MFI adjustments needed for increased compression ratio on 911T?

wanderboy99 · 01-14-2022, 04:12 PM

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.

Richey · 01-14-2022, 07:42 PM

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 .

Jeff Alton · 01-14-2022, 08:42 PM

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.

manbridge 74 · 01-14-2022, 09:45 PM

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.

Richey · 01-15-2022, 04:22 PM

Too high for pump gas !

RobFrost · 01-15-2022, 05:25 PM

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.

Sent from my SM-G988B using Tapatalk

wanderboy99 · 01-15-2022, 06:30 PM

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.

Jeff Alton · 01-15-2022, 06:57 PM

Quote:

Originally Posted by Richey

Too high for pump gas !

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.

Jeff Alton · 01-15-2022, 06:58 PM

Quote:

Originally Posted by RobFrost

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.

Sent from my SM-G988B using Tapatalk

Yes, while it is apart, have the cam ground to a spec that works better.

Also, measure the CR, don't rely on the published number.

Jeff Alton · 01-15-2022, 07:04 PM

Quote:

Originally Posted by RobFrost

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.

Sent from my SM-G988B using Tapatalk

Yes, while it is apart, have the cam ground to a spec that works netter.

Also, measure the CR, don't rely on the published number.

MST0118 · MST0118's Garage

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.

wanderboy99 · 01-16-2022, 03:40 PM

Quote:

Originally Posted by MST0118

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.

I should be able to report back here in 3-4 months when the rebuild’s done. Wish me luck.

Thank you