Intake porting/polishing

[exdeath]

Eh? For intake flow that is not under pressure, I'm still going to go with that manifold being terrible. You would want the throttle in the middle.

You've seen the FEA results for this intake?

 

[yottabit]

You want a textured surface in the intake ports to promote turbulence/swirling as the air enters the cylinder to promote air fuel mixing.

+1

If it's direct injection then that's another story

I'm not sure how the story changes with carb/TBI vs MPFI

Like anything else, when it comes to aerodynamics/fluid mechanics, it's very hard to predict whether something is good or bad just by looking at it.

 

[phucheneh]

Note that the plenum tapers down away from the TB inlet. This compensates for the inlet being on one side. It's a perfectly valid way to design an intake manifold, and when properly designed it will not lead to different flows to different cylinders.

I say this having seen side-entry and top-entry intake manifolds modeled first-hand and having used both on FSAE cars.

I'll take your word for it. I mean, it makes sense...it still just seems 'sub-optimal,' though. In the best of cases, wouldn't it still be engineered for certain conditions? As in, flow equalizes when the engine is sucking down 'x' amount of air? It's just hard to think that the last cylinder or two wouldn't be frequently getting a little less air as you're opening and closing the throttle and changing engine RPM and load.

But I'll confess that I never quite understood designs with a big plenum chamber, anyway. If the air is all moving together at the same velocity, how are you going to efficiently feed the cylinder on the end? Wouldn't the air for it need to accelerate to cover the distance? It always seemed like the most efficient intake design would be a lot like an efficient exhaust design in reverse. I.e. equal length runners smoothly diverging from a central point.

 

[Jeff7181]

I'll take your word for it. I mean, it makes sense...it still just seems 'sub-optimal,' though. In the best of cases, wouldn't it still be engineered for certain conditions? As in, flow equalizes when the engine is sucking down 'x' amount of air? It's just hard to think that the last cylinder or two wouldn't be frequently getting a little less air as you're opening and closing the throttle and changing engine RPM and load.

But I'll confess that I never quite understood designs with a big plenum chamber, anyway. If the air is all moving together at the same velocity, how are you going to efficiently feed the cylinder on the end? Wouldn't the air for it need to accelerate to cover the distance? It always seemed like the most efficient intake design would be a lot like an efficient exhaust design in reverse. I.e. equal length runners smoothly diverging from a central point.

It doesn't move at the same velocity in the plenum. Where it narrows, the velocity increases.

 

[yottabit]

It always seemed like the most efficient intake design would be a lot like an efficient exhaust design in reverse. I.e. equal length runners smoothly diverging from a central point.

I believe your correct to be honest, however I believe for some reason (maybe what JCH13 was talking about) the "penalty" for having a log or box-style intake is a lot less than having a log-style exhaust manifold would be (probably because of the higher velocity, pressure, temp. of the exhaust gas)

The nice thing about the box style intakes is the packaging and manufacturing is easy, and you can also easily dial in the proper amount of intake volume you are looking for for torque at the RPM you want, and have very little flow restriction.

Looking back at old hot rods and top fuel dragsters, they use something more like what you are describing, like a "high rise" intake manifold that has long header like passages for each intake port.

 

[JCH13]

It specifies for exhaust ports, and I agree. Intake ports/runners which have and air/fuel mixture flowing through them are quite different.

"A mirror finish of the port does not provide the increase that intuition suggests. In fact, within intake systems, the surface is usually deliberately textured to a degree of uniform roughness to encourage fuel deposited on the port walls to evaporate quickly. A rough surface on selected areas of the port may also alter flow by energizing the boundary layer, which can alter the flow path noticeably, possibly increasing flow. This is similar to what the dimples on a golf ball do. Flow bench testing shows that the difference between a mirror finished intake port and a rough textured port is typically less than 1%."

This is what porting and polishing is all about. You can change the texture of an internal surface and cause the air stream to start bending around a curve before it smacks into the back of the curve and is forced to bend. It's the difference between a clumsy "ugh ugh me make hole bigger make more power" port and polish job vs. a high end port and polish job that's done with some thought and testing.

*shrug* if the flow difference is really 1% or less, then clearly the only real concern is fuel evaporation. I can argue that either way in my head, so I just don't know. I can only guess that with different configurations/engines the ideal surface roughness changes slightly.

I also have a feeling that we're in violent agreement here... heh

 

[JCH13]

I'll take your word for it. I mean, it makes sense...it still just seems 'sub-optimal,' though. In the best of cases, wouldn't it still be engineered for certain conditions? As in, flow equalizes when the engine is sucking down 'x' amount of air? It's just hard to think that the last cylinder or two wouldn't be frequently getting a little less air as you're opening and closing the throttle and changing engine RPM and load.

But I'll confess that I never quite understood designs with a big plenum chamber, anyway. If the air is all moving together at the same velocity, how are you going to efficiently feed the cylinder on the end? Wouldn't the air for it need to accelerate to cover the distance? It always seemed like the most efficient intake design would be a lot like an efficient exhaust design in reverse. I.e. equal length runners smoothly diverging from a central point.

Note that basically only one cylinder is drawing in air at any given point, so it's not exactly like the first 3 cylinders are going to consume all the air and leave none for the 4th.

Here's an interesting post on FSAE.com:

http://fsae.com/eve/forums/a/tpc/f/125607348/m/94210167

Very similar intake designs, but one is capable of about 10hp more (out of 70-90hp, generally speaking). Very subtle changes can make a very big difference, so common intuition means about nothing, unfortunately.

I believe your correct to be honest, however I believe for some reason (maybe what JCH13 was talking about) the "penalty" for having a log or box-style header is a lot less than having a log-style exhaust manifold would be (probably because of the higher velocity, pressure, temp. of the exhaust gas)

The nice thing about the box style intakes is the packaging and manufacturing is easy, and you can also easily dial in the proper amount of intake volume you are looking for for torque at the RPM you want, and have very little flow restriction.

Looking back at old hot rods and top fuel dragsters, they use something more like what you are describing, like a "high rise" intake manifold that has long header like passages for each intake port.

All very good points, especially the point about penalties. The flow rate difference between the intake and exhaust is staggering. The exhaust gas is 3-4x as hot as the intake charge (in absolute temperatures), so triple or quadruple the flow rate right there basically... plus the products of the burning fuel (less important).

Design benefits from specific runner lengths and the helm-holtz resonant frequency of the plenum also matter a lot. There are many, many things to consider...

It doesn't move at the same velocity in the plenum. Where it narrows, the velocity increases.

This.