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always wondered why exhaust valves were smaller than intake valves - it seems that a back pressure would result (intake air volume is greater that output air volume unless temperature is involved) but have no idea why one would want that. any comments appreciated. i almost flunked thermodynamics so that is probably why i don't know.
 

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Well, I'm not the only one apparently to also wonder this but was too embarassed to ask.

Most modern cams have larger lift/duration for exhaust; does this compensate for smaller valves?

I do recall from somewhere that a certain amount of backpressure is needed, is this also the reason for smaller exh valves?
 

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I believe the exhaust side of the Ford stroke (and heads) have always been a weak spot. That is why a split cam produces better results on a Ford. I know Edelbrock has built this technology into their cams. Basically the duration is longer on the exhaust stroke, holding the exhaust valuve open longer, to clear the chamber better.

But why not just make better valves in the first place? I'm sure there's a reason somewhere.

Greg.
 

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A big part of it, is in the fact that not everything comming in is going out, thus the exhaust valves don't necessarily need to be as big as the intake.

You need the bigger intake to get adequate air/fuel mixture into the compression chamber. You then compress the mixture and ignite it. During the ignition, a chemical reaction takes place converting the air/fuel mixture to energy (some of which is lost in heat), and exhaust gases. Therefore, the exhaust gasses (which is all that needs to come back out) does not equal the volume of fuel mixture comming in.

That being said, the exhaust valve really should be bigger on small block fords *LOL*.

Edited:
For a real world example: Look at a log when you burn it. Once the chemical reaction has taken place (the chemical reaction occurs as the log burns), the result is a much smaller matter, as a lot of the matter is converted to engergy (heat in this case) during the chemical reaction.
 

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Unless someone has changed the basic tenets of the universe, matter(fuel-air,firelog)is not destroyed in a chemical reaction, only in a nuclear reaction. As far as I know we don't have any cars powered by nuclear reactors.

As a matter of fact, due to the much higher heat content and a decreasing volume, the exhaust gases in the combustion chamber would be under a much higher pressure and would escape the chamber at a much higher velocity. Due to this the exhaust valve can be smaller and still let out the correct amount of exhaust.

Or not.
 

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Intake air has 14.7 psi pushing it into the empty cylinder. Toward the bottom of the power stroke, the gases are still under 100+ psi. This pressure moves the exhaust gasses out quite well. Exhaust valve timing is set to take advantage of this residual pressure since the power stroke ends about 1/4 revolution past TDC.
 

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The actual reason is that you only have about 14 lbs (atmospheric pressure) pushing the intake mixture into the cylinder and past the intake valve. The exhaust, on the other hand has the pressure left over from combustion to force it out past the exhaust valve. The pressure is not as high as the combustion pressure because work has been done with this energy by pushing the piston down, and much of the heat energy has also been removed by the cylinder walls and the chamber, going into the cooling system and released through the radiator. The pressure forcing the exhaust out the exhaust valve is increased by opening the valve earlier when there is more pressure availible - the reason for the different cam profiles.

There are also other subtle factors involved like header design. A properly built header will use the exhaust aspiration effect on one tube to lower the pressure on the next cylinder up in the firing order. This lower pressure will let the exhaust flow out of the chamber quicker and easier. One thing to always remember is that it is the pressure differential that creates flow.

A common myth is that engines need backpressure, that's BS. That's another subject though......... *G*.
 

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Doesn't the myth about engines needing backpressure rise from concerns about having port and header sizes too large (or too small) that do not optimize exhaust flow pulse velocity and scavenging.
 

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Maybe the exhaust gases are more agitated to get out of there and just move faster than the intake charge. G1967T has the idea....

It is a known fact that intake charges need more room to work with before the valve opens up to feed the cylinders....

The larger intake port and valve sizes also help keep things in proportions.....A larger intake will yield more air and fuel...thus better horsepower (this is almost right)

Usually the ratio is 3 to 2 for intake vs exhaust sizes....

it is also known that spent gases expand due to their thermal nature......post power stroke.....

So a smaller exhaust valve is incorporated to keep the strokes balanced (no burping back into the intake side....) boss engines are notorious for this due to their kinked exhaust ports and huge valves.

They are also smaller because backpressure is needed to support some low end torque as well as keeping things nice and hot (for the environmental freaks)

Here is a person who can really answer your question in a scientific
and hands on approach. He's pretty smart.

[email protected]
 

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One thing to always remember is that it is the pressure differential that creates flow.
-------------------
This is what I was trying to get to.......I couldn't have said it better myself Whisperer!!!!

:)
 

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Backpressure, when present, is certainly a factor in exhaust tuning. The size and length of the tube determines the pulse speed and travel down the tube at any given RPM. The target is to have the pulse exit the individual tube into the collector at the same time as the previous firing cylinder is exiting the collector. That creates a vacuum at the end of the individual tube which (for our discussion here) "sucks" the exhaust out of that cylinder. A properly tuned header, with the engine at the RPM the header was designed for, will have a negative pressure pulse hit the exhaust valve at the same time it starts to open.

Backpressure after the collector will dampen the pressure waves.

The common myth of "You need backpressure" comes from people uncapping their headers at the drag strip and having their car run slower. Well sure it does, You just leaned the carb mixture out substantially. If you then up-size your main jets accordingly to get back in tune, you will always run faster then you did before you uncapped the headers.
 
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