Boost question: I've seen 22-24 as a decent max, but is that a fueling or turbo wear limit?
07-20-2007, 10:34 AM
#11
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many different factos are in the game here
but i think the most important one that makes most limit their boost around 22psi is shaft speed.
the relation btw shaft speed and psi is like hp and speed. If you have a 50hp and you can go 100mph it doens't mean if you have 100hp car you can go 200mph. So if you shafts speed for 20psi is let say 10k rpm(i know its much highier) and you want 25psi the speed might go up a lot more than 12.5k rpm.
ALso another factor is CFM at a very high psi the turbo looses its efficiency by not producing as much cfm as necessary so loosing power. CFM psi and air temp are all tight together.
this explains a little about cfm.<ul><li><a href="http://en.wikipedia.org/wiki/Actual_Cubic_Feet_per_Minute">http://en.wikipedia.org/wiki/Actual_Cubic_Feet_per_Minute</a</li></ul>
the relation btw shaft speed and psi is like hp and speed. If you have a 50hp and you can go 100mph it doens't mean if you have 100hp car you can go 200mph. So if you shafts speed for 20psi is let say 10k rpm(i know its much highier) and you want 25psi the speed might go up a lot more than 12.5k rpm.
ALso another factor is CFM at a very high psi the turbo looses its efficiency by not producing as much cfm as necessary so loosing power. CFM psi and air temp are all tight together.
this explains a little about cfm.<ul><li><a href="http://en.wikipedia.org/wiki/Actual_Cubic_Feet_per_Minute">http://en.wikipedia.org/wiki/Actual_Cubic_Feet_per_Minute</a</li></ul>
07-20-2007, 10:57 AM
#12
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Some interesting reading, and help pull together some facts but it left me vauge on some points
So because our air is thinner, we will have to move a great volume of air (spin faster) to achive the same pressure as a car in identical conditions other than elevation. But I still wasn't clear on whether 20psi has the same 'density' as 20psi at sea level. It kinda reads that way, but I know that cars make less power here so that cannot be right?
The flip side is that it's easier to spin a turbo here because of the low air density, but obviously the same RPM's won't net you the same pressure, so it really doesn't matter how easy it is to turn.
So if it's an RPM to efficency issue, you wouldn't be able to generate the same pressure here as at lower elevations. Man, there is a lot to know! If standard atmospheric pressure is 14.7 and it's around 11.7 here it's not a flat standard is it? Like if 1000rpms generated 23psi at sea level it would make 20psi here?
If it wasn't for fan efficiency it seems like you would need at least 3 psi more than a similar car at sea level to get the same performance, but I don't think it's that simple. Lordy, I need a drink.
The flip side is that it's easier to spin a turbo here because of the low air density, but obviously the same RPM's won't net you the same pressure, so it really doesn't matter how easy it is to turn.
So if it's an RPM to efficency issue, you wouldn't be able to generate the same pressure here as at lower elevations. Man, there is a lot to know! If standard atmospheric pressure is 14.7 and it's around 11.7 here it's not a flat standard is it? Like if 1000rpms generated 23psi at sea level it would make 20psi here?
If it wasn't for fan efficiency it seems like you would need at least 3 psi more than a similar car at sea level to get the same performance, but I don't think it's that simple. Lordy, I need a drink.
07-20-2007, 11:11 AM
#13
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Actually answered one of my own questions by reading further following links
The correction for altitude is 1.22 at 5500', so if you wanted the same air density as a car running 23 psi at sea level you would need to run 28 psi here. Compound that with the fact that it has the turbo has to move a lot more volume to achive the same pressures and you can see why cars make so much less power here!
If you use the same correction factor, if 23 psi is the best for efficiency at sea level would that mean given the same temp and humidity around 19 psi would be the most efficent pressure here?
That same logic a step further would tell you that a Stage 3- car at 16/17 psi at sea level would make the kind of power a Stage 3 car running 20/21 psi makes here. That pretty much lines up with 1/4 mile times I've seen for various S4's outside of Colorado....
Did I jump over some logic rules with that math?
If you use the same correction factor, if 23 psi is the best for efficiency at sea level would that mean given the same temp and humidity around 19 psi would be the most efficent pressure here?
That same logic a step further would tell you that a Stage 3- car at 16/17 psi at sea level would make the kind of power a Stage 3 car running 20/21 psi makes here. That pretty much lines up with 1/4 mile times I've seen for various S4's outside of Colorado....
Did I jump over some logic rules with that math?
07-20-2007, 12:10 PM
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More good reading! In a nutshell then...
It looks like if at sea level we decide that 23psi is the "optimum" for efficiency (debatable I'm sure, but we'll assume that) then given the same temp/humidity in Denver we should really only run about 20psi for safe, dependable results. However, as the temps drop and air density increases you can bring pressure back up. Obviously the stock software is designed to handle those adjustments for you, but using a MBC standalone we bypassed those adjustments.
Understanding what I have then learned, a large, effictive intercooler would help reduce temps and thus allow the turbos to spin at a lower rpm (or reduce volume) to reach the same pressure. Since our climate is so dry, it seems like some form of water injection would help, as well as spray for the intercooler(s) with evaporative cooling being so effective here.
Thanks for all the help and enlightenment, doens't make me any kind of expert but it certainly helps to have a basis of understanding!
Understanding what I have then learned, a large, effictive intercooler would help reduce temps and thus allow the turbos to spin at a lower rpm (or reduce volume) to reach the same pressure. Since our climate is so dry, it seems like some form of water injection would help, as well as spray for the intercooler(s) with evaporative cooling being so effective here.
Thanks for all the help and enlightenment, doens't make me any kind of expert but it certainly helps to have a basis of understanding!
07-20-2007, 01:34 PM
#20
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Just running pump gas here, and we backed the FPR way off from before. Nigel has the logs
That might have been at 20 psi, I can't rememeber for sure but I know that we had PLENTY of room in them when we took it to where it sits now.
Of course I'm now going to back it down to 20 psi after what I've learned today. Better safe than sorry!
Of course I'm now going to back it down to 20 psi after what I've learned today. Better safe than sorry!