I know that this belongs in the exhaust section, but I like this forum

I was currious so I was doing a few numbers on the exhaust for our L's to figure out how to tune an exhuast. I just need to know what's a typical exhaust temp at WOT. Also, if my impression is correct I want to size the length of the exhaust system ( or whatever will cause the rebouding shock, muffler) to make its natural frequency equal to the RPS at peak HP? Or do i want to place it just before/after. Input would really help. I couldn't find anything useful with a quick search.

fn = (n/2)*(c / (L + 0.6a))

a= radius
assuming c = ((gama)*(R/m)*(T))^1/2 and T = 800K I got a needed exhaust length of about 127". Any tuners or someone with experience wana tell me if my assumptions are correct.

Also a thought. The idea of tunning one's exhaust is to promote flow out of the cylinder by providing a lower pressure when the exhaust valve opens. So I had an idea kind of like a reverse supercharger. Why couldn't one have a forced exhaustion system where you basicly have a supercharger pulling the air out of the exhaust system and providing a constant vacuum in the region between the headers and the "exhaust-charger". I am sure that this idea is not unique so why did it fail if anyone has heard of it?

Spade$

 

My exhaust temp gage gets a reading from the #5 cylinder, exhaust tube, of my headers.

It runs 1000 degrees cruising around the streets at 40-60 mph...1200 degrees if I give it 3/4 throttle going down the freeway at 70 or so............1400 degrees max at WOT down the track.

I have never seen it get above 1400 degrees. The #5 cylinder is suppose to be the hottest cylinder.

My mods are in my sig below to give a comparison for yours.

Hope this helps,


Rocks

 

Am I safe in assuming that's deg F and is where is the EGT sensor... really close to the top of the header I'd hope with temps like that. Hmm now i need to figure a temp from the entrance to the exit and int that... with T = 1400 F i get a tube length of 144.9in.... that's damn long

 

The bung for the EGT is usually placed about 2-3" from the exit of the exhaust port.

The main reason for not 'reverse supercharging' an exhaust system is mainly emissions. While creating an exhaust system that 'super-scavenged' and drew exhaust out of the cylinder at an exaggerated rate sounds good in theory you've got a number of factors to contend with.

In addition to drawing out all of the burned exhaust gasses you would also be pulling out some of the unburned charge mixture...tough to pass emissions with that. Commonly referred to as over-scavenging...

You've got the opening and the closing of the valves to deal with. You could go with an aggressive profile on the exhaust lobe to deal with part of the emissions problem (quickly shutting the exhaust valve at the point that all burnt exhaust gases have been extracted and immediately closing at the opening of the intake valve) but steep cam lobe profiles tend to be hard on the valvetrain. You could look into a cam profile that had no overlap, thus you would only have one valve open at a time...but you would have reversionary pulses in the intake and exhaust tracks to deal with.

What it sounds like you are trying to do is create a 'two-stroke' four-cycle motor. Two strokes are probably the best example of 'reverse supercharged' exhaust out there. By their very nature they rely on nothing more than exhaust scavenging and engine vacuum to both remove the burnt exhaust gases and draw in the intake charge. Two strokes are notorius for making crazy power but that's mostly due to the fact that they can be revved sky-high because there's no valvetrain to float limiting rpm's and you also don't have the weight of the valvetrain to turn, etc. However, they are also notorius for being huge polluters...lots of unburned hydrocarbons escaping from their pipes. To the point that two strokes are no longer allowed on U.S. streets (the last two strokes were the Yamaha RZ and RD series of bikes). Bimota tried to make a clean two-stroke a few years ago with their V-Due bike. It basically looked like a diesel in that it had injectors that would inject the intake charge into the cylinder as the piston came up and closed off the exhaust port. In theory it sounded good and the emissions were reasonable but the V-Due has since gone the way of the DoDo.

Also, the rebounding shock is going to vary with a number of factors...including the actual muffler itself, material used in the construction of the exhaust, ambient temperature, etc. You are rarely, if ever, going to have a constant frequency to work with. It may only vary by small amounts from day to day but that will be enough to throw some additional variables into the equation. And do you really want to tune it for peak hp (which occurs at about 53-5500 rpm) or do you want to tune it to the most often used portion of the powerband? You're going to have to delve much deeper than your above equation if you want to do this right...I can think of a few laws of thermodynamics and their associated equations that you might want to take into account.

If you can figure out a way to make this work than you have indeed built a better mousetrap my good man.

 

Thanks Struck that clears up my question. Damn, yet again I think of something not-so-novel.

This was just an inital calc. I'll used my thermo once i learn a little more gas dynamics (next semi for me ) I wasn't really planning on doing anything serious with this. I might use some MHD code that i have access to if i want to fine tune anything I might make.