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Registered
Join Date: Sep 2019
Location: Philippines
Posts: 49
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MFI adjustments needed for increased compression ratio on 911T?
Hello everyone.
I’m having my ‘73 911T (with MFI) engine rebuilt. We’re increasing CR to a maximum of 9.5:1 with stock T cams. My question, will this increase in CR warrant any adjustments on MFI settings? Thanks in advance for your help. |
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Registered
Join Date: Apr 2007
Location: OC
Posts: 824
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I don't think your engine builder has a clue . an engine has a theoretical CR and an actual based on the camshafts . An engine with a theoretical CR of 9.5 to 1 will have an actual CR that will be too high with T cams .
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Registered
Join Date: Jan 2003
Location: Langley,B.C.
Posts: 12,000
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Too high for what? Yes, the cam is far from optimal for a higher (though not high) CR.
At any rate, your MFI will always need adjustment anytime any engine parameter is changed.
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Turn3 Autosport- Full Service and Race Prep www.turn3autosport.com 997 S 4.0, Cayman S 3.8, Cayenne Turbo, Macan Turbo, 69 911, Mini R53 JCW , RADICAL SR3 |
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Functionista
Join Date: Jul 2011
Location: CO
Posts: 7,717
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A good question.
I’d agree with Jeff, setting your CO at idle and 2500 rpm partial load after all other basic CMA checks are done is needed. I’ve never checked mixture on a 911 engine before and after a rebuild with compression change but from doing many air cooled motorcycle builds I think you’ll find that fuel requirements will be somewhat less. Was usual to go down around one main jet size.
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Jeff 74 911, #3 I do not disbelieve in anything. I start from the premise that everything is true until proved false. Everything is possible. |
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Registered
Join Date: Apr 2007
Location: OC
Posts: 824
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Too high for pump gas !
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Registered
Join Date: Jul 2021
Posts: 689
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Any reason you're not having those T cams reground to an E profile? This is a no brainer if you ever open up a 2.2. Can't speak for the 2.4 but I would imagine the same.
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Registered
Join Date: Sep 2019
Location: Philippines
Posts: 49
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I'm from the far east and the garage is fairly new and this is their 5th 911 engine rebuild. It's the only shop I trust since they do have a remote experienced mechanic from the US that's making all the calls. He did mention that prior to any MFI adjustments they will be taking CO measurements and do CMA's. Also, they recently rebuilt a 2.4 motor that's been bumped up to 2.5 with T cams and a higher CR, it runs great! But this one had carbs and mine has MFI, thus my question.
Thanks everyone for all your replies! I will also be forwarding these questions you brought up. |
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Registered
Join Date: Jan 2003
Location: Langley,B.C.
Posts: 12,000
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Really? I would not think so. Unless you have perhaps tried this exact combo....
9.5 with even mild EFI cams is fine on pump gas.
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Turn3 Autosport- Full Service and Race Prep www.turn3autosport.com 997 S 4.0, Cayman S 3.8, Cayenne Turbo, Macan Turbo, 69 911, Mini R53 JCW , RADICAL SR3 |
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Registered
Join Date: Jan 2003
Location: Langley,B.C.
Posts: 12,000
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Quote:
Also, measure the CR, don't rely on the published number.
__________________
Turn3 Autosport- Full Service and Race Prep www.turn3autosport.com 997 S 4.0, Cayman S 3.8, Cayenne Turbo, Macan Turbo, 69 911, Mini R53 JCW , RADICAL SR3 |
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Registered
Join Date: Jan 2003
Location: Langley,B.C.
Posts: 12,000
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Quote:
Also, measure the CR, don't rely on the published number.
__________________
Turn3 Autosport- Full Service and Race Prep www.turn3autosport.com 997 S 4.0, Cayman S 3.8, Cayenne Turbo, Macan Turbo, 69 911, Mini R53 JCW , RADICAL SR3 |
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Registered
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Report back on any MFI tweaks because of this. I don’t think that bumping compression will affect things too much with your pump if you’re keeping the same T cam and throttle bodies/stacks. Maybe 1 click on main rack but maybe not. Let us know.
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Registered
Join Date: Sep 2019
Location: Philippines
Posts: 49
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Quote:
Thank you |
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