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Anthony
 
Join Date: Jul 2019
Location: Anchorage, AK
Posts: 16
911SC Small Ports w/ 964 Cams? Good Idea?

Completely rebuilding 1981 911SC Engine. Upgrading to 3.1L P&C's (97mm, 9.5:1 CR) - Deal from Henry at Supertec which was too good to pass up! Also regrinding the cams to 964-spec. Upgrading to SSI's and new muffler. Keeping stock CIS injection.

This car will be almost 100% street. So I would like to maintain good low-to-mid range torque, but an overall power/torque boost is the goal...

I'm afraid the 964 cams may reduce the low-end grunt, while the smaller ports will limit the upper end - essentially holding back the full range... Am I being overly paranoid? I've also considered slightly advancing the cam timing to allow better performance at the low-to-mid range.

Question is whether or not I should keep the small ports that come stock on '80 - '83 911SC's? Or backdate to the '78-79' larger ports? Pros/cons of either option?

Any tips/thoughts would be greatly appreciated,
Thanks!
-Anthony

Old 09-13-2019, 03:12 PM
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That has been discussed.

Check out these threads. https://www.bing.com/search?q=porsche+911+SC+cams+ports+dynomometer+pel ican+site%3aforums.pelicanparts.com
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Old 09-13-2019, 03:44 PM
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Quote:
This car will be almost 100% street. So I would like to maintain good low-to-mid range torque, but an overall power/torque boost is the goal...

I'm afraid the 964 cams may reduce the low-end grunt, while the smaller ports will limit the upper end - essentially holding back the full range... Am I being overly paranoid? I've also considered slightly advancing the cam timing to allow better performance at the low-to-mid range.
964 Cams aren't the choice for a street car. As you alredy mentioned, the powercurve wil rise and you'll have to rev you engine over 6000 Rpm till you'll feel the benefit. You could try compensate that "a little" by advancing the cam timing to 1.4-1.7 mm like the SC cam spec, BUT as the cam comes with higher valve lift, the risk of valve to piston collision you should really take into account.
But in your case the ports are the real "bottleneck" as the engine will need a huge amount of air due to the bigger 3.1 conversion and ... due to the significant higher RpMs needed for the benefit of the 964 cams.

So for your 1981 model you would need to switch to the bigger inlet manifolds and ... to the heads with bigger ports, beside 3.1 and 964 cams.

For Street choose SSIs, a Dizzy from 123 Ignition with 6-pin CDI connection for optimal individual ignition timings, and be shure you are running a flaweless 100% working Lambda System - perfectly CO adjusted where the AFR for acceleration will be optmal at 12.5:1
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Own digital ECO build
All about the 930/16 and 930/07 Lamba based US SC models:
http://www.elfertreff.de/showthread.php?t=26670
(its in german english will follow, use google translator meanwhile)

Last edited by AndrewCologne; 09-13-2019 at 04:52 PM..
Old 09-13-2019, 04:50 PM
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I had same concern as you when I did my rebuild. Similar to yours except 3.0L.

I decided to go with Dougherty’s DC15 cams. Sport SC. They are kind of an in between of stock and 964.

They are still low end for the street and seems to give really good power in the 3500-5000 range. I don’t go much past 5,000 so I have really experimented upper range. But they are a nice upgrade.


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Old 09-13-2019, 05:02 PM
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Here is a Dyno comparison you might be interested in. It is a comparison of a small vs big port SC with CIS. Big port is 8.5 compression and small port (blue) is 9.3. Both had SSI exhaust. With small CIS you will not make much power above 6K RPM. The Big port SC motors also have larger diameter runners as well so you would need to source bigger runners and airbox to convert to larger port.



You will get a little gain from the larger displacement (3%). SSI and a good muffler will make nice gains over stock exhaust.

The dyno runs below are from two similar cars. Big port SC motors with higher compression late SC pistons{ 9.3 to 1), SSI and sport muffler (dual outlet). If you compare it with the Big vs small dyno you can see the large port still makes less torque below 4K even with more compression.



You should probably see what Cam options would be best to maximize your torque below 6K as your small port heads and CIS will not support making power over 6K.

thanks,

john
Old 09-15-2019, 12:39 PM
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why don't you try first with stock cams ? I did and for slow street use they give great torque out of corners. Would not change. Car is 3.2SS, twin spark, small port w/SSIs and it made more sense for me to work with what I had. Very pleased with result, great below 5000. Great in fact already at 2500 rpm. No need to downshift to overtake. I don't think 964 cams would be a better choice for my engine and usage. I did try DC13 cams and found some loss of bottom end torque and no useful gain at top

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Last edited by trond; 09-17-2019 at 12:31 PM..
Old 09-17-2019, 08:37 AM
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