airflow - is more or less better?

[Matt Timion]

I've been thinking about this for a while now. I guess this goes along with the question of Horsepower and fuel economy that I raised not too long ago.

So, back to the question.

Is it better to give MORE air to your engine, or less air? Some people say that adding a good exhaust manifold and a wide exhaust pipe will give them better mpg (one guy claims a 50 miles-per-tank increase).

but if this is the case, why do all economy builds (I can only think of Hondas right now - the HF, VX, CX, etc.) have smaller than normal intake manifolds? Am I really to believe that Honda's engineers got it wrong when they build their engines? Should they have put massive intakes instead of tiny ones on these engines?

I'm totally baffled by it. Everyone keeps saying "more air, more air" to increase MPG. All that I can see from that though is more power.

So, tell me why I'm wrong, or tell me why i'm right.

[SVOboy]

<a href=http://community.foe.co.uk/cgi-bin/yabb/YaBB.pl/YaBB.pl?board=online_actions;action=display;num=10 60791111 target=_blank>Exhaust something</a>
This is not really about exhaust size, but reducing back pressuring (same thing bigger size does) to increase mpg.

[abeja]

It isn't more air that is increasing fuel economy, it is reducing pumping losses through restrictive exhaust/ intake points.

[SVOboy]

Originally Posted by abeja

It isn't more air that is increasing fuel economy, it is reducing pumping losses through restrictive exhaust/ intake points.

So then there would not be much difference between a 5 inch and 2 inch exhaust on my 92 mpg b2, since a 2 inch cat back is sufficient to reduce back pressure? And could you recommend some cheaper headers/exhaust crap that will still serve the purpose of reducing back pressure even if they aren't super race quality?

[kickflipjr]

Fro mpg the best setup would have the best wind speed velocity. Big exaust pipes slow down air velocity. It's too bad they dodn't make performance intakes in 1 3/4" or 2" size. that would be better then the standard 3"

[SVOboy]

Originally Posted by kickflipjr

Fro mpg the best setup would have the best wind speed velocity. Big exaust pipes slow down air velocity. It's too bad they dodn't make performance intakes in 1 3/4" or 2" size. that would be better then the standard 3"

Is there some way to figure out hp versus exhaust size versus speed to figure out what be would optimum?

[kickflipjr]

One side affect of free flowing intakes/exaust is it might take longer to warm up. My old car (92' sl1) would never go past the 1/3 temp mark in the winter.

I think that the stock size exaust pipe is best for mpg. You might get more HP is high rpms but, you lose HP at low rpms.


http://www.spswebpage.com/tech/index.php?articleID=a5207vs5223

^
on this site they gained several HP by removing the muffler. (but they don't show the dyno sheet under 4000rpm

[dfoxengr]

a smaller dia intake and exhaust will not get you power, lol

[SVOboy]

But will a smaller intake and exhaust gain you mpg or will going larger increase both power and mpg?

[dfoxengr]

a 3" exhaust on a stock civic wont help you. it's all in what your motor needs.
nor will a 1" intake on the same car help you at ALL
you need to "design" your complete engine setup to where all parts work together.