We often run into questions on boost and if it’s enough for the tune, if waste gate needs adjusting, or some having problems with sensors or mechanical parts causing a loss of boost, or an over boost condition. Here we will go over some of the common problems, fixes, and how boost works on your XPT.
Also see this article, RZR Turbo & Pro XP 1127 Boost Codes Explained: https://aftermarketassassins.com/tech-blog/f/rzr-turbo-over-boost-code-explained
Boost: What is boost? Well as you probably know; it’s pressure created by the turbocharger. For basics on how a turbo works google is your best friend. Boost in an XPT and most modern turbo systems is controlled electronically with a boost controller. This is a little black box with 1 vent line and 2 black lines running to the turbo (one to the waste gate, one to the compressor housing). This controller is controlled by the ECU and has it’s own very specific mapping to make it work correctly. This manipulates the pressure going to the waste gate by bleeding off pressure as needed. This works with your MAP sensors to determine boost pressure based on atmospheric pressure.
Your ECU will command a certain amount of pressure. Some may think we command say 16.5psi, and that’s where it should be no matter what. We give averages of boost pressures, but many things can vary boost outcome. This is because boost is controlled in what we call absolute pressure. Absolute pressure is what we want to see for total pressure combined with your atmospheric pressure. At sea level your atmospheric pressure is 14.7psi. The air around you has this amount of pressure. So lets say we are commanding 30psi of total pressure in our tables; the turbo would then make up the extra pressure of 15.3psi, which is your boost pressure. Take that same machine commanding 30psi of absolute pressure and go to 4000ft where your atmospheric pressure is 12psi, and that same tune would create 18psi of boost. In short, boost will always vary depending on your atmospheric pressure and that’s okay, because you will get the same amount of air… some just start with more to begin with.
Turbo Efficiency & High Altitude: Say we command 30psi absolute pressure and at 1000ft we are at 14psi of atmospheric pressure. This would create 16psi of boost. Now we take that same setup to 10,000ft altitude and our atmospheric pressure is 10psi. Our boost pressure would then be 20psi. Making 20psi is much harder work for the turbo, and not feasible especially for the XPT turbo. It then falls out of the peak compressor map and will probably not create that much boost, or get way too hot trying, and create very little power. We have atmospheric compensation tables to adjust for this… really they are caps to keep boost in line with where the turbo can run best. This is why sometimes you will see a little less boost at high altitude as we try and keep turbo RPM’s roughly the same. Our tunes do a much better job of allowing more boost at high altitude than factory, but your waste gate many times needs to be further adjusted at higher altitudes than lower altitude. Turbo efficiency is a huge deal at any altitude. Many tuners will just command a huge amount of boost and let the turbo do what it can. This is terrible on components as it creates all kinds of heat, and no sustained power.
Waste Gate: This goes right along with turbo efficiency. On a traditional old school turbo system your waste gate spring pressure is all you have to control boost. You want 10psi of boost? Then you get yourself a waste gate with a 10 PSI spring and dial in the waste gate rod to bleed off any pressure over 10psi. It’s easy to get confused with electronically controlling the waste gate… thinking your spring pressure no longer matters, but it DOES! The factory has enough spring pressure in the XPT waste gate to hold back the boost the factory creates, but start turning it up and it’s not enough. Even though the boost controller will try and close the waste gate to create more pressure, the spring is too weak and the waste gate actually gets forced open from the exhaust pressure. This is why for many of our tunes we need you to adjust your waste gate rod shorter. This puts more preload on the spring so it will not open when we don’t command it to. You machine might make the commanded boost without adjusting, but the turbo is working harder because it’s losing pressure out the waste gate, and that does not make power. Always adjust your waste gate to our recommended settings for the tune. Waste gate springs can wear and sag just like any other spring. From time to time or on older waste gates, you might need to adjust again or replace them. Most of the times the WG is bottomed out inside the coupler when adjusting. If you need more adjustment the heim joint end can be removed and shaved down for more adjustment. Just be sure to mark where you were originally. See our waste gate adjustment video in the tech section on how to. Remember you don’t need a mity vac or pressure pump to adjust the WG. You can just use a series of zip ties from the WG arm to your sway bar.
Over Boost: This is a problem on many XPT’s. The most common issue for this is the electronic boost controller (black box on the rear firewall, 2 lines running to the turbo) being clogged. The vent line simply goes to atmosphere and is prone to getting clogged with dirt. When this happens it cannot bleed pressure off like it should, and will shoot over target boost. You can carefully remove the line going to the waste gate actuator (bottom line on the boost controller) and blow through it with compressed air. Many times this will solve the issue, but sometimes the controller must be replaced. Too low of RPM caused by improper clutching or worn clutches can cause overboost. It will load the motor too hard, creating a boost spike. A blow off valve not working properly or with too much spring pressure can also cause boost spikes off the throttle. Too tight of a waste gate adjustment can also cause this issue, as well as a sticking waste gate. Improper ECU tuning can also cause this issue which is why data logging is important.
Under Boost: Not getting the boost? Normal boost should be within 1psi of target which you can view manifold pressure vs. target boost if you have the PVCX and data log. If you are lower than this and not getting anywhere near the boost you should, it’s time to start digging. Here are the things to check: waste gate too weak or sticking? Use an air pump with a low PSI gauge to push 6psi at the waste gate actuator. This should start to move around 5-7 psi depending on adjustment. Any lower and you probably need to replace it. Is your turbo shot? Remove the intake tube and take a look at the compressor wheel. You can usually tell if it’s been hitting the housing. You can also use a needle nose pliers to wiggle the shaft and check for excessive play. These turbos do have a good amount of play in them when cold, but when hot have much less. I’d suggest checking them after running it (be careful, hot!) just to make sure. Boost leaks are another very common issue. If your charge tube is not sealing correctly on the turbo or throttle body it will definitely cause an under boost or no boost condition. If your blow off valve is not working properly that will also cause a lack in boost pressure. MAP sensors not working correctly or having an issue with the wires going to them will cause issues as well. Swapping these sensors with known good ones is the best way to test, or data log if you have the Powervision. You have 3 MAP sensors. Your atmospheric pressure sensor located before the turbo in the intake tube. The other 2 are the same sensor and measure boost/heat before and after the intercooler. One is in the charge tube and the other is in the intake plenum. The electronic boost controller could also be bad causing a low boost condition, or the boost lines running to it. The boost lines run from your intake plenum, then T off with one line going to your boost controller, and the other to your blow off valve. If any of those lines are blocked or pinched it will cause a low boost condition. An air restriction in the airbox with a clogged filter, collapsing intake tubes… ect can also cause low boost. The last cause for low boost can be caused by improper clutching. Boost works off engine load, when the engine is loaded it will create boost. Unloaded it won’t. The simple version; if you don’t have enough clutch weight, it may not create the proper amount of boost (normally not off by a huge amount, 1-2 psi low). Keep in mind that target boost vs actual boost will many times be off by roughly .5 to 1.5 psi as a cushion for over boost protection.