Will installing an intake on a Civic Vx affect MPGs?
I have a VX and I'm thinking about putting an intake and exhaust on it. How bad will it affect the MPGs? I was thinking the extra unrestricted air would be good for the engine and might increase HP as well as MPG.
Though intake piping isn't nearly as critical as the actual intake runners, increasing diameter might increase overall flow, but it lowers velocity. What this means is that air isn't accellerated to as high of speeds, meaning the air is moving slower. What this will basically do, is raise the RPM of the peak efficiency of the intake system slightly. You'll loose low RPM effiency, but gain upper RPM efficiency.
But again, intake piping before the throttle has little effect compared to adjusting the runners after the throttle. Though removing too much volume from the intake piping before the throttle can massively impact throttle responce/drivability. Why? Because the computer senses the Throttle move and adds fuel before the MAF or MAP senser can tell the ECU the difference in actual air comming in (this is call transient enrichment). With less piping volume the air takes less time to start entering the throttle, causing you to go lean, and if you're running just an open throttle body there will be very little direction for the air before it gets to the throttle, so once you get above 2k rpm or so there will be lots of turbulence, causing the engine to go rich from less efficent air flow.
My conclusion: Don't spend money on those fancy over the counter intake systems, they're barely (if any) different than what you can build at home. Make your own intake piping and see how it affects mileage. If you like the results and want something a little more "official" then go ahead and buy one.
Gollum is 100% correct and obviously an individual in the know. As for the exhaust system: I have experienced gains both in performance and mpg by building free-flowing exhaust from the CAT out. You can see one of those exhaust systems at http://fabmaster.pictiger.com/albums click on the "Honda HF to VX" album then go to page 5. It shows in there. Click the first picture and it will enlarge. Also, in the Gassavers How to/DIY forum in the thread CRX HF to VX Conversion is the written description of the exhaust.
I do not recommend putting a header on the VX. The alteration of the factory scavenging effect of that exhaust manifold may adversely affect mileage because larger exhaust ports may slow exhaust pulses on such a small engine and increase the chance for reversion (exhaust flowing backward.)
My VX had and Intake on it when I bought it, I removed it to put a stock intake an box on, the power curve is much smoother and there there seemed to be a noticeable difference in low end power...I think if you switch the intake and decrease the low end power you will have to in turn use more power and more fuel...I'd stick with a stock intake...
As for exhaust, mine came with a muffler shop system on it, no resonators just straight from the car back to a muffler, I like how it sounds not raspy and not overly loud, I never had a stock system on the car so I can't complair but I don't think you will see any major negative effects from changing the exhaust just don't go overly large...2 1/4 ideally maybe even 2 1/2...I know Dan has an Apexi WS2 on his VX and it sounds nice and he gets fairly decent mpg numbers still...HTH GL
I appreciate the feedback guys. I'll probably stick with the stock intake and go for just the exhaust, but nothing large or loud, just with a good sound to it and possibly improvement in performance and mileage.