Re: Uppdates on the fastest Sy´s in the world?
this is just a little of what I have found looking around some forums....this guy (an ME at an aerospace company in CO) is speaking of the Bandimere raceway in Colorado (5000+ ft.)
****************************************************************************
he first thing to understand is what your engine power really is. I once heard someone say you could tune a car solely on cylinder pressure. The basis for this was that cylinder pressure is the source of all power, and nothing else matters so long as knock and EGTs are not causing damage. Well, that may be close enough to true for a n/a car, but not a turbo. See, the power you get out of your car is not the power your engine produces. What you get is (The power your engine makes) minus (The power your engine consumes). That's a big difference. If you take a 5hp hit to the power your engine produces but make it spend 10 hp less doing so, you come out ahead, right?
Now, for an N/A car, there's not a lot you can do. You can open up the intake and exhaust and make the engine work less hard to suck air in and expel the exhaust. A short ram intake will make the engine expend the least amount of energy sucking the air through the intake, so the power your engine consumes will be minimized. A CAI will be a bit of a longer straw for your engine, but it might put more dense air into your engine, which will increase the power your engine produces by raising total cylinder pressure.
Now, let's look at a turbo car. Sure, the bigger the MAP, so long as it is sufficiently cooled, it will always produce more power. More air in, spark it, burn it, the cylinder pressure is insanely high. Great, right? Here's the problem - exhaust. Your valves open, and the engine has to work to push that exhaust out. Now, without a turbo that's fairly easy. With a turbo, the engine is pushing the exhaust through a turbine. If it's way outside of the efficiency range of that turbo, it's like it you walked up to a turbine at a power plant and tried to spin it. It's hard! So, sure, you can keep turning up the boost, but there's no point. Your engine will continue producing power, but it will consume that power and more. What that means is you're running the life out of your turbo and your engine for the purpose of moving slower.
You can still do things to reduce the amount of energy your car consumes for a turbo car. When we do exhaust, uppipe, and silencer removal, that's what we're doing. Smoother pipes, etc. Even the lighter pulleys and whatnot. Your engine isn't producing more power, it's consuming less energy in the process. You don't start making more power until you increase boost, engine managment, and that sort of thing.
Now, the turbo. I'm going to try to explain this without many numbers. A turbo is a very simple device. It doesn't know gauge pressure. It doesn't know absolute pressure. It only does ratios. Imagine if you ask a guy to make $10 for you. If you give him $10 seed money, that's kinda hard. If you give him $1000, that's easy, right? A turbo works hard based on the pressure that is coming out of it divided by the pressure going into it. So a turbo at sea level (14.7psia) would work the same amount to produce 14.7psig as a turbo up here (12.2psia) would to produce 12.2psig. It would be a pressure ratio of 2.0 in both cases. Keep in mind the absolute pressure coming out of the turbo is atmospheric pressure plus gauge pressure. So that means with a 2.0 pressure ratio you'd have 14.7+14.7=29.4psi at sea level. At high altitude that would be 24.4psi... sucks, don't it? That's what sucks for us. But even so, THE TURBO DID THE SAME AMOUNT OF WORK TO DO 12.2PSIG UP HERE AS IT WOULD TO DO 14.7PSIG AT SEA LEVEL!!!
Now, that's not being entirely honest with you. We measure pressure after the intercooler, and there is a drop there. We also are neglecting the intake, and there is a pressure drop there. You need to account for those things, and they usually make it even worse for us altitude folks. I'm not going to get into the details of that.
So here's what I'm saying - you can't run the same boost pressure as people at sea level for any given turbo. There is a little uncertainty because compressor maps are generally shown in volume flow rate, and we're dealing with 20% less dense air.
Intercoolers are also less effecient up here so charge temps will be slightly higher all else equal as well (ie lower manifold density for a given boost level)
Here at altitude the extra work necessary to make the equivalent absolute manifold pressure changes the dynamics of the entire system.
Think of it this way. You have two co-dependent systems. The internal combustion engine which happens to be used as a handy hot gas generator to power the turbocharger, and a turbocharger.
You demand (X+y) work out of the turbocharger to make a sealevel equivalent absolute manifold pressure. This work is greater than the work (X) it would have to do at sea level due to the lower ambient air pressure. It must also run at a higher pressure ratio to reach that higher pressure. To run at a higher pressure ratio with a lower density inlet air pressure on the compressor, it has to spin the compressor significantly faster. This usually puts the compressor on a point to the right and above the point it would be operating at on the compressor map at sealevel.
The work necessary to spin the compressor comes from the turbine which is driven by the exhaust manifold exhaust gas pressure expanding to the local air pressure.
So now you have the following problems:
The compressor must run at a higher pressure ratio -- that means it is less effecient (usually). So you have the extra work necessary to develop new higher pressure, plus the extra work necessary to make up for the loss in effeciency. You also must spin the compressor faster (due to lower inlet density and higher pressure ratio), so you need to have more exhaust gas velocity to achieve this level of performance and slightly higher exhaust gas pressure to create the extra torque on the turbine wheel to do this extra work.
The engine is also exhausting into a higher pressure in the exhaust manifold so its VE is lower (less effective boost ie manifold pressure vs exhaust gas pressure) It is as if our exhaust valves were slightly smaller than the same engine at sealevel. To compensate, to reach an equivalent boost level, you need a bit more manifold pressure than a simple 1:1 equivalent to the sea level manifold pressure to compensate for the lower VE of the engine.
Result --- the turbine spools slower all else being equal than it does at lower altitude. To optimize for altitude, you would want a turbine and housing A/R that spins up almost too fast at sea level, and at sea level would be running below its max effeciency point (pressure ratio wise). You also want a larger dia turbine or A/R housing that would make more torque with less back pressure, but if you do this you loose exhaust velocity.
That is why the PE1820 and some of the hybrid turbos frequently did not live up to expectations up here. They spooled too slowly to give ideal performance. They suffered from the supra syndrom, and never really got to full boost during a drag strip run, and took longer to re-spool on each shift.
Now the hot gas generator (the engine) sees the effects of these demands by the turbo in that boost rises more slowly at altitude (so you need to run more agressive boost controller settings). This slower onset of boost is probably the primary reason our 60' times are about .05+ second slower than equal cars at sealevel. We simply do not make as much torque as quickly, and loose more spool on each shift, then they do. You need to be extra careful to maintain exhaust gas temp up here to maintain velocity and quick spool up. Also exhaust gas system back pressure is even more important up here.