Motorsports Village

EX port
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Author:  rick360 [ Sat Dec 15, 2012 11:12 am ]
Post subject:  Re: EX port

randy355 wrote:
Would a bigger ex. valve reduce ex. duration requirments?
Anything that increases true valve curtain area should reduce duration requirement. Steeper seats will reduce curtain area yet that seems to make more power even though it seems to want more duration as a result.

randy355 wrote:
A larger ex. valve would flow more even if the throat was left the same size, so if throat speed is important, a larger ex. valve should be a plus because of increased throat speed.
It will increase bench velocity, but will it change velocity in an engine? If the cylinder has the same amount of gas to exhaust and the same throat area to move it through the average velocity will be the same. You would change the velocity through time ... more velocity sooner which would mean less later. Not sure that would be best.

Adger Smith wrote:
Did the tighter lash on each cam make more power because of seat timing, Tighter lash, or did it reduced valve gear deflection??
The ex lobes weren't especially fast ramps so it's hard to think it is deflection and with 250ish duration the cylinder pressure would be pretty low by the time the valve is opening. Very thick wall 3/8" pushrods.
It can be very difficult to tell "why" things happen in a race engine. Many times people "think" they know why, but they don't really "know". But it's fun to try to explain things anyway.


Author:  Adger Smith [ Sat Dec 15, 2012 12:04 pm ]
Post subject:  Re: EX port

My opinion of Ex speed is you don't have to creat a port (around the valve) that enhances velocity because when and if the ex valve is opened at the right time the ex gases are leaving the port at super sonic speed. A port should be sized for the volume to increase as the speed slows down. The gases expand as they cool due to the density of the heated gasses.
One more thing about the ex opening you had. You stated that it responded to the ex lash being tightened with every cam. Then you stated that the ex valve opened when the cylinder pressure was low. That could be showing that the ex valve needed to be opened when the cylinder pressure was higher. As you dump ex early and take advantage of the velocity it creats more pull through and enhances moving the intake charge in during overlap. The placement of the lobe in the ex cycle could have been an issue and responding with thighter lash was the result.

Author:  Slowpoke [ Sat Dec 15, 2012 7:25 pm ]
Post subject:  Re: EX port

Adger Smith wrote:
The gases expand as they cool due to the density of the heated gasses.

It's probably a dumb question but, can you explain in more detail?
I thought gases would expand due to an increase in heat and/or a decrease in pressure.

I don't understand some of the things you guys talk about,
but I do enjoy trying to follow along, and I usually learn something. =;


Author:  shrinker [ Sat Dec 15, 2012 8:32 pm ]
Post subject:  Re: EX port

The main problem with the exhaust gasses, occurs when the piston has to pump them out. The first stage (which is ex valve opening) the gasses flow supersonic through the port. When you hear an engine that has a cracking exhaust note thats a good supersonic flow. Dull /bass sounding engines dont get supersonic flow. When there dull, they have substantial lower efficiency at exhausting and they have high concentrations of exhaust residue in the cylinder and in the intake manifold. If you dont get it out supersonically you have to pump it out with the piston, That takes energy to do and it results in higher cylinder pressure during the exhaust stroke, that causes the exhaust to go up the intake on overlap.
The volume taken up by the exhaust gasses increases once they are let out of the cylinder and the flow rate becomes subsonic. So the first stage of supersonic flow is at constant volume then the volume increases once the speed of flow decreases to subsonic velocity, thats when the gasses expand and the pressure drops dramatically.
If the power stroke is long enough or the ener conversion efficiency to the crankshaft is high the flow may not go supersonic, its the energy that is remaining in the cylinder that gets released when the valve opens that determines if the flow is supersonic or not. The valve rate of opening and the low lift flow of the port also influences the extent of supersonic flow. With proper control of the intake charge condition a tuner can change these factors of supersonic flow etc for an engine. Some engine designs can actually benefit from intake reversion and poor supersonic flow, but the engine should be fixed correctly not patched up by those techniques.

Author:  zums [ Sat Dec 15, 2012 10:05 pm ]
Post subject:  Re: EX port

ive never heard of exhaust gases expanding as they cooled, if that was so you would have to use larger tailpipes after a muffler in a full system to keep power up, ive found the opposite to be true, eg 3 " system than slightly smaller od after the muffler with no e/t penalty

Author:  Adger Smith [ Sun Dec 16, 2012 1:34 am ]
Post subject:  Re: EX port

You are too far back in the system to worry about the hot ex gasses cooling. It has to do with particle density and by the time the ex gasses get to the muffler it is all below the temps that particle density is a consideration. What you are working with is the velocity trace or flow from the muffler out and what you say is exactly what happens after the muffler. Have you ever studied or had the theory of "stepped headers" explained? Ever watched the Space Shuttle take off. Check out how quick the ex gasses expand in that puppy. Notice those big bells hanging off the backend of it with those white hot gasses coming out? Humm. I bet they got it wrong, it needs some straight pipes.. on the other hand jet engines squeeze the expanding gasses for more thrust when they kick in the after bunners. That is more of a velocity thing for thrust with expansion taking place after the jet nozzle . The temps actually play a part in that application.
Good, short explantion, Shrinker
BTW we are dealing with Internal combustion engines in our race cars. Rockets and Jets are more often classified as External combustion engines.

Author:  shrinker [ Sun Dec 16, 2012 6:53 am ]
Post subject:  Re: EX port

I think your both confusing the issue or miss understanding one another. Gasses at a pressure thats higher than ambient will expand until ambient is reached. The energy required to expand in volume until ambient pressure is achieved is supplied in all cases by kinetic energy, eg temperature. In other words as a gas expands the temperature drops. IF the gas is an exhaust gas from a combustion then there is also the possibility of continued combustion as it expands. The consequence of that is the temp wont follow the rules but the energy for the expansion is still coming from kinetic energy, its still the same energy transfer. Its just that its not a closed system.
When you compress a gas it heats up when you let it go it cools and expands. Simple.

Author:  Adger Smith [ Sun Dec 16, 2012 10:51 am ]
Post subject:  Re: EX port

That is possible because I should have said Energy Density instead of Particle Density. Remembering all the terms used back in Engineering School 40 years ago gets harder ever day... :~0
It is all about compressed gasses. Starting in the combustion chamber and going right out the ex pipe. I've always thought it had the wrong name. It should be Chemical Reaction Chamber. After all combustion is just another form of a chemical reaction. Fire!

Author:  shrinker [ Sun Dec 16, 2012 5:03 pm ]
Post subject:  Re: EX port

yeah I hear ya on the memory thing!
i call an engine a chemistry preparation machine, many times I see chemical switching happening in a burn, its fun.

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