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Increasing compression in 2.7?
A local 911 engine builder claims to have good experience from removing material from the cylinder heads to increase the compression from 8,5:1 to 9,5:1. He uses shims to compensate for the removed material in the valve train. This way I could still use the low milage cylinder/piston set in the car.
Does anyone here have experience with this procedure? It will be done on a set of spare cylinder heads that I have lying around so that the originals will remain untouched. The plan is to combine this with 964-profiled cams to hopefully get around 200hp from my otherwise stock 2.7 1977 911S (SSI/Dansk headers and exhaust). |
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04-28-2021, 07:58 AM
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sounds like smoke and mirrors
the only way you can remove material to raise comp ratio is to fly cut the head That would be a lot of cutting, shims reduce comp ratio the thicker the shims get. the 964 cam is a poor choice for performance , it is an emission cam . just some thoughts Ian
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Kermit, 73 RS clone, Just Part of the Team Chris Leydon ,Louis Baldwin ,Peter Brock ,Riche Clark Jerry Sherman ,Rob McGlade ,Donnie Deal Hank Clarkson ,Craig Waldner ,Don Kean ,Leroy Axel Gains |
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04-28-2021, 05:47 PM
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He giveth and he taketh away. My example may not be yours exactly, but I put a 2.7 kit in my 2.4 motor, 8.5 to 1. Drove it that way for a number of years. Went back in, cut the heads .040'', had to cut the pockets a bit, (MFI engine with 4.5 S cams) and raised it to 9.5 to 1. I had to massage the left side cam idler arm a bit and made stainless steel lash caps for the tensioner pistons so that the piston didn;t ride too high out of the tensioner body. That extra bit of compression is worth it. Bob
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04-29-2021, 05:23 AM
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Current: 1975 911S --Chocolate brown Past: 1967 911S --Bahama Yellow 1990 C2 Targa --Silver 1973 914 2.0 --Delphi Green |
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04-29-2021, 05:31 AM
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Yes, I plan to keep the CIS, so that limits the choice of cam profiles. Dougherty Racing also has some which are supposed to work with the CIS.
The procedure described by @r lane is similar to what my engine builder suggests, I guess. |
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04-30-2021, 12:00 AM
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I wonder how a set of non CIS style 9.5 pistons would do in a CIS motor? I'll be in that same boat soon with my stock 2.7, and have been thinking about SC cams and a compression bump..
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Current: 1975 911S --Chocolate brown Past: 1967 911S --Bahama Yellow 1990 C2 Targa --Silver 1973 914 2.0 --Delphi Green |
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04-30-2021, 05:28 AM
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Here is some info on CIS performance .
https://forums.pelicanparts.com/porsche-911-technical-forum/1073886-3-0-rebuild-performance-advice.html Ian Page 1 post # 19. specs on engine Page 2 post # 22 Dyno sheet
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Kermit, 73 RS clone, Just Part of the Team Chris Leydon ,Louis Baldwin ,Peter Brock ,Riche Clark Jerry Sherman ,Rob McGlade ,Donnie Deal Hank Clarkson ,Craig Waldner ,Don Kean ,Leroy Axel Gains Last edited by icarp; 04-30-2021 at 11:16 AM.. |
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04-30-2021, 07:48 AM
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04-30-2021, 09:05 AM
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