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?
"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>
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?