At least your man enough to admit that. And yea,when someone says that I need to "learn something" and " I have no clue how exhaust works on an engine" I do have a tendancy to get "touchy". This board is to share info and to enjoy not to flame (when flaming is not warranted that is) I hope your post brings on more info.

 

There are really 3 reasons why long tubes show marginal gains.

#1 The stock exhaust system isn't that bad

#2 The flow restriction in a lightning pretty much starts right at the exhaust valve.

#3 The long tube headers are incorperated into an exhaust system that is full of compromises. Even the JDM system...wich in my opinion is the best money can buy right now.....has compromises.

You said that this has nothing to do with flow velocity or air speed...may I suggest you go get your book and keep readin.
Dale

 

Ok, now that some guys have let off some back pressure, let's resume class...

Scavenging on a steet car IS augmented by a properly formed collector. A Top Fuel dragster uses open pipes angled, and at the proper length, to use the airflow past the car to 'pull' the exhaust from the pipes. Our stock L mufflers augment scavenging with their internal crossover.

 

Mondo1, I read your post wrong. At the time your post seemed ridiculous, and a semi-flame seemed in order. Sorry about that.

Anyways, on a supercharged engine the supercharger thoroughly scavenges the cylinder. This means that the header tubes just aim the flow of spent gases. This is why blown cars without collectors work so well. The reason for the small gain with long tube headers is that there is less flow restriction with them. I don't know the diameter of the primaries of the long tube headers, but the larger the better. The same holds true for the rest of the exhaust system. This wouldn't be the case if it weren't SC'd though, and we'd be having a much different discussion.

Bad as L, #1, #2, and #3 are correct. As far as flow velocity or air speed, I think that we're trying to say the same thing. hugh, we must have the same book. hehehehe

 

air1
just checkin
Dale

 

The main reason exhaust is not critical is that you have a blower filling the cylinders with about 9 to 15 pounds of boost. Top Fuel cars use zoomie headers for down force only! They add about 1000 pounds of downforce. The first funny car to use them was Mickey Thompsons blue car. If I remember right it was in 1967 and driven by Danny Ongais. A set of longtube headers would probably show more gains on a regular 5.4 without the blower. It is not that they do not work, it is that they are not needed as much with a blower. There is some gains to be made in exhaust, but not as much as a unblown engine. Everything we do to these trucks helps, some things more than others. When you add everything up these trucks make some serious power and turn some incredible times! Skip

 

Well said skip.

The reason that I started this thread in the first place was because some were stating that the L needed backpressure, and the way to get the "proper" backpressure was with the correct muffler. I just wanted everybody to realize that that is the wrong logic. Be free!

 

cool!...im free with my no cat cutouts!

 

Look man, I ain't fallin for no banana in ma TAILPIPE!!!!

 

Reversion, what is it? It is simply the exhaust pulse flowing backwards momentarily during the overlap phase of the camshaft at low cycling rates. During the overlap phase the piston is pushing out the last of the exhaust gases and prior to reaching top dead center (T.D.C) the intake valve and the exhaust valve is still closing. At this point in the engine cycle both valves are in the open position. At high cycling rates the inertia of the incoming intake charge and the outgoing exhaust pulses keep the exhaust flowing in the proper direction. But at low cycling rates, as the piston is pushing out the last of the exhaust gases the intake valve opens and some of the spent exhaust charge is pushed into the intake manifold. As the piston reaches T.D.C. and begins the intake stroke the exhaust valve is still not completely closed. As a result, the piston pulls from the intake and exhaust valves simultaneously causing the exhaust gases to flow in a reverse direction. This is normally not a problem until you add water into the exhaust stream. Reversion can be severe enough to stall the engine, add water to the oil, rust the exhaust seats, etc. This effect only happens at idle speeds, but remember that during shut down the engine encounters the greatest reversion.
Port angles, bend angles, tube lengths and diameters are all important in achieving top performance results from today's motors.
http://www.ssheaders.com/header.htm a good link for exhaust basics....with Forced induction things are a bit different.

 

That is an excellent source of information for normally aspirated engines. As you mentioned SC'd engines are different. Essentially boost blows the exhaust out.

 

Question: How does boost blow the exhaust out when the intake valve is closed? True the other pistons are being"forced" down on the their intake stroke, which 'speeds up" the exhaust stroke of the spent gasses on the other pistons, but is it REALLY exiting under boost?

 

Mondo1,
I looked into this a little. I'm not sure exactly how it works on the L. I think that the cam profiles have the most to do with this. From everything that I could find the boost actually blows most of the exhaust out. There is a book that I need to find that's supposed to explain this in detail. I think the the overlap in the valves is slight so the exhaust valve isn't actually closed for a very small period. It does seem to be common with all positive displaced SC'd engines though.

Any SC CAM designers out here?

 

does anyone happen to know the stock grind on our cams?

 

Ask Timburntire. He's done a few cam swaps and knows the specs.