TonyG

Where are you measuring the pressure?

PorscheLars

4-2-1 where 2 goes into one. just before WG pipe. 90 degre to the flow.

Tuned it today wit 1.0 (14.5PSI) and it realy pull hard. I dialed it back to 11 and its feel that this pressure gives the same power as before 3" exhaust and maf etc...

TurboTommy

If I'm reading PorscheLars correctly, his ratio is around 1 with full boost in the mid 2k RPMs. So, I could see that my ratio is probably in the ballpark with my free flow exhaust and full boost around 3.6k. I can't see that my porsche shop are complete idiots.
Valve overlap is also an issue with good positive ratios; too much and raw fuel blows right through into the exhaust.

Jeff Lamb

PorscheLars, thanks for sharing your data. I think that measuring backpressure in the exhaust crossover pipe is very useful in the area of understanding whether or not the turbo is properly sized for the application. It sounds like you are seeing roughly a 1:1 ratio for boost pressure to exhaust back pressure at a certain boost pressure and rpm.

I plan to also measure the pressure in my exhaust between the turbo and the cylinder head. Along these lines, I am wondering if you could describe the exact type of pressure sender you are using and exactly how it is installed? I know that you mentioned you are taking your pressure reading at the point where all exhaust pipes merge into 1 (and before the wastegate), however, I am wondering if you have the pressure sender basically screwed right into the crossover pipe? If so, how does it resist the high heat without failing. If the sender is not directly screwed into the main exhaust crossover pipe, can you give us some more details on how it is installed?

Thanks,
Jeff

dand86951

Jeff, the easiest way is to hook a pressure gauge up to the CO2 sniffer tube. It is far enough away that the heat shouldn't cause a problem. You may want to use a silicon 1/4 inch hose with adapters to a gauge. Other wise you would have to weld a fitting into the crossover. If you want to watch it while driving then get the hose long enough to bring into the car.

TonyG

dand86951

The entire cross over pipe glows red hot (as well as the headers, and the turbine section). We're talking well over 1400F

To measure at this location, you have to use copper piping for a foot or so, then you can go to silicon hose.

Jeff Lamb

I agree with what Tony has written. The crossover pipe gets very hot and would destroy most pressure tranducers if they were screwed directly into the pipe. This is why I was wondering exactly which pressure transducer PorscheLars is using and exactly how he has hooked it to the exhaust.

Tony, do you have a pressure transducer hooked up to your crossover pipe? If so, which one are you using?

Thanks,
Jeff

Jeff Lamb

I just re-read Tony's reply and it occurred to me that he is probably talking about not using a pressure transducer at all. It sounds like he is talking about just running from the exhaust to a short copper line to a silicone line that runs into the cabin of the car. This silicone line would then be hooked up to a standard mechanical boost gauge.

Tony, is this the setup you are describing? If so, what is the size of the pressure drop due to the dramatically reduced heat of the exhaust and long line length between the exhaust pressure source and the gauge?

I was thinking about using a pressure transducer to drive an electrical gauge so that I could also datalog the exhaust pressure readings. This is why I was asking about the pressure transducer type configuration.

Jeff

Ethan_76

Jeff,

I believe many people use the method Tony described to measure backpressure and the reduced temperature at the sampling point shouldn't influence pressure really. If anything, it'll add a slight amount of latency to the reading, but the cooler temps shouldn't really cause a "vacuum". You typically see your highest pressure towards the upper RPM band, when the engine is under the highest amount of load and if you measure it in a higher gear, the latency is negligible for all practical purposes.

Regards,

Ethan

dand86951

As I was saying just hook the silicone hose to the CO2 sensor which is already over a foot away from the crossover. Then with six feet of silicone hose the temps will work out ok for a manual pressure gauge or if you want a high quality transducer would work. I know the silicone hose will stand it as my CO2 pipe broke at the mid point several years ago and I just spliced it with silicone hose for a temporary fix, and it has not shown any sign of deterioration. The exhaust gases do get hot, probably closer to 1600F at full throttle, but physics says they want to keep going the direction they are and so they don't really want to shoot up the sampling tube and fry everything in sight. If you look at the factory setup, there is a rubber(probably silicone) cap on the sampling hose and mine is original after 17 years.