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I've been building a 3.0 fitted with domed Ross pistons, Elgin Cam grind 280/270, Webers and Pre-74 exhaust. I'm at that point of down selecting the CR and threads I've looked at here give me pause. So I'm asking for suggestions...
Intended use is street with desired low to mid torque in the 3000 - 6000 rpm range. Since I'm socal, it's 91 gas unless I swing by John Wayne for some AVGAS to mix (Nyet.) Shown below are my calcs for CR. They indicate that significant shimming is required to get the value below 10. I did let George at Ollies talk me into added the second plug port even though I didn't plan on twin-plugging during the build. Given gas and intended use, should I consider it? ![]() ![]()
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Gary 71 911T Miss Demeanor / 2013 Audi Q5 Hundeführer / 1995 993 Miss Adventure |
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What lobe centers are the cams on?
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John Dougherty Dougherty Racing Cams |
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Hi John,
110. You did these last July from SC CIS cams.
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Gary 71 911T Miss Demeanor / 2013 Audi Q5 Hundeführer / 1995 993 Miss Adventure |
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Hi Gary,
Seems to me like 10.5 and the twin plugs. IF you shim the barrels out to drop it to 9.8-1 you will lose the quench, which might create more problems then it cures. BTW AVgas is designed to atomize at high elavations... not good for an engine operating at sea level. You should be able to find a Unocal station with 100 octane. You can spike the 91 with a little 100 and see what happens.
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John Dougherty Dougherty Racing Cams |
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Irrationally exuberant
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I ran some numbers and I get different figures than yours. (I could be wrong.) Judging from my figures it looks like you would need to increase the combustion chamber volume by 10cc to get the compression down to 9.5:1.
That's quite a bit. Looks like to do it you could: (A) Increase deck height to 2.2mm (which is crazy IMO) (B) Reduce piston dome volume 10cc (is there that much meat?) (C) Increase combustion chamber volume 10cc "B" looks like the only one with a chance of working. ![]() -Chris
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'80 911 Nogaro blue Phoenix! '07 BMW 328i 245K miles! http://members.rennlist.org/messinwith911s/ |
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Doesn't want/need a 3.6L
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Quote:
There is a Unocal that sells 100 unleaded at the pump off of the 5 freeway and La Paz in Mission Viejo. There is also a Texaco that offers high octane leaded and unleaded in the City of Orange. I am sure there are quite a few others, but these I am familiar with. Ralph |
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Author of "101 Projects"
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9.5 with single plug,
10.5 with dual plug for us octane starved Californians... -Wayne
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Wayne R. Dempsey, Founder, Pelican Parts Inc., and Author of: 101 Projects for Your BMW 3-Series • 101 Projects for Your Porsche 911 • How to Rebuild & Modify Porsche 911 Engines • 101 Projects for Your Porsche Boxster & Cayman • 101 Projects for Your Porsche 996 / 997 • SPEED READ: Porsche 911 Check out our new site: Dempsey Motorsports |
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The pocket for the second plug should add a little volume to the chamber but whittling down the top of the piston would certainly be the best solution.
Chopping a little off the top of the dome in addition to sinking in some deep valve pockets might be able to give you the volume you need. |
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Alrighty then. Thank you all for your input. My course has been slightly adjusted to aim towards 10.5 CR supported by dual plug ignition. I will do some higher fidelity measurements, so today I'm off to buy some hollow-core solder. Thanks again - stay tuned.
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Gary 71 911T Miss Demeanor / 2013 Audi Q5 Hundeführer / 1995 993 Miss Adventure |
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Air Medal or two
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I am a pigg.....keep the c/r up and solve the problems
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D troop 3/5 Air Cav,( Bastard CAV) and 162 Assult Helicopter Co- (Vultures) South of Saigon, U Minh Forest, Delta, and all parts in between |
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