Fuel Consumption

Sinjin · #1 (**permalink**) 10-03-2010, 05:16 PM

Driven my recently finished DRB Cobra for 300 miles.
It has a stroked 351W - Now 427
Dynoed at 520hp / 540ftlbs Torque
The motor runs beautifully but boy is it thirsty.
It has a Speed Demon 750CFM.Mechanical second stage actuation.

Average consumption to date 3.5MPG (USgallons).

The intended use for the car is 70% street -30% track day.

Just wanted some feedback from the club on the fuel usage similar motors (like the ROUSH 427's).
I dont have the car to compete in Economy runs but it would be nice to enjoy cafes and wineries insread of fuel stations.

rustyBob · #2 (**permalink**) 10-03-2010, 06:18 PM

What trans. and rear gears do you have ????

209 · #3 (**permalink**) 10-03-2010, 06:24 PM

I average maybe 12 to 14 and that's with the air off. 209

Zoom This · #4 (**permalink**) 10-03-2010, 06:52 PM

Need more info like jet sizes pri and sec. Timing, a/f ratio. Either the mpg calc is in error or the motor is way off tune. I have a 418W 550 hp and get 8-10 in town and 18-20 on freeway at a steady 70 mph. Sounds like you need a day at the chassis dyno.

Sinjin · #5 (**permalink**) 10-03-2010, 07:39 PM

Thanks for the response

Zomm this -
Primary 76 Secondary 86 -
At 3500RPM AF left 11.94 AF Right 12.32

Rusty Bob
Tremec TKO 600 - 0.86 Final Drive jag 3.31 Dif

209
Those figures would be ideal for me

Rick Parker · #6 (**permalink**) 10-03-2010, 08:14 PM

Sounds rich, might be washing the cylinders........get it leaned out. Look at your plugs.

WardL · #7 (**permalink**) 10-03-2010, 08:15 PM

I have averaged 13 mpg and have a range from 12 to 15 mpg. Roush 427 SRTW, 512HP, 17" wheels, 3.46 rear ratio and 0.82 OD. At 70 MPH I'm turning 2,550 RPM. I just found out my ignition timing total advance is 32 degrees at 4,200 RPM. 16 Deg at 900 RPM.

209 - what kind of cruise control do you have?

bobcowan · #8 (**permalink**) 10-03-2010, 09:14 PM

Quote:

Originally Posted by Sinjin

At 3500RPM AF left 11.94 AF Right 12.32

There's the answer right there. It running way too rich. Get it on a dyno and have it tuned correctly.

Also, check your timing curve. Retarded timing costs you in fuel and power.

Got the Bug · #9 (**permalink**) 10-03-2010, 10:28 PM

Quote:

Originally Posted by WardL

I have averaged 13 mpg and have a range from 12 to 15 mpg. Roush 427 SRTW, 512HP, 17" wheels, 3.46 rear ratio and 0.82 OD. At 70 MPH I'm turning 2,550 RPM. I just found out my ignition timing total advance is 32 degrees at 4,200 RPM. 16 Deg at 900 RPM.

209 - what kind of cruise control do you have?

I've got a Roush 402SR and my mileage is exactly the same.

209 · #10 (**permalink**) 10-04-2010, 07:44 AM

WardL,
I'm running a Rostra unit my installer adapted to the car. Unit was for Honda and Mazda. Model or reference # 250-1483 Cliff

STL Mark · #11 (**permalink**) 10-04-2010, 08:29 AM

Could it be that your speedometer is not correct and recording more miles than you are actually driving?

Zoom This · #12 (**permalink**) 10-04-2010, 08:56 AM

bobcowan is right. You are rich. Next time you're on a dyno look at your a/f ratio at 2000-2500 rpm under light load, 4th gear. Should be up around 13.5-14.5:1. I think your jetting is too rich on primaries and way too rich on secondaries. I'm going to guess that your jetting should be closer to 72-74 pri. and 76-78 secondaries to get a good a/f ratio. Get the primaries dialed in first under light load lower rpm, then fuss with secondaries under full throttle high rpm to get your overall a/f ratio right. I have an MSD 6AL ignition that is set for 20 initial advance and 32 total all in by 2200 rpm. Car runs like a top. 10.83:1 compression, cam is .565 lift and 232 duration on intake with 110 lobe separation.

Sinjin · #13 (**permalink**) 10-06-2010, 01:20 PM

Hi Zoom This

Have ordered new jets - Start with 74 Primary and 78 Secondary. I have an MSD 6AL and MSD Distributer and will confirm which timing curve is being used - I think it is 32 degress at 4200 but will check with timing light - as opposed to yours all in at 2200.Will post the results -
Thanks once again to all for your contributions

Michael C Henry · #14 (**permalink**) 10-08-2010, 11:48 AM

I have seen allmost 13 MPG coming home from RENO. My milage is best when cruising in 4th gear.That doesn't happen around town often. I'd hope it's better than 3.5 MPG.

YerDugliness · #15 (**permalink**) 10-08-2010, 12:38 PM

Another reason to get a small block.

I get 24 mpg on the highway with my 0.63 overdrive T-5. I can pass almost anything on the road without shifting down to 4th gear. What I can't, I certainly can by downshifting. Those who say they will burn rubber if they floor it in 4th at 65 can just eat my dust as I walk away from them. I've always said, too much torque=too much tire smoke (and, apparently, too little fuel efficiency).

Cheers from Dugly

jwd · #16 (**permalink**) 10-08-2010, 05:13 PM

Quote:

Originally Posted by Rick Parker

Sounds rich, might be washing the cylinders........get it leaned out. Look at your plugs.

With the advent of unleaded, alcohol enriched & oxygenated gas, reading plugs hasn't been an accurate way of reading A/F mixture for decades.
To the OP, Demon carbs. are junk on the street. If you want better throttle response, performance and gas milage, get a Holley Street Avenger.