# Do superchargers like back-pressure?



## indignation (Mar 5, 2003)

I am going to supercharge me 2.0L some day so I want to make sure I get an exhaust system and intake that will work best. Should I go the most high flow system I can find, or is a little back pressure needed. What system should I get? Thanks


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## Scott F. Williams (Oct 14, 2000)

*Re: Do superchargers like back-pressure? (indignation)*


_Quote, originally posted by *indignation* »_I am going to supercharge me 2.0L some day so I want to make sure I get an exhaust system and intake that will work best. Should I go the most high flow system I can find, or is a little back pressure needed. What system should I get? Thanks

It is a common myth that certain engines "like" or need any amount of back pressure at all. However, all engines need max flow/ exhaust gas velocity across a varying rpm range. The trick is in getting the benefits of free flow while maximizing scavenging and minimizing back pressure throughout this range.


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## indignation (Mar 5, 2003)

*Re: Do superchargers like back-pressure? (Scott F. Williams)*

what do you mean by scavenging?


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## Scott F. Williams (Oct 14, 2000)

*Re: Do superchargers like back-pressure? (indignation)*


_Quote, originally posted by *indignation* »_what do you mean by scavenging?

Simply put, scavenging is the extraction of exhaust gasses from the engine's cylinders. Generally though, the term is used when discussing tuning the extraction to provide to vent exhaust gases more efficiently. That is, more efficient exhaust scavenging translates to faster revs and increased torque and horsepower in a particular rpm range.
Read this page and be forever more enlightened. You'll be a hit at dinner parties. But... I warn you, exhaust tuning is so misunderstood that you'll inevitably cause arguments. The backpressure arguement is a classic example.






















http://www.accesscom.com/~knliao/exh_theory.html


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## mrkrad (Nov 9, 2000)

*Re: Do superchargers like back-pressure? (Scott F. Williams)*

dont worry about all that mumbo jumbo is you have more than 2 valves per cylinder








lol


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## indignation (Mar 5, 2003)

*Re: Do superchargers like back-pressure? (mrkrad)*

anyone else care to input


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## JettaRed (Aug 14, 2000)

*Re: Do superchargers like back-pressure? (indignation)*


_Quote, originally posted by *indignation* »_anyone else care to input

I had a Magnaflow on my supercharged 2.0. Magnaflow is essentially 2.25" straight thru piping. (You can see daylight through it.) I lost a lot of low-end performance and had minimal high end performance gains--not enough to make it worth it.
Regardless of clinical calculations, my butt dyno said it was worse. I put a stock system back on (GLI muffler with TDI mid-pipe) and regained my performance. So, argue all you want about any back-pressure being bad, I'll take a little.
BTW, the Magnaflow cat-back had the poorest evaluation during some magazine's (European Car?) taste test a while back. That was with a 1.8T, though.


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## mrkrad (Nov 9, 2000)

*Re: Do superchargers like back-pressure? (JettaRed)*

Thats interesting, because wouldn't a supercharger making a static amount of FLOW given a specific rpm? So would it really care?
If it sees 500rpm its going to make XX CFM
I don't think they like or dislike backpressure.
The output of your final horsepower from the motor may change however.



_Modified by mrkrad at 8:45 AM 5-3-2003_


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## Scott F. Williams (Oct 14, 2000)

*Re: Do superchargers like back-pressure? (JettaRed)*

Some folks tend to miss the point that backpressure inevitably occurs when an exhaust is tuned for maximum gas velocity. Maintaining or increasing the gas velocity means restricting the tube size and, in turn, that increases the back pressure. However... the backpresusre itself is always a hindrence to making power at any engine load and rpm. In other words, if one could maximize gas flow, gas velocity, and minimize backpressure at the same time, optimal levels of power would be made at any engine load and rpm.


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## Daemon42 (Feb 9, 2001)

*Re: Do superchargers like back-pressure? (mrkrad)*


_Quote, originally posted by *mrkrad* »_Thats interesting, because wouldn't a supercharger making a static amount of FLOW given a specific rpm? So would it really care?
If it sees 500rpm its going to make XX CFM


Not true of a Centrifugal S/C btw.. A positive displacement supercharger moves a measured amount
of air with each revolution and the power limiting factors are how much heat it generates from compression
and how much air it can flow at high rpms. Centrifugal superchargers flow exponentially more
air with increasing rpms, but they also have an open path from intake to outlet through the
compressor vanes and if the back pressure in the *intake* is high enough the total flow will stop, 
such as when the throttle plate closes. If the exhaust is a restriction the engine will still flow 
less air even with the blower. The boost pressure may be very high in this case, but boost != total flow. 


_Quote »_
I don't think they like or dislike backpressure.


As stated already the backpressure is a side effect of a different goal, which is
to increase the exhaust gas velocity in the exhaust manifold with moderate diameter piping. 
If there's a high velocity exhaust flow, then at the end of the exhaust stroke a slight
vacuum is created in the exhaust manifold that sucks the last of the exhaust
gases out of the cylinder, so it can get more clean air in there on the next
intake stroke. This makes for good low end torque. At high revs you want a free flowing
exhaust because peak horsepower really is about how much total air you can move
through the engine. This is going to hold true regardless of whether the engine
is blown or not. It's always a balance between low end torque and high end power.
Dunno why more exhaust systems don't have flapper valves to change the flow
characteristics (look up Yamaha's EXUP valve). 
On a side note.. Turbos like massive exhausts because they're all about creating
a high pressure differential across the turbo compressor. High pressure on 
the manifold side, low pressure on the exhaust side creates the fastest spool
up times. Getting the turbo to spool quickly at low rpms is more useful than
the small percentage gain in torque to be had from maximizing scavenging.
And of course at high rpms the big exhaust simply flows more air and that's
good for a turbo too. 
ian


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