Free flow exhaust
 

I was reading an older book by David Vizard and he mentions that a free flowing exhaust will improve performance and economy by reducing the "pumping losses" of the motor. Has anyone tried exhaust mods to increase performance? He also mentions that a high compression ratio produces better economy. Perhaps this is why the Festiva is so efficient, it has a 9.7:1 compression ratio stock. I wonder if a higher than stock compression ratio would offset the increase in price of higher octane fuel that would be required (or water injection)?

I've heard this and that about exhaust. But I've never seen any evidence besides the general BS remarks that people make without any evidence of clear thought.

Anyway, most hondas are 9-10 compression, it's part of building an efficient engine, I think, and raising the CR raises the volumetric efficiency of the engine or whatever, so I can see how it would increase FE also, though this has never been tested to my knowledge, so it's hard to say if it does indeed work xor if it'll offset the cost in gas (or the losses from adjusting timing).

I am not experienced with exhaust modifications, but I have read that 'too much' is not good at all. The engine is designed to work around a certain amount of back pressure from the exhaust. If the valves do not have this, a poor running condition or worse fuel economy may result...

'Everything in moderation'

stock backpressure is decent...
 

Yes I'm with XFi on this. My Geo had an exhaust break in the flexpipe (in the downpipe) and it immediately resulted in less FE. Last year I went from 29mpg to 27ish..
It has the "4AFE" 1.6L DOHC and was sounding like a go-cart so I had to have a universal flexpipe welded in.
Then I went back to 29ish and even hit 30mpg.
Anyway, that leak brought that backpressure down which it needed. Its not like it was a computer thing either, I didn't have the 200bux for the repair immediately, so I ran it for three tanks broke...

The Integra has needed a new exhaust for a couple of years now -- it basically just rattles and leaks a bit. I'm still at a loss with the backpressure debate. I'm pretty certain that a stock-design header that's lighter in weight and Mandrel bends in the piping would increase efficiency, but it gets complicated beyond that. I'll probably just stick with the stock design replacement when the time comes.

RH77

the real key is maintaining exhaust velocity. if its too small its like blowing through a straw, if its too big itll slow down too much, like blowing through a toilet paper cardboard thing, man im tired.

anyway, it totally depends on the setup. also any cr bellow 10:1 is low these days.

the argument for a higher compression ratio aiding in fuel economy is that in theory the reciprocating mass is the same but there is more bang coming out of it, hence more efficient, but there also is work in compressing the air...

It really depends. When playing with your exhaust you can change your torque curve, and ideally, reduce the rpm you make peak torque at. By doing this you will gain a bit of volumetric efficiency since you can get to peak torque (where your engine is sucking an atmosphere of air, and maybe more) faster to minimize pumping losses. Otoh, you can screw your torque curve and reduce engine efficiency... It really depends on the engine. As for compression ratios, they used to be raised to increase efficiency, but now that variable valves/ignition have been introduced, it can be set lower with no drop in FE.

exaust systems should be tuned for the engine, and it's operating speeds, and this includes the header, both the langth, and diametor of the tubing used makes a differnce in how the engine is going to run, a well designed exaust system provids minum amount of back presure, and almost pulls the exaust out of the engine thru the momentum of the continuse exaust being pushed out, like thisisntjared said, it's like either blowing thru a straw, or a sewer pipe, you want the exaust moving thru there at an ideal speed, to small and it takes alot of effert to move, to large, and it just sort of lingers bouncing around creating weird shock waves that make it louder, and do creat back presure, and that is where alot of these after market headers cause problems as well, they are too big! yes, they are a nice smooth transition, but they are to big to keep the gasses moving unless you reve the hell out of your engine.

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Rick,

I think you are saying that the exhaust behind your converter is rusted out. In my opinion, that is the only section you can safely "play" with. You can adjust the stock sound of your vehicle by going larger diameter extension pipe and all sorts of mufflers safely without affecting your FE.

I think the part of any stock system that really defines the FE is the manifold to downpipe to converter area.

With that said, don't some users here have data before and after they removed OR upgraded converters(to free flow) or added headers? Maybe if I scan thru all the gaslogs here I'll find something...
Will

Ryland nailed it, it really takes "exhaust tuning" for your intended RPM range to create the right system.

I had replaced my stock muffler on the S-10 with a more 'straight through' design and used a down pipe. Was cool for a couple days, then it seemed like the engine compensated and lost torque; put a pipe back on and things seemed better, so still had the pipe at about stock dimensions, but with a freer flowing muffler. Didn't do any 'testing' though. bummer.

and yeah, my Yaris Compression is like 10.5