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Hi Tom!

Funny you mentioned flow testing. I did quite a bit of flow visualization on my stock head.


Didnít use the synchro meter to measure flow rate. I am not sure my vacuum can pull enough to make the intake passage the limiting feature. But I can try!

Here was my set up



Here is after green and orange paint. You can see the impact of edges. And you can see how the valve stem blocked the paint.


Last edited by VFR750; 12-31-2017 at 04:51 AM..
Old 12-31-2017, 04:46 AM
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Mike

I have used the shop vac method to do the front to rear synchronization of my carbs before installation. It worked really well.

Iíll bet you can meter the flow through the head by installing the valve and and controlling the amount it is opened to get a reliable reading on your syncromometer. Not sure it would be accurate enough to show the difference in porting the head though. Worth a try I would think.
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Old 12-31-2017, 05:41 AM
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I'll be very interested in seeing if you can get valid results from using a shop vac. I have tried this in the past without getting any usefull data.

One thing to keep in mind is just increasing the amount of flow through the head will not neccsarily inprove your performance. What you want is to increase, or maintain, the VELOCITY of the gasses through the port. In other words there needs to be a ballance between port velocity and flow.

If you only open up the size of the port, while everything else remains the same, the velocity will be reduced. That is one reason why large port motors have poor low end power, due to poor volumetric efficiency, or port filling, at low RPMs.

On a 911 motor, enlarging the port is not rocket science. The port shapes and valve sizes are set. Porsche through years of experience and testing supplied motors with, ususlly, appropriate port sizes.

Now days all one has to do is look at what port sizes were used by Porsche and use that as a guide.

Funny that the SC motors first used 39mm ports, which are on the large end of what one would consider useful, then changed to 34mm ports, which were smaller than what the 2,7s used.

To split the difference, only makes sense to me. As long as you don't alter the shape of the port, or go beyond the size Porsche used, I don't believe there is any down side to porting. Keep in mind the the valve size is the same either way.
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Old 12-31-2017, 10:46 AM
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Gordon - my suspicion is that Porsche changed the 3.0 from the big port/big runners to the small port/runner setup in 1980 in the US due to some kind of regulatory fuel mileage or perhaps emissions issue. The Euro SCs kept the big ports, and got a large bump in compression - to 9.8:1. The US SCs got 9.3:1, which was better than the previous 8.6 or so. But that's a guess.

Ludvigsen notes that the US models didn't get the 9.8CR because US premium wasn't up to the task, but doesn't mention the ports. Aichele doesn't mention port size, nor does Frere. But the Euro SCs are distinctly faster than their US counterparts on the race track.
Old 12-31-2017, 04:52 PM
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The interesting thing about it is that the HP rating for the US models did not change when the port size was reduced.

Somewhere there is a post comparing dyno results between the two heads on similar motors. Obviously the larger port made more power up at the top of the RPM range.

A friend of mine used to have a '68 with a Euro SC motor. It was quite a rocket.

My current motor has the large port heads. Personally, I would have liked to have started with the small port heads which I would have opened up to 37mm, which I think would be optimal for the type of motor I want. That said, I don't think I am giving up much on the bottom end with the 39mm ports I have.

FWIW, I wanted a torquey, responsive motor since my primary objective is pulling out of slow turns at an AX. I am happy with the way my motor turned out.
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Last edited by Trackrash; 12-31-2017 at 05:55 PM..
Old 12-31-2017, 05:50 PM
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Here are some old dyno results I have. This is three 3.0L CIS engine configurations. All had early style heat exchangers. Big port, big runner 8.6 compression (RED), Big port big runner 9.3 compression (Blue) and later small port, small runner 9.3 compression(green). Torque is helped quite a bit by compression. Big port motor gives up very little on bottom but make better top end power. Small port small runner dies above 5000 RPM. Best combination for stock parts on 3.0L is Euro 9.8 pistons and big port CIS, big runner heads.

john


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Last edited by targa72e; 01-01-2018 at 12:47 PM..
Old 01-01-2018, 12:42 PM
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Those are the grafts I was referring to. Thanks John. Very interesting that there seems to be no loss of torque at the lower RPMs with the larger ports. I wonder if it has anything to do with the long CIS intake manifolds.
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Old 01-01-2018, 03:46 PM
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Quote:
Originally Posted by Trackrash View Post
Those are the grafts I was referring to. Thanks John. Very interesting that there seems to be no loss of torque at the lower RPMs with the larger ports. I wonder if it has anything to do with the long CIS intake manifolds.
I'm sure it has everything to do with the CIS manifolds being the flow bottleneck, not the ports themselves. If they're running CIS I'm sure they saw no point to porting the heads to 39mm
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Old 01-02-2018, 05:24 AM
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Quote:
Originally Posted by lvporschepilot View Post
I'm sure it has everything to do with the CIS manifolds being the flow bottleneck, not the ports themselves. If they're running CIS I'm sure they saw no point to porting the heads to 39mm
Actually what I meant was with the large port motor and the long runners that the CIS uses may have been the reason for no loss of torque on the lower end of the RPM scale. My thinking is a long runner will aid bottom end power even with the larger runners of the 39mm port motor.
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Old 01-02-2018, 08:12 AM
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Here is another collection of runs. The red and blue graphs are the same motor with different intakes. This motor was 8.5 comp with big ports. The blue graph is with a 3.2 Carrera intake manifold and programmable injection (no MAF). The Carrera manifold has large long runners and has lower restriction compared to CIS. It made much more top end power but lower midrange torque compared to CIS. The Blue graph is the same motor with stock CIS re-installed, no other changes. You can see it looks pretty much the same as the other 8.5 big port motor I posted. Green graph is stock 3.2 US Carrera, more compression, larger ports and more displacement than 8.5 comp 3.0L but in general same power curve shape. With the larger runners on the Carrera manifold you are staring to see a trade off of low end torque vs high end power above 4000 RPM.





john
Old 01-02-2018, 12:00 PM
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Blue is the clear winner over red in the "under the curve" power.
Green looks really nice.

So the verdict,, open up the ports to 39?
Old 01-02-2018, 01:13 PM
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