Is my MPG way off?
 

Hello again; good to be back in the Weber crowd.

Nut much traffic in here now, but I'll pu out my Q anyway.

Car is 2400 lbs w/ driver. 347 CUI, 2.02 inlet valves, 306 deg (adv) cam with 0.6" lift.

Running at highways, 45-60 mph. One toe at the throttle, accelerating as little as possible, I get 14,9 MPG.

At last setup, with injection setup, I got 13 MPG.

Should it be higher than 14,9 with the Weber setup?

If I want it lowered with current engine setup, what would be the first steps to try out?

I have thought of two things:

1- Be sure I have a thermostat. If not, the engine will operate at a low temp on the highway- not good. (I don't remember if installed the stat; have to drain to have a look this winter.)

2- Change to a smaller idle jet. As throttle is barely open, marching on in 2000- 2500 rpm, I'd guess it's suckin from idle/ transition holes?

Please correct me if I'm wrong.... I went bigger on idle jets to get rid of a transition- miss. And I did! (Luck or other changes might be the reason. I did build a new linkage like this
http://www.clubcobra.com/forums/webe...ml#post1184203 and with a good return- spring, all has been really well after.)
,

I'm tempted to tame the engine with a smaller cam, but IDK if that would make a ton of difference.

Thanks
RS

You're doing lots better than I did with the Roush 427IR. I got 9.

I have 347, Edelbrock 600 cfm, small ford heads, B 303 cam, T-5 trans, 3.23 rear . And we just did a 800 miles trip , we averaged over 22mpg , Ed

RS,

Your question is similar to one someone else (not using Webers) asked a day or so ago. This is a tuning problem! Get a pair of wideband sensors, one for each engine bank and one of the many handheld data logging gizmos available today. You will be able to make some pretty good driveability and fuel economy adjustments to the Webers.

As luck would have it, Engine Masters did an excellent session on tuning carbs. Admittedly, the Webers use slightly different names for some of the same circuits and probably some circuits the Holleys don't have. All that stuff notwithstanding, the principles are the same and translate fairly well across to your Webers.

The Engine masters session you want to search out on the Motortrend Channel or other channels with the same series is Season 7, Episode #6. They use an LS3 with a Holley 950 XP as their test bed to go about smoothing out the AFR across the engine operating range. Their approach is quite good, easy to understand, and, most importantly, pretty easy to duplicate.

The Engine Masters gang used an engine dyno. You don't need to. If you have access to a chassis dyno, that would be nice but not necessary. You can do the same job in the car, especially if you bring along an extra set of hands and use up a little bit of brake pad material.

Smiles per gallon

What is the gear ratio in the rear?
What transmission are you using, 4spd, 5spd, 6spd, automatic?

Bill S.

that's nice, I have 8 MPG ;)

My average is 7-8 :)

On a bad day, mine can sometimes drop as low as 21 or 22 mpg! It all depends on how hard you work the engine. On the highway and in 5th OD I can easily get over 25 mpg sometimes better ...

:LOL: Better than I get with 289, 6mpg maybe 8mpg if wind is at my back.

Jeff,

Well, I just calculated mine with the 289 and on a race weekend I am getting 2.5 mpg and the fuel that I am running is $12 per gallon!

RS, as others have pointed out, you need to take a look at the whole system. You cannot isolate one factor to account for the operation of a whole. Also, Webers are different than one or two four barrel carbs. They are not progressive like four barrels, it's all eight in all the time. A car with a four barrel will spend part of the time operating on just two barrels.

There is some great advice above. When I tuned mine we used a AFR gauge on each header tube to get the idle right and set the main jets. You probably do not need that, but have an AFR gauge on each bank as pointed out previously is a good idea.

Good luck.

Jim

Jeez, my son's Z4 just made it coast to coast on one tank of gas. Of course it was in the back of an Intercity Auto Transport big rig.:rolleyes:

:lol::lol:

Who cares as "Bird of Prey" said "Smiles per Miles" If we were worried about MPG, none of us would own Cobras ;) Jim (1795), those 2.5 MPG are just the best HUH?? Plus, the smell of Unburnt or Burnt 109 :) When racing the Alfa I bought 1/4 drums of VP-109 (16Gals) and thought the big bore group were crazy buying 55Gals at a time :) Cheers Tom.

I would spend more time making sure the set-up is running correctly and the car is tuned right instead of gas mileage.

It is a Cobra. The only time I worry about gas mileage is when I have 2 gallons left in the tank and the next gas station is 30 miles away.

When you get the tune correct, i.e. the AFR where the engine wants it (and it should be), both mileage and engine torque will improve. The bigger payoff is driveability and engine longevity. Excessively rich AFRs will produce less torque, less mileage, and increased gas/oil dilution, which will work to your disadvantage with bearings and the loads they carry.

What is your target? These engines got lousy mileage in their 60s versions too. Well tuned adaptive fuel injection will help but you're not going to avoid a EPA gas guzzler tax today.

When I got my 2004 Ram 1500 with the 5.7 hemi and it got 12MPG in city I considered that the Cobra was doing about the best it could.

You may like to read up a bit here about BSFC:
https://help.summitracing.com/app/an...hat-is-bsfc%3F

BSFC = lbs. of fuel used per hour ÷ horsepower
For example, a 300hp engine using 150 lbs./hr. of fuel would have a BSFC of 0.5.

Your average Cobra needs less than 30 hp to cruise at 65 mph. You need to place your most efficient rpm at that speed.

And the A/F ratio needs to be right at that rpm, too. If you had a Holley carb you would want to jet the primaries accordingly and make sure your power valve is closed.

Your cam with .600 lift should have quite a bit of overlap where fuel consumption is higher at lower rpm. It's throws out unburnt mixture. (That cam's most efficient rpm is higher than that of a mild cam). Higher rpm however comes with more friction.

What I am saying it, if your highway speed rpm is too low for the cam, you might use less fuel in a higher gear!

My car's rpm was 2.500 at 65 mph and it would have benefitted from 2.000 rpm at 65 mph (i.e. a 5th gear). But perhaps not 1.500 rpm when the cam was still off.

What messes with fuel consumption of our V8s is a radical (low vacuum) cam where the power valve is not matched to keep the power circuit closed. This should only open at WOT (when vacuum is, again, low)

i get 7 in town 12 on the highway money well spent

Which that engine combo, you should be happy with an average of 15MPG.
Remember that takes into account ALL engine running conditions for that tank volume analysis.
Look at your idle air jet sizing, and run an AFR wideband for tuning.
Gary