strait talk about e -pipe diameter vs. flow...

[jeepbob]

Matt,
I think you have spent too much time on simulators, desk top dyno is not the most accurate tool in the world but it is a fun toy. When you are ready to come out and play with the big boys, we can show you how it is done. Some of the guys on this site have seen my junk and how it works and thier past posts have told the story. You get worked up way too easy and it is way too fun to do it, try lightening up a little. As far as an open mind I try more stuff than most people even dream about which is why I am going to switch to Pontiac power in my Wag (desktop dyno'ed at 570 hp and 693 ft/torque but will be detuned by going to dished pistons). As far as reading a book my automotive library makes a librarian jealous (did you not read my first post?) and I have read them all. Practical experience is also worth a thing or two and I have 40 years or working on and driving compitition cars, 4x4's, big trucks, and bikes. I also spent many years in the engineering dept at GM and even some time at AMC engineering. I also do not take my own judgement as being 100% correct as I am always bouncing ideas off of people with huge amounts of automotive experience. I even learn a few things off this forum. Every now and then I can even answer a question.
BTW my offer still stands, so in the words of Mills Lane "Lets get it on!"

Pete, whenever you get down here we WILL hit the trails and I will want to come up to the upper this summer. My cousin just bought a pump with which to fill the pit from the swamp/pond.

[Jerk]

so you've got all this "real world" experience and you're still ignorant...sad. I really have more important things to do than "get it on" with some one who laughs in the face of expert gleaned knowledge, note: I'm not claiming to be an expert, I just listen to them, especially if they have been in the performance industry for 40 years! what would you hope to prove from "getting it on" anyway? I didn't build my j20 to go thrash it, its a daily driver, that hauls materials/tools/me to job sites and occasionally goes off road/plays in the mud/snow. you on the other hand apparently build specific use only vehicles, example: where did you get a pontiac engine with those specs? obviosely not stock, and therefore, why not use an amc v8, you can build them almost as strong and there is no adapting needed? just my opinion.
regardless of where you got it, it ain't gonna be a streetable engine, mine is. anyway you look at it, what I said about pipes and performance has been proven time and time again, but, hey- you remain ignorant, that's your choice. far be it for me to try to enlighten anyone around here. **** it!

[jeepbob]

You just don't get it do you, I gotcha again.
You are way too easily riled up.
BTW the pont motor is not stock but is driven on the street. The reason for going to it over an AMC motor is cost. It costs way more to buy the AMC parts to build 450 to 500 hp than I have in this motor. If I had to build this motor from scratch it would be a $4000 motor, to do the same to an AMC would be $5000 (I have priced it out). I only have to freshen this motor up and I will put new pistons in it and still have less than $750 in it.

[Dive 30]

OK . . . ignorance is about to open it's mouth . . . so everyone duck and cover.


Where is torque generated? I know the power stroke of the motor. I understand the intake side. More air=more O2 to burn. More fuel=more fuel to burn. So on the intake side, the more fuel/air mixture (as a side question, why is the ideal fuel/air ratio 14.7:1, the same as 1 atm [14.7psi]?) we cram into the cylinder, the more we have to burn, thus the bigger the bang, thus more power.

But on the exhaust side, the benefit I see to a tuned exhaust is cylinder scavenging. You create a negative pressure on the cylinder, thus it sucks out more exhaust and pulls more air/fuel in, thus bigger bang, thus more power.

But isnt' the actual torque of the motor generated when the bang happens and the fuel/air mixture ignites? Thus the motor is closed to the negative (or ambient in Matt's ideal) pressure on the exhaust side, right? Or is valve overlap such that at low RPM a high velocity exhaust will actually suck fuel/air out of the cylinder (or maybe reduce compression?) reducing torque? Whereas more restrictive exhaust (read mid velocity) will scavenge the cylinder at lower RPMs without depleting the fuel/air mixture, right? But a more restrictive exhaust will be just that (restrictive) at higher rpms causing exhaust gas to remain in the cylinders thus reducing power at higher RPMs? Thus why VVT flattens torque curves. You can have the right amount of scavenging for a larger RPM band right?

help me out guys
Phil

[ March 31, 2002: Message edited by: Dive 30 ]</p>

[Crazy_Jeepman]

Well, I see this subject is as controversial as Stick shifts versus Automatics, Ford versus Chevy, and so on. I think when one person believes in an idea that is great, and when you put the idea to function even better. However trying to make everyone believe as you do, will not fly well here, or in the real world. Like the saying you can lead a horse to water but cannot make it drink. I see no reason to argue over the theories of the size of pipe to use. Matt its awesome you have a wealth of info and like to share your thoughts and ideas but to get all bent out of shape RESTRICTS (PUN INTENDED) your line of thinking and the initial purpose of your thread on this subject. I think you meant to share with everyone what you feel to be true and would work and that is great. However some of us old farts will not take the info as gospel truth, and Sorry I do not. However some will and may or may not like the results. I have no Desktop Dyno's nor do I read books unless I am after specific info, I do not have time to read, play desktop Dyno, cause I am doing the building and rebuilding all the time. I came from a long line of busted knuckles, greasy finger nailed and oil grease matted hair type life forms. In this case of the exhaust system advice or info your choice I would not want to be insisting this is what will work, someone go out and spend big $$$$ on a system on your recommendation and seemingly well researched info, only to have it not perform as intended. I do not know if it would be the way to go or not, I think it is my choice to say no it will not work on my rig and I will continue to use my OEM small piped system and go Jeepin like I have for years. Matt do not get defensive or offensive on this subject. I have many ideas that have been shot down by others because it was considered a poor idea or the info was incorrect. I never take it to heart, c'mon this is how friends and Jeeps are made, suggestions and criticism, nothing is gained by getting mad! I would like to share this web site for those who are confused now. This is the info I believe and consider fact for the Exhaust specs for any of my rigs. I will not argue this any further. Matt you have lead us all to the water however not all of us are thirsty, but thanks for sharing. Here ya go Click the Exhaust section in the following or read the whole article good info.
<a href="http://home.att.net/~jroal/perftheory.htm#Exhaust" target="_blank">
Remember, Lets all have fun no matter what you decide to pass gas through!!!!!!!</a>

[ March 31, 2002: Message edited by: Crazy_Jeepman ]</p>

[jeepbob]

Truce.
I am going wheelin, see ya all in the mud.
Matt have a happy Easter, same to every one else!BTW its run what ya brung and we all pull each other out! Try not to dig in too deep and have fun.

[Jerk]

<blockquote>quote:</font><hr>... on the exhaust side, the benefit I see to a tuned exhaust is cylinder scavenging. You create a negative pressure on the cylinder, thus it sucks out more exhaust and pulls more air/fuel in, thus bigger bang, thus more power.

Phil, this is true with TUNED headers that are run into each other through various means, some have "x" pipe between the two header collectors, some have "H" pipe in the same place, some ,like the '66-68 or 69 ford gt40s had a 360 degree system where the odd firing cylender on each side was run to the other bank and helped scavenge even more. This actually gave an exhaust sound more like a 6 cylendar since it was a smoother sounding. Also regular 4into 1 and sometimes tri-ys can be tuned to their respective bank(head) and you get the shock wave of one exhaust pulse arriving at the collector just when the next exhaust pulse enters the runner leading to the collector, thus "pulling" with it the next pulse, this is only effective within certain rpms and therefore, you must tune runner length as well as diameter to the rpm /power band that the engine will be running in. confused yet? [img]smile.gif[/img]

But isnt' the actual torque of the motor generated when the bang happens and the fuel/air mixture ignites? (This is only part of it, the middle part. This is the belief that too many people fall into, where they think "If I put a huge duration cam in my engine and leave everything else alone, I'll get radical power!" its a scale, all must be balanced- such is life)

Thus the motor is closed to the negative(or positive as the case maybe in smogged vehicles) (or ambient in Matt's ideal) pressure on the exhaust side, right? (sometimes this is true, it does depend on alot of variables. valve timing and the like as to when the exhaust valve closes...to be continued...)

Or is valve overlap such that at low RPM a high velocity exhaust will actually suck fuel/air out of the cylinder (or maybe reduce compression?) reducing torque?

This is an interesting question, as it can be true also, and false- if you have a long duration cam, you will, at low rpm (idle-2000 rpm lets say) have your intake valve opening and exhaust valve closing, causing not only the 2 gasses to intermingle fresh a/f mixture with spent exhaust gasses in the cylender, but also in the intake manifold and exhaust header, this is why most drag cars with lumpy cams sound really cool/sick when they idle, the plugs cannot ignite exhaust gas a second time so the engine sounds like its going to die. this problem alleviates itself as rpm climbs and the engine then makes a hellacious effort at turning rubber into toxic smoke! if you have a short duration cam, you will more likely not allow the exhaust system to "see" the intake system through the cylender. short duration cams also have a very smooth idle and run very well untill they hit high rpms, say 5000 rpm in a typical v8 where they really have no power, as opposed to the drag car that has rough idle and makes most of it's power at and above 5000rpm. Now, saying that, I'll say this, the exhaust pipe diameter, flow characteristics make almost no difference at idle or low rpm, idle to 2000 rpm. what it does do though, with headers, is allow for some extra scavenging throughout the entire rpm range of the engine, most noticeable at higher rpm, 2000 to 5000 rpm in the typical v8.
open pipes, larger diameter, just allow the pumping of the piston in the exhaust stroke to work that much less to free the cylender of spent gasses, the exhaust pulse from the burning(exploding more accurately) gasses doesn't exit itself into the cylender under its own force, but is pushed out by the piston which is of course tied to the crank making its work the result of a different piston's power stroke. now where the argument seems to be, apparently, (I don't really understand the thinking too well) is that larger pipe allows the exhaust gas to slow down and presumably it gets tired and finds a starbucks some where and waits to find a muffler to hang around in, while it drinks a mocha latte', then all the rest of his gas buddies do the same until you've got a traffic jam around the coffee bar and no one leaves. The idea that exhaust gasses moving fast like water down a waterfall is completely rediculous, it can be compressed (unlike fluid) and will back up or slow if restricted.) However, since even though the pipe is larger, and the gasses may not be moving as fast at a given rpm, there is still the same amount of them, just moving slower. this is good since it will allow more gas to be packed into a pipe (ristricted somewhat just by the nature of being a confined tube for containment or conveyance)without it being over packed and back pressuring the system as quickly, which would cause the piston's exhaust stroke to sap more power from the power available to spin the crank and make motion in the old drivetrain thus motivating the vehicle. In the smaller pipe, the gasses have to move faster and are more dense because they are RESTRICTED by pipe diameter and this will be more easily packed by the normal running of the engine, the longer the pipe the more it wil restrict also, although to a somewhat less extent.

Whereas more restrictive exhaust (read mid velocity) will scavenge the cylinder at lower RPMs without depleting the fuel/air mixture, right? (once again, depleting f/a mix is really only a concern with long duration cams)But a more restrictive exhaust will be just that (restrictive) at higher rpms causing exhaust gas to remain in the cylinders thus reducing power at higher RPMs? (this is acttually more likely, but like I said more likely if using a long duration cam, the exhaust valve is usually closed sooner not allowing this in most street driven v8s with a mild performance cam.

Thus why VVT (I don't mean to seem dumb now, but whats vvt? valve to valve timing?) flattens torque curves. You can have the right amount of scavenging for a larger RPM band right?

To answer this and hopefully wrap all this together, I'll say this: larger rpm band meaning broader power band (torque at low rpm and hp at high rpm instead of both at only high or only low rpm)yes, if you allow a larger exhaust pipe, you allow more flow, this translates into not only more axhaust gas, but the efficient handling(quicker exitting) of more gas, but it works on lower rpm as well as (designed for) higher rpm. Losing the exhaust gas and freeing the space it occupied for more exhaust gas is what big pipes are all about. this is only the exit side of the engines power circuit though, intake manifold, carb and intake port/valve in combustion chamber, is the beginning. spark, compression ratio/pistons, exhaust port/valves/combustion chamber, headers/pipes/ mufflers/cat, are the end. the cam is the brain behind the intake valve to the exhaust valve, everything else thats bolted on must be matched to it all and also to the vehicle and purpose.

help me out guys
Phil

Hope that helps phil. -Matt
[ March 31, 2002: Message edited by: Dive 30 ][/QB]<hr></blockquote>

[ April 01, 2002: Message edited by: Matt W ]</p>

[jeepbob]

VVT= variable valve timing and it works by changing the valve timing as the revs of the motor change to allow the torque curve to be changed.
Some headers for higher performance engines have a venturi in them to speed up the air flow and create an anti reversal area in the pipes to increase the scavange effect. This allows the the spent gasses to be drawn out of the chambers more effeiceintly. This style header works at higher revs and has little effect at low speeds.
180* headers combine cylinders from both banks to get the best possible scavange effect but again are only used on race motors.
This debate could rage on for months but until you actually get out and try different size pipes on the same rig will you actually find out what works best on it. On race cars to find the tune length you paint the exh pipe and go run the car. Where the paint burns off that is the tune length but you have to add 28 to 36 inches of pipe to it. The amount of pipe after the tune length will not make much difference as long as it is kept to the 28 to 36 inch point. A header gasket can take as much as 20 hp from a race motor.

[Don S]

NOW you got me thinking 'bout the simplicity of the single stroke engines.....At least the exhaust port is equil to the cylinder bore and with no valve stem to reduce the flow........Then, of course, there is the Ram-air intake box driven turbo...Putting 14.7 negetive lbs of pressure on the exhaust system. This does require a flame retarding system at the end of the tail pipe........I just love April Fools Day.....you know....I celebrate it every day........ ...ds..

[Jerk]

ah, thanks for the heads up on the vvt thing bob.

And I couldn't remember if it was 180 or 360 degrees, I figured if I got it wrong, someone would correct me, at least I got the function right..yes? as far as them only being used on race cars- sure, that's really the only place you would need the marginal advantage they would give. I know a couple of local dirt roundy-rounders out here and one of them used to drive Porches (not professionally, just drove 'em, and hard- he always did pretty good until some dingus put him in the wall or spun out in front of him and poor old Dave got caught in the middle of the dogpile everytime! I don't think the guy finished a season in 3 years simply because of getting his car munched. ) Any way, the other guy, Jodie, had some 180 degree headers and still couldn't compete w/ Dave, all else was pretty equal, both in a "mod" class, open wheels, tobular frame, 5 link rear suspension, using 400 chevy blocks and alcohol for fuel... and both were amatuer drivers, I asked Jodie if his tuned headers gave him any real advantage before he tried em, he said he hoped he could keep up with dave...after they ran a few races, he was pretty much of the mind that that wasn't gonna happen. heh- heh, he sold the car shortly after.
Now as to this header gasket thing using 20 hp...how does that work? curious. -Matt

[jeepbob]

On motors designed to run with no back pressure, and I am talking the high dollar Nascar and NHRA stuff not street motors, The cams, intakes combustion chambers, ports, and exhaust systems are designed to begin drawing air/fuel mixture thru the intake before the piston starts its down (intake) stroke. A leaking header gasket or broken header at the head allows outside air to be sucked into the system. This does 2 things, it decreases the amount of mixture drawn into the cylinder and also cools the exhaust which slows it down. That makes the whole system less efficeint and drops the hp.
On our street motors which are designed to have some back pressure, when the pressure is removed after a point the engine will go lean (non EFI) and you can not enriched it no matter what size jets you put in the carb. An EFI motor will go rich as the O2 sensor reads the lean mix and opens up the fuel flow to the point where the motor has to go rich. Primary header tube size has a rule of thumb also as you want to go just a little larger than the exhaust valve by about a 1/4". However you must also experiment here as if your motor never sees high rpm then a slightly smaller primary tube will be more efficient and create more low end power. I am by no means an exhaust system expert but have tried a few things here and there and found that what is good on a race car or even a light street car may not work on a 4wd or tow rig. Some times these lessons did not come cheap either as either broken parts or expendisure of cash to buy different ones because of disapointment were the result.
BTW the May 1999 issue of 4x4 Power magazine has a real good article on how exhaust systems work and they actually used 4 or 5 different vendors in the writing of it with no endorsements. There is even a chart on what size pipes to use with the power and size of the motor, although it looks like the recomendations were made for cars and not trucks which tend to run about 1/4" smaller for more low end power.

[ April 03, 2002: Message edited by: jeepbob ]</p>

[Jerk]

hm, interesting. I agree btw about going with slightly larger runner tubes than the exhaust port, it helps stop reversion greatly, its just hart to find them for less than $300! for a jtruck anyway. and with our goofy dogleg ports, its a pain to bend tubing to fit the pattern w/out screwing it up, I imagine. I'm not so sure about the leaning out deal but hey, I've never heard the opposite either, it just seems slightly...um shall I say: "far-fetched", again I claim to be no expert here either, just read alot of what people have done and found proveable, repeatable advantageous gains with. - Matt