I've been thinking lately about intake manifold design applied to a forced induction engine. The accepted Helmholtz resonation theory just does not seem like it would yeild a power increase in a FI engine, since the air is always going to be under pressure, how can negative pulses take place in an intake?

Generally, pressure waves will travel 4 times back and forth during the duration of the intake valve being closed. Once the valve is closed, there is no longer a force acting on this pulse correct? It just more or less "bounces", or resonates in the intake untill the valve once again opens, and continues this cycle. Would this still happen on a FI engine? You would think when pressurized air hits a close intake valve, with all the additional force behind it (from the rest of the compressed air in the intake or runner) that it would not allow for a negative pulse to back its way out of the intake. Sure, there would be some turbulance near the valve as the runner fills with more and more air with nowhere to go; but wouldn't it just increase in pressure untill the valve opens again?

What im getting at, is there has to be a better way to design intakes for a FI engine. Forcing air into a plenum, hitting the back wall, then finding its way down to ports just doesnt seem efficient. I understand the principles of the plenum system for a NA engine(at least i think i do ), but these priniciples just dont seem like they would apply in an FI engine.

I'm thinking an intake design similar to a backwards header, with a throttle body on the "collector" end. So air can be forced in, smoothly, without restriction and turbulance cause by hitting a "boxy" plenum. The runners would have to be rather long, which generally is not good for a high rpm engine, but on a FI engine, would it still hurt high rpms torque?

I'm really just thinking out loud here, anyone have any input?

some good reading>> http://www.grapeaperacing.com/GrapeA...ionsystems.pdf

 

If what you say were true then we'd all be driving cars with LT1 style intakes as the packaging and machining is easy. Think L98, 5.0 Ford, LSx, 5.7-6.2 Hemi, 5.2-5.9 Magnum and others that run a very long runner intake. L98 and 5.0 Ford having some extremely long runners from plenum to valve and both make extraordinary low end power.

With FI you still drive under part throttle and any change you make to manifolding will make an appropriate change to the curve even under boost. You aren't changing the velocity of the intake charge with FI, just the density so ram tuning will still have an effect. You are just feeding the engine with a higher pressure atmosphere.

 

You are correct, on a NA motor, long runners promote low end power, so when at part throttle, the long runners i had in mind would also promote low end power, which is good while not in boost. But once positive pressure was attained, i would think it would be far less turbulant charge of air.

How is velocity not changed with an increase in pressure? if volume of the port remains the same? I understand the air becomes more dense, but with that i would think came higher velocity. What are you basing this on? not saying your wrong, but i dont understand...haha

 

So you think a FI manifold should look similar to this?


I personally think this is a good design based on the characteristics you may be looking for:

 

kinda the idea i had, but i was thinking a good ballanced header with a merge collector, flowing backwards.

 

Velocity is locked to displacement and piston speed.

 

even when under pressure?

 

Yep, forced induction is a 'density increaser'. By definition that means there are more molecules per cubic inch. The little buggers are closer together but not necessarily moving faster. Think about a line of people walking into a building at 3 mph with everyone 4 feet from the next person. A certain number of people enter per minute. Now put the people 2 feet apart, but still moving at 3 mph. By increasing density you have increased the 'people flow' without increasing the velocity.

To stretch the analogy a bit further: assume the people are going thru a door that is cycled open and closed by a timed mechanism. The long line of people leading up the door will be stopping and starting at the door, but a block away the pace wil be quite uniform. There won't necessarily be a 'people wave' reflected back from the door, but the distance between individuals will decrease and increase as the door cycles. There's a chance that those stopped awaiting the door opening will be gently pushed thru the door by the crowd still moving behind them. This will happen with both densities, but the pushing should be stronger with the higher density.

My strange view of the world...

 

Explaining technical ideas using "real life" situations doesn't really give you a strange view of the world. It's when you start explaining real life situations using car terms that people start to look at you wierd Just part of being a Gearhead I guess

Great explanation though!

 

Another thought....

It's all about pressure. Air moves from high pressure to low pressure it's really not like a stream of water being shot into the intake thru the TB/carb like you are visualizing in this sentence "Forcing air into a plenum, hitting the back wall, then finding its way down to ports just doesnt seem efficient."

Part of understanding motors is "seeing" air.

FWIW almost every engine simulation out there doesn't use the Helmholtz resonation theory in their calculations, due to the pressure principal I stated above.

NA engines get increases in pressure due to that, but FI engines don't since there is so much constant pressure in the intake port anyways.

Bret