better flowing exhaust = better MPG???

[red civic standard]

I am working on a project car and motor that's sole purpose is attaining the highest MPG possible. I will be using a Civic VX motor with a VX gearbox in a 92 Civic DX coupe.

Do you think a better flowing header/exhaust would improve my MPG or hurt my MPG?

[SVOboy]

You bastard! I just saw this on h-t and was about to invite you over here, this is a better question for the general tech section, and has been asked there once but no one knows. I talked to an expert and he told me it really depends case to case, so you'd just have to see. Tell us more about your set up though, ;-)

[Matt Timion]

there was a thread we started on this a while ago.

http://www.gassavers.org/forum_topic/modifying_exhaust_to_increase_mileage.html

In theory it would help, but then again HOnda's engineers have always had small intakes and small exhausts for their fuel efficient cars. most of the people who say to go big have never built for fuel economy before.

It would be nice if someone tried it

[n0rt0npr0]

I've done this and it don't work. I put together an experimental exhaust and I ran it for four tankfuls. I admit I do not know how much more free flowing it was, but it was shorter and it had 1/4 inch more diameter to it.
I lost a lot of low end torque when I personally did it, and that torque is what gets you nice city MPG's
My MPG fell by an average of 3 (45%city - 55%hwy at the time)
~Will

[SVOboy]

Yeah, I forgot to mention that bigger exhaust = more high end hp, less low end torque.

[red civic standard]

Originally Posted by SVOboy

You bastard! I just saw this on h-t and was about to invite you over here, this is a better question for the general tech section, and has been asked there once but no one knows. I talked to an expert and he told me it really depends case to case, so you'd just have to see. Tell us more about your set up though, ;-)

The car is a 1992 Honda Civic DX 2 door coupe. It currently has a 1.5 litre non vtec engine and DX transmission. I have added a K and N drop in filter and NGK spark plugs and spark plug wires. It currently manages 38-41 MPG on the freeway.

Over the winter I am rebuilding a civic VX motor. This is a 1.5 litre VTEC motor that honda designed for good gas mileage. It has roller rockers and other cool stuff that help with MPG. This will be mated to a Civic VX transmission (longer gears). The engine will be tuned with real time programmable software through a laptop computer.

The car will recieve a front lip, a 1-2 inch drop, camber kit with full alignment, full underbody paneling, rear diffuser, and some super skinny 13" tires.

I am currently debating the modifications that will be done to the motor. I will certainly stick with NGK plugs and wires and the K and N drop in filter. I have at my access a 99-00 Civic Si exhaust (direct bolt in) which has larger diameter piping.

[SVOboy]

That's a lot like my proposed set up except now I have a 91 crx dx. I will be doing an hf tranny and the vx mini-me that'll give me a higher compression ratio for a more efficient engine. What's your mpg goal?

[red civic standard]

My goal is 50 MPG average for all of next summer. A lofty goal, but you have to shoot for something.

The car gets 38-40 MPG now. I figure that all of the little mods that I do to the engine, transmission, and vehicle will net me an additional 10 MPG.

[Matt Timion]

Originally Posted by red civic standard

My goal is 50 MPG average for all of next summer. A lofty goal, but you have to shoot for something.

The car gets 38-40 MPG now. I figure that all of the little mods that I do to the engine, transmission, and vehicle will net me an additional 10 MPG.

You do realize that I'm pretty much doing the same thing to my car, right?

well, not really, but I am planning on the following:

• d15b vtec-e engine (jdm version of d15z1)
• Civic HX rims and LRR tires
• CRX HF transmission (since I don't have a hydro tranny in my 89 Civic I cannot simply switch to the VX tranny like you can).
• MPFI swap
• K&N air filter
• NGK wires
• unknown high performance plugs

I currently get about the same mileage as you (38-40) and I'm hoping to break 50 after the engine swap and wheel replacements.