Hey guys. I've had my Invidia Gemini in my car for quite a long time and my gas mileage goes down from 17MPG average to 15MPG average. So it is now time for me to get a Plenum spacer in order to get some more WHP and get more mileage. I believe my cat-back system supports mid to high range power. I dont know which plenum spacer is best to go along with my cat-back.

Here are my options:
5/16" Motordyne Iso thermal.
5/16" Motordyne Copper Iso thermal.
1/2" Motordyne Iso thermal.
5/8" Skunk2 Spacer.

Since my cat back is best from mid to high range power, which one of these Plenumer spacer do you guys prefer? Mileage is my first choice, HP is second. Thanks for your input.

 

5/16" is by far the most popular and it lets you keep your engine cover on. I generally recommend Iso Thermal but if you drive in very cold weather get the Copper Iso Thermal.

 

Thanks for the info. Between 5/16" and 1/2", which one is best for mileage? and which one is the most suitable with my exhaust?

 

Wait you want increased mpg and power?? HAHHAHAHAHA

 

I've heard that Plenum spacer will add more HP/TQ and gain MPG. But correct me if I am wrong

 

Even if it happened to give a bit of gas mileage, why bother putting it on if you want more power which would suggest you would be getting on the gas more as well, which would overall lead to a drop in mileage?

 

Any gains in mileage, if any, will be so minimal it shouldn't influence your decision, especially when choosing AMONGST plenums. In the event that it does create any gain in mileage, it would probably only be under low-throttle conditions and once you put on the MD spacer, you're just gonna want to go WOT more often which is gonna hurt your mileage.

Plus, like the previous poster said, kinda hard to make more HP and burn less fuel doing so.

 

Spacer is what I have in my mind at this point. I dont know if that is the right decision or not. My G is my daily drive and I dont wanna go to the gas station every other day lol.

 

My point is, you're worrying about something that is a non-issue. If it impacts mileage, it will be so neglible, in either direction, that you won't notice it.

It's not like you're swapping in a V8.

 

Thanks for all your inputs. I decided to go with Motordyne 1/2" Copper Iso thermal.

 

i think you're letting MPG get to your head. ease up on the gas pedal and you'll see increase MPG. especially from a stop! controlling your right foot will beat any advertised product for MPG.

 

^ that is absolutely right. I am trying not to go above 3k RPM. But a spacer would help to save gas a bit more.

 

Hmm why this choice over the 5/16"? 1/2" is really only useful when you are going all the way up the rpm range and in my, and many others, opinion not a great choice for a DD or basically non track car.