Ive seeing alot of threads regarding people force feeding their rides, heres a tech article on turbocharging your ride from August 2005 issue of Super Street. Note: Due to the Size of the article I am only posting the juicy parts of the article ie. the technical aspect of choosing and turbocharging your vehicle. 1. Finding ideal target hp gain for your vehicle. Beware of differences between inertial chassis dyno whp and tq-cell-based computed crankshaft hp. Most turbo oriented caluculations are for crankshaft hp from tq measured on a water brake engine dyno. Crankshaft hp will always be higher than chassis dyno hp due to conversion factors, and heat losses into drivetrain and tires. 2. Convert Traget HP to required turbocharged airflow under pressure A good rule of thumb is that every 10hp you make with a gas engine requires a pound of air per minute delievered into the intake manifold. For example, to transform a 100hp NA engine into a 200hp turbo engine, youre going to need a turbocharger capable of flowing 20lbs of air per minute at a realistic boost pressure. Youll need to need to compute the basic engine airflow rate in CFM as follows *Airflow=(cubic inches displacement X rpm X 0.5 X volumetric efficiency(V.E))/1728 *0.5 is there because a four-stroke engine only breathe every other revolution, while 1728 converts cubic inches to cubic feet per minute *If you were to insert 85 percent for V.E for a typical 2.7L Toyota truck engines 165CID X 0.5 X .85/1728=264 CFM *At 80 degrees ambient temperature at sea level 264CFM converts to pounds/minute follows: lbs/min=CFM x.07 *Therefore 264 X .07= 18.48 lbs of air per minute *Using the rule of thumb this 2.7l engine should produce approximately 185 NA HP. 3. Verify Traget Boost Pressure On a street car, target boost pressure will be the lesser of the boost needed to make target power or (more likely) the engines boost detonation limit. Increase boost enough and A/F mixture in the combustion chambers will begin to explode instead of burning smoothly, which is very bad for the engine. There is no point of having a trubo that can push abnormal amounts of air beyond the point of the engines boost detonation limit, if your fuel system cannot deliever the fuel required to safely run your engine at that boost. Additional boost limiting factors include fuel supply constraints and engine calibration limitations. With premium street gas(typically 93 octane for most and 91 for you Cali folks), figure a maximum of 10 psi of boost w/o an intercooler and 15 psi of boost with an IC. Highest boost pressure will be feasible with pent roof combustion chambers, really efficient intercooling, higher available octane gas, excellent engine management with a high quality calibration using the best anit-dentonation countermeasures. 4. Convert Target Boost PSI to pressure ratio. You need a pressure ratio to work with compressor maps that plot air flow at various pressure ratios and compressor speeds. If youre working with target boost pressure, youll need to convert boost pressure to a pressure ratio, which is the percentage of one atmosphere above nothing at all delievered at the compressors outlet. For example: 10 psi of boost + 14.7(atmospheric pressure)/14.7=1.68 boost pressure ratio. Which means BPR of 1.68 equals 68 percent higher pressure than NA. 5. Convert Pressure Ratio to Density Ratio Unfortunately, a turbo engine system will not usually flow as many CFM as youd predict just by multiplying the pressure ratio times the NA airflow of the engine in CFM. Take in mind that density ratios vary because of turbos thermal efficiency and efficiency of the intercooling system. Most street turbos operate in the thermal efficiency range of 55-85percent. Lets go back to the 2.7liter Toyota engine. If you were to boost that engine to 7.5 psi without an intercooler, the pressure ratio for that engine will be (7.5+14.7)/14.7, which yields a pressure ratio of 1.5. Using the stock airflow of that engine of 18.5lb/min(264 CFM), at a BPR of 1.5, you might expect the airflow under boost would increase to roughly 28lb/min. However, if this particular turbo with no IC is just 70% efficient, the density ratio at sea level is really about 1.3 which equates to 24lb/min of airflow. Bottom line: when Turbocharging your ride, youll need to start with a bit more of turbo to compenste for turbo efficiency or better yet inefficiency. 6. Select the RPM range of your engine for maximum compressor efficiency(Youll need access to your turbo compressor efficiency map). Youll typically want to match your tubos maximum compressor efficiency with your engines most useful part of the rev range. This will require alot of knowledge about turbos, better yet tap a turbo expert at Garret, Precision Turbo, etc. Rather than simply acieving maximum peak power, think of the areas that will greatly help your vehicle acceleration. 500HP at 10,00rpm is useless compared to a 300hp at 4000-6000rpms in a street application. But if you are trying to maximize your peak power, remember that a forced induction system will always shift peak power to a higher part of the rev range. But with the days of electronic boost controllers, users will be able to pinpoint when the peak boost will be created anywhere on the rev range based on rpm, gear, and other factors. You might place peak boost at lower rev range to amplify tq(help with acceleration) but youll need to pull the boost back at higher speed to limit peak hp to stay within certain fuel contraints or to maybe to protect your drivetrain. For example, ultra power dragsters limit boost at lower gears to manage tire spin. 7.Select a compressor. Youll notice that a compressor map will resemble a 3d topgraphical contour map of a hill. This map describes the compressors efficiency at various combination of airflow rates and boost pressure. To the left of the map also are danger zones at the east and west of the map. This zones illustrates the surge and choke zones of the compressor. Big turbo tends to surge at lower revs with huge turbo lags and usually no usuable power gains. Hybrid turbo is a solution if you are looking for a big compressor ie t3/4 hybrids whichs utilizes a smaller turbine but with a bigger compressor to eliminate turbo lag. If your engine has a really good bottom end, you might choose a big turbo to create big peak power but to accentuate your bottom end as it creates full boost. But with a engine with a weak bottom end and a strong top end ie Hondas B series/H22/F20C, you might want a smaller turbo that spools quickly and build boost quickly to maximize bottom end power and let your engines top end take the rest of the way. Either way a small turbo will run out of steam at the upper reach of the rev range and a big turbo will spool slowly but their airflow at higher rev range will eclipse the smaller turbo counterparts
Posted on: Fri, 14 Mar 2014 08:15:13 +0000