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Turbo; JH or PL?



Larry sez: "An engine doesn't "work" to drive a turbo, you should know that.  At least not in the sense that an engine "works" to drive a supercharger.  A turbo is driven by waste energy."
True to some degree and quite a common conception.
On an NA engine most of the exhaust pressure in the cylinder is released very shortly after the exhaust valve opens. The exhaust stroke consists mostly of pushing the remaining fumes out against the exhaust system back pressure, on the order or a psi on a decent exhaust system.
On a typical turbo engine the exhaust pressure required to drive the turbo is roughly 2 times the boost pressure. If you're running 15 psi boost then the piston is pushing roughly against a 30 psi backpressure on the exhaust stroke to force the exhaust past the turbo. A nice 250 lb force the piston needs to push against! The net effect is the engine "working" against this backpressure and loosing power as a result. It is also a substantial heat input to the piston.
This is one reason why a bigger turbo will make more power. The turbine housing is bigger, represents less of a restriction and the engine doesn't have to work as hard forcing the exhaust past the turbo. Course you also get slower turbo response.
So, while a turbo is driven by "waste energy" it isn't simply free. The engine does additional work to drive the turbo and parts of it do run hotter, critical parts like the piston and exhaust valve in particular that are surfaces forming the combustion chamber. The fact of the additional heat means the engine is more likely to detonate.

Larry sez: "EVERY engine works against multiple restrictions.  Those restrictions are what prevents every engine from developing unlimited/infinite power.  Every engine produces heat."
Again, generally true. OTOH, forcing 30 lbs/min (think about that for a minute. 30 pounds of air when a cubic foot weighs .08 pounds!) through a hole designed for optimum flow at 15 lb/min takes a lot of work. The engine needs to work more to force air through restricted passages. The more boost, the more work the engine does driving the turbo to force the air through passages not designed for that amount of flow.
Again, in particular the exhaust system needs to be modified to handle the flow generated. It's not only a turbo response issue. For a given boost pressure the turbine will need a certain amount of pressure drop across it. Increased exhaust system pressure relates directly to increased pressure that the piston needs to force out past the turbine.
It's worth pointing out that none of these restrictions are a linear relationship. Doubling the flow results in the back pressure more than doubling. This is true of the exhaust system, flow past the turbine and flow through the inlet ducting and ports.

Larry sez: "
First you rail against higher boost levels, then you claim that the reason for NA modifications is to allow those same higher levels of boost.   Make up yourmind."

Oh, I have nothing against higher boost. I'm simply trying to point out that increasing boost has less and less returns the further you go i.e there are limits to this theory. If you're at 5 psi, then by God, you're right! Almost any engine can take more boost than that and it is indeed an easy way to more power.
If you're at 15 psi, there's a good chance that trying to increase the boost will deliver no more boost,  no more power or will cause detonation or damage to the engine, UNLESS you modify it to accommodate the additional heat loads and reduce the airflow restrictions and therefore the work the engine does driving the turbo.

Any engine needs to be designed as a system. All the parts need to be properly sized to work with each other. No sense putting a 1" exhaust on a 2.0L 16V engine.
And the same thing applies to a turbo engine. Just as the turbo needs to be sized so that the compressor is operating in an efficient range, the rest of the engines parts need to be sized to carry the additional flow generated by the turbo.
Do you need to do it? Nope. You can just turn up the boost until the engine can't make any more boost, makes no more power or blows up from the extreme heat load from trying.
Or you can start modifying the engine, reduce it's work load by properly sizing the ports, the IC, the intake ducting, the exhaust system and installing a free flowing exhaust manifold or a cam designed to work with a turbo. Reducing all the restrictions reduces the work required to spin the turbo for a given boost. Reducing the work load to drive the turbo reduces the heat load to critical parts and the likelihood of detonation and lets you run additional boost should you desire to "just turn up the boost". 

And in closing, one little example about how "just turn up the boost" doesn't work.
1979 Scirocco, IHI turbo, no cat, otherwise stock exhaust system.  The engine would not make more than 5 psi boost!  You could do whatever you wanted with the wastegate and 5 psi was IT! Replacing the exhaust system with a bigger diameter pipe and free flowing muffler allowed the turbo to produce more boost.
Personal experience on my 79 Rocco that would eat GTI's for breakfast way back in 1983! ;^)

I guess if your theory that the turbo was driven only by waste energy was correct then none of this would be true. Unfortunately, the engine does have to work to drive the turbo. There is no free lunch.
Dan


  From: LEF 
  To: Dan Bubb ; David Utley ; scirocco-l@scirocco.org 
  Sent: Saturday, January 06, 2007 10:32 PM
  Subject: Re: Turbo; JH or PL?



    Improving the exhaust is a particular area where modification can improve a 
    turbo's performance.
    A turbo is driven by not only heat input but pressure differential across 
    its turbine. Reducing exhaust back pressure improves turbo response and 
    compressor outlet pressure with no additional work done by the engine to 
    drive the turbo.

    An engine doesn't "work" to drive a turbo, you should know that.  At least not in the sense that an engine "works" to drive a supercharger.  A turbo is driven by waste energy.

    the maximum boost 
    the engine can run is limited due to all the heat created by the engine and 
    turbo working against multiple restrictions.

    EVERY engine works against multiple restrictions.  Those restrictions are what prevents every engine from developing unlimited/infinite power.  Every engine produces heat.  

    Just turn up the boost until it fails then declare that this crappy VW short 
    block can only sustain 200 Hp!
    Yep, you can crank up the boost to make more power. But only to a certain 
    level.
    The standard performance improvements for an NA engine apply to an FI engine 
    in that they not only improve performance but also allow the engine to 
    operate at a higher boost level due to reduced internal stresses and 
    internal heat.

    First you rail against higher boost levels, then you claim that the reason for NA modifications is to allow those same higher levels of boost.   Make up yourmind.

    You can get all the boost you can possibly want..and more...with no NA modifications.  You can get so much that the engine is hurt.  That's my whole point.    I believe you can agree with that if you'll let yourself.

    larry
    sandiego16v