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I've been bitten by the stroker bug.

 
Old 03-02-2018, 05:03 PM
  #16  
Kevin in Atlanta
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Originally Posted by GregBBRD View Post
The 968 pistons and a GTS stroke crankshaft give one a connecting rod over 6" long.

The rod angle is really nice.....
Good to hear.
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Old 03-03-2018, 04:48 AM
  #17  
The Forgotten On
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^^ 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.
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Old 03-05-2018, 01:04 PM
  #18  
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Originally Posted by The Forgotten On View Post
^^ 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.
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.
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Old 03-05-2018, 06:53 PM
  #19  
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Originally Posted by GregBBRD View Post
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.
+1
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.
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Old 03-05-2018, 07:04 PM
  #20  
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Originally Posted by GregBBRD View Post
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.
Interesting, I didn't know the issue was that severe.
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Old 03-05-2018, 07:56 PM
  #21  
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Originally Posted by The Forgotten On View Post
Interesting, I didn't know the issue was that severe.
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....
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Old 03-05-2018, 07:59 PM
  #22  
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Originally Posted by GregBBRD View Post
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....
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.
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Old 03-05-2018, 08:40 PM
  #23  
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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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Old 03-05-2018, 09:09 PM
  #24  
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Originally Posted by The Forgotten On View Post
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.
It would have been really difficult to do, so it would have certainly affected the price.

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.
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Old 03-06-2018, 06:37 AM
  #25  
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Originally Posted by GregBBRD View Post
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....
Itís 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.

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. Itís 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.

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Old 03-07-2018, 02:57 AM
  #26  
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Originally Posted by Marti View Post


Itís 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.

And be prepared to "undo" any 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.
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Old 03-07-2018, 10:44 AM
  #27  
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I wish all owners had strokers , really livens up the drive big time
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Old 03-07-2018, 10:54 AM
  #28  
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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.
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Old 03-07-2018, 11:02 AM
  #29  
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Originally Posted by Hacker-Pschorr View Post
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.
PM sent.
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Old 03-09-2018, 04:38 PM
  #30  
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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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