New intake but bad gas mileage

I have a 2000 Dodge Durango with a 360 magnum. I put an air gap dual plane intake on it which is supposedly supposed to make more power (which I definitely felt) and improve gas mileage. Unfortunately, gas mileage is not so good. When I put it all back together I had trouble getting the fuel injectors to line up just right without modification to the brackets. I also depressurized the fuel lines in the process. The manufacturer said I should have it looked over in case I did something wrong. I don’t think I did because everything works fine, the check engine light isn’t even on. I’m just wondering if the injectors not being just right could be the problem or depressurizing the fuel line can make it look like I am using a lot more fuel until I fill it back up. If anyone has any other thoughts please let me know.

P.S. The manufacturer mentioned a vacuum leak but didn’t specify anything. So I will check all my vacuum hoses but I don’t think thats it.

You’ll need to wait to refill it to get an accurate gas mileage reading.

Yeah, if the injectors aren’t lined up right that could definitely affect your gas mileage. Failure to get a good fine mist shot into the intake ports affects the ability of the gas to fully combust, combust at the right rate, and possibly even to do so at the right point in the power stroke. Any of these would mean you’d be turning less of the energy in the gas into usable power and your mileage would suffer.

By the way, if you’re getting more power to where you can “definitely feel it”, you’re doing so by using more gas. Despite what the ad may have claimed, you cannot add a performance modification to your engine that both gives you more power and improves gas mileage. There is no free energy out there.

Hey, thanks for that response. It at least gives me something to try. About gas mileage and power increase though. The way I was understanding things was that if you improve the engines ability to breathe you can have better mileage while driving normally and more power when you put your foot down. The power increase I felt was high end, so only above 2500 or so rpm. Like if you have a better flowing muffler or lack there of. Is my logic sound or do I sound like an idiot, haha.

The new intake manifold likely allows greater airflow, which to maintain the correct combustion will require more fuel. You’ll get more performance, but you’ll also burn more gas. More performance means more fuel and that means less mpg.

The effect is similar to a turbo charger but to a lesser extent. A turbo (or a super charger) pumps more air into the cylinders but more fuel is required as well to have the correct mixture for combustion. Turbo’s give good performance when the petal is pushed down hard. When the car is coasting along the excess air not needed for combustion (because the car doesn’t need much power coasting along) is diverted via a waste gate back into the exhaust.

Your manifold puts more air into the combustion cycle all the time, while the turbo does it “as needed”. This means you’ll likely notice a significant decrease in your mpg. Turbo’s decrease mpg too, but mostly when they are driven hard so the mpg penalty isn’t as great.

Keep the old manifold for awhile. If you decide the extra performance isn’t worth the extra cost for gas you’ll have to option of going back to the standard set up.

Your logic is sound, however that improved ability to breath means ability for the intake stroke to draw in more fuel mix. It does not equate to obtaining more power from the same amount of fuel.

Yeah, enhancing the ability to breath in will affect your power curve. Obstructions and turbulance (another type of obstruction) have more pronounce effects at higher flows, and reducing them will thus have a more pronounced effect at higher RPMs.

Yeah, makes sense. I’ll probably have someone with more experience make sure those injectors are right. If everything’s in order and gas mileage is still atrocious then Hughes Engines are a bunch of lying SOB’s and I’ll put the old one back on. Haha. Thanks man.

Do some tweaking and whatever you can to be sure the system is working properly. Then you can check the mpg and decide if the manifold stays or goes. If the injectors are “off” a bit that can degrade mpg quite a bit.

To really know what is going on you needed to take very precise mpg readings for a month or so before making the switch over to get a base mpg. In addition a good exhaust gas analysis to determine you base of the combustion by products coming out the tailpipe. Put on the new manifold and get it set up correctly. Then run your mpg’s over several tanks of gas for a month and retest the exhaust gases. Then you can tell if you are on track.

Sometimes seat of the pants “performance” comparisons are “placebo” effect. You can test the results scientifically if you can make before and after dyno runs or elasped times at the drag strip to confirm the seat of the pants impression. What a lot of companies rely on is our lack of access to the kind of sophisticated equipment needed to verify the claims they make against the results we get in real world usage.

That is too true. As for testing mpg’s, can I just get one of those little hand held, plug into your CPU things. Specifically what product would you suggest buying for testing mpg’s. I remember the “A/C vs. windows open” episode of Mythbusters they used a lap top. As for exhaust gases I assume you mean get it IM tested now and then in a month.

The only way you can check miles-per-gallon without a lab setup is to track your mileage over a number of tankfuls. You’ll need to compare that to what you were getting before, assuming you kept those records.

I don’t know what IM testing is, but of you’re referring to tailpipe emissions testing that will tell you how effectively you’re burning the fuel you’re using, but is a better indicator of an optimum setup than of fuel mileage, especially of you’re not on rollers. You may be burning more efficently at idle but not at higher RPMs, Fuel mileage is also effected by variables like how the new setup affects tranny shift points. Your power curve changes, your vacuum profile changes, your tranny may shift differently.

But I support the emissions testing to help determine if your setup is good. How effectively you’re burning the fuel is a good thing to know when making these changes.

Be careful when extrapolating from Mythbusters definitive data. They’re more show than science. And they’re typically focused on one specific thing to the exclusion of others that might be relevant for other knowledge.