Polished intake mani gains?

[LV-DSM]

Just polished the runners on my intake manifold this weekend. I was bored. Anyone notice any gains from just doing this?

 

[echo]

not sure, but you should really resize your sig. i believe the limit is 300x300.

 

[HPF_MK3T]

Well, did you port or did you polish the runners.

If you polished the runners you may have gained and lost. Polishing the runners causes the air to flow smoothly through the runners, but loses any of the turbulence or swirling that is caused. This turbulance is needed to help with fuel atomization when the air/fuel mixture enters the combustion chamber. Improper fuel atomization can cause improper fuel burn or incomplete burn, which in turn can cause torque/hp loss but mainly at low rpm's and a decrease in fuel mileage. The other side of polishing the runner would help increase air velocity by reducing turbulence, giving you a bit more top end power, but not much more without making the runners larger.

A good way to go would have been to port the manifold and leave it somewhat rough to the touch, porting the manifold will also cause a bit of low rpm power loss due to the decreased air velocity in the larger runners, but increased performance due to the extra flow at higher rpm's. The rough surfaces helps with fuel atomization.

Intake manifold power gains are similar to scavenging power gains/losses in an exhaust system. In an exhaust system, the previous exhaust pulse pulls the next pulse out as it travels through the exhaust system. Then there is the point where the RPM's are high enough where the exhaust has to push it's way out, so you want more flow instead of more scavenging, so the benefits of a larger system help out. So, going larger lowers the scavenging affect, lowering low end power but helping with top end flow/power. Going smaller helps with the scavenging affect and low end power, but suffers on top end flow/power. Of course, on a turbo motor the rules are much different when it comes to exhaust, but I'm just using it as an example.

For intake you're looking at it the same way. With the stock manifold the runners are sized pretty close to the ports in the head. So the velocity of the air going into the port is the same throughout the runner. Open the runner and the velocity slows down making the pulse from the previous cylinder work harder to pull in the air at lower rpms, suffering on bottom end, but gaining up top with the improved flow. Hope that helps.

 

[rubbersidedown]

i know this is off topic but i have a quick question since you bring up exhausts. Would a 2.5" dp going to a 3" be a good compromise between just a full 2.5" and a full 3"? Or should i just stick with going the same diameter all the way through?

 

[HPF_MK3T]

Well, on a turbo motor 3" is the size you want to shoot for, and the sooner you can do that after the turbo, the better. The best upgrade would be to replace the turbo elbow or "o2 housing" with a straight through unit and downpipe in one piece, since the factory elbow has a 2 1/2" outlet to the downpipe and the price for one of these units isn't much more than a regular downpipe. Your goal is to get all of the exhaust out of the turbo with the least amount of backpressure.

Thanks,

 

[rubbersidedown]

well i really wanted to retain a little low end power and i was wondering if the 2.5" to 3" would do that? or would the power retained not be large enough to even bother?

 

[marshall]

I'm gonna disagree a bit about needing rough surfaces for the runner walls. In my opinion this theory is a holdover from carb manifolds and doesn't hold true for FI applications. The air doesn't meet the fuel until the top of the intake valve - that's a pretty turbulent area, and right behind that you've got all the turbulence being created in the cylinder. So, cleaning up the runners walls seems like a good idea to me, I went one step further and polished my intake ports as well..

Just my opinion,
Marshall

 

[HPF_MK3T]

rubbersidedown - on a turbo motor the exhaust diameter between 2.5 and 3" really isn't going to make a big difference when it comes ot low end power since you will always have a turbo in the way, where the exhaust pulses will have to go through that. You want to be able to get the exhaust out of the turbo once you start getting the exhaust flowing so you want to minimize backpressure beyond the turbo itself.

marshall- on FI and any kind of vehicle, you want some turbulence shortly before the fuel supply and after. Since the manifold is a dry manifold, it shouldn't make much difference if you polished it. You do not want to polish your intake ports, though, because that causes fuel to puddle up on the walls, creating large droplets of fuel which are harder to burn, which is why fuel atomization is important--smaller droplets are easier to ignite. The rough walls cause turbulence around the edges, keeping the fuel off of the walls. Even if the air is more turbulent once it enters the combustion chamber, once the fuel atoms are combined into a droplet, they will not be separated.

 

[CBgst]

So basicly your saying that extrude honing the mani is bad because it hurts the feul atomization. If so why do people do it?

 

[HPF_MK3T]

Because with increased flow and runner diameter by extrude honing, you get more top end power. But extrude honing smooths and opens the runners, not just smooths them.

With ported intake ports, there will be increased flow at high rpm's, so puddling is lessened because of the air velocity and less turbulence. At lower rpm's, where flow is slower, the puddling occurs, affecting low end power and mileage, especially off idle where air velocity is minimal.

Since the intake runner is a dry manifold, polishing it will change bottom end little to none, and top end power will be little. So, unless you've ported the manifold, port matched the intake ports to the manifold, cleaned the bowls, and removed any burrs or casting imperfections, power gains will be minimal with just polishing of the intake runners and intake ports.

Soooo Anyway, I never said that polishing or porting the manifold was bad. I never said that extrude honing the manifold was bad. I was just mentioning the ups and downs of porting and polishing. and why.

So I will shorten my original post.

Polish runners - lose very little bottom end and mileage, gain little top

Polish Intake ports - lose very little bottom end and mileage, gain little top

Port runners - lose some bottom end and mileage, gain a bit up top

Maybe I should have just put "Not Really"

 

[LV-DSM]

Wow. Thanks for all the info. Can't wait to get the car back together. I'll let you know if I notice a difference.

 

[Sellersbs]

Does anybody know of any places that will port and polish my intake manifold??

 

[rubbersidedown]

i'd suggest just having it extrude honed as apposed to port&polish. but that's probably just me.

 

[loreak]

how much does it cost to extrude hone