anyone read david vizards book on exhaust and backpressure? its got some interesting insight on the way exhaust works.

 

dude you gave me a good laugh thank you

but just a simple thing here
basic design and operation from a honda motor isn't any different then a chevy motor

both are based on a 4 stroke design


but once again
thank you for the good laugh almost had me on the floor laughing


also for the little import crap comment
goes to show you something about why those motors have a bad name with people like this running around

 

There is difference between fact and opinion. Backpressure being bad or good one way or the other is a fact. If it were the color of a car being debated, then yes comments should be respected, but the rampant belief of misinformation is ever-present and should be smitten wherever and whenever possible. It isn't that anyone has a personal beef with anyone believing the myth, just a problem with them trying to present it as fact. Sometimes flames are what it takes to enlighten...

Jared

 

Good first post newbie!!! Your gunna fit right in here new guy. Welcome to TGO.

we need a :werd: smilie. and one of these

Dead on.... mis-info can spread like wild fire. You wouldnt belive the mis info I hear about stuff every day...

 

I agree for a noob this guy already has my respect

good job jaredi

but are you sure you haven't been here under another name

just by that one post you seem to have been here before

 

The force is strong with this one... He will grow up to be one of the strongest jedi third gens ever to walk TGO.

sorry, just read the Start Wars III spoiler... movie is gunna be hot.

 

also you notice people are so strong about their idea as to what is right or wrong

but for the life of them CAN'T explain it

they just know it is right and that is all there is to it


lets see this guy bonz explain why lack of backpressure would hurt import "crap"

 

My ICQ name actually is JeDI.

I just hope I made my debut a little better than this guy.

Most of the time it is just something they heard that has no factual base. It never gets questioned in their mind because X person said it works this way, or X person said they had a car one time that did X because and on they go never questioning, just accepting as fact. And then it gets really bad when they are asked to explain and can't. Then they take it as a personal attack, when it's not... Just sort of an invitation to think about some ideas that may not be right... It's happened to me, this thread has made me think, and rx7 speed's post has made me understand exhaust much better than before.

Threads like these are the ones people learn from, flames or not.

Jared

 

one of those people that does make it sound like a personal attack is me
granted kinda aggressive with how I put things but most know it is just to be taken lightly and that there are times the pot likes me to stir it


but yeah it does seem that way with someone questioning why they think it is that way and it turns into an attack

eh oh well


but hey still for a noob your fitting in too fast lol
think your more of a veteran here so far then some other people who have almost 1000post on there belts

 

If I don't say alot it makes people think I am smarter than I am

I have noticed that here. I have seen some people with over 500 posts that don't know what they're talking about, things that even I know (in all my three weeks reading this board) that they have yet to learn. And their post count is that high? I don't understand... Maybe he was just being sarcastic or something...

Jared

 

I'm curious about the logic that gutting or removing the cats will make the car run worse. Unless your car is running an OBD-II diagnostic system, there's no post-cat oxygen sensor, so whether or not the converter is there should make no difference in how the car runs.

Also, for whoever is using the old "drag cars use open headers" argument, there are 2 things you forgot to consider. The first is that drag cars definitely do not operate in the same powerband as a typical mild street car. A large diameter primary tube and a shorter length, like an open header, is more optimized for high-RPM racing situations. Although running your engine open header might not reduce your hosepower output, it's not really going to help it much either.

The second factor is the exhaust length. Once again, drag cars may run open header, but the primary tubes on those headers are probably half the length of your entire exhaust system. Answer me this, if the engine was supposed to have absolutely NO backpressure at all, why would a Dragster even run headers in the first place? Me and a buddy had the same question, so we ran a test mule engine on a stand with no exhaust manifolds. The wondrous result: we burned all the exhaust valves and we pretty much toasted the heads. And for the record, I've also run my car with open headers and there was no measurable performance gain. Although I can't lie, it sounded so good I almost hated to put the new exhaust on.

While I'm at it, I guess I'll actually say something related to the original topic. The cat is removed on my car, and it didn't shave seconds of my ETs or make it spin tires by tapping the gas pedal. But unless you've been replacing your converter every 50,000 miles like you're supposed to (doubtful), then chances are removing it will make the car breathe easier, a clogged converter will make the car run like crap. Removing or gutting the converter will give the exhaust a deeper note, and replacing the oxygen sensor at the same time could actually make a decent driveability increase. My two cents, maybe I'm not 100% correct, but I know I'm not completely wrong either.

 

Drag cars run headers in the first place because if they didn't the entire car would be engulfed in a ball of flames. Gutting the cat can cause turbulance in the exhaust by going from small to big back to small and the exhaust gasses bouncing around inside the now hollow cat isn't all that great. The fact still remains, any type of backpressure will cause a decrease in both hp and torque. You can't argue it, it's a fact. BACKPRESSURE IS BAD! Umm, you ran an engine with no exhaust components what so ever???? WTF were you thinking man. No offense, but it's pretty common sense that you don't do that. Headers/exhaust manifolds serve a purpose. They are there to keep the thousand degree exhaust gasses that shoot flames out from destroying everything in the area. No wonder your results were negative.

 

Psscht, lighten up a bit guy. Like I said, we ran the test mule engine (read: it was going to the junkyard anyway, so who cares about it) on a stand. No one was looking for real world results that way. But hey, it only makes sense that you'd ignore the couple of facts I threw in and target a stupid joke test that me and a couple buddies did when we were about 17. Like I was saying, the length and diameter of the tubing has plenty to do with it. You wouldn't run a 1300-cfm mono-blade throttle body on a stock 305 TPI, would you? Much less impedance to flow there, only problem is that an opening that big would slow down the velocity of the incoming air. Same rules apply to exhaust. If you run open headers or some ridiculous 4-inch diameter exhaust pipe when you just plain don't need it, you're going to slow down the exhaust flow. So instead of repeating BACKPRESSURE IS BAD over and over, maybe you can just chalk it up to "use a little bit of thought and some clever tuning." The whole reason there are different diameter exhausts and different sized primary tubes is because there's an optimum balance between pressure and flow. Completely eliminating the backpressure altogether isn't quite the miracle answer to everything. Just think about it.

 

ok backpressure doesn't cause flow though
it reduces flow

also when flow goes up pressure at the valve goes down
this is due to scavenging
think about differences in high pressure vs low pressure zones
high pressure moves to a low pressure zone
the greater the difference the faster the movement
so in my mind that says lets reduce the pressure in the exhuast as much as possible

now getting the right sized exhuast system will speed it up without inducing much if any pressure and actually reduce the pressure at the exhuast valve opening


as for why you wouldn't want to run without an exhuast system
partly noise
mainly scavenging

there would be none becuase your exhuast pulse wouldn't be able to create a low pressure zone behind it to help pull out the next pulse

 

I guess no one reads david vizard anymore.
according to the book, having some pressure is necessary to optimize the velocity of the exhaust gas to exert a negative pressure "pull" on the cylinder during overlap, increasing cylinder fill although backpressure is present in the exhaust system.

this is directly from the book:
"As we know air flows from one point to another by virtue of a pressure difference...
"this difference is normally associated with the piston travelling down the bore thereby inducing fresh charge...
"the better the head flows, the less suction it takes to fill the cylinder. for a race engine, the pressure difference between the intake and cylinder should not exceed about 10-12inches of water (.5 psi). "
"...for most cost conscious motors, 20-25 inches is the limit of decent power is to be achieved (street motors, about 1 PSI).
"from this we can say that at most the pistons travelling down the bore exerts a suction of <b>1PSI on the intake port.</b>

"on the most successful single 4-barrel drag race motors, a well tuned exhaust will exert a suction of <b>6-7PSI</b> on the cylinder. because this occurs during the overlap period most of this suction is applied, via the open intake valve, to the intake port. from this it vcan be seen that the exhaust system draws on the intake port as much as <b>500%</b> harder than the piston travelling down the bore. this demonstrates that under these circumstances, that the exhaust is the primary element of induction, not the piston travelling down the bore..."

"<b> IS bigger better? NO! </b> if the pipes are too big scavenging will suffer"

and it also goes on to explain how equal length headers are a waste of cash, how exhaust pressure is important, etc...
you guys should read it somtime, it will clear this mess up, and educate a few non beleivers.

 

Alright, now lets first start out by defining backpressure. Back, as stated in the dictionary, is "to cause to move backward." Pressure, as defined in the dictionary, is "a constraining force or compelling influence." So backpressure in essence means to cause a constraining force to move backward. Why in the world would you want a constraining force to move backward against the flow of exhaust gasses??????? The answer is you wouldn't! Please do not confuse backpressure with velocity. The backpressure debate is old and tired and has been proven several times over that you do not in any way need it. You can argue all you want, but I'll just go look at the tens, if not hundreds of thousands of dyno graphs and #'s that have proven this very fact. BACKPRESSURE IS BAD!

 

i never said backpressure even once in the thread that i just wrote.

only pressure. you always have pressure in exhaust, no matter what. even if you say "no theres no pressure because its reading 0PSI" your still wrong because here on earth at sea level 0psi is still 14.7PSI atmospheric.

so no matter what, pressure is present, and important.

 

The reason you have to use manifolds or headers on an engine, yes even on the stand, is so that when the exhaust valve closes, it doesn't get a massive rush of cold air over it. That's bad because it will cool it off so quickly, and it happens so often (..open, close, open, close...) that it can cause the valve to lose its seal because it can burn, warp, crack, or do other things that are, in general, bad.

Back pressure is BAD on ANY 4 stroke engine. Our engines are 4 stroke engines.

I think this "backpressure is good" myth came from the fact that a 2 stroke engine (think Nitro RC cars here) needs backpressure for it to run properly and make lots of power. That is why they have a tuned pipe that is specially designed to force unburned fuel back into the engine by backpressure. The backpressure thier exhaust systems make also keeps the pressure in the cylinder(s) up as well. On a 2 stroke engine, this increases both power and fuel economy.

We, however, have an exhaust valve to do that for us, so why restrict the exhaust system?

If you wanna go fast, set up the exhaust to have as close to no backpressure as possible.

THAT is the reason that headers, high flow cats, high performance mufflers, and dual exhaust systems will make MORE HORSEPOWER!

 

Let me get this straight.

Say I wanted to measure the existence at any point of "backpressure" in the exhaust of any typical passenger vehicle. I would then have to put sensors before and aft any possible restictions in that system, i.e., bends, areas of increased size or decreased size, to measure the pressure before and aft these areas. CORRECT????????????

So by the rationals that I have read so far: Starting from the headers, if you could make an exhaust system, w/o mufflers, that was completley straight, the the pressure, or lack thereof, would be the same, at the headpipe connection and the same near the tailpipes.??????????????

Does the fact of the pipe itself containing these exhaust gases create "backpressure"????????????????

Just a few questions I would like to get answered.....
.......eventhough I think I know the answers.....he he

 

If they did a poor job building the exhaust system (think GM here) then yes, the pipes themselves will create a very small amount of pressure.

But a properly built exhaust system with smooth, clean bends and made of the right size pipes, will make so little pressure that it would never be any kind of problem.

 

running something to mesure pressure in my exhuast right not very far from the exhuast port at 7000rpms WOT in about 3rd gear I get about 3.5lbs of pressure or so

and I think can get it a little lower then that when the muffers (read rusted falling apard stupid chambered like design) are replaced


also yes the pipes do create pressure in the system

but also think about this
pressure at the valve vs pressure at the rest of the system
you are going to have pressure in rest of the system no matter what
less pressure everywhere the better in my eyes as long as velocity doesn't suffer (ie going to 5 inch pipes with 2" primary on a 305) but the high the velocity the lower the pressure at the valve will be

ie you might be pulling 3psi of pressure when taking a reading at your mid pipe but yet exhuast valve might be seeing suction of about 1psi
note numbers are not exact just throwing them in for referance sake

kingtal
i don't think that pressure is wanted but rather it is going to happen in the exhuast system regardless
by putting pipes on you make pressure in the exhuast, bends make pressure, but the end result is LESS PRESSURE AT THE VALVE. so again pressure itself isn't wanted but a byproduct that can't be avoided

also mr dude you say that backpressure is needed b/c during overlap the intake charge gets sucked through the exhuast

but here is a counter

backpressure will put a load on the motor which waste energy, reduces scavenging due to pressure differential, possible reversion, cause hotter running exhuast valves, cause contaminated a/f charge due to burnt gas not being fully expelled

so is it really worth it to save a little bit of fuel?

second i don't think that the exhuast is going to suck out THAT MUCH a/f on a 4 stroke engine
the time of overlap is very small and that is partly what a byprudoct of scavenging is a little bit of wasted a/f

 

Whew!!!!!!!!!!!!!!!!!!!!!!!

 

I've got a true dual setup on a firebird with some sound and noticeable power differences. I'm with you on unrestricted exhaust but i'm also with the backpressure somewhat. You dont need "Backpressure" though, what you need is to improve your exhaust scavenging. H pipes, crossover's and highflow cat's and mufflers all have to work in unison. It isnt just horsepower some people want, some people want an aggressive sound too.

 

now I know this is old and has already been taken care of but it amuses me when people say this.


if you resist something more it makes more power? maybe when that resist the motor from turning by putting agreater load it waste energy.

but lets follow this train of thought for a moment of some other cheap methods to produce more torque.

add weight to the car
drive with the breaks pushing in a little bit
make your car the shape of a van
put the gear oil in your engine
hell for that matter put on a brake at the end of your crank snout

all these are going to put more of a load on your engine and make it work harder do you really think any of these are going to create more torque though or make you faster?

 

yes yes old post still I know but shiz it's been a while and I'm bored

you bring up one point of a car on a cam with a lot of overlap. most cars with lots of overlap have little use at low rpms either way. low rpm for them is a moot point.

also with a cam on huge amounts of overlap usually the motion of the piston going down in the cylinder is going to prevent too much intake from going out the exhaust when it hits TDC. usually the overlap part is a little BTDC to a little ATDC at that point any intake charge that does make it out the exhaust isn't going harm matters much anyway as it should in effect speed up the intake charge as a whole and help it out with filling the cylinder at later parts. if anything with a high overlap cam I would be more afraid of the other thing from going on. since the intake opens while the piston is moving up for the first part pushing the exhaust back up into the intake stream at which point it would dilute the intake charge with a built in EGR effect. this woudl be further problematic if you had a system with decent amount of backpressure


woot woot caffine soap. think I am going to buy some

 

again read above

if nothing else if I hook up a weedeater motor a v8 to try to resist the motion of the motor does that mean I get more power?
or

maybe if I did that to my mazda I would get more low end torque.

woot woot off to figure out how to get the weedeater motor to spin the other way then my rx7 motor now

 

see what you started. you woke up the sleeping import guys and now the world is going to end like godzilla put on japan for a while there

 

i agree with you on the hondas theres one in my drive thats my sons that takes on any v-8s and wins

 

I'm sure there are a few v8's that would beat your sons bike