ROG100

Just recently had Sean R&R my intake manifold.
New everything apart from the ISV. Dave's SharkVent system plus my own intake spacers and new 24 Lb Ford 4 hole injectors.

Fuel compsumption has improved significantly according to the digital readout.
I am seeing a circa 20% to 25% improvement particularly around town.
15 mpg to 18 mpg.
I am also checking by the old fashioned method of number of miles between tank fulls.

Why is this???
Am I running leaner - not a good thing.
Is the digital mpg only set up for reading the stock setup?

Just wonder what the experts think.

Jim Mayzurk did exactly the same at the same time on his GTS and is also seeing the same improvements.

WallyP

To over-simplify the operation, the fuel consumption shown is calculated by adding up the open time of the fuel injectors.

If you use larger injectors, they are open less time, so the computer thinks that you are using less fuel.

Does the paper method show the same improvement?

DR

Hi Roger,

We see the same thing but even more dramatic mileage increase when we install 30 and 42 Lb injectors with our SC systems..

Alas it is not real.

The fuel mileage calculations are based on a the stock injector sizes, when you install a larger injector the duty cycle is reduced and the car thinks it is using less fuel and therefore gives you a higher MPG readout.

aggravation

My guess, and this is just total guess before the experts chime in, is the computer is basing it's calculations on 19 lb injectors but in your car the fuel is flowing through 24lb injectors. The O2 sensor will tune the mix correctly but nothing is in the software to correct the mpg calculations....

PorKen

Rog,

You and Jim just put in the 24#'ers, and the LH adapted to it, or did you SharkTune?

If you are getting better mileage with the stock chip, I would guess it's because you have a little more gas on acceleration/low rpms, where before it might have been too lean. With better torque you get that big ol' pig of a GTS auto rolling easier?

Tom. M

I've got the 24's in my GT same thing. Better perceived mileage on the dash..but real measurements shows about the same as normal (20 plus on highway). The stock LH handles the 24's with no problem. When I ran the 30's, the LH had a hard time during startup and it would take several tries and lots of pedal action to get it to idle. When it warmed up though the 30's ran fine with the stock brain (Note..I've also got the adj. fuel pressure regulator).

dprantl

+1. My 928 shows ~30% better on the digital dash than it really is with 30lb injectors. Has anyone found a way to mess with the digital dash to fix this? Maybe there is an adjustment variable that can be set somewhere?

Dan
'91 928GT S/C 475hp/460lb.ft

Jim Morton

Like others have mentioned... my "digidash" is wacky with the 30# injectors. Car ran rich until using Sharktuner.

Now, after some fuel and ignition map adjustments, I am getting 2-3 MPG better highway mileage than I did on stock maps with simply changing the injector size.

For around town, using the tuning... well let's just say that mileage is suffering !

:^)

the flyin' scotsman

Roger.................Sean messed with the FI system so that its now messing with your head it thinks its a SMART car now.

Seriously, since my own intake R&R with the OEM injectors cleaned/balanced/rebuilt, all gaskets replaced and the TPS/ISV cleaned checked for full operation I've experienced a smoothness that the engine never had before (in my ownership), ease of starting and a slight improvement to fuel economy.

The '88 MY doesn't have the digidash so the MPG information is not front and center..............I just look at the fuel guage but any help in conserving costs so we make it to the big event this summer is very welcome

Tom in Austin

Roger, did you say you can see a difference by the old-fashioned miles driven/gals to fill up? If so, the improvement is real whatever the dig dash says.

Would a new/cleaned MAF/injectors/etc. produce the measured improvement? It stands to reason the work you did would improve gas mileage if the car runs better than before.

ROG100

Bummer - I thought I could save enough money on gas to buy another 928.

So what about the "call for range" and miles to empty - are they out of whack as well???

Have not done enough miles on a new tank of gas to know but I suspect the experts are right 8>)

123quattro

Running lean gets you better fuel economy, but less power. You get less power for a given charge of air so the throttle needs to be opened more to get you back to the original power level. This decreased throttling of the engine lowers the pumping loss and gives better efficiency. It isn't implemented in production cars because you have to have a regenerative NOx strategy and NOx is difficult to deal with in general. That, and the potential for lean misfire goes up, which increases emissions and spikes the temperature at the cat, or turbo, whichever comes first.

andy-gts

my 24 lbs injectors did show an improvement from total mpg of 19 to 24 mpg on 2 tanks of gas by actually calculating the numbers. I had Mo do the intake R&R and injectors due to a raptor sc install. I figured it was going from single to 4 holes on the injectors with better atomization. Just my
.02
andy

ROG100

Well I was thinking of going back to 19Lb so I can enjoy my digidash.

However why can't we find a way to overcome the problem and fix the dash?

Alan, where are you Alan????

byrdman454

What are the benefits to using 24lbers versus 19lbers on a stock S4 or GTS?

Roger, do you have something done to your GTS I was not aware of?