Intake Question

huskerpt · Post by **huskerpt** » Sat Jul 03, 2004 5:16 am

I need to explain something before I ask my question, so please bear with me. I have a 2001 Dodge Dakota Quad Cab with the 4.7L V8. The only thing that I have changed on the truck is a drop in K&N filter. I routinely get 18-19mpg on the interstate. The truck is only rated to 18mpg. I post on a Dakota forum and the guys with the exhaust, cold air intake, etc... seem to get worse mileage than I do (for the most part). Finally, to my question. I have put 1600 miles on my vibe in the 2 weeks I have owned it. On the 500 mile trip we drove yesterday on the 2 fill ups I was getting 33 mpg. Is putting an exhaust, CAI on going to hurt fuel mileage or will it stay about the same (If I can keep my foot out of it)? Secondly, is there any benefit from the drop in filter for the Vibe?Sorry about the lengthy post.

huskerpt · Post by **huskerpt** » Sat Jul 03, 2004 5:17 am

About the fill ups yesterday, one of them was 6 gallons and the other was 10.7 gallons for 340.7 miles.

ragingfish · Post by **ragingfish** » Sat Jul 03, 2004 5:21 am

More often than not, performance modification negatively affect your mileage, regardless of whether or not you put the pedal down. These engines are (theoretically) tuned for optimal balance of fuel economy and performance. By putting on, for example, an air intake, you're bringing more air into the cylinders. The engine has to balance out the air/fuel ratio, so more fuel has to be used to properly combust. Thus, more fuel gets used.Not sure about how the exhaust works...but I'm sure someone else can chime in and answer that...

blk182@n7 · Post by **blk182@n7** » Sat Jul 03, 2004 5:50 pm

how did this turn into a dodge discussion? performance upgrades will not always have a neg affect your mpg. depending on what it is. i changed the spark plugs, intake, exhaust, and my mpg chnaged from 330 to 13 gal to right around 375 to 13 gal on the hwy.basically you want cold air in and hot air out. colder the air in - the more power.

Smokin' Rubber · Post by **Smokin' Rubber** » Sat Jul 03, 2004 6:00 pm

I don't really believe that stuff about the backpressure... how the hell can having more pressure help the engine? Makes no sense since you would think that the easier it is for the car to push the air out the less it would have to waste power doing that and the more it could put to the drivetrain and wheels.

blk182@n7 · Post by **blk182@n7** » Sat Jul 03, 2004 6:06 pm

I agree with that

cohocarl · Post by **cohocarl** » Sat Jul 03, 2004 7:43 pm

Quote, originally posted by Smokin' Rubber Â»I don't really believe that stuff about the backpressure...I'll agree to a point. I can see how there may be a scavaging effect that would help the exhaust out of the cylinder with the correct size header pipe. But generally a stock exhaust system could be less restrictive. On the other hand, Devlop's motor has alot of back pressure and he's making alot of hp. I had a custom rear exhaust, and an Injen CAI and I was regularly getting 32mpg. Both would probably give you better mileage if you drove exactly the same as w/o, but that CAI has such a beautiful sound when you get up around 4,000rpm or so.

Smokin' Rubber · Post by **Smokin' Rubber** » Sun Jul 04, 2004 4:25 am

LOL find the diagram of how the valve routing for the exhaust works on that link it looks so ineffecient, the pipes in that Outback's muffler are looped (as in they go in a complete loop)... and as for the back pressure thing on Develop's car.... maybe he'd have more if the pressure on his engine was relieved Someone find some kind of evidence that back pressure is good and I may concede but otherwise I'm going to mount 100 mm exhaust on my future cars lol now that is free flowing my friends

redlava · Post by **redlava** » Sun Jul 04, 2004 9:46 am

I have both an exhaust and an intake on my Vibe and I have noticed that my city milage has gotten slightly worse after the mods. Probably around 24 MPG compared to 26 or 27. My highway milage has not really been affected at all. On my recent roadtrip to Michigan for the supermeet I was getting 36 MPG. I have over 11,000 miles on my car now so it is well broken in. Just as long as you don't have a lead foot, which is hard sometimes after the mods , you probably won't notice too much difference.

Smokin' Rubber · Post by **Smokin' Rubber** » Sun Jul 04, 2004 7:59 pm

Quote, originally posted by silverawd26 Â»Ok.. This part I really messed up on... The right information:Too much backpressure will cause a loss of power. LOL poor silver we forgive you, but are you still saying that no back pressure at all is bad?

goodvibe · Post by **goodvibe** » Sun Jul 04, 2004 11:29 pm

Actually, some restriction in intake or exhaust can help with torque at lower RPMs but is always bad for maximum power ratings. It keeps the gasses from escaping past the valves before they close for combustion when the valves are open longer because of the motor spinning slower. Everyone here with an intake, true catback exhaust or both makes less power at 2k rpm than they did before but possibly much more above 3k. As far as mileage is concerned. There are going to be variables because of the fuel curves matching your mods and how well the stock bits are designed(good in our case). You could find yourself running either a bit leaner or richer at different throttle positions or RPM than before but less restriction means more air running through the engine and more gas with it to make the extra power. Mileage should drop with mods but if you're running leaner it could go the other way. On our cars, CAIs tend to run leaner at normal driving conditions so that we can make the most power at high RPM full throttle where we are a bit rich. It's more complicated but assume everything leans a touch. It's caused some CELs on installs. TRDVIBE probably had this issue with the TRD intake before Unichip. If I was getting better mileage after an intake and exhaust install, it wouldn't always thrill me.

Smokin' Rubber · Post by **Smokin' Rubber** » Mon Jul 05, 2004 8:14 am

So its like the whole debat with turbo sizes. Larger ones make more total horsepower but are weak at low RPM's because they require more RPM's to build up pressure. Kind of an interesting thought

huskerpt · Post by **huskerpt** » Mon Jul 05, 2004 10:16 pm

Completed the trip yesterday and after sitting on the interstate for 1 hour (stupid construction) the GT got 32 mpg on the fill up. Not to shabby for a car that has 2200 miles on it. I ran somewhere in between 75-80mph. I can't complain.

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