http://home.earthlink.net/~mmc1919/venturi.html

 

Hey tuke, did you do all the welding on your exhaust?

 

Bigger is better for both the exhaust and intake.
A 6.9 at 3300 RPM displaces 401 CFM of air.
So you need as little restriction as possible to get 401 CFM to the engine on the intake side.
After heat expansion of the exhaust gasses we need to get rid of more than 401 CFM of exhaust with no restriction, I am going to guess in the 600+ CFM range when pulling a big hill.

Turbo's multiply the CFM required both in and out.
I need to get about 1003 CFM to the engine and and close top 2000 CFM out when I am pulling hard.

At 70 MPH my exhaust goes up for about 2 feet above the stacks when I mash it hard on a hill. That is pressure pushing it up that high and I am running dual 3" stacks.

When I am running the engine hard working it, the tips of my stacks will melt plastic instantly. So there is not much cooling of the exhaust happening in 15 feet of pipe.

If you have a stock Ford exhaust, with your engine idling go back to the engd of the exhaust. Put your hand down and see how far away from the exhaust you can feel the pressure of the exhaust. I could feel mine about 6 feet away. At idle you are putting 79 CFM through the engine with a 6.9. Going to 3" pipe from the stock 2.25" ID pipe was like putting a four barrel on a gas motor.

 

Agemenon its all clamps no welding could be welded I guess.

2new + Cheaper you guys are to much lol Bernoulli's Principle? Thats great. Alls I can tell you is when the hammer goes down it sounds sweet. Bigger pipe moves more exhaust, straight through muffler no restriction. Ed's a happy camper.

 

yeah, I'm sure we both realize that bigger is better on our exhaust. I guess we are just "exhausting" our knowledge of the subjects and principles(physics) that apply to exhaust. There are perfectly logical explanations for everything. But reality supercedes theory. Cheaper and I can debate all day, but only proven facts, dynos, and 1/4 mile times will tell the real story.

Cheaper, your A O K in my book.


sorry to jack your thread tuke. as you can see in my posts, once I get typing, it's hard for me to stop.

i envy your simplicity of the subject.

bigger is better!

 

I never said it didn't. You've misunderstood what I am trying to say. When it is under pressure - like coming out of the cylinders - expanding gas cools - everyone knows that. If it didn't refrigerators wouldn't work. What I did say is that AS it cools it occupies less volume. I'm talking about AFTER the pressure release - when it is back at or near atmospheric pressure - which is essentially the pressure level it at as soon as it is ejected into the exhaust system. From that point on - as it travels down the exhaust system it continues to cool dissapating the heat it carries from the combustion process into the metal of the pipe. The more it cools the more dense it becomes and the less volume it occupies. As the volume of gas shrinks, if the pipe size stays the same, then the gas's velocity slows. Hence having the pipe size decrease slightly makes more sense than having it increase in size.

Like I said, that isn't even what I was talking about. I'm talking about AFTER the intial pressure release - when the gas begins its travel down the exhaust pipe and begins cooling and contracting.

Of course.

The one thing you're missing with your convergent duct illustration is the fact that there is a vacuum at the convergent end. We're not just talking about laminar flow in a duct. We're comparing a vacuum chamber (inside of the air cleaner) being fed by ambient air pressure (stock intake) with that same vacuum chamber being fed by a convergent duct that is being exposed to a high speed flow of air - essentially scooping the air.

With a laminar flow inside a duct, the relative pressure at the outlet end of the convergent duct IS lower than the pressure ahead of the convergence. However in this case that pressure drop is insignificant compared to the vacuum in the chamber at the end of the duct. With the stock intake the vacuum is still there, but the pressure created by scooping the high speed flow of air isn't.

So it amounts to being not so much an increase in absolute pressure (which is what I believe you thought I was saying) but rather a significant increase in the relative pressure (or actually lack of vacuum) inside the vacuum chamber at the end of the duct.

If that "scooping air" principle didn't work there'd be no reason to put those forward-facing intake ducts on fighter jets to feed the engines. The ducts could just as easily be placed anywhere and facing in any direction - whatever was most convenient.

 

And yeah, I can debate and discuss physics principles all day. Its the Engineering education coming to the surface.

In the end what works, works. The rest is all just talk.

And like 2New, I'm fairly fast at typing, so creating these long posts doesn't take long. That makes me kind of a "keyboard blabbermouth"

Anyway, there was a point to my original question. I have 90% of what I need to fabricate/install a 3" exhaust with 2.5" downpipes and smooth flowing Y-pipe.

However, what I'm debating is whether it would be beneficial for the two 2.5" pipes to Y into a piece of 4" pipe, then reducing to 3" just ahead of the muffler. I'm pretty much s stuck with the 2.5" downtubes and the 3" muffler and tailpipe, but everything ahead of the muffler is up for debate.

The thing that prompts my question is the fact that all the systems you buy seem to go the other way - bigger pipes downstream instead of upstream. In my original post I asked the big question; WHY? Is there some benefit? Or would the decreasing pipe sizes downstream be better - like with gassers?

 

I meant to say that there wasnt a humungo effect of the ducts. there is a gain yes, bellmouths on jet engines are there for a reason. they help the flow enter the engine more smoothly as well as grab a little more air. I was just saying the effects arent containing super powers, although every little bit helps.

you also must rememeber, and maybe you do, the fact that our engines are not vacuums. They are just an air pump that creates a "lower pressure" for a higher pressure to fill. This pressure differencial is converted into velocity as the higher pressure of the atmosphere tries to enter the lower pressure zone. The engine does not suck air in, but simply gives the atmosphere an easy place to go. the names of our brake system and vacuum lines is technically wrong. they should really be called lower pressure lines, not vacuum. since vacuum is defined in negative in hg, not a pressure differential.

I know we are on the same page, and will stop beating the dead horse. I must finish my theology paper on "predestination vs. free will"

later,

jeremy

 

jeepers this would cause some type of back presser would it not if so the this is not what we want on these idis or any other motor from my exp

 

cheaper, you could just go with a dual inlet and dual outlet muffler. then you could convert your 4 " into two 2.5" and then into the muffler. you then wouldnt have the restriction of smaller pipe. Then you can either come out the other end with the dual 2.5" and put it back to 4" or just make it into duals after that.

I dont see why you are trying to put a 4" exhaust into a 3" muffler though... why?

 

LOL! I originally thought about going the dual 2.5" route. Then I got a smokin' deal on a brand-new complete ATS 3" exhaust system.

DUDE! What have we been talking about?!? I'm trying to figure out if there is a benefit to running the two 2.5" down pipes into a 4" pipe (instead of a 3" pipe) before reducing to the 3" pipe for the muffler. The same idea I've been talking about - using bigger pipes closer to the exhaust manifold and slightly smaller pipes further back - to keep the exhaust gas's velocity up. You know - for better exhaust scavenging?

 

ohhhh, oh, I see. I think the only effect you would see is a smooth transition for the y pipe. as far as physics and adiabatic bla bla is concerned, I dont think it the smaller diameter would help. The fact that the transition of the y to a 4" then to a 3" would smooth out the bottle neck effect. Kinda like 4 lanes quickly going into three, instead of giving them time to merge. Maybe bad illustration.

I think your idea of 4" to 3" is beneficial and I concur. The exhaust flow would be more smooth.

funny, we posted the crap out of this thread, just because I didnt understand your question...

well, "Big gulps huh?...See ya later!"

jeremy

 

Not a problem it's kinda like watchin' these

Cheaper if I could've went from 2.5" to 4" I would've done that. I don't know why Dynomax didn't do that. It makes no sense to me to go from 2.5" to 3" back up to 4". From what I've been reading the last few years these beast's don't need any backpressure the just need to breath.

 

Exactly - I figure the Y to 4" to 3" would have to be better than just the Y to 3" - which is really my other alternative with the stuff I have. It would have to be better than going from the Y to 3" to 4" - which is what most aftermarket systems seem to do. They seem to disregard the gas velocity/scavenging effect completely.

That we did - but it made for an interesting and stimulating conversation (at least for the two of us. May have been boring to others, but OH WELL!)

BTW, you are correct that there is no actual vacuum at the intake manifold - at least not in the same sense that space is a vacuum. I was using the term in the common vernacular. To be more precise there is only a "vacuum" relative to the ambient atmospheric pressure OUTSIDE the air cleaner. As you said though, this is really not a vacuum, but rather an area of lower pressure.

 

So I'm guessing that the dynamax system you put on was a undoubtedly fine investment wasnt it?