| Message |
to push the limits further without the frear of immediate destruction. In order to play, you gotta pay - if you want to run 15 psi of boost or 30 psi of boost, inherently you are going to introduce more wear on the system as the power output increases. Look at the Supras making 1000HP. They aren't running 24psi of boost - more like 35+psi of boost to get there, and using large single turbos that operate at higher efficiencies simply due to their size. Little turbos gotta spin fast to move a lot of air at high pressure - much faster than one large turbo by like 3X as much. The higher speed of the smaller turbo adds more heat to the air at a given flowrate and pressure. The PSI number is just as important as the airflow, surge limits, and choke limits. You can't just look at one single parameter within the compressor map to make any sort of judgement on what is best and what to expect. In fact, you can't really look at all of these elements of just the compressor map and be able to make a determination on what to expect - there are far too many other variables involved in the vehicle. Exhuast manifolds improve the flow of gases from the engine to the turbine - this minimizes backpressure as the RPMs increase, which allows the engine to continue to move the exhaust gas out of the cylinder so as to fill it with clean intake air and fuel. The rest of the exhaust plays a role as well in terms of turbine efficiency since a choky exhaust will create less pressure drop through the turbine. On the intake side of things, there are quite a few more parameters to look at. Dual intake vs. single intake: The dual intake presents less restriction for hte turbo's compressor, and since the compressor is a pressure multiplier, the less restriction going into the compressor, the easier its job is to compress the air, which means better efficiency as it wont have to spin as hard to do the same work. Just after the turbocharger, the intercoolers serve as very good efficiency boosters, most operating at around 70-75%. The effect this has on the system is no different than simply using a compressor with much greater efficiency. Adding internal breather mods like cams, valves, portwork, combustion chamber valve deshrouding, etc etc, are simply volumetric efficiency boosters for the engine. You aren't changing the displacement of the engine with these mods - it still remains at 3L, but you are improving the volumetric efficiency of the engine. As for any engine dynamics associated with modifications in the valvetrain, ports, manifolds - the engine is simply an air-pump. There are modifications you can make to it that will either improve airflow in specific areas at the detriment to other areas (tuning the "sweet-spot"), OR, there are changes that will improve the entire overall efficiency of the engine. When serious work is done to the ports, combustion chambers, manifolds, larger valves, higher lift cams, etc, you are improving the volumetric efficiency of the engine, but not the displacement. Displacement will stay the same unless you bore or stroke it and boring to 87.5m or 88mm has very little effect on the displacement when speaking in terms of compressor maps. Changing port volume is simply tuning the sweet spot. Larger ports have slower velocities, which will detriment top-end power, but improve low end. This makes really a small difference with forced induction engines as compared to doing things like using larger valves or cams with higher lift since the pressure within the manifold forces the air into the chamber. Larger valves and higher lift cams are going to produce more overall power as it improves the VE of the engine across the board. Duration and port size are just sweet-spot adjusters, and play much smaller roles in forced induction engines. Having smaller port size creates better cylinder filling, but only at higher RPM. This can hurt bottom end performance in NA engines as compared to a ported runner. Nissan used larger ports on the TT heads to try and make up for the lag in the turbochargers, but as soon as the turbos get spooled up, the detriment due to the larger runner is overcome by the pressurization many fold. When compressor map RPM lines are plotted(as seen in the map that was in my post the other day), it is always assumed at 100% volumetric efficiency to show the maximum absolute values of displacement that the engine could possibly produce. This is done to allow one to make a decision on what wheel to use and what they could expect. We know that with all the upgrades I listed above, we will improve the volumetric efficiency of the engine, but we won't exceed the rpm plots on the map as they are already assuming 100% VE. Since no engine will actually operate at 100% VE over its full RPM range, the linear line plots in corrected form would actually have a curve to them - the curve itself representing the VE of the engine. So yes, these breather mods are putting you further to the right in the compressor map, but not beyond the linear 100%VE RPM line plots. With the testing I will be doing, we will be able to construct a real and accurate plot of how the engine breathes through the map. We will be recording P1, T1, P2, and T2 (pre and post compressor readings; P1 and T1 will be the measured atmospheric conditions for that day, P2 and T2 will be the measured values at the inlet of the intercooler - same as the compressor discharge) P3 and T3 will be used to gather information on the intercooler efficiency, which isn't really part of this immediate discussion. But, if we know P1, T1, P2, and T2, I will be able to plot the exact points in the map that the VG is using through its RPM range. What we are going to find, I believe, is that although the compressor is running at lower efficiency (which only means that the discharge air is at a higher temperature), the intercooler is going to make a dramatic effect on the efficiency of the system as a whole. I think you may be overlooking this fact when trying to make reason on what boost level is best when looking solely at the compressor map and assuming that is what the engine is seeing. Discharge temps at 30psi will likely be somewhere around 330F, but the intercooler will knock that down to somewhere around 140F. The intercooler is an efficiency booster and it does an excellent job. Mathematically speaking, when we look at the intercooler discharge temperature and calculate the efficiency of the system up to that point using the above values, we would measure an efficiency of around two-hundred and seven percent. If it weren't for the intercoolers, the kind of power we have been observing in these cars would be only a dream. So just keep this in mind when trying to figure what is the best boost pressure to run based solely on the compressor map - it really only tells you if the given compressor is going to operate at the demands of your application, not the exact trends of engine performance as there are many other factors involved. The intercoolers significantly affect the actual efficiencies that the system is operating at, which in turn, may help you to understand why you can actually run 30psi of boost on these turbos and it will continue to make more power up to this point. Add to that the benefit of water injection, and you have one heck of an efficient system. :) On Dee's car with the sport600s, we were pushing about 25 degrees of timing advance at full boost of around 28psi with water injection and Sunoco race fuel. If we were running C16 that day, we would have been able to run a degree or two more timing, and it would have made even more power. Think of it this way: a naturally aspirated bigblock chevy making some 500horsepower, high compression, and race fuel, will run a timing advance of around 35-40 degrees. If we could push our engines to run somewhere around 30-35 degrees of timing advance without detonation, they would make more power all the way up. But we can't do this because we are limited by the fuel. If we could run something like alcohol or nitro in these engines, we would have to run somewhere around 60 degrees of timing advance no problem and it would make stupid power. The additional timing advance is a component of the fact that those fuels burn much slower - it is analogous to race fuel in our engines as this higher octane fuel doesn't burn as quickly. To sum these points up, you can't determine just by looking at the compressor map what the given compressor is going to do for your application. The compressor map's practical use really goes no further than to allow one to make the determination on if it fits their application. Given the fact that we are going to be testing this turbocharger through quite a dynamic range of its capabilities and it is already on the car, we will be able to gather the data that will show that although you aren't in the most efficient region of the turbo's capacity, you will still make more power due to the fact that you can flow more air at greater pressure. THe intercoolers, the piping, the rest of the system, along with proper tuning, will allow this, as well as finally come together to produce a system that isn't going to grenade "tomorrow". But, like I originally said, you gotta pay to play. The turbos aren't going to last as long at 30psi as they will at 15psi. You dont even have to look at a map to make this determination. That's where the person needs to make the decision for themself as to what they want to get out of the car - all we can do is simply supply the information to them as accurately as possible to allow them to make the right choice for them. Good reading your post - I hope to have clarified some things for you without sounding condescending, which is not my intent at all. -Take care
[ ashspecz.com ]
[ agpowers@bellsouth.net ] Enthusiasts soon understand each other. --W. Irving.
Are you an enthusiast? If you are out to describe the truth, leave elegance to the tailor.
Albert Einstein
|
 |
