Gt-12

[kimcrwbr1]

i remember being told before that when your building headers that all the runners should be the same length is there any truth to that or is it another wivestale?

I am just guessing here but being in air movement HVAC the pipes are not long enough to make much a difference it is the turns where you get the most turbulance (resistance) inside the pipes. In general a 90 degree angle adds about 5 feet of pipe for long turn and 10` for short turn. I think the key would be back pressure to equal out the static pressure in each pipe allowing the fuel to keep the valves and pistons from burning up. In refrigeration we use all long turn 90s and only allowed so many, air and gasses are a fluid measurement and when your working with 200 to 450 psi your moving alot of (cfm) cubic feet per minute. The back pressure will equal out the amount of pressure from each individual pipe. Of course I`m bored.

 

[gwaii]

actually in header design it's a different principle at work-it's the timing of pressure waves from the exhaust pulse,they travel at the speed of sound,and when they reach the end of the primary tube they create a negative pressure pulse.the idea is to place the tube next to one that is going to have the exhaust valve open next so the negative pressure pulse will reach that cylinder as the exhaust valve opens,helping to pull the exhaust out of that cylinder.

if all tubes are arranged to be in the right location and of the right length,all the cylinders will help scavenge each other at a predetirmined rpm.this negative pressure wave will also help pull the next intake charge into the cylinder at the valve overlap period,slightly increasing volumetric efficiency.

this is part of the reason for the specific pairings i made on my headers.

 

[CHKNFKR]

I've read about scavenging and all, the part i don't understand how one determines the appropriate length and arrangement to maximize this effect?

 

[gwaii]

it's some basic calculations...in the hot exhaust,the speed of sound is about 700mph.this stays constant regardless of the rpm.at your power peak rpm,it's easy to calculate how much time there is between one cylinder's exhaust pulse and the next.figure out how far you can go at 700mph in this time,and the tube length will be half this(down one tube,and up the other).


this is a very simplified version of the concept,but it gives the basic idea.

also,as long as the inside walls of the tube are smooth and a constant diameter,it doesn't matter how many bends and curves there are,the pulse will not be affected.this is why headers on grand prix cars can be so convoluted,and i just like the look of it.

 

[alwaysFlOoReD]

as long as the inside walls of the tube are smooth and a constant diameter,it doesn't matter how many bends and curves there are,the pulse will not be affected.

That's counter-intuitive.
Good to know, though.

Richard

 

[CHKNFKR]

Sounds simple enough.

I need to stop reading your posts, they give me far too many ideas and actually make me think i can do things i never would have considering trying out myself.

 

[gwaii]

the thing to remember here is that it's not pressure in a gas that we are talking about here,it's a shock wave like the ones you see coming off big explosions.contain that shock wave in a tube and it will follow the walls of the tube.this is the same principle that wind instruments like bugles and trombones use.in the trombone the length of the tube is changed while playing to alter the timing of the pressure wave coming out the end-and as such the pitch of the tone.

equal length headers will be tuned to a specific rpm,and give a narrower power band with a higher peak.unequal length headers will give a broader power curve with a lower peak.for most of what we do,we want that broader power curve.the high peak is better in formula one where every extra hp is needed,even if it means constant shifting to keep in that narrow power band.

 

[slow96gt]

I had a customer earlier today with a cargo container he had made into a sewing shop for his wife.


All I could think when I saw it is "Wow, that looks like a mother-in-law suite Gwaii would build"

i lived in one of these for a year while i was overseas lol

 

[gwaii]

Sounds simple enough.

I need to stop reading your posts, they give me far too many ideas and actually make me think i can do things i never would have considering trying out myself.

that's the idea.you learn more by trying things you didn't think you could do that by sitting around wondering how it's done.

 

[kimcrwbr1]

actually in header design it's a different principle at work-it's the timing of pressure waves from the exhaust pulse,they travel at the speed of sound,and when they reach the end of the primary tube they create a negative pressure pulse.the idea is to place the tube next to one that is going to have the exhaust valve open next so the negative pressure pulse will reach that cylinder as the exhaust valve opens,helping to pull the exhaust out of that cylinder.

if all tubes are arranged to be in the right location and of the right length,all the cylinders will help scavenge each other at a predetirmined rpm.this negative pressure wave will also help pull the next intake charge into the cylinder at the valve overlap period,slightly increasing volumetric efficiency.

this is part of the reason for the specific pairings i made on my headers.

It is kinna like I was saying only completely different.

 

[gwaii]

gas pressure in the tube does come into play as well,but it plays a smaller part.it determines the tube diameter,which is a function of cylinder volume.

 

[martin]

I can see this turning into another math lesson, if we're not careful.

 

[TheTopher]

what's wrong with him going over math? the normal people get a headache and the nerds like me get to learn something. Seems great in my eyes.



By the way, congrats on over 1000 posts in the thread.

 

[gwaii]

By the way, congrats on over 1000 posts in the thread.

woo hoo!


getting pretty close to 25000 views,too

 

[martin]

There are books devoted entirely to intake and exhaust design and they have enough math in them to give Einstein a headache. The other thing to consider is that while gwaii is typing out complex equations and explanations he is not working on the car.