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D-Shape ports on my Polk Head, WTF?

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definitiveno

15+ Year Contributor
1,237
7
Sep 8, 2004
Reno/Sacramento, California
Has any one ever bought polks stage 4 cnc head before? Were your exhaust ports D-Shaped? I have been 5000 miles away from my car for 1 and a half years now so I haven't seen the parts coming in or any thing but I got a call from robert at roadrace today and he wants to know why my head has D-shape ports on the exhaust side. My response was obviously I didn't know cause I wouldn't have bought a tubular manifold was I aware of that.


Nowhere on their website does it say they put D-shape ports nor is there pictures and nor have I ever read about polk putting out D-shaped ports before. Honestly ive NEVER seen them on a DSM period. Is this common now or am I getting hornswaggled? It's always some thing with the POS car I swear to god :notgood:
 
Honestly ive NEVER seen them on a DSM period. Is this common now or am I getting hornswaggled? It's always some thing with the POS car I swear to god :notgood:

Back when Bj's was still in business all of his heads had D shaped ports. Shep went deep 9's on the D shaped ports as well. Not sure how common they are anymore however.
 
The point of porting a head is for air velocity and horsepower. Remember that.

D shaped ports were actually engineered for high air velocity AND horsepower. This allows for a lower and powerful RPM band. Excellent for the street all the way to the strip. This is why you see 2g heads having great street capablities (smaller ports = higher air velocity) while the 1g head flows great on the track (big ports = higher RPM flow capability)

Higher air velocity mixes the air and fuel better than a slower air charge. It also keeps the air swirling in the chamber better, which is known to keep the combustion process very happy while it burns more efficiently. One of the best things about air velocity is your VE (volumetric efficiency) will actually go up = The air can get in faster. That means it takes less boost to make more power.

When it comes to your head, they probably ported it like that to get the exhaust out faster increasing your VE and spooling that turbo that much faster.

Bigger is not always better, but most DSMers dont realize that ;)
 
Thats all wonderful, I didn't know that fiery gases preferred to travel in D-Shape pipes, excellent. However, ALL THAT IS COMPLETELY NULLIFIED WHEN YOUR MANIFOLD DOES NOT SEAL. No turbo spool, no rpm band, no torque, no all those other positive things you were referring to......

Can you refer me to a builder that makes D-Shape mani's for a dsm? Ive been in the market a while, almost 4 years, still have never seen one personally.

If D-Shapes are going to give me more power but completely limit me from finding parts that work with it then there's not going to be any benefit to me at all.


Ive bought 2 heads for this car already, the first grossly overported and would not seal a manifold more or less work right, and the 2nd which was this one. If I have to buy a third I will probably part this POS out. I can not handle DSM shops any more. Where's the pull all your hair out and light yourself on fire with gasoline smiley when you need it.
 
the flat spot of the "D" shape is facing down correct? iv never done any d shaped porte but i will do them on my neon head (sohc) but lets not go there. the only reason they are going to be that shape is because im going to raise the roof so much that the end result is going to look similar to a "D". have you ever thought of porting the manifold to match the head? what kind of turbo manifold do u plan on running? log, ram horn, mini ran, top mount, bottom mount etc??? i may be able to help you out there.


mike
 
I bought an SLS manifold back when they were still 750, it's a tubular stainless jobber, it took me literally 8 months to get from them. I already feel like an complete moron waiting 8 months for their mani now I will feel like an even bigger one if the son of a beach wont work.
 
I will feel like an even bigger one if the son of a beach wont work.

So have you tried it yet?? They'd have to port pretty high up to not seal against the manifold.

The reason D ports work well is the porting is usually on the roof not the floor. As the hot exhaust gases exit at high velocity it can't easily make the turn down along the floor anyway. The momentum of the gases want to escape in as straight a line as possible, which from the valve seat, is basically up. Porting the floor does nothing to help more gas escape but actually slows down the velocity due to the larger volume of the port.

Porting the roof is where you can get material out of the way, increasing flow, with only a minimal increase in volume. Sometimes welding the port floor HIGHER really aids in flow as it's all dead space anyway.

If they know what they're doing a D port will be beneficial to flow.

Rick - '91 GSX :dsm:
 
So have you tried it yet?? They'd have to port pretty high up to not seal against the manifold.

The reason D ports work well is the porting is usually on the roof not the floor. As the hot exhaust gases exit at high velocity it can't easily make the turn down along the floor anyway. The momentum of the gases want to escape in as straight a line as possible, which from the valve seat, is basically up. Porting the floor does nothing to help more gas escape but actually slows down the velocity due to the larger volume of the port.

Porting the roof is where you can get material out of the way, increasing flow, with only a minimal increase in volume. Sometimes welding the port floor HIGHER really aids in flow as it's all dead space anyway.

If they know what they're doing a D port will be beneficial to flow.

Rick - '91 GSX :dsm:
I spoke with the people at polk yesterday and they told me their D port fits the stock gasket. I had only spoke with robert at roadrace who explained "from the looks of it" he wasn't sure if it would seal. If the polk guys are correct it should work fine. I don't know man, building this DSM has turned me into a worry wart cause ive had so many problems with so many vendors. Parts that don't fit, parts that take too long and then some parts that never show up.........ughhhh. I will let you guys know how it goes in the next week.


Ive heard too many good things about roadrace, I hope they are still a good shop as ive heard that in the passed.
 
Well first lets all try to remember some of the basic rules of physics.
1: Airflow ( in this case) will follow the path of least resistance
2: Curves or turns should be kept to less than 20 degrees.
3: The ideal gas law is PV=nRT
Where
P= absolute pressure
V= the volume of the vessel
n= the number of moles of gas
R= the universal gas constant
T= the absolute temperature

Lets also remember that all porting increases the ports volume.

The defnition of velocity is the rate of change of position along a straight line with respect to time.

The exhaaust port has a positive pressure within it, therefore the short side radius (the ports floor) will matter.

Pressure contained within a vessel will equalize itself within that vessel at roughly the speed of sound. (preaty darn quick)

Therefore there is positive flow along the ports floor.
 
I have a BJ's head with D ports. The gasket covered the lower "D" portion so I trimmed it to be the exact same size as the port. Then I ported the exhaust mani to the gasket. I have not satrted the car yet but I don't forsee any issues.
 
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