My Porsche 911 Forum
You are not logged in. Please Login.

Posted: Jun 16, 2009 9:39:00 pm

so what is back pressure and if we need some or not? if so how much ?

answers on a post card please? :happy07:

Posted: Jun 17, 2009 6:16:35 am

That's an awesome question.  I havent seen anyone ask that for ages.  This is a very complex issue but you can talk about it in general terms.

So here goes:

This is less of an issue these days as modern DME's can compensate for back pressure.

You have to start with the optimal air fuel ratio that an internal combustion engine is seeking to run at.  This is called stoichiometric combustion and is combustion at an air/fuel ratio of approximately 14.1:1.  i.e., 14.1 parts air to every 1 part fuel (measured by mass).  If a car is running lean the ratio would be higher say 15:1 or 16:1 etc.  If its running rich then the fuel ratio might be 12:1 or 13:1 for example.

Now, valve burning occurs as a result of a very lean-burning engine.  The reason why the engine is burning lean to begin with is that the reduction in exhaust backpressure is causing more air to be drawn into the combustion chamber than before. Earlier cars with carbs often could not adjust because of the way that backpressure caused air to flow backwards through the carburetor after the air already got loaded down with fuel, and caused the air to receive a second load of fuel. Once these vehicles received performance mods that reduced backpressure, they no longer had that double-loading effect, and then tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if backpressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.

Modern 911's don't have to worry about these effects, because modern engine management will measure that the engine is burning leaner than before, and will adjust fuel injection to compensate. In effect therefore, reducing backpressure really does two positive things:

(1) The engine can use work otherwise spent pushing exhaust gas out the tailpipe to go faster: and
(2)  the engine breathes better making for a more efficient engine.

Of course, the the engine managements ability to adjust fuel injection is limited by the physical parameters of the injection system  but with exhaust backpressure reduction, these limits won't be reached.

With cars that dont have lamda sensors and doent measure airfuel ratio, then the airfule ratio will be mapped into the engine management system,  This will be done for a stock intack, fuel injection, exhaust configuration.  Ideally therefore, with a ne exhaust so should the engine be mapped to compenstate.

Posted: Jun 18, 2009 9:51:32 pm

so with that in mind , i have standerd headers a dansk straight through pre silencer and a dansk spot back box,
what do you recon pete, i am gona get it re mapped when i get the chance.
thought it was a interesting question.

Posted: Jun 22, 2009 1:21:26 pm

Its a very interesting question.  Alot of people dont realise how important the exhaust can be to the overall set up.  Dansk are pretty good IMHO and you wil see some power gains and even better gains in sound.  A map to optimise is always the best, but that costs a few quid. 

If you look around can you find a chip that is aleady optimised for an exhaust and cam package and will save money against a live ecu remap.

MyPorsche911  · · Forums
Powered by myUPB v2.2.6  ·  © PHP Outburst 2002 - 2021
Creative Commons License