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Originally Posted by GregBBRD
(Post 14842235)
The 968 pistons and a GTS stroke crankshaft give one a connecting rod over 6" long.
The rod angle is really nice..... |
^^ Avoid massively porting the intake side of the head. It flows extremely well up to HP you wont achieve in a street engine and the loss in port velocity is no bueno.
Clean and refine the port castings and port match it to the intake manifold and call it a day. For porting the intake, all you need to know is 2 words. Extrude hone. |
Originally Posted by The Forgotten On
(Post 14843528)
^^ Avoid massively porting the intake side of the head. It flows extremely well up to HP you wont achieve in a street engine and the loss in port velocity is no bueno.
Clean and refine the port castings and port match it to the intake manifold and call it a day. For porting the intake, all you need to know is 2 words. Extrude hone. If this is left alone, the air flow hits the edges of the port entry, greatly disturbing flow. If the intake port is enlarged to "match" the intake size....well, this is the last place you want to remove metal....both flow and velocity through the port will suffer. Extrude hone is pretty much a "no win" situation, because of port exit enlargemwnt. |
Originally Posted by GregBBRD
(Post 14848360)
The problem with Extrude hone is the same as it has always been. ..the port "exit" which meets the heads gets enlarged the most. This will leave this area 2-3mm larger than the port in the head.
If this is left alone, the air flow hits the edges of the port entry, greatly disturbing flow. If the intake port is enlarged to "match" the intake size....well, this is the last place you want to remove metal....both flow and velocity through the port will suffer. Extrude hone is pretty much a "no win" situation, because of port exit enlargemwnt. avoid enlarging the intake manifold port any larger than the head port. You can match the intake to the head, you are likely to see some opportunity due to probably the casting process. |
Originally Posted by GregBBRD
(Post 14848360)
The problem with Extrude hone is the same as it has always been. ..the port "exit" which meets the heads gets enlarged the most. This will leave this area 2-3mm larger than the port in the head.
If this is left alone, the air flow hits the edges of the port entry, greatly disturbing flow. If the intake port is enlarged to "match" the intake size....well, this is the last place you want to remove metal....both flow and velocity through the port will suffer. Extrude hone is pretty much a "no win" situation, because of port exit enlargemwnt. |
Originally Posted by The Forgotten On
(Post 14849248)
Interesting, I didn't know the issue was that severe.
Now that I'm a bit smarter than then, the loss of power could have certainly been because the manifold surface was too smooth, increasing the laminar air flow issue. Wow. That was a long time ago.... |
Originally Posted by GregBBRD
(Post 14849348)
In the late 1980's, we were tasked with making the 3.2 liter Carrera engine make more power for track use. Those manifold were terrible---where the plenums attached to the runners. At first, we cut the manifolds in half, did port work, and welded them back together. Then we tried the Extrudahone process. These manifolds all made less power than the stock ones did. This is where were we first ran into the manifold port exit getting so huge....so we blamed the loss of power on this issue.
Now that I'm a bit smarter than then, the loss of power could have certainly been because the manifold surface was too smooth, increasing the laminar air flow issue. Wow. That was a long time ago.... I wonder how that would have affected your results. |
with using 968 pistons in a 928 build is there any given "go-nogo" point for the shinyness of the piston skirts? how can you tell if the piston "coating" is still good to use with the alusil bore?
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Originally Posted by The Forgotten On
(Post 14849355)
The flow issue is an interesting one for sure. Having an intake port too smooth causes turbulence, and a lot of it. I guess it's why I see aftermarket intake systems for new 911s given divots inside like the outside of a golf ball.
I wonder how that would have affected your results. With the 3.2 liter plenums, we blasted the surfaces after porting and polishing.....before they were welded back together. This whole process took considerable time. |
Originally Posted by GregBBRD
(Post 14849348)
Then we tried the Extrudahone process. These manifolds all made less power than the stock ones did. This is where were we first ran into the manifold port exit getting so huge....so we blamed the loss of power on this issue.
Now that I'm a bit smarter than then, the loss of power could have certainly been because the manifold surface was too smooth, increasing the laminar air flow issue. Wow. That was a long time ago.... When changing something you need to think through whether you could be going backwards or forwards before you make the change. My small example is the nub situated at the exit of the intake manifold which I thought long and hard about removing, trying to think why it was there, was it design for performance or due to the manufacturing process. It’s a very minor thing when you consider the size of the port. My conclusion was that it was related to the manufacturing process, it was to small to be a performance aid and I doubted that Porsche would have been able to design this into the part in the first place. I removed it. |
Originally Posted by Marti
(Post 14850227)
It’s not an obvious approach to leave the surface inside a port rough, most people would think that smooth is better - that is until you learn more about aero and surfaces. When changing something you need to think through whether you could be going backwards or forwards before you make the change. Surface finish air flow changes do show up on a flow bench. Unfortunately, there is no "hardfast" rule for what works. Some ports like to be smooth, some ports like to be rough. There's a fair amount of trial and error when "working up" a new (new to the guy doing the work) cylinder head. |
I wish all owners had strokers , really livens up the drive big time
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There is still one stroker left to be completed identical to Kiborts motor.
Bored out block, Carillo Rods, and Moldex crank (designed by Twin Turbo Todd) are sitting in a storage room at my friends shop. Block will need to be Nakisil coated & pistons, rings, wrist pins made. The Rennlister sitting on these parts has very little interest in ever finishing the project. |
Originally Posted by Hacker-Pschorr
(Post 14852872)
There is still one stroker left to be completed identical to Kiborts motor.
Bored out block, Carillo Rods, and Moldex crank (designed by Twin Turbo Todd) are sitting in a storage room at my friends shop. Block will need to be Nakisil coated & pistons, rings, wrist pins made. The Rennlister sitting on these parts has very little interest in ever finishing the project. |
I've been told wear on the ring lands can be an issue with used 968 pistons.
Is there some mid range stroker combination that doesn't need custom rods and crank (using 968 pistons)? |
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