gibsanez

No. All you did was state regurgitated information as fact, without anything to back it up. Backpressure is good because it's designed into the engine. What does that even mean? You probably think that large exhaust loses backpressure don't you?

It's called Bernoulli's principle. The larger area of the pipe, the slower the velocity. It's a law of physics. By going to too large of an exhaust, you actually CREATE backpressure. Have you ever heard of exhaust scavenging? If you can create enough exhaust velocity, you'll actually promote a slight vacuum at valve overlap during TDC. You can actually start sucking air/fuel mixture into your combustion chamber BEFORE the piston starts is intake stroke.

Please, with facts, explain to me how making my engine work harder to push the exhaust out of the cylinder and into the atmosphere is GOOD for my engine and horsepower.

Would some kind of restriction of air going into my engine be good too somehow? Why do you think those whale dick PWJDM intakes have such a bad throttle response? Or even a 70mm throttle body on a 1.6? Larger area, slower velocity. Our engines are air pumps. They can only suck in as much as they can suck in, or push out for that matter. Making it harder to do either just doesn't make sense.

esean93

wouldnt you have to tune your fuel delivery if you are running a open header setup?the engine runs lean with low back pressure right?im running open header because my stock exhaust doesnt bolt up and im saving money for turbo.my engine harness doesnt have a o2 plug so im guessing itll run richer.im probably close to 14.7:1 ratio though.

gibsanez

You should get a dyno tune no matter what exhaust setup or bolt on's you have. It's the ONLY way to optimize all the extra air or fuel you're giving your engine. Period.

JdmOdz

iTrader: (2)

Interesting..

gibsanez

Backpressure = resistance to flow.

Why would you want resistance to flow in your exhaust? Why would you want your engine to work harder to push the exhaust out?

Makako

I just peed myself. wow... particles. sucked back in and engine catch fire.. in all the years i've been doing this wow... thank you. that really made my day and will be the joke of the week!.. amazing!

Actually I'm surprised to see how no one had tried to prove the ( sucked back in ) theory.

Maybe it has a big part to do with Exhaust Cam timing, where the valve stays open after the exaust stroke while the piston starts it's downward intake stroke. yet... the rushing intake air pushing all the chamber exhaust gas out wont push it out? damn.. why did engineer's make it that way. oh yeah.. so you have nothing pulling back into the chamber once your exhaust valve closes!! BING BING BING>>>>

i just peed agian.

GoLowDrew

Ok, guys, I thought we can put it behind us as we are going OT from the OP's thread. Here is a quick link.

http://myspecv.com/f/t4739-qr25de-pr...screw-faq.html

The bottom line reply one can post would be "That is a Nissan engine. Not Honda." So we are going way off subject. The OP can do what he wishes. Sorry if you can't agree with my replies.

jdm_bones

iTrader: (5)

Man, you have to love all the false information on this forum....The truth is that it is bad for your engine...This is because once your engine becomes hot, it runs the chance of burning valves, and piston rings..Because during back pressure, your engine will suck in cold air, this cold air hitting a hot surface will cause other serious problems like bent valves, oil consumptions, and even blowing a motor if it is not taken care of in time...Do yourself a favor and atleast run a test pipe with an extended pc. of pipe...This is of courses so you dont go breathing in all the CO fumes...and you don't lose your hearing! But save yourself some money, and just buy an aftermarket Hiflow CAT...It's better for you and the envoirnment...
-Rick

Rotten Egg

guess i should sell my cold air intake lawlz...no wonder everyone is leaving this site..

miguel329

hollow the cat...

static-x

my cat is hollowed out. has been for 5 yrs on a catback thermal exhaust. stock engine. would you say thats good or bad

letsdrive

ok to sum it up.....open header wont hurt your engine but is loud and annoying.
backpressure is bad...sure youll lose torque in the low rpms w/ a larger exhaust, but youll gain in the top end.
honda-tech has a lot of misinfo!!

gibsanez

LOL, backpressure with OPEN HEADER? ARE YOU SERIOUS?

Please explain to me how an open header with an engine constantly pushing exhaust out if it has ANY form of backpressure, enough for cold air to travel up a piece of tubing over 600 degrees, and then in your words, "bend a valve".

You even said it yourself. You love all the mis-information on this site. And you just added to it.

Seriously, look at the facts. Engine, pushing hot exhaust out of a short open header running at a couple hundred degree's, somehow creates backpressure and sucks up air and yet it stays cold enough to make it all the way to the valves and then bend something.



How's this? Big exhaust loses torque eh? How's a dyno chart of a 2.5" exhaust VS a 3" exhaust on a 2.0L K series?

https://honda-tech.com/forums/all-motor-naturally-aspirated-44/who-says-3%22-too-big-k20-dyno-2341715/

letsdrive

well....a k series 2 liter is another story. i was referring to the smaller 1.8 and 1.6 bseries. you give some low end torque and response to get some up top.

Ron454321

Turbo Subarus have had problem with the cat breaking up and blowing back into the engine, may be a little different story due to the forced induction but true nonetheless. I removed the pre-cat and replaced the main on mine because of this.

gibsanez

With a properly sized exhaust, you don't lose torque OR horsepower. Look at the dyno chart. He went with a larger exhaust, and made more horsepower and torque, everywhere in the powerband.

There is a point of diminishing returns. There is a sweet spot when it comes to exhaust area, and it changes with each single engine setup. So what am I trying to say?

Bigger is better, To a point.

That means that no, 5" exhaust on a D15 isn't a good idea. Neither is 1".

patriot

iTrader: (2)

1)gibsanez is right. you just added to the top-secret vault of wrong.
2)subarus dont blows cats into the engine. it's the precat in the upipe that goes up and takes out the turbo. usually doesnt go much further because the metal shavings from the toasted turbo circulate through the engine and thats what really does the damage.
3)the open header myth about burning valves is typically due to the fact that when you change the exhaust system that drasticly you change the air flow and therefore air/fuel mixtures. with a freer flowing exhaust you typicly run leaner...and that's when you burn valves.

89s1

and lean does = hot.

patriot

iTrader: (2)

5" on a 7.3L diesel is the sweet spot, lol.

gibsanez

If one more retard comes in here swinging misinformation without facts or theories about backpressure or exhaust systems, I'm just going to completely drop it and leave honda-tech. Some people refuse to listen.

en00be

zcar4me

I read something that made sense long ago. An exhaust system, and intake for that matter, is a lot like drinking a milkshake. To drink the most of your milkshake with the least amount of effort, you want a large straw. But not so large as to where you can't suck up any milkshake.
It made sense to me.

And the reply above me is hilarious^^^ I miss that show.

ddd4114

Dynamic pressure is reduced, but what about the head loss?

With a well-designed exhaust system and camshaft, the pressure drop is actually petty large.

gibsanez

My understanding of most of the aspects of Bernoulli's law are amateurish at best. The only experience I've personally had with head loss is in dynamic pumps in an industrial setting. I can honestly say that I don't understand what you're asking.

ddd4114

The head loss in a pipe or tube is due to major and minor losses from friction. The turbulence of the fluid, the surface roughness, the number of bends and bend radii, etc. all contribute.

In this case, increasing the diameter will decrease major losses (due to friction) because Reynolds number (a measure of turbulence) drops, and the ratio of surface roughness to tube diameter also drops. With my question, I was implying that dynamic pressure is only one part of the equation; you have to account for frictional losses as well.

However, it was sort of a cheap question on my part since Bernoulli's equation doesn't really apply to exhaust systems. It only applies to flow that is steady, inviscid (frictionless), irrotational, an incompressible - none of which apply to flow through an exhaust system. There are additional terms that you can include in Bernoulli's equation to account for head loss and compressibility, but it still isn't the best approach. You really need an engine simulator to figure out what's going on, and it will probably use the Navier-Stokes equations (primarily) to calculate pressure, velocity, etc.