Posted By RichC on March 16, 2006
A fellow Volkwagen TDI driver emailed me a question about my fuel mileage, and in particular if I “noticed a change in MPG when I switched to the Sprint 520s? What about smoke?” He’s talking about the fuel injectors that I originally put in my Volkswagen Jetta TDI to increase performance.
I answered basically “No” and “No” to just the mild Sprint 520 or PP520 injectors. No noticable drop in fuel economy or increase in smoke …. even without adjusting the IQ (Injection Quantity) with the VAGcom interface and software.
My theory goes like this … if the fuel you add to the cylinder gets complete combustion (equal btu’s), the engine in your car (m) will provide an equal amount of work (W) sending you down the road with the same velocity (v). (v = speed with direction)
Of course there are a few other variables but I’m assuming you drive you car exactly the same …. equal acceleration and speeds. (hard to do when you have new found power!)
Its when you change your driving habits (because you will with more ‘available’ power) that is you’ll notice a drop in your fuel mileage OR if you install overly large injectors that spray excessive fuel into the cylinders … noted by black smoke (incomplete combustion wasting fuel). Even with a chipped car, if done correctly, and the fuel and air balance is kept appropriate, the mileage will not drop, trusting you drive conservatively. I do notice that my mileage has drop just a little but then that’s because 1) I use the extra power too often, and 2) I ‘foolishly’ opted for even more aggressive injectors than the Sprints or PP520s and have a Rocketchip Euro III setup at a recent RallyVW Tuning session. My new setup over-fuels considerably anywhere past half throttle and the only way I see to correct this is to add more boost. (bigger turbo: VNT 17, 20 or 22) I can eliminate smoke by running biodiesel at near B100, but still am overfueling under hard acceleration and wasting fuel.
I still manage 46-47mpg on the highway at 75mph so long as I don’t drive aggressive … otherwise I’m going to be in the low 40s. But once you drive a TDI with more power there is no going back … the ‘torque pull’ is addictive. I maintain a pretty accurate lifetime cost of ownership … on a spreadsheet if you care to check it out.