Fuel efficiency/MPG - MFI and other

Danish Dynamite · 05-09-2009, 11:28 PM

How are you guys MPG?

I am running a 1972 911 2.4S with a 3.0 liter MFI engine with high compression pistons

I am finding it hard to get much better MPG than 12 (or 5 km/Liter (Denmark))

I am thinking about swapping to 46mm PMO's in the future for better MPG..

/ Christian

RoninLB · 05-10-2009, 12:58 AM

17 mpg @ steady 70 mph

David E. Clark · 05-10-2009, 02:09 AM

I'm with you Christian. I get about 12-15 MPG on my 1973 911E running 89 octane gasoline (what we call "mid-grade" in the States).

Even so, I encourage you to stick with your MFI system. If you are set up correctly - which admittedly is sometimes a challenge - you will lose performance, especially throttle response, by switching to PMO's. Economically, it will take forever to break even on costs even with the gas prices in Denmark; and you won't be one of those cool

, slightly "off"

MFI people anymore!!

Here in the States the PMO's would cost you between $1,600.00 and $2,300.00, not including installation. Your mileage might increase by 10% -25%; most people report increases closer to 10%. With gas at $5.93 per gallon in Denmark you'd save about $.60 - $1.50 every 18 miles. Unless your car is a daily driver this would probably be years of driving.

signature65 · 05-10-2009, 09:24 AM

My 73T MFI gets about 12mpg also. Refreshing the top end now and am going to CMA my MFI to see if I can do any better.

Flat6pac · 05-10-2009, 09:28 AM

Stock 3.0 I would get 20mpg at 75+mph
3.2, stock, I would get 25mpg at 75 or 80

Bruce

Brother · 05-10-2009, 09:48 AM

Holy crap.

Where does the fuel go? It is only a 2.4L engine.

BURN-BROS · 05-10-2009, 10:03 AM

I wouldn't expect any more than 15 mpg. You can tune them to get better fuel mileage but you take a big step back in driveability. Just enjoy what you have. It is special.

David E. Clark · 05-10-2009, 11:43 AM

Quote:

Originally Posted by Brother

Holy crap.

Where does the fuel go? It is only a 2.4L engine.

Car and Driver tested the 1972 911s in May 1972 and the "S" got "14-17 mpg on regular fuel." They were never made to be fuel efficient!

Brother · 05-10-2009, 12:09 PM

I guess I was expecting something more like 16-20 such as listed under the T.

Danish Dynamite · 05-10-2009, 01:04 PM

I am happy with the engine. It really pulls strongly and has an incredible sound with the air cleaners! It only has 2000 miles on it since rebuild, so I am going on a dyno soon. I am expecting it to run to rich - it makes a lot of black smoke when I hit the throttle.

Here is a fun little video from a recent track day. Look for the yellow 911 with a turbo rear wing (Swedish previous owner molested the nice 2.4S)

http://www.youtube.com/watch?v=vFpPq4C-et8

I was hoping to get a few more miles pr. gallon by running a bit more lean, though I won't comprimise the drivability and power in the engine.

David > I see your point! Spring has just hit Denmark on full power and I love to drive my car, but 3 visits to Shell @ $70 @ week is bit annoying..

But I do love the MFI. It really is something special

RSTarga · 05-10-2009, 01:39 PM

You must be running rich. I have a 2.7 MFI 8.5 compression and get 12 in city and 15-18 on the highway depending on how aggressively I drive. Fuel mileage is so dependent on how you drive. I don't think going to carbs will help your consumption, maybe make it worse. Check your timing and advance and your mixture.

layzee · layzee's Garage

I did a roadtrip in mine about 600 miles in my 2.4E, returned 22mpg UK which is about 18MPG US, that was with a lot of WOT over mountain roads.

RoninLB · 05-10-2009, 02:46 PM

I've been a carb nut since the 1960s

I want MFI on my next 3.4

Eagledriver · 05-10-2009, 07:31 PM

If you are taking your car to the dyno you'll be able to check the AFR's and adjust the MFI for better economy. You'll need a long skinny tool to adjust the rack. Once you know what you have you can adjust the rack to run richer at the track and then lean it back out for street driving.

-Andy

Frank 993 · 01:01 AM

Quote:

Originally Posted by RSTarga

Check your timing and advance and your mixture.

+1

I do not know the specifics of your injection system but I understand that mixture IS adjustable therefore you can make it as economic as it can be what you need is:

1- Get a wideband air fuel monitor a and adjust your idling and cruising mixture to 15 to 1 full throttle 12.5-13.

2- Adjust the timing for better efficiency, the gains here can be substantials especially at low speed and idling where with about 15 degree initial + full vacuum advance your idle will jump up quite a bit be cafe full about where it end up in high RPM low vacuum conditions though.

The above method when applied to a carbureted and turbocharged 6.75l auto transmission 2300kg Bentley gave me 6.5km/Liter.

Happy tinkering

Danish Dynamite · 05-11-2009, 11:58 AM

The engine specs are:

930 engine block
95 mm cylinders
JE pistons 10.5 comp
Compression adjusted to 10.2
Carillo rods
3.0 Euro Carrera crank
Carrera 3.2 heads with new valves and guides. Harder valvesprings. The heads were machined with a special flange for MFI
GE 60 cams
MFI setup with throttle boddies and stacks from a 2.2 S
KN aircleaners
Dansk Stainless steel heat exchangers
Stainless steel 2 in 1 out muffler.

We have only adjusted the fuel pump be driving the car and trying to hit the right spot.

David E. Clark · 05-11-2009, 12:42 PM

Quote:

Danish Dynamite:
The engine specs are:
...

JE pistons 10.5 comp
Compression adjusted to 10.2
...

Have you adapted a variable compression ratio system for adjusting the compression? If so, how does it operate? Ignition timing? Variable camshafts? How are you monitoring the compression changes? Do you have the same compression ratios throughout or are you adjusting the compression ratio in the individual cylinders? This is very interesting.

Flieger · Flieger's Garage

OK, smart @$$

JE pistons do not always spec out to be their advertised compression ratio, even when special ordered. Their shape may not work with your combustion chamber volume and deck height to create their intended compression ratio.

You must check your actual compression ratio when building a modified engine. Too high a compression ratio could result in an engine confined to race fuel.

He probably got "10.5:1" JE pistons and installed them so that compression was 10.2:1

David E. Clark · 05-11-2009, 01:53 PM

Quote:

Originally Posted by Flieger

OK, smart @$$

JE pistons do not always spec out to be their advertised compression ratio, even when special ordered. Their shape may not work with your combustion chamber volume and deck height to create their intended compression ratio.

You must check your actual compression ratio when building a modified engine. Too high a compression ratio could result in an engine confined to race fuel.

He probably got "10.5:1" JE pistons and installed them so that compression was 10.2:1

I wasn't trying to be a smart a$$! I assumed that the answer was much simpler as you suggested, especially given the small difference in ratios, but in seven years on this board I've heard from many people with impressive technological savvy (and the financial means to experiment!). Variable compression ratio technology has been around for decades. I was actually hopeful that Danish Dynamite might have figured out a way to adapt this technology or some other technology to his/her car - especially given the concern for gas mileage expressed in the original post.

kenikh · 05-11-2009, 03:45 PM

Jeff Higgins' 3 liter MFI motor w/ twin plugs and 10.5: 1 CR gets pretty close to what you are seeing.

You might be running a little rich on accel/decel, but better safe than sorry, right? My guess is that PMOs won't be dramatically different - it most certainly won't pencil out from a cost perspective (fuel savings vs. cost of carbs/install/tuning).

Enjoy the electric throttle response - this isn't your daily driver after all, right?