Why is the exhaust valve smaller than the intake? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 02-14-2011, 10:27 PM
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Why is the exhaust valve smaller than the intake?

This doesn't make sense to me. The intake charge is a smaller volume than the combustion gases. That is after all why the piston goes down after combustion. So it would seem that if you have to move a larger amount of gas out, then a larger exhaust valve would be in order. So why is the intake larger? Yes I am noobie, deal with it! And thanks for the info!

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Old 02-15-2011, 12:23 AM
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Unless the engine is supercharged, there is only 15 psi to push air into the engine's cylinders, on the intake stroke.
On the exhaust stroke, there is still was more pressure inside the cylinder when the exhaust valve opens, than the 15 psi atmospheric, and the piston is actually pushing the exhaust out of the cylinder.
Long story short, it is easier to get the exhaust out, than it is to get the air in. You only have so much space to put the valves in, and making the intake bigger improves the total airflow through the engine
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Old 02-15-2011, 05:11 AM
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Exactly what DANIELC said...
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Old 02-15-2011, 05:32 AM
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the intake charge is larger, you have fuel and air mixture going in, burned up mixture exiting, anytime you burn something it gets smaller, does it not?
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Old 02-15-2011, 06:40 AM
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Quote:
Originally Posted by jaymz
the intake charge is larger, you have fuel and air mixture going in, burned up mixture exiting, anytime you burn something it gets smaller, does it not?
Uh, no it doesn't.
The burning (exploding) air/fuel mixture expands, which is the basic operating principle of the internal combustion engine.

EDIT:
I think you were confusing the end result of combustion with the effect of combustion.
Example:
Burn a big stack of wood, end up with a small pile of ashes.
That isn't accounting for the immense volume of BTUs that were released during the burning.
Capice ...???

Last edited by retromotors; 02-15-2011 at 06:50 AM.
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Old 02-15-2011, 06:54 AM
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Think of it in terms of energy. Basic law of physics is energy is neither created or destroyed, only changes forms. X amount comes in intake valve. Once ignited, most of the energy is used to push the piston down. The leftover energy (exhaust gases) are expelled out the exhaust valve. You are not creating energy by igniting the mixture, only converting it to heat, basically. Since most of the energy was used in pushing the piston down, there is less that has to go out the exhaust valve compared to what came in. The sum of the energy used to push the piston down and the leftover that has to exit can never be more than the amount of energy that came in.
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Old 02-15-2011, 07:25 AM
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I am going to suggest that the intake charge is smaller in volume. Why, because it is colder, and at least some of the fuel enters the cylinder in a liquid state, and it has not fully vaporized yet.
The exhaust gas is hotter and therefore occupies more volume.
But again, residual leftover pressure in the cylinder is much higher that the atmospheric pressure that pushed the air in to the cylinder in the first place.

It may very well be that early in the last century, when engineers were looking to get more power out of an engine, someone just tried to make the exhaust valve bigger, on a test engine, and found out it made less power than an engine with the same size valves, or they tried an experiment with an engine with bigger intake valves, and found a power increase.
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Old 02-15-2011, 07:56 AM
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Cold or not, part of the reason the intake charge volume is larger than the exhaust is because some of that volume is taken up with fuel-it's not all air. That's why a carbureted, alcohol-fueled engine has bigger intake ports (and often intake valves). It takes nearly twice the volume of alcohol as it does for gas. There is no fuel in the exhaust leaving the cylinder (theoretically), plus it is forced out by a pressure greater than the atmospheric pressure can draw through the intake port and valve, as DanielC said. So a smaller exhaust is sufficient. Plus, as stated, there is only so much room for valves in a head.
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Old 02-15-2011, 08:05 AM
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Early engines were actually designed according to SS's theory. They put more effort on exhaust than intake. I remember reading about a team (Kenz and Lesley - play with the spelling) that would take early Ford (I think) flatheads and reverse the cam among other things redirecting the entire flow chemistry as designed allowing more intake flow and letting the exhaust take care of itself as the higher pressure after combustion exits easier than atmospheric forces entry.
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Old 02-15-2011, 08:31 AM
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I feel dumber for reading all of this previously posted crap.

atmospheric pressure is not what causes air to enter into the cylinder, it has nothing to do with anything asked.

The volume of the intake and exhaust is continuously changing through each cycle of an engine, again irrelevant.

The PRESSURE differential on the exhaust stroke is much greater than the pressure differential on the intake stroke and that is the primary reason for the valve sizing as well as the flow requirements.
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Old 02-15-2011, 08:52 AM
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Reminds me of the question, If fuel was supplied why would the engine not run in reverse rotation.
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Old 02-15-2011, 09:03 AM
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I suppose the next question is "why does my engine run better at sea level that it does at 5,000 ft"? It would crack me up to watch east vs west shoot out's in Tucson Az. and the guy's from the coast would get their butt's kicked!
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Old 02-15-2011, 09:07 AM
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Quote:
Originally Posted by sqzbox
I suppose the next question is "why does my engine run better at sea level that it does at 5,000 ft"? It would crack me up to watch east vs west shoot out's in Tucson Az. and the guy's from the coast would get their butt's kicked!

It is because the air density changes, same thing can happen at a single location with a temperature change.

BTW, some engines have been configured to run both clockwise and counterclockwise with only changing a few parts.
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Old 02-15-2011, 09:12 AM
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Quote:
Originally Posted by spinn
Reminds me of the question, If fuel was supplied why would the engine not run in reverse rotation.
I've seen winged sprint car's that do that. Theory was to counter the engine's torque coming out of the corners keeping the left front tire on the ground.
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Old 02-15-2011, 09:36 AM
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Wow, there are a lot of people posting in this thread who apparently slept through physics class.

First, on a normally aspirated engine, atmospheric pressure is the ONLY thing that moves air into the engine. The piston moving down on the intake stroke reduces the pressure inside the cylinder, and since that pressure is now lower than atmospheric, the DIFFERENCE in pressure between the two is what fills the cylinder. Yes, I'm ignoring second-order effects like reversion pulses.

Second, as noted way above in this thread, the piston pushing the spent mixture out on the exhaust stroke, coupled with the buring of the mixture, causes the inside of the cylinder to be considerably above atmospheric pressure. The result is that the higher delta pressure requires a smaller valve for the same mass flow.

And please note that it isn't volume of the intake or exhaust charge that matters as far as valve size is concerned, it's mass flow.
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