Crazyquik

Well I guess its time to start asking tech questions <img src="images/icons/smile.gif" border="0">

When you hear about people using tuned collector lengths or using different lengths off the collector to move the power band, how do you determine what length to use. Is there a mathmatic formula (I found one for intake runners, it would probably work with exhaust too) or what? I'll post if it you want.

I've heard the old racers trick is to put some extensions on there and paint them with really cheap paint, where the paint stops burning off is where you should cut the pipe, but thats probably just a myth, right?

J.

JaSSon00

talk to someone at slp, they seem to have figured out the tuned header issues... <img src="images/icons/grin.gif" border="0">

GeorgeC

"Tuned" headers and tuning collector lengths are two completely different things...

Although I haven't tried it on my car, I too have heard the old "paint and cut" method. You could start with some 18" collector extensions. Paint them first with some bright color of your choice (so it's easier to tell exactly where the paint begins to burn), and start trimming them. Obviously, this will only work if you plan on running open headers with no Y-pipe or other exhaust system on the car.

I run Dynatech race headers w/ 12" Dynatech slip-fit collectors. I haven't experimented with any collector extensions yet, but open headers are worth 1mph and ~.07sec or so on my particular car. I have a pair of 1-chamber Flowmaster race mufflers which I can install/remove in 5 minutes. I belive part of that gain comes from my crankcase evacuation system, which works much better with open headers vs. any restriction on the exhaust side.

[ November 13, 2001: Message edited by: George Coffey ]</p>

Nine Ball

iTrader: (38)

Yep, the old "paint trick" is what the guys at the track use on their trailer cars. Like George said, it doesn't work for running with a catback, just open header collectors.

Another way to do it is use a crayon and draw a long line down the collector and make a single pass to see where it stops melting. Thats about where you need to cut the collector off.

Tony

Crazyquik

Ok, well why is that? It just seems very illogical that where paint stops burning is the optimal length.

Maybe you can help us out Tony, since I'm sure you took at least one class in fluid dynamics. Or at least tell me if this formula has an obvious hole in it.

Exhaust scavenging is greatly aided when a negative exhuast pulse travels back up the header, and if the exhaust valve is open when the pulse gets there there will be a greater gain. Speed of the pulse will vary due to air temp and quality, so lets go with about 1300 fps, about the speed of sound.

Everyone agree so far?

Lets say the engine is running 6000 rpms, and the exhaust valve opens once ever 2 revolutions (720 degrees of crank movement). We'll use a T1 cam, so it stays open 221 degrees, and then it will be closed for the next 499 degrees. So at 6000 rpm it takes about .01 seconds for the engine to make a revolution. Now 499 degrees is about 1.39 to 1.4 revolutions, right? Now if all this is right, it takes about .014 seconds from the time the exhaust valve closes to when it opens again.

Does this seem correct?

So multiplying the speed of sound by .014 gives you 18.2 feet, but since the pressure have has to come back its more like 9.1 feet. Of course that would be kind of longish. If you divide 9.1 by 2, that means the pulse will move down and back thru the exhaust tract twice, but will return back to the port at the correct time. That would be 4.55 feet or 54.6 inches.

Now if the headers have 30 inch primarys (Ghanns do, what about FLPs?, TTS are like 26-28 on one side and 27-32 on the other arent they?) then for 6000 rpm with a T1 cam, a 24 inch extension would be optimal.

Now, everyone tell me what I did totally wrong, because I know there has to be something bad there. Like the speed of the pulse, wouldn't it slow from 1300, which was kind of assumed anyway? It wouldn're remain constant for sure.

I guess how the cam is degreed could mess up the calculation too couldn't it?

J.

99 Black Bird T/A

iTrader: (4)

I think the pulse will resonate a little...sort of like a musical note played on a stringed instrument...this will make the calculation for the length a little more complicated.

big hammer

to get it perfect is pretty complicated. Especially when you start dealing with stepped headers

stockA4

You did it again lol

big hammer

99 black bird has been a saint here for almost 20 years!

G Atsma

Whoa! A stealth 20 year thread exhumation....

stockA4

​​​​​​What he's doing with that little 250,000 mile car, that's the right thing to do man, right thing to do, ppl need to understand that the LS1 f-body is still a lethal platform, even an old tired one and still the best place to learn about going fast for so many reasons and this guy does nothing but strengthen my resolve to race the **** out of my cars for as long as I can. Stock ls6 intakes right down to the maf screen for the win!!! Lol

stockA4

Just curious because I'm planning to install longtubes with cut-outs not far behind the collectors on my 376 really soon, if anyone has ever actually cut down there header extensions "where the paint stopped burning off" and actually gone faster? I can't stop thinking about it haha

RB04Av

Lots of things work the same way. Not just exhaust systems. Anything like a pipe that has a resonance in it behaves this way. In the case of exhaust, the temperature of the gases varies according to the pressure; so the cool spots on the pipe are where the pressure is lowest and the flow rate is the highest. (those 2 things vary inversely to each other within the same flow) Cutting the pipe to the open atmosphere there joins the exh stream to the atmosphere at the point that it's most ready to exit, so to speak.

Can't speak to "going faster" as in, a back-to-back test; it's just physics.

You do H- or X-pipes the opposite way; try to locate them at the hot spots. That way they conduct some of the high-pressure gas in one side to the other side, to share the flow most effectively.

Old Geezer

iTrader: (3)

https://www.jalopyjournal.com/forum/...aders.1079666/
Chew on......

G Atsma

I still remember those.... did they work??

wannafbody

iTrader: (2)

That's an interesting concept for headers. That would act as an expansion chamber.

G Atsma

I wonder if that would kill any scavenging effect.

OLDWS6

I'm not sure how well those things worked but I remember the original Ramchargers running a '46 or '47 Dodge coupe at the old Detroit Dragway with an intake that mounted t 4bbl carbs on a log manifold with at least 18 in runners. The exhaust system consisted of 8 individual pipes ending in reverse cone megaphones.

https://www.allpar.com/racing/pix/hi...and-mighty.jpg

G Atsma

I still remember that! Gotta love the innovation for the time!