high flow exhaust = 3-5% mpg gain?

[Blu3Z3rg]

The reason, whether it's right or not, would be that the exhaust gasses get burnt a second time in the cat and therefore should take up less volume. Do a cat back and you'll see some power and mpg increase.
If you do a catback with 2" all the way through, maybe with a straight through muffler of some sort (flowing proper cfm isn't the only thing, a smooth flow is necessary) then you will see your best gains for the money.


Remember, exhaust works with FLUID dynamics. Meaning, think of water flowing through it - is it smooth, or will the water get chopped up and sloshed around?

[anotherls]

Cool way of looking at it, and it makes sense, I want something quiet and the old one is rusting, so if I can put one together cheap I will do that but I will wait a little longer, probably cheap and easy to find 2" parts i hope.

[mcrews]

Originally Posted by Blu3Z3rg

The reason, whether it's right or not, would be that the exhaust gasses get burnt a second time in the cat and therefore should take up less volume. Do a cat back and you'll see some power and mpg increase.
If you do a catback with 2" all the way through, maybe with a straight through muffler of some sort (flowing proper cfm isn't the only thing, a smooth flow is necessary) then you will see your best gains for the money.
?

can you show the data you input to come up with the 2" size .

[anotherls]

Originally Posted by mcrews

can you show the data you input to come up with the 2" size .

I think it was because of the exit size of my catylatic convertor, also in the other exhaust threads there is a link which I cannot post becuase I dont have enough posts yet, but it shows you how big of exhaust pipe you need for what horsepower levels. The car I drive is 140hp and it is on the border i believe for 2'' to 2 1/4'' pipe, and the way I drive my car 2'' should be perfect.

[mcrews]

here are the factory specs on your car:
Engine & Performance

Base engine size: 1.7 L
Cam type: Single overhead cam (SOHC)
Cylinders: inline 4
Valves: 16
Torque: 110 ft-lbs. @ 4500 rpm
Horsepower: 115 hp @ 6100 rpm
Turning circle: 34.1 ft.
Base engine type: gas
DriveTrain

here is the basic chart:
Pipe Diameter (inches) .....Max HP Per Pipe ......Max HP For A Dual Pipe System
1 1/2 .................................78 ....................................155
1 5/8 ................................. 92 .......................................185
1 3/4 .................................108......................................... 217
2 .................................144 ...........................................289
2 1/4 ...............................185................ ......................... 371
2 1/2 ................................232............... ............................. 463

at 6k rpm, the BRAND NEW Civic made 115 hp. THat is an unsustainable rpm over more than 5-7 minutes on a used car.
given a 10 yr old car, the MAX hp is maybe 100.
I'm not sure what 'how you drives' means, but you are on an mpg thread , which implies low rpm.
factually, you need to be at 1 3/4 MAX and in reality, 1 5/8.
If you have a scangauge that is correctly calibrated, you can see the hp being produced at cruise.

What most people seem to do is think that the pipe should reflect the max horsepower of the engine, and that is incorrect. the pipe should reflect the hp that you use 70% of the time. Not the hp you use less than 5% of the time.

[mcrews]

Originally Posted by SiVX

Can someone with a stock vx please crawl under the car and give me a pipe diameter? or circumference if thats easier for u... the vx I bought recently has an aftermarket pipe from the exhaust manifold/cat. then it has a 2007 SI muffler added. and yes i have googled and tried to look up replacement parts for the sizing but they dont give me all that info wherever i find parts.
I've read all the stuff about less back pressure at higher RPMs. I plan on maintening those lower RPMs for lean burn and FE. Seems to me trading it off for a stock would be the thing to do??? I'm confused lol
I still don't have enough info for the pipe calculator. Dont know where to find valve opening BTDC or closing ATDC or duration...
Thanks!

sivx,
your civic new produced max 92hp at 6000 rpm.
you need a single pipe in the 1 1/2 diameter
based on the chart above.

[anotherls]

Originally Posted by mcrews

here are the factory specs on your car:
Engine & Performance

Base engine size: 1.7 L
Cam type: Single overhead cam (SOHC)
Cylinders: inline 4
Valves: 16
Torque: 110 ft-lbs. @ 4500 rpm
Horsepower: 115 hp @ 6100 rpm
Turning circle: 34.1 ft.
Base engine type: gas
DriveTrain

here is the basic chart:
Pipe Diameter (inches) .....Max HP Per Pipe ......Max HP For A Dual Pipe System
1 1/2 .................................78 ....................................155
1 5/8 ................................. 92 .......................................185
1 3/4 .................................108......................................... 217
2 .................................144 ...........................................289
2 1/4 ...............................185................ ......................... 371
2 1/2 ................................232............... ............................. 463

at 6k rpm, the BRAND NEW Civic made 115 hp. THat is an unsustainable rpm over more than 5-7 minutes on a used car.
given a 10 yr old car, the MAX hp is maybe 100.
I'm not sure what 'how you drives' means, but you are on an mpg thread , which implies low rpm.
factually, you need to be at 1 3/4 MAX and in reality, 1 5/8.
If you have a scangauge that is correctly calibrated, you can see the hp being produced at cruise.

What most people seem to do is think that the pipe should reflect the max horsepower of the engine, and that is incorrect. the pipe should reflect the hp that you use 70% of the time. Not the hp you use less than 5% of the time.

Thanks for the info I did read that chart off of the max hp I have the ex which was 127 (thought it was 140 that must have been my old one), the way I drive is never over 2800 and definately never over 3000 rpm, I use the torque but not so much the hp, to keep speed is about the only reason I use hp, which I have teh ultragauge so I will have to keep an eye on it. Do you think the bottleneck in the exhaust is a problem though, or something not to worry about, I have been getting very good mileage with the good weather, since I got the ultragauge I have averaged about 42 mpg, which is a pretty good start for me, it was just weird to see the change in pipe sizing while I was hunting down that exhaust leak.

[Frank Lee]

There are diameter changes all over the place in exhaust systems. Seems like many mufflers have smaller diameter outlets than inlets and they work. It could be that the velocity changes resulting from the diameter changes don't hurt flow like we imagine they do.

[niky]

Depends on how that diameter change is implemented. But for a stock exhaust system, you won't feel the difference at low to medium rpms as much as you would at high rpms.

And a ten year old Honda D17? Should be making +/- 5 hp compared to stock. Those are reasonably robust little motors.

Still have issues with that pipe chart, because I've seen more power than that out of a 2" pipe... but we're not after maximum power here, anyway.

[jtbo]

I have read backpressure here and it really is more or less a myth with four stroke engines, two stroke engine does need back pressure but with four stroke fuel injected engine there is no use for that really.

The truth about exhaust backpressure and torque

Good exhaust manifold for economy is built so that length of pipes matches cruising rpm, each pipe should be equal length too, it is made so that previous exhaust pulse 'draws' next one out, so that cylinder fills up easier with fresh stuff and with length of pipes you can match effect to be most at any given rpm. There are surely lot of calculators for this too, haven't bothered to look that much.

It is similar myth to one where intake manifold should never be polished, that was because with carburetor fuel was mixed better with air when there were some turbulence and fuel did not get stuck to intake walls when there was rough finish, but with port injection there is no need for such, so intake can be polished for maximum air flow.

There are lot of myths with cars and especially with performance. Many performance aspect also increase efficiency of engine as basic idea of adding performance is to get more air into cylinder, performance minded will put more fuel to the mix, one that likes to get more mileage out from car wants more air into cylinder to burn engine bit leaner, however be careful not to overdo that, another benefit of better flow trough engine is that it will burn cleaner.

I did read from somewhere how somebody seriously was worried others messing with intake pulse ram effect by making changes to airbox, now there is one misconception again, that stuff should be left inside to intake plenum, certainly airbox has nothing to do with that effect, but so strong are beliefs instead of scientific facts that beliefs live and spread more.

Diesels are much simpler in there as they are made to burn lean, they are made to 'knock' so those are not easily broken by going really lean.