not catch a very important detail (that's where the VR custom work comes in). They were no longer able to make the same power as they already made with a true VR Stage 3 turbo, and they experienced knock. You guessed it, backpressure went up

Let's see now who's going to be testing backpressure and advertising it as a selling pointe? LOL

Markus951

How do you make a system to measure backpressure? Isn't the testpipe on crossover pipe getting hot and will melt the hose attached to it? What materials have been used there, specially the hoseconnection between gauge and test pipe on the crossover?

Markus

ross255

Markus,
on the cross over there is a test port, used in the US to measure emmisions. You would have to make some kind of metal pipe, as you probably dont have one already.

Laust Pedersen

The 2-3 ft test tube is stainless steel, which is capped under normal conditions and therefore does not get hot, since there is no exhaust flow.

Blowing the cap however, as I once did on the track, will make the tube hot enough to burn through a heater hose, if it leans against it.

A pressure measurement does not require flow. See top left corner of the picture.

Driftomagnifico

You can't compare manifold backpressure between two un-identical cars. The backpressure produced by the exhaust will not produce a linear change in the backpressure produced by the turbine!

Backpressure affects turbine efficiency which directly affects the backpressure produced in the manifold due to a reduction in mass flow. This is because backpressure reduces the pressure ratio at the turbine.

I can't believe no one has even mentioned turbine efficiency in this thread, it is the sole reason for the K27 compressor affecting backpressure using the same K26 model turbine. The turbine has an efficiency curve, and generally is designed to work in correlation to the same shaft RPM as the compressor wheel. When you put on a bigger compressor wheel you can run the turbine efficiency off it's chart creating excessive lag, and excessive backpressure, ala GT2540R.

GPF

Hi Markus,

I went to a hydraulic/air fittings place with the blanking nut that caps the test port off and explained what I wanted.

They sold me some very high temperature braided hose and swaged a connection on that has the same internal threads as the blanking nut.

I got them to swage a connection on the other end so I can couple a hose to run it to a gauge when I want to use it, and ran it up into the engine bay to the rear of the exhaust manifold.

It looks like aeroquip but obviously isn't, and the inner material is made out of some very high temperature polymer - sorry, can't remember which. Since installing it I have just left it attached and it's been fine.

Regards

Graham

pete95zhn

Just came back home,I installed a boost gauge with a hose to my crossover test pipe and made some test accelerations.

Result: Boost 1,2 bar, pressure at x-over 1,8 bar at 6000rpm. Ratio 1:1,5

Turbo: KKK27/6 hybrid,compressor housing K27/3LEP with 52x76mm Holset wheel ,turbine housing original /6 with KKK 62,5mm wheel modified to 60x66mm ,A/R 0,6 .
Exhaust: Full 3",including downpipe.

EDIT: Same results from dyno runs,now both logged and printed.