porting theory: valve seat approach angle
11-15-2010, 11:03 AM
#1
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Join Date: Jun 2001
Location: Hinesville, GA
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porting theory: valve seat approach angle
Just found a topic that I am debating while porting a set of Vortec heads. No, not LT1 but they use the same port design.
On OEM Vortecs it's practically impossible to hit 90% or even 85% on the throat when porting without cutting into the valvejob using stock valve sizes. It's also hard to open up the intake runners that much in the pinch. (had to weld one before) Most of the meat you can remove is in the roof...as I have done. My heads are headed towards my new 383. The rest of the work came out great, even managed to port a teardrop around the back of the exhaust valveguides. My nicest set of heads yet.
Problem I have deciding on is this: By the time I blend the bowls and clean the ridge up under the seat, the approach angle from the bowl to the seat is getting close to 90 degrees with no real radius for air to flow around leading up to the seat. The long side of the port is what is closer to 90 degrees, where the short side has a little more radius to work with. These heads are still using 1.94/1.5 valves though they are backcut ferreas.
Engine is a street engine running TPI and I've kept that in mind while porting. I have not removed much metal at all except in sizing the intake runners which still have the same port opening dimensions as stock. I'm worried about the flow path slamming air into the back of the valve and causing turbulence upon entering the chamber due to the approach angle being too steep, though I'm sure it flows better than the stock port since the ridge is gone. Nowadays it seems as porters tend to prefer a radius leading up the to the seat (which makes it hard to get close to 90% so I'm assuming it's intended for low RPM cylinder fill and velocity on street heads) Any thoughts?
I have a couple junk 062 heads I'd like to play with but no flow bench to verify results and none local to here.
On OEM Vortecs it's practically impossible to hit 90% or even 85% on the throat when porting without cutting into the valvejob using stock valve sizes. It's also hard to open up the intake runners that much in the pinch. (had to weld one before) Most of the meat you can remove is in the roof...as I have done. My heads are headed towards my new 383. The rest of the work came out great, even managed to port a teardrop around the back of the exhaust valveguides. My nicest set of heads yet.
Problem I have deciding on is this: By the time I blend the bowls and clean the ridge up under the seat, the approach angle from the bowl to the seat is getting close to 90 degrees with no real radius for air to flow around leading up to the seat. The long side of the port is what is closer to 90 degrees, where the short side has a little more radius to work with. These heads are still using 1.94/1.5 valves though they are backcut ferreas.
Engine is a street engine running TPI and I've kept that in mind while porting. I have not removed much metal at all except in sizing the intake runners which still have the same port opening dimensions as stock. I'm worried about the flow path slamming air into the back of the valve and causing turbulence upon entering the chamber due to the approach angle being too steep, though I'm sure it flows better than the stock port since the ridge is gone. Nowadays it seems as porters tend to prefer a radius leading up the to the seat (which makes it hard to get close to 90% so I'm assuming it's intended for low RPM cylinder fill and velocity on street heads) Any thoughts?
I have a couple junk 062 heads I'd like to play with but no flow bench to verify results and none local to here.
11-25-2010, 12:39 AM
#3
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Join Date: Aug 2007
Posts: 573
Re: porting theory: valve seat approach angle
Imagine running through the port as fast as you can, then a little faster where your starting to fall over, the last thing you want is the wall to not be there and you fall over, then have to get up and restart your fall into the cylinder. As straight as possible is the key to flow.
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