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Fast Friday #6: Injectors, MAFs, and Power pt. II

How to go faster, stop quicker, and turn harder. Chips, exhaust, larger turbos, bigger/slotted/drilled rotors, high performance brake pads, manual boost controllers, performance shocks/struts/springs, airbox mods and more! Also discussion on HID and Xenon lights, aftermarket foglights and other exterior lighting.
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Volvo Repair Database Fast Friday #6: Injectors, MAFs, and Power pt. II

Post by matthew1 » Thu Aug 09, 2012 11:05 pm


Fast Friday #6 is by... Lucky!

Injector sizing and MAF sensors, Part 2

This is part 2 of 2. See Injector sizing and MAF sensors, Part 1.

Support Your Supporting Components
As performance levels rise so too do the necessary supporting components of the engine to reach the power goals you might have. Most fuel system upgrades are pretty straight forward and only consist of a fuel components I.E. Fuel pump, Injectors, and fuel pressure regulator. For the most part the Walbro series of fuel pumps are a popular cost effective option when it comes to replacing the factory pump. The 255lph pump is an easy pump to fit to your typical FWD Volvo model with the exception of a few AWD cars where the pump adapting takes a bit more work.

The fuel pressure regulator can be upgraded to higher pressure regulators from various options on the market whether they be same as stock but higher static pressure or aftermarket setups with a variable pressure adjustment. Either way with respect to both pump and regulator if the stock systems are working well there’s not generally a need to replace them unless you’re seeking higher than ~350bhp.

... And Your Injectors
Now injectors on the other hand are a bit more particular and you want to select injectors properly to avoid lean fuel mixtures and poorly spent dollars. We already talked last week about MAF sizing and how quickly the stock MAF can be maxed out but what’s important to recall is that a larger than stock MAF sensor will cause the ECU to detect less air passing the sensor and subsequently incur a lean condition across the board. For the S90 MAF at 2.75” ID it represents (stock MAF=2.4375 ID) a similar size increase as Green injectors do over stock injectors so while some folks will install the larger MAF and larger injectors as a pair to offset one another it’s not a 1:1 ratio and re-tuning is typically still necessary.

Math Time: Work Backward From Your HP Target
Now for some math: To determine the size of injector you need you must first determine you BHP target. For the car in question let’s assume we are looking to achieve 350BHP. Secondly we need to understand the Brake Specific Fuel Consumption (BSFC) of our engine. BSFC represents how much fuel in lbs/hr is required to reach the brake horsepower target. For Volvo turbocharged engines from 1994 and later the BSFC varies a bit but is typically calculated at .57 BSFC. The calculation for total fuel requirement is then BHP x BSFC and the product in this case is 199.5 lb/hr fuel flow. From here we divide by the number of injectors the car has, 5 in this case, which then equals 39.9 lbs/hr per injector. Technically this would be the max injector we would need but that’s estimating the injector at 100% operation on time. Not really feasible and most tuners will tell you that you should calculate for an 85% on time maximum. So let’s add in a 15% buffer 39.9lb-hr / .85= 46.94 lbs/hr

Another way to calculate for injector size can be Max HP ={injector size x number of injectors x max duty cycle}/BSFC. Bear in mind that real word environments change and what works on paper can be a bit different when on the dyno however given the math above this estimates on the liberal size so you would end up with slightly larger than necessary injectors rather than injectors that were almost big enough but not quite.

Tune in Next Week
Next week, let’s talk ignition!

Robert Lucky Arnold
ARD Tuning

~ ~ ~

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1997 850 T5, MSD ignition coil, Hallman manual boost controller, injectors, R bumper, OMP strut brace [gone]
2004 V70 R [gone]