Hello. Just wondering why intake valves have more diameter than exahust ones.....
Thanks
Intake vs. exhaust valve diameter
- Thread starter mjoekingz28
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To minimize pumping losses.
Because once burned there's less to go out so a smaller valve is all that's needed. It's also forced out by the rising piston.
Descending piston has to pull air/fuel in; I think the fancy term is that there is but atmospheric pressure to fill the void made by the falling piston.
But when the piston is moving upwards, it is a positive pressure which can well exceed atmospheric pressure. Actually its well above that, what with the hot and still expanding gasses.
Net result: much easier to get air out of the engine rather than in. Since one is limited in valve size, it is better to sacrifice exhaust diameter.
Now, add forced induction and things change. But even still, I bet there is higher pressures in the exhaust than in the intake.
Chris142.....Nope. there is a rule of physics called the conservation of mass.
Supton: Nah. The piston acts like a pump.....it will pull any air in that it needs. If it can't, you'll see your air filter or intake collapse. Any and all ambient atmospheric pressure is irrelevant.
Just a theory, I have no why to prove it...but I think the exhaust valve is bigger because exhaust gases are MUCH hotter than the intake charge. As the temperature rises, so does volume. So there is actually a larger volume of gas that needs to exit than what entered. Hence, the larger exhaust port.
Originally Posted By: Phishin
Just a theory, I have no why to prove it...but I think the exhaust valve is bigger because exhaust gases are MUCH hotter than the intake charge
Good point about exhaust valves being smaller due to heat issues. I'm sure much of the heat is being removed through the small sealing surface area; and a larger diameter valve is going to have the center all the hotter. That said, sodium filled exhaust valves is an old trick to aid heat flow out of the valve into the head, I think it dates back to the 60's.
Piston certainly acts like a pump, but it's not that good. 15-20psi at idle/decel? That doesn't take out the air filter nor any lifters, and clearly leads to very little air/fuel in the cylinder.
Was reading the other day about how some of the earliest engines used a very weak intake valvespring and *no* camshaft, relying solely on atmospheric pressure to open the intake--apparently this was part of the impetus behind the F head (intake valve in head, exhaust in block).
In general, engines exhibit better performance when intake valve size is optimized. Exhaust valve size is not as critical.
It's not unusual for the intake valve to be much larger than the exhaust valve. And the result is better volumetric efficiency.
Interestingly, this (intake valve area vs, exhaust valve area) formula works well for Turbocharged/supercharged engines also. Audi, for example was using engines with 3 intake valves and 2 exhaust valves. Ford used 2 intake valves and one exhaust valve in many engines with good results. Also, some 4 valve diesels use intake and exhaust valves that are very close in size.
There have been some experiments with extremely high RPM F1 type engines trying to use intake and exhaust valves that are quite close in size. Guessing this is just simple geometric size optimization.
It's easier to get the exhaust gasses out than to get the intake gasses in.
Keep in mind that the exhaust valves start to open BEFORE the piston is on it's way up for the exhaust stroke, and cylinder pressure is still very high. Leading to efficient evacuation of the cylinder. Many don't really understand that the piston does not force most of the exhaust gas out. Most of the exhaust gas pressure escapes the instant the valve opens. The engine does not do much work pushing out the remaining volume, as the pressure is quite low in a non turbocharged engine (with a relatively free flowing exhaust system)
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Originally Posted By: mjoekingz28
Hello. Just wondering why intake valves have more diameter than exahust ones.....
Thanks
Originally Posted By: robertcope
Originally Posted By: Phishin
Just a theory, I have no why to prove it...but I think the exhaust valve is bigger because exhaust gases are MUCH hotter than the intake charge
The other big factor is that the correct exhaust port size to maintain proper exhaust velocity and scavenging is always smaller than the intake side of things.
There is no reason in the world to have an exhaust valve that is oversized for the port it is feeding.
Oversize the valve with a correct port, and you get nothing and potentially lose something.
Oversize the valve and port, and you have an engine that's a total dog.
That's the purpose of the 3V heads in the Ford engines. The single exhaust valve allows for high port velocity at lower rpms, to give the engine the widest, flattest torque curve possible.
Originally Posted By: Phishin
Chris142.....Nope. there is a rule of physics called the conservation of mass.
Supton: Nah. The piston acts like a pump.....it will pull any air in that it needs. If it can't, you'll see your air filter or intake collapse. Any and all ambient atmospheric pressure is irrelevant.
Having built more than a few 350+ hp mouse's with mild cams, if you want to make power, then think of the engine as a simple pump. The ONLY force moving air into the engine is the atmospheric pressure. ( = 1 BAR+ varies with altitude; the column "weight" of the "bowl" of air we swim in 
Any cylinder head layout has limited area to place an operational Intake and Exhaust valve AND the spark plug. Typically the intake valve area will be the limiting factor to achieve max airflow IN. as you only have 14-15psi "pushing" the air in. On the Ex side you are exhausting high pressure hot gasses which would require less valve area to evacuate the cyl adequately - THEN you have header scavenging to resonate "pull" the remaining gas out at the intake ex open overlap (yes both valves are open at the SAME time around TDC finishing the EX stroke. Now FORD has gone too far in the past with 351C heads and others so there is a tipping point where the exhaust will become too small for max torque at RPM. it’s a balancing act especially for a street engine were you want more than a 1500- 2000 RPM range of operation ( racing say 5000-6500) street torque peak tune to run and operation range at 3250-5250.
Any cylinder head layout has limited area to place an operational Intake and Exhaust valve AND the spark plug. Typically the intake valve area will be the limiting factor to achieve max airflow IN. as you only have 14-15psi "pushing" the air in. On the Ex side you are exhausting high pressure hot gasses which would require less valve area to evacuate the cyl adequately - THEN you have header scavenging to resonate "pull" the remaining gas out at the intake ex open overlap (yes both valves are open at the SAME time around TDC finishing the EX stroke. Now FORD has gone too far in the past with 351C heads and others so there is a tipping point where the exhaust will become too small for max torque at RPM. it’s a balancing act especially for a street engine were you want more than a 1500- 2000 RPM range of operation ( racing say 5000-6500) street torque peak tune to run and operation range at 3250-5250.
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When the exhaust valve opens, cylinder pressure is 100+ psi, and the exhaust basically blows out, equalizing cylinder pressure to exhaust pipe pressure before or a little after the piston reaches BDC. Then the piston pushes the remaining exhaust gases out. Traditionally, the design ratio has been to make the exhaust valve 80% of the intake valve diameter, but lately, the trend has been more toward 75%. Then the exhaust cam duration is increased to make up the flow deficit.
Originally Posted By: supton
Good point about exhaust valves being smaller due to heat issues. I'm sure much of the heat is being removed through the small sealing surface area; and a larger diameter valve is going to have the center all the hotter. That said, sodium filled exhaust valves is an old trick to aid heat flow out of the valve into the head, I think it dates back to the 60's.
Originally Posted By: A_Harman
When the exhaust valve opens, cylinder pressure is 100+ psi, and the exhaust basically blows out, equalizing cylinder pressure to exhaust pipe pressure before or a little after the piston reaches BDC.
It is always harder to pull vacuum then to build pressure. Look at any other system be it hydraulic or pneumatic, the pressure lines (exhaust) are always smaller in diameter than the intake or return lines. That is why the intake pipe is much bigger than the exhaust pipe. Valves follow the same principle unless the engine has other means to pre-compress the incoming air.
Originally Posted By: Chris142
Because once burned there's less to go out so a smaller valve is all that's needed.
Originally Posted By: Phishin
Chris142.....Nope. there is a rule of physics called the conservation of mass.
Supton: Nah. The piston acts like a pump.....it will pull any air in that it needs. If it can't, you'll see your air filter or intake collapse. Any and all ambient atmospheric pressure is irrelevant.
Just a theory, I have no why to prove it...but I think the exhaust valve is bigger because exhaust gases are MUCH hotter than the intake charge. As the temperature rises, so does volume. So there is actually a larger volume of gas that needs to exit than what entered. Hence, the larger exhaust port.
Originally Posted By: TFB1
Originally Posted By: Phishin
Chris142.....Nope. there is a rule of physics called the conservation of mass.
Supton: Nah. The piston acts like a pump.....it will pull any air in that it needs. If it can't, you'll see your air filter or intake collapse. Any and all ambient atmospheric pressure is irrelevant.
Just a theory, I have no why to prove it...but I think the exhaust valve is bigger because exhaust gases are MUCH hotter than the intake charge. As the temperature rises, so does volume. So there is actually a larger volume of gas that needs to exit than what entered. Hence, the larger exhaust port.
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