Konstantin

Hello
anyone here has messured the Backpressure before teh Turbo and after the Turbo?
I want to test it tomoorw and I want to know what is teh stock value.
I still have teh stock kat converter and a 3 L 8V engien so It would be interesting to compare

Konstantin

fast951

Konstantin, you are not looking for a fixed value, what you want to know is the ratio between intake pressure (boost) and exhaust pressure (back pressure). Both values are a moving target, but the ratio should stay withing a range.

NZ951

I tested the pressure after the turbo, there is a thread somewhere...

nine-44

I think I read it was supposed to be no more than 2-3 times the boost pressure, closer to 2 times preferably. That is measured before the turbo. You should be fine to hook up to the CO test pipe that is stock with silicone hose to a regular pressure guage. Good luck

NZ951

Andy, try closer to 1 preferably, and no more than 1.5:1

mark944turbo

"no more than 1.5:1"

Duh you want as much intake pressure as possible and as little exhaust, but where is this number coming from? It depends totally on the engine and turbo when you will stop making power and start running into trouble. Throwing numbers around is useless until you have real data.

NZ951

Where are the numbers coming from? How about tuners like JME and turbo specialist Vitesse. Happy with those sources?

special tool

NZ - that kid works for JME

Konstantin

I mean at stock pressure 0,85 bar boost how much is the exhaust pressure before and after the Turbo

Konstantin

mark944turbo

Konstantin, the only real numbers I have seen for backpressure are in an old post by Laust Pederson. He measured them by videotaping guages, the exhaust one he had hooked up to the emissions testing port in the downpipe. But his car was certainly not stock, he saw up to 60psi in some cases, with the stock turbo. If you find any stock data, let us know. It would probably vary a lot due to the wearing out of the wastegate springs, but I would be interested to see it.

NZ951

Then I just he feels stupid about asking now

JM and I were talking about cams a while back, and it came up in discussion... hence how I know JM's thoughts on it.

mark944turbo

NZ, nobody, no tuner, is capable of telling you your exact ratio. Yes they can tell you what ranges work on engines they have seen, but it remains useless info unless you state what engine and turbo you are talking about. The reason I am telling you this, even though you probably dont think you are disagreeing with me, is because the 1.5 - 1 ratio you posted is unrealistic for most people in this forum (at least those that want as fast a streetcar as possible).

The engines that JME builds are 99% for the track, where you dont care so much about spoolup, peak backpressure to intake pressure ratio will be much lower when a turbo is selected for this application. The turbo is selected to be most efficient at the top of the rev range, but our cars must make a compromise of all rev ranges for street performance.

A good street turbo that is properly selected will have its best ratio somewhere earlier, then it will decay as revs get higher, up to 2:1 and beyond by redline. It will continue to make more power with a ratio over 1.5 to one. Its not a bad thing. Do you see what I mean?

fast951

Once you go over 2:1 it's out of control and knock becomes an issue..

pstickne

What do the relative backpressures have to do with knock?
Isn't knock directly related to predetonation?
I mean, what uncontrollable correlation is there between the ratio and combustion compression, temperature and the volatility of the gas being used?

fast951

When backpressure is high that exhaust gases don't exit or even go back into the combustion chamber (during overlap period). You have very hot gases in the combustion chamber which causes predetonation.. Similar to running high boost without an intercooler...