port matching intake manifold

[mk3ukr]

Tomorrow I will be fitting my new intake manifold with 4 mm phenolic gasket. I want to fabricate locating pins to prevent any movement of manifold flange relative to head ports.
My question is: Is it OK to enlarge slightly head port entry to create reversion dam or it works only for exhaust side?
To port match intake with engine in car is big PITA
Thanks
Vladimir, 89T

 

[figgie]

well IF you do decide to port match. Do not do like in the exhaust and smooth it out. The finish has to be somewhat rough.

 

[rakkasan]

well IF you do decide to port match. Do not do like in the exhaust and smooth it out. The finish has to be somewhat rough.

If you have a carb'd car, that is true as you need the turbulance to keep the air/fuel mixed. With a fuel injected car you don't need the turbulance so smooth porting is desired.

 

[figgie]

If you have a carb'd car, that is true as you need the turbulance to keep the air/fuel mixed. With a fuel injected car you don't need the turbulance so smooth porting is desired.

wrong answer. You do need the turbulance to help keep the fuel atomized otherwise you start wetting the walls and that is never good

 

[lzalusky]

wrong answer. You do need the turbulance to help keep the fuel atomized otherwise you start wetting the walls and that is never good

There should not be any fuel in the intake system as the fuel injectors are in the head a couple inches away from the intake valves. Fuel atomization is not the issue with a smooth finish on the intake manifold. I have read that a rough finish on the inside of a pipe will create a turbulent boundary layer between the air and pipe wall, actually allowing better velocity and flow than a smooth surface. I would probably just polish the inside of the intake manifold though.

 

[mk3ukr]

If you have a carb'd car, that is true as you need the turbulance to keep the air/fuel mixed. With a fuel injected car you don't need the turbulance so smooth porting is desired.

I remember reading somewhere intake ports are made rough to create turbulance near the port walls. At low load/rpm it has effect of smaller runners and improve low rpm torque. At high load/air velocity this effect cancelled and you have bigger runners for good high end HP. This explanation makes sense to me. But AFAIK all tuners agree that intake ports have to be roughened up.
Guys, my main question was is it OK to make head ports entries slightly larger then manifold ports. Like reversion dam on exhaust side. To port match manifold + head + 4mm gasket is major PITA when engine in the car
Thanks
Vladimir, 89T

 

[89mkthree]

ok stupid question. what is this "reversion dam" thing? ive been planning to port my head and manifolds for a whilea and ive never heard of this.

 

[figgie]

ok stupid question. what is this "reversion dam" thing? ive been planning to port my head and manifolds for a whilea and ive never heard of this.

in it most simple explanation

the hole the air is going to is bigger than the one it came out off.

 

[IJ.]

in it most simple explanation

the hole the air is going to is bigger than the one it came out off.

ROFLMAO! Simple is good :biglaugh:

 

[89mkthree]

i like simple, thanks guys! now what does it do exactly, i mean why do it?

 

[IJ.]

Stops the boundary layer from flowing backwards into the cylinder on overlap (reversion dam is a clue)

 

[hotsup]

Over many years of seeing port matched & polished manifolds (intake & exhaust) & cyl. heads the surfaces have always been polished to "as smooth as glass" finish. So i don't know why it should, all of a sudden, change from that. :dunno:

 

[supra90turbo]

reversion = reverse
dam = wall that holds shit in

reversion dam = wall that stops air from flowing backwards.

 

[figgie]

Over many years of seeing port matched & polished manifolds (intake & exhaust) & cyl. heads the surfaces have always been polished to "as smooth as glass" finish. So i don't know why it should, all of a sudden, change from that. :dunno:

you are highly mistaken
even the v8 guys know never to polish the intakes. 80 grit is the highest grit recommened on the intake side,

http://www.sa-motorsports.com/diyport.shtm

Step 2.

This is the guide I use, please notice the v8 they are working on

 

[tkvtec]

That's a nice guide. Thanks for posting that. I just wish I knew of that when I had my intake manifold off.

 

[supra90turbo]

Figgie: Post that up in the tech section! I would, but it's your finding.