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Message: [QUOTE="KCTurbos, post: 936101, member: 13852"] Depends on how smokey you want it to be on a race tune. I have a couple of guys running our KC 68mm stage 3 turbo and pull max weight with no problems. The key is going to be in the tuning and RPM band. There is no way around it... bigger turbos make power and run more efficiently later in the rpm band. The guys that I know running bigger turbos with larger injectors usually have no problems as long as they do 2 things. First don't ever town a race file. Even if you can keep egts in check you need to worry about timing also. So make sure to get a good tow tune written. I love the option of making the tow/haul button lock out OD... which leads to the next item. Keep your RPMs up. With a bigger turbo and bigger injectors your truck is not going to like being around 1500-1700rpms. If you tow max weight in that area then you will roll coal and have high egts until you get the turbo above 1800-2000rpms. What we recommend is just have a tow file written to lock out OD using the tow haul button. That way you can control where your rpms are at. I do the same thing on my 7.3 powerstroke. I run a larger single non-vgt turbo and pull max weight all the time. If going up a hill i just use the OD button to keep the rpms above 2000rpms. 190/75s and our stage 3 has shown to clean up the injectors on a race file to almost nothing. 600hp+ has also been hit by guys that like to hit up the track and/or go to the dyno. On the other hand the smaller the turbo the more efficiently your turbo will operate at lower rpms. So if you stuck with a stock turbo then towing at 1600rpms is not as bad... but on a race tune you are going to run hot, smokey, and not have the same power potential. We had a truck with 190s only make about 500hp on a stock turbo. When we added the KC 68mm it gained 133hp with just a turbo upgrade (numbers were backed up at the track). Bigger turbos and smaller turbos operate different at different areas of the compressor map. too many people are led to believe that a bigger turbo will act exactly like a smaller turbo. This is not true!!! Off the line can be helped but locked in OD at lower rpms it will be easier to notice the difference. Tuning can help. Have the tuner make sure it does not lock the tc into OD so easy and so low. Gears will also help. Raise that RPM level more Smaller tires will help. This raises the rpms and takes a little load off the motor Air flow mods will help like a ported intake, aftermarket air filter, up pipes, manifolds, etc... They will actually improve the volumetric efficiency which will cause the turbo to operate more efficiently at lower rpms. Much of this has to do with the efficiency area of the compressor maps (different turbos operate differently at different area of the comp map)... It is something easier explained over the phone... but I will try here. I am not sure if you understand all the different terminologies but for this discussion I would focus on pressure ratio, Air flow, and the efficiency islands. A simpler way to think at it is pressure ratio = boost, air low = rpms, and efficiency islands = temps (those are not exactly what they mean but it helps to understand better). The higher the pressure ratio the higher the boost The higher the flow the higher the rpms the higher the efficiency the lower the temps and the better the turbo will run and spool up here is a general map. You can see that the closer to center of the map you get the more efficient it is. This also equates to lower temps. The farther you get from the center, the higher your air temps get, which raises your egts. Also the quicker you get on the map and get to the efficient areas of the turbo, the quicker the turbo will spool up and make efficient power. [IMG]http://i280.photobucket.com/albums/kk170/chstfish/Compmapexplained_zpsdf1d78d9.jpg[/IMG] Here is a map comparing the pmax to the stock turbo. The green area is where we spend most of our crusing and towing time (around 1600-1800rpms). You can see that it is very close to the center of the efficiency islands of the stock turbo and that it is way out of efficiency of the larger pmax turbo. Which means that at the same psi, same boost, same rpms... the stock turbo will be putting out cooler air. Also you can see that the smaller turbo can put out more boost at a lower rpms without surging (that would be the top left areas of the compressor maps) I know this is not your exact situation but it illustrates the same points as to what we are talking about. Smaller turbos operate better at lower rpms and lower boost. The green area = 1600-1900 rpms the red area = 1800 - 2100 rpms (this is where you need to be towing on a bigger charger) [IMG]http://i280.photobucket.com/albums/kk170/chstfish/cruising_zps687fb550.jpg[/IMG] Here is another picture of where we spend most of our time at 3/4 pedal - WOT on a spirited run. You can see that now you are almost dead center on the efficiency island of the pmax and now the stock turbo is way out of its efficiency range. This is one of the reasons why a bigger turbo like the pmax or KC 68mm stage 3 will will feel so much faster on the high end, spool faster above 2000rpms, and lower your WOT egts,. [IMG]http://i280.photobucket.com/albums/kk170/chstfish/WOTmap_zpsdbff942f.jpg[/IMG] There is a lot more that goes into it than just this... but it should give you and idea of what I am talking about. You will also note that I did not plot just one little tiny point on the maps... that is because our boost and rpms fluctuate so much with tire size, speed, tunes, volumetric efficiency, etc... so some people might experience slightly different results but the general principles are the same. Bigger turbos will make way more power Smaller turbo usually lack power on the top end bigger turbos generally takes longer to spool/light Smaller turbos operate better at lower rpms bigger turbos generally equals higher cruising temps bigger injectors generally equals higher cruising temps bigger tires generally equals higher cruising temps and spool later higher elevation generally equals higher cruising temps Now before anyone nit picks my calculations.... I did not really make any. I just guestimated to show general theory and not exact plotting points. Also I know every map is different but these ones were easy to find and illustrate the principles were are talking about. I hope that helps a little bit. [/QUOTE]