Intake porting/polishing

[Jeff7181]

An old school mate of mine posted a picture of his progress porting/gasket matching and polishing a new intake manifold for a 390FE that he's building.

I commented that he shouldn't go crazy with polishing the intake runners. He disagrees because the engine in his current rock crawler "screams" and he polished the intake runners smooth as glass.

I realize the actual effect is going to be pretty small, but can anyone think of a reason to polish the intake runners on a carbureted engine to a near-mirror finish? I've never used anything finer than an 80-grit cartridge roll on intake runners, even when port fuel injection was to be used.

*EDIT* Also, he pointed out that the 390FE is somewhat unique in that the intake ports in the heads are quite short, and the runners in the intake are longer. In my opinion, if he properly matches the ports, it doesn't matter where the intake ends and the head starts as the idea behind gasket matching is to remove the "seam" where the intake and head meet. Am I wrong about any of this?

 

[Zivic]

An old school mate of mine posted a picture of his progress porting/gasket matching and polishing a new intake manifold for a 390FE that he's building.

I commented that he shouldn't go crazy with polishing the intake runners. He disagrees because the engine in his current rock crawler "screams" and he polished the intake runners smooth as glass.

I realize the actual effect is going to be pretty small, but can anyone think of a reason to polish the intake runners on a carbureted engine to a near-mirror finish? I've never used anything finer than an 80-grit cartridge roll on intake runners, even when port fuel injection was to be used.

*EDIT* Also, he pointed out that the 390FE is somewhat unique in that the intake ports in the heads are quite short, and the runners in the intake are longer. In my opinion, if he properly matches the ports, it doesn't matter where the intake ends and the head starts as the idea behind gasket matching is to remove the "seam" where the intake and head meet. Am I wrong about any of this?

Seems like a lot of work for what would be small gains. Just boost it and forget all the port/polishing shens

 

[MixMasterTang]

I don't see how it would hurt in anyway, so really not worth arguing about.

 

[phucheneh]

What would the benefit of rough intake runners be? Or are we just talking about the difference between 'smooth' and 'smooth as glass'?

I would say seamless transitions and the removal of the general 'cast' surface texture would net most all of the gain. Going from a smooth surface, like if it was CNC'd, to a mirror, is probably very little if even measurable. But it surely won't hurt.

You'd have to talk to someone who builds race engines for a living to really get a valid response, though. Even then, there's a lot of opinion if they've never actually observed the difference on a dyno with no other variables.

 

[jlee]

Intake design in general will make a much more substantial difference. Example: http://atsracing.net/Parts/Details/E-IMAN-SR

 

[phucheneh]

That would be an absolutely terrible intake design for an NA car.

 

[twinrider1]

Decades ago the prevailing thought was the intake ports needed a little roughness to help keep fuel atomized.

I agree with you that port matching is more important.

 

[JCH13]

I looked up some article on surface roughness and it's effects on flow velocity. It would seem that it does have a noticeable impact on performance. Pressure losses go up with surface roughness, as does heat transfer. Thus a rougher a surface will get more heat and less air to the engine.

Surface roughness can also trap fuel and keep it from getting to the engine, apparently. Again, this effect would be magnified in a carb'd engine.

Now, I don't know what grit should be used, but in the articles I read I saw a very strong correlation between smooth surfaces and high flow velocities. I would think #80 grit would be pretty darn rough. I personally wouldn't go to a 'near mirror' finish, i.e. #1500-#2000, but I would probably finish everything with #400-#600. I say this as a guess, not an expert opinion.

Like MixMasterTang alluded to, smoother is not going to hurt, and it definitely helps. I won't argue if the improvements are worth it or not though, that's a case-by-case decision IMO.

His point of the relatively long intake runners in his engine simply means that he's polishing a large portion of the whole intake port, so his efforts yield bigger results.

Though the fact that your friend is using a carb on a rock crawler isn't a good indication of a well-thought-out decision making process.

FWIW I see around the internets people claiming anywhere between +10% to WHP to power losses depending on how good the port+polish job is. Though I think that has more to do with screwing up the "port" part and less to do with the "polish" part.

That [ATS racing intake manifold] would be an absolutely terrible intake design for an NA car.

That style of intake design is just fine for NA engines assuming the runner lengths are reasonable and the plenum volume is reasonable given the rest of the engine's design.

It's also important that the runner-to-plenum connection look like the following to have a good flow transition.

 

[JCH13]

Not that wikipedia is an exhaustive source of information, but here's what it says:

"It is popularly held that enlarging the ports to the maximum possible size and applying a mirror finish is what porting is. However that is not so. Some ports may be enlarged to their maximum possible size (in keeping with the highest level of aerodynamic efficiency) but those engines are highly developed very high speed units where the actual size of the ports has become a restriction. Larger ports flow more fuel/air at higher RPM's but sacrifice torque at lower RPM's due to lower fuel/air velocity. 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%. The difference between a smooth to the touch port and an optically mirrored surface is not measurable by ordinary means. Exhaust ports may be smooth finished because of the dry gas flow and in the interest of minimizing exhaust by-product build-up. A 300 - 400 Grit finish followed by a light buff is generally accepted to be representative of a near optimal finish for exhaust gas ports.
The reason that polished ports are not advantageous from a flow standpoint is that at the interface between the metal wall and the air, the air speed is ZERO (see boundary layer and laminar flow). This is due to the wetting action of the air and indeed all fluids. The first layer of molecules adheres to the wall and does not move significantly. The rest of the flow field must shear past, which develops a velocity profile (or gradient) across the duct. For surface roughness to impact flow appreciably, the high spots must be high enough to protrude into the faster moving air toward the center. Only a very rough surface does this."

Basically some smoothing is good, but too much can be bad in some respects.

 

[Jeff7181]

What would the benefit of rough intake runners be? Or are we just talking about the difference between 'smooth' and 'smooth as glass'?

I would say seamless transitions and the removal of the general 'cast' surface texture would net most all of the gain. Going from a smooth surface, like if it was CNC'd, to a mirror, is probably very little if even measurable. But it surely won't hurt.

You'd have to talk to someone who builds race engines for a living to really get a valid response, though. Even then, there's a lot of opinion if they've never actually observed the difference on a dyno with no other variables.

I worked on 1000+ HP alcohol and gas engines years ago and I was always told NOT to use finer than 80-grit cartridge rolls when opening up or just cleaning up air passages that also pass fuel. That's still smoother than the surface of the intake left by the casting process, but no where near mirror finish.

A rough surface helps to atomize fuel and avoid what I've heard referred to as "fuel splatter" (basically atomized fuel condensing on the surfaces, especially around bends).

 

[JCH13]

The theory there is that a rougher surface has more effective surface area than a smooth surface. Like crumpling up a piece of paper, the paper still has 8.5x11in of surface area, but takes up less space on your desk. This lets fuel evaporate off of the walls of the intake more easily.

Though I think the rule of thumb for a 1000hp engine will be the exception, not the rule.

 

[Jeff7181]

The theory there is that a rougher surface has more effective surface area than a smooth surface. Like crumpling up a piece of paper, the paper still has 8.5x11in of surface area, but takes up less space on your desk. This lets fuel evaporate off of the walls of the intake more easily.

Though I think the rule of thumb for a 1000hp engine will be the exception, not the rule.

The same physics apply whether it's a 120 HP 1.6L I4 in a Kia Rio or a 1000 HP 572ci V8 in a dragster, the effect is just on a smaller scale.

Actually, now that I think about it, I wonder what the effect would be from polishing the entire intake path to a mirror smooth finish and applying NeverWet.

 

[Tsavo]

What works for one engine won't necessarily work on a different engine design.

I'd think the design of the 390FE has been fettled enough by now to separate the hocus pocus from stuff that makes more twist.

 

[JCH13]

The same physics apply whether it's a 120 HP 1.6L I4 in a Kia Rio or a 1000 HP 572ci V8 in a dragster, the effect is just on a smaller scale.

Actually, now that I think about it, I wonder what the effect would be from polishing the entire intake path to a mirror smooth finish and applying NeverWet.

I understand that the same physics apply. My point is that making 1,000hp using alcohol has a metric fuckload (it's a technical term) more fuel than 120hp in gasoline and thus the optimized trade-off of flow velocity vs fuel evaporation rate is probably different.

Neverwet would be interesting, assuming it can stand up to the corrosive effects of alcohol/fuel and the fuel didn't just pool up and cause hydrolock!

 

[Jeff7181]

What works for one engine won't necessarily work on a different engine design.

I'd think the design of the 390FE has been fettled enough by now to separate the hocus pocus from stuff that makes more twist.

Physics is physics. No engine works better with liquid gasoline dripping into the cylinders.

 

[JCH13]

Physics is physics. No engine works better with liquid gasoline dripping into the cylinders.

No crap.

Tsavo's point is still valid though.

I bet the intake velocities in your 1000hp 572 engine example are MUCH higher than those in a 390, and you're using a lot more fuel to boot. This means that more fuel is being 'splattered' out on every turn. Thus the intake on your 573 has to be able to recover and re-evaporate much more fuel than the intake of a 390. I'm not saying one extreme or the other is the right answer, but certainly the right answer changes from engine to engine because each engine will have different requirements and different underlying characteristics.

 

[Fritzo]

The theory is is the smoother the surface, the more flow and therefore more power. Companies like Ferrari polish the hell out of their manifolds for this reason. An extra couple of HP never hurts

 

[ISAslot]

When you say new intake I'm assuming it's an aftermarket aluminum one? The stock cast one is quite rough (and heavy!), but if it's an aluminum it's probably not rough enough to make a noticeable difference.

 

[phucheneh]

That style of intake design is just fine for NA engines assuming the runner lengths are reasonable and the plenum volume is reasonable given the rest of the engine's design.

It's also important that the runner-to-plenum connection look like the following to have a good flow transition.

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.

That design would be akin to putting a V8 carb above the water pump.

 

[Jeff7181]

The theory is is the smoother the surface, the more flow and therefore more power. Companies like Ferrari polish the hell out of their manifolds for this reason. An extra couple of HP never hurts

They also use port (maybe direct now) fuel injection which means no fuel passes through the intake runners and thus there is no fuel "deatomization" penalty for increasing the flow by 1 or 2%.

 

[Jeff7181]

No crap.

Tsavo's point is still valid though.

I bet the intake velocities in your 1000hp 572 engine example are MUCH higher than those in a 390, and you're using a lot more fuel to boot. This means that more fuel is being 'splattered' out on every turn. Thus the intake on your 573 has to be able to recover and re-evaporate much more fuel than the intake of a 390. I'm not saying one extreme or the other is the right answer, but certainly the right answer changes from engine to engine because each engine will have different requirements and different underlying characteristics.

I get what you're saying... the right answer doesn't change, it's just a question of to what degree it matters.

Given the fact that polishing requires more work and will likely yield zero benefit on this particular 390 FE (especially considering he opened up the ports A LOT to match the gasket so he's already reducing the velocity of the incoming air) and nobody will ever see it once the engine is assembled I don't see the point.

Also consider that the 390 FE is not a high revving engine and full throttle conditions in a rock crawler are brief at best so it's much more important to have an optimal mix at (relatively) low velocities and part throttle conditions where peak flow is of no concern.

I just don't see a polished intake runner on a low RPM, carbureted engine where full throttle conditions are rarely seen being anything but detrimental.

 

[JCH13]

The 'answer' being 'how rough to make a manifold' certainly does change. Your experience was being told that 80grit is the only way to go. The sources in the Wiki article say that 300-400grit with a buffing operation is generally ideal. I can't find a single piece of evidence to say that one of those is always right and the other is always wrong. I can certainly come up with a lot of arguments about why one works better in one situation.

I'm not arguing that a mirror-like finish is ideal, but if there is a lot of anecdotal evidence suggesting that it does work well then we would be foolish to ignore that. Perhaps there is something else at work that we do not understand.

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.

That design would be akin to putting a V8 carb above the water pump.

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.

 

[Jeff7181]

The sources in the Wiki article say that 300-400grit with a buffing operation is generally ideal.

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.

 

[exdeath]

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

 

[Fritzo]

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

Wouldn't the configuration of the ports take care of this?