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I'm mostly interested in taking the 3.4 and getting the most out of it. I love how quick it revs. The 3.8, with or without the X51 option just doesn't rev as quickly.
Hmm. You mean the 3.4 seems like it pulls towards redline better, or do you mean it revs more quickly in neutral? The 3.8 takes the 3.4 and bores it, keeping the crank, intake, etc. If it feels less responsive it may be down to software? I'd be tempted to try a GT4 if I were you- slightly lighter flywheel and perhaps different software might address your issue? With all e-throttle engines there is a lot going on between your right foot and the throttle plate...
Originally Posted by 997s07
If I remember from my one and only fluid dynamics class, the pressure drop from those larger diameter runners (?) would cause some nice air intake velocities. Wouldn't that help with faster filling of the cylinders at any RPM? How does that diminish low end torque?
Intake manifolds don't have steadily moving air. Instead the air starts and stops abruptly as the intake valves open and shut. This seems a bad thing for flow, but designers have learned to tune the intake to resonate at these frequencies. Ideally air pressure waves hit in time with valve openings to cram air into the cylinder and then close the valves before it escapes. Done correctly this can achieve a volumetric efficiency of over 100%, meaning the air pressure inside the just closed cylinder is higher than ambient. Simple fluid dynamics theory would suggest this is impossible, that you'll always get some pressure drop, but at specific RPMs this is not the case.
Air velocity is required for this resonance to work optimally, as you're effective using the air's momentum to pack the cylinder. Thus intake designers worry about both port flow and air velocity, and intake runners and ports can easily get too big.
Look at a GT3RSR's intake ports and you'll find them huge compared to a street or Cup GT3. At near 10,000 rpm the RSR's intake valve is open for a much shorter duration, requiring much more flow to fill the cylinder. Thus despite the larger runners the air achieves similar velocity to the street GT3's at 7k rpm. Unfortunately this means that at low RPM the RSR engine is always going to be limited, despite heads and runners that flow extremely well.
Hmm. You mean the 3.4 seems like it pulls towards redline better, or do you mean it revs more quickly in neutral? The 3.8 takes the 3.4 and bores it, keeping the crank, intake, etc. If it feels less responsive it may be down to software? I'd be tempted to try a GT4 if I were you- slightly lighter flywheel and perhaps different software might address your issue? With all e-throttle engines there is a lot going on between your right foot and the throttle plate...
Yes the 3.4 pulls to redline quicker. In neutral the 3.8 and 3.4 are not different in how quick they reach higher revs. I haven't had a chance to try a GT4 yet. But I will see if I can make that happen.
I'm sure piston weight makes a difference in revving but the difference between the 3.8 and 3.4 piston shouldn't be that big.
Originally Posted by Petevb
Intake manifolds don't have steadily moving air. Instead the air starts and stops abruptly as the intake valves open and shut. This seems a bad thing for flow, but designers have learned to tune the intake to resonate at these frequencies. Ideally air pressure waves hit in time with valve openings to cram air into the cylinder and then close the valves before it escapes. Done correctly this can achieve a volumetric efficiency of over 100%, meaning the air pressure inside the just closed cylinder is higher than ambient. Simple fluid dynamics theory would suggest this is impossible, that you'll always get some pressure drop, but at specific RPMs this is not the case.
Air velocity is required for this resonance to work optimally, as you're effective using the air's momentum to pack the cylinder. Thus intake designers worry about both port flow and air velocity, and intake runners and ports can easily get too big.
Look at a GT3RSR's intake ports and you'll find them huge compared to a street or Cup GT3. At near 10,000 rpm the RSR's intake valve is open for a much shorter duration, requiring much more flow to fill the cylinder. Thus despite the larger runners the air achieves similar velocity to the street GT3's at 7k rpm. Unfortunately this means that at low RPM the RSR engine is always going to be limited, despite heads and runners that flow extremely well.
This is intriguing. So the intakes are optimized for one specific RPM (RPM range?)? It'll be very difficult from what I recall of FD to have different resonance frequencies within the same system. Am I correct in thinking this?
What about the headers? I'm guessing they should follow the intake resonance frequencies.
I think you're probably sensing the shape of the power curve. A 3.8 and particularly an X51 is going to accelerate harder and hence pull through a gear faster. However we're very sensitive to changes in acceleration, and hence a car that makes more torque as revs climb can feel quicker because it's accelerating harder the closer it gets to redline...
Originally Posted by 997s07
This is intriguing. So the intakes are optimized for one specific RPM (RPM range?)? It'll be very difficult from what I recall of FD to have different resonance frequencies within the same system. Am I correct in thinking this?
What about the headers? I'm guessing they should follow the intake resonance frequencies.
A simple tube will have multiple spaced frequencies it resonates at, but in practice there will be one range that's the best. However modern Porsche induction systems are far from simple tubes. They include resonant chambers with flaps that can be reset to create multiple resonant frequencies. The lumps and bumps in the torque curve are largely the result of switching from one resonance to another.
The most extreme version of this may be BMW's continuously variable length intake manifold. Yes exhaust has its own resonant frequencies and works in similar ways, hence the effectiveness of "equal length headers", etc. More on the the subject here: http://www.autozine.org/technical_sc...e_exhaust.html
This is complex and very carefully tuned stuff, hence bigger is certainly not always better.
I think you're probably sensing the shape of the power curve. A 3.8 and particularly an X51 is going to accelerate harder and hence pull through a gear faster. However we're very sensitive to changes in acceleration, and hence a car that makes more torque as revs climb can feel quicker because it's accelerating harder the closer it gets to redline...
A simple tube will have multiple spaced frequencies it resonates at, but in practice there will be one range that's the best. However modern Porsche induction systems are far from simple tubes. They include resonant chambers with flaps that can be reset to create multiple resonant frequencies. The lumps and bumps in the torque curve are largely the result of switching from one resonance to another.
The most extreme version of this may be BMW's continuously variable length intake manifold. Yes exhaust has its own resonant frequencies and works in similar ways, hence the effectiveness of "equal length headers", etc. More on the the subject here: http://www.autozine.org/technical_sc...e_exhaust.html
This is complex and very carefully tuned stuff, hence bigger is certainly not always better.
to early to discuss 991R.
who is AP? i do not know why his words are particularly important.
i trust no one. i dont even trust myself.
when it shows up, it shows up.
But you need to get on the list now, unless you got a 918. Or a Purple RS to trade.
Originally Posted by mooty
^ larry is way funnier than i.
and he is a better drinker as well.
We're all better drinkers.
Originally Posted by 997rs4.0
Will be interesting to see what this 991R turns out to be. My first impression was that it would be similar to what the cayman R was to the cayman.
Then I thought, with PDK only for 991gt3 there is a slot to fill. But, with the 991.2 gt3 going to be MT option there is really no need for a 991R.
991R Hopped up C2 with the right unavailable options for GT3 and RS with the 7 speed manual.
"you need to get on the list now, unless you got a 918. Or a Purple RS to trade."
===> i do have a barney inbound ;-) but i dont think that will get me a 991R
"We're all better drinkers."
===> really, i saw you quit after ONE SIP of root beer! wait until i bring out the korean dish water drink. you would die while i gulp done gallons at a time.
"you need to get on the list now, unless you got a 918. Or a Purple RS to trade."
===> i do have a barney inbound ;-) but i dont think that will get me a 991R
"We're all better drinkers."
===> really, i saw you quit after ONE SIP of root beer! wait until i bring out the korean dish water drink. you would die while i gulp done gallons at a time.
ROOT BEER - Are you f**king serious?
in the rest (oh wait) ... in the civilized world, ROOT beer is admisitered by dentists you swill it about it your mouth and spit it into a little suction cup...
ROOT beer ... never enter a bar in Scotland and ask for ROOT beer ... if you
want to leave with your life or your ***** John ...
ROOT beer ... are you 12?
dont forget to put the toilet seat back up when your finished dear
ROOT beer ... never enter a bar in Scotland and ask for ROOT beer ... if you
want to leave with your life or your ***** John ...
===> that's why i haven't visited your homeland... in fear of my life.
ROOT beer ... never enter a bar in Scotland and ask for ROOT beer ... if you
want to leave with your life or your ***** John ...
===> that's why i haven't visited your homeland... in fear of my life.
I bank on this pattern of behavior in my line of work (root canals). Many of my clients need my services because of avoiding regular dental care, and for that I'm quite appreciative as Porsche's are expensive. :-)
^ root canal in USA is $$$ but pretty easy. oral surgeon put you to bed.
back home was cheaper....
they use local anesthesia and a LOT OF MUSCLE and YANKING action and i can hear LOTS DRILLING...
after 2-3 root canals as kids, i learned to brush often.....
^ root canal in USA is $$$ but pretty easy. oral surgeon put you to bed. back home was cheaper.... they use local anesthesia and a LOT OF MUSCLE and YANKING action and i can hear LOTS DRILLING... after 2-3 root canals as kids, i learned to brush often.....
You are smart about the brushing. An ounce of prevention.....
You know how 4 out of 5 dentists recommend this or that? I'm the 1 out of 5 dentists who doesn't recommend brushing. It's bad for business. :-)
10-04-2015, 11:14 AM
