Just curious to know why there’s a 10% reduction on exhaust ports typically. Why aren’t they the same size? I bet the answer is flow, but I don’t understand it. Can someone shed some light on the subject?
Why are Exhaust ports 10% of Intakes?
- Thread starter 340inabbody
- Start date
-
Exhaust is under great pressure vs intake which (without a blower) is under a vacume
So the pressure can push the same volume of air through a smaller hole.
At my house I needed more nat gas outlets but my houses supply pipe size would not support it. The plumber had the gas company install a 5 psi meter (vs the standard 2.5psi meter) then added a regulator at the point of distribution. This effectively increased the capacity of the supply pipe to accommodate the additional outlets.
So the pressure can push the same volume of air through a smaller hole.
At my house I needed more nat gas outlets but my houses supply pipe size would not support it. The plumber had the gas company install a 5 psi meter (vs the standard 2.5psi meter) then added a regulator at the point of distribution. This effectively increased the capacity of the supply pipe to accommodate the additional outlets.
360duster
Well-Known Member
i think since there is only so much space available in a head, the port that relies on atmospheric pressure is made bigger. Regarding the exhaust port we´re always talking about positive pressure, not so with the intake port.
Michael
Michael
OldmanRick
Well-Known Member
Wet flow vs dry flow?
Inertia
Well-Known Member
Wood/fuel/air volume before burn, - vs ash, after burn.
Thanks guys. Your comments back my research. I am guessing the actual ratio of valve sizes were optimized for power and efficiency via iteration on something other than a flow bench since it had to take in considerations of temperatures and scavenging etc you would only see on an engine dyno.
I suppose this has a large impact on overall efficiency but I haven’t seen much on this ratio of head design in any papers.
I suppose this has a large impact on overall efficiency but I haven’t seen much on this ratio of head design in any papers.
I don’t think it’s a one size fit all I think there’s just so many darn variables. The shape of the combustion area desired RPM range, efficiency physical size limitations. I found this comment from somebody on another forum that I thought was interesting. From Speedtalk.com
This is easier to read, hopefully
Darin MorganShow Guest
The 75% intake to exhaust flow ratio is NOT correct and never was. I think some engineer pulled this out of thin air.
The engines RPM range, size and intended use as well as other design criteria will be the deciding factor as to valve size and placement but trying to hit some preconceived flow ratio with total disregard for other more important tuning variables is not an intelligent or well thought out approach. I will admit that the industry is full of these little snippets of pseudo wisdom but one should not believe everything they hear. When you try and optimize the valve area in an unlimited engine combination you always sacrifice exhaust valve size for intake valve size. Its much easier to cam and manipulate the exhaust system to evacuate the cylinder ( dynamic blow down and cam shaft events) than it is to manipulate the intake tract. Current Pro Stock and Comp Eliminator engines have intake to exhaust (size in diameter) ratios of 70% and flow ratios of only 58% to a maximum of 61%. These engines are producing 2.78HP/CID at over 5800fpm piston speed so the that kind of shatters the long held belief that the exhaust should flow 75%-80% of the intake. Could we make more power if the exhaust flowed more. No, we tried that. Could we make more power if we increased the exhaust size and kept the same discharge coefficient? Yes it does but there isn't any room left for a larger valve because we just stuck this big intake in there so its really a mute piont. I know thats a simplistic explanation of what is going on but I think it drives the piont home.
Darin Morgan
-Induction Research and Development
-EFI Calibration and Tuning
Reher Morrison Racing Engines
1120 Enterprise Place
Arlington Texas 76001
Phone 817-467-7171
Cell 682-559-0321
Reher Morrison Racing Engines
Darin MorganShow Guest
The 75% intake to exhaust flow ratio is NOT correct and never was. I think some engineer pulled this out of thin air.
The engines RPM range, size and intended use as well as other design criteria will be the deciding factor as to valve size and placement but trying to hit some preconceived flow ratio with total disregard for other more important tuning variables is not an intelligent or well thought out approach. I will admit that the industry is full of these little snippets of pseudo wisdom but one should not believe everything they hear. When you try and optimize the valve area in an unlimited engine combination you always sacrifice exhaust valve size for intake valve size. Its much easier to cam and manipulate the exhaust system to evacuate the cylinder ( dynamic blow down and cam shaft events) than it is to manipulate the intake tract. Current Pro Stock and Comp Eliminator engines have intake to exhaust (size in diameter) ratios of 70% and flow ratios of only 58% to a maximum of 61%. These engines are producing 2.78HP/CID at over 5800fpm piston speed so the that kind of shatters the long held belief that the exhaust should flow 75%-80% of the intake. Could we make more power if the exhaust flowed more. No, we tried that. Could we make more power if we increased the exhaust size and kept the same discharge coefficient? Yes it does but there isn't any room left for a larger valve because we just stuck this big intake in there so its really a mute piont. I know thats a simplistic explanation of what is going on but I think it drives the piont home.
Darin Morgan
-Induction Research and Development
-EFI Calibration and Tuning
Reher Morrison Racing Engines
1120 Enterprise Place
Arlington Texas 76001
Phone 817-467-7171
Cell 682-559-0321
Reher Morrison Racing Engines
Umm, no. This is a misunderstanding of the chemistry and physics. Combustion ADDS oxygen to fuel — this chemical reaction releases the energy contained in chemical bonds, but the total mass & volume remains the same (for all intents and purposes). If 2/3 of the mass of of the air/fuel mix was converted directly into energy, by Einstein's equation you would have a thermonuclear bomb-sized explosion in your combustion chamber. Energy equals mass times the speed of light squared.
TT5.9mag
Two atmospheres are better than one
When Darin Morgan speaks, you should listen. We all should.This is easier to read, hopefully
Darin MorganShow Guest
Post Sun Oct 15, 2006 7:01 pm
The 75% intake to exhaust flow ratio is NOT correct and never was. I think some engineer pulled this out of thin air.
The engines RPM range, size and intended use as well as other design criteria will be the deciding factor as to valve size and placement but trying to hit some preconceived flow ratio with total disregard for other more important tuning variables is not an intelligent or well thought out approach. I will admit that the industry is full of these little snippets of pseudo wisdom but one should not believe everything they hear. When you try and optimize the valve area in an unlimited engine combination you always sacrifice exhaust valve size for intake valve size. Its much easier to cam and manipulate the exhaust system to evacuate the cylinder ( dynamic blow down and cam shaft events) than it is to manipulate the intake tract. Current Pro Stock and Comp Eliminator engines have intake to exhaust (size in diameter) ratios of 70% and flow ratios of only 58% to a maximum of 61%. These engines are producing 2.78HP/CID at over 5800fpm piston speed so the that kind of shatters the long held belief that the exhaust should flow 75%-80% of the intake. Could we make more power if the exhaust flowed more. No, we tried that. Could we make more power if we increased the exhaust size and kept the same discharge coefficient? Yes it does but there isn't any room left for a larger valve because we just stuck this big intake in there so its really a mute piont. I know thats a simplistic explanation of what is going on but I think it drives the piont home.
Darin Morgan
-Induction Research and Development
-EFI Calibration and Tuning
Reher Morrison Racing Engines
1120 Enterprise Place
Arlington Texas 76001
Phone 817-467-7171
Cell 682-559-0321
Reher Morrison Racing Engines
Earlie A
Well-Known Member
This is correct. The only way that mass is converted to energy is in a nuclear reaction. In the combustion chamber, the mass flow going out the exhaust valve is equal to the mass flow coming in the intake valve, assuming no losses to the crankcase (blow by). The power comes from the energy released in burning fuel, so the chemical potential energy of the exhaust is much lower than the chemical potential energy of the intake charge.
The ‘law of conservation of mass’ and ‘law of conservation of energy’ in thermodynamics explain many of these principles.
What causes the piston to go down....
A huge increase in pressure in the combustion chamber from the combustion of the air fuel
The piston goes down then back up. As it's going up the exhaust valve is open and a lot of that pressure is released and the mechanical energy of the piston pushes out the rest.
Bottom line... There is greater pressure ( much greater than atmospheric) in the combustion chamber on the exhaust stroke, this higher pressure can push more volume out through a smaller opening. The lower pressure atmosphere needs a larger opening to push the same volume of air fuel into the chamber.
You put a blower on an engine to force more air fuel into the combustion chamber in the same amount of time. And through the same size valve opening
A huge increase in pressure in the combustion chamber from the combustion of the air fuel
The piston goes down then back up. As it's going up the exhaust valve is open and a lot of that pressure is released and the mechanical energy of the piston pushes out the rest.
Bottom line... There is greater pressure ( much greater than atmospheric) in the combustion chamber on the exhaust stroke, this higher pressure can push more volume out through a smaller opening. The lower pressure atmosphere needs a larger opening to push the same volume of air fuel into the chamber.
You put a blower on an engine to force more air fuel into the combustion chamber in the same amount of time. And through the same size valve opening
Last edited:
Indeed
David Vizard got me thinking about this because he often emphasizes that the exhaust valve size is crucial for optimizing engine performance. He stated that the exhaust valve should be large enough to ensure that the engine can expel exhaust gases efficiently, but not so large that it compromises the velocity of the flow. He often balances the valve size with the port design and the engine’s intended RPM range to achieve the best performance. He likes formulas, but doesn’t have a formula for this one. So it really coalesces with what Darin Morgan says. I think there’s a lot more to this then meets the eye. A lot of volumemetric thermal dynamics going on.
The other thing that got me thinking about this is how everybody has different methods of porting a head. I wonder how many people really know what the hell they’re doing. One that impressed me last night was on Engine Masters looked at a big block and the head was ported and gained about 100 hp. Just from porting! This was Steve Dulcich motor and he did the porting.
The other thing that got me thinking about this is how everybody has different methods of porting a head. I wonder how many people really know what the hell they’re doing. One that impressed me last night was on Engine Masters looked at a big block and the head was ported and gained about 100 hp. Just from porting! This was Steve Dulcich motor and he did the porting.
A guy I know speaks to him all the time, he has him on speed dial.
Intake is high density, low energy, dependent on the piston speed, atmosphere, or boost.
Exhaust gas is low density, high energy, which is dependent on the amount of fuel/air delivered by the inlet system, and the efficiency of combustion of the delivered fuel/air.
Of course, NOS changes the usual N/A inlet efficiency... so it can alter the ratio for a given combo adding it.
Of course, anytime Darin speaks, one should shut up & listen....take notes when necessary.
Exhaust gas is low density, high energy, which is dependent on the amount of fuel/air delivered by the inlet system, and the efficiency of combustion of the delivered fuel/air.
Of course, NOS changes the usual N/A inlet efficiency... so it can alter the ratio for a given combo adding it.
Of course, anytime Darin speaks, one should shut up & listen....take notes when necessary.
Steve did the porting but his “Buddy” a one “Mr, Brice Movey” do the touch up on all of the porting, head and intake, IIRC.
Carrying on…
What David Vizard and Darin Morgan are stating is the valve size ratio is a variable based on many parameters than just the head ports themselves such as what the engine is designed for and doing in what rpm range in what vehicle etc….
This is why no exact ratio or sizes are given. To many variables.
The biggest reason, in a nutshell and probably over simplified is on the intake side, the fuel takes up more space being a (cool) liquid and the exhaust is a (hot) gas. As also stated earlier, under pressure. So when the exhaust gas exits the exhaust valve, the burnt gas exits very rapidly with little to slow it down where as the liquid gas incoming I figure has more weight.
TT5.9mag
Two atmospheres are better than one
I imagine the fuel used, and whether or not the engine is supercharged, turbocharged, or on nitrous will have a very large effect (pure speculation on my part) on determining what valve size ratio will be most effective. But as always this type of thing only ever gets experimented with at the highest forms engine building/racing so to most of us it makes absolutely no sense to worry about it.
Well another reason I started thinking about this was that maybe I want to put a larger intake valve on my J head. That would definitely change the ratio configuration. What would that mean? So I started asking questions. It’s how I learn.
TT5.9mag
Two atmospheres are better than one
Learning is great and the quest for knowledge is important. That’s how we all move forward. But let me give you something else to consider, no single engine built with J heads, (ok maybe some of the guys racing F.A.S.T) will ever benefit from testing at this level or be at the point of hp/cu in where testing at this level will infuluence the outcome of said build. If you want bigger intake valves in your J heads put bigger valves in your J heads, just know that on mild stuff you can go backwards on airflow just doing that.
The incoming gas/air charge has the same mass(weight) as the exhaust. However the exhaust gas is less dense. Think hot air ballon
Dan
There is a post asking about fuel line size on a 440 6 pack. The final word was 3/8 from tank to pump and 5/16 from pump to carbs.
Illustrating that fluid (or air / fuel mixture) under atmospheric pressure needs more cross section area to flow the same amount of fuel (or air / fuel mixture) as a pressurized line (fuel pump or combustion )
Illustrating that fluid (or air / fuel mixture) under atmospheric pressure needs more cross section area to flow the same amount of fuel (or air / fuel mixture) as a pressurized line (fuel pump or combustion )
Earlie A
Well-Known Member
Once the intake charge is in the cylinder and the valves are closed, the only thing that influences density is piston position (assuming no leaks past the rings or valves). Combustion does not affect density. Temperature and pressure go up tremendously with combustion, but unless the piston moves the density in the cylinder does not change.
-
Similar threads
