Fuel Economy Question...

[Hot Rod To Hell]

Ok guys... I have a question for you. My car got better gas milage with a more "hot rodded" motor in it than it does now, why?

Old Engine: <font color="blue"> 13mpg </font>
406 SBC, 11.6:1, 825cfm Race Demon carb, 1" open spacer, ported Edelborck Super Victor intake, CNC ported Brodix Track 1 heads, Lunati Solid Roller (268/270 @.050, .632/.632 lift)

Current Engine: <font color="blue"> 9mpg </font>
406 SBC, 9.5:1, 750 Holley dbl Pumper, stock edelbrock Victor Jr, Ported GM 487 casting heads, still 1.94/1.50's, Comp Cams 292 Magnum Hydraulic Flat tappet cam (244/244 @.050, .501/.501 lift)

The rest of the combo:
3000lb car.
Both engines run full roller rockers, both engines use the same MSD Pro Billet Dizzy and 6AL box, same 1-3/4"x31" headers. The car has 3" dual exhaust w/"X" pipe &amp; Flowmasters, a th400 with a converter that flashed to 5200 behind the old motor (haven't checked with the current mill), a 9" with 4.11's, and 30 x 13.5 tires.

The only thing changed was going to a "mild-er" motor, why did it LOSE 4 mpg? http://www.jalopyjournal.com/ubbthreads/images/graemlins/confused.gif http://www.jalopyjournal.com/ubbthreads/images/graemlins/crazy.gif

[Unkl Ian]

Can you put the bigger carb on the new motor and try it again ?

[Fat Hack]

Efficiency at a given RPM...the "wilder" motor was likely more 'dialed in' to your converter and gear...and thus, it delivered better economy numbers in that specific vehicle.

[Nimrod]

I think the higher compression ratio probably had a lot to do with it.

[Hot Rod To Hell]

BTW Hack, I forgot to ask...how'd this motor sound when I left Jays the other night?

[BigDdy31]

[ QUOTE ]
Efficiency at a given RPM...the "wilder" motor was likely more 'dialed in' to your converter and gear...and thus, it delivered better economy numbers in that specific vehicle.



[/ QUOTE ]

I'd go mostly with Hack on this one but if that Holley is a true double pumper, those can really squirt some fuel. Like one of the other fellas said, try your old carb again to see what happens. I've never run a Race Demon before but maybe they are just more efficient than the old Holley design.

[Ayers Garage]

I think it's the higher compression of the older engine making a big difference.

Hell, my daily driver has 17:1 compression and gets 20 mpg. Definitely no where near apples to apples comparison though.

[Baumi]

[ QUOTE ]


Hell, my daily driver has 17:1 compression and gets 20 mpg. Definitely no where near apples to apples comparison though.

[/ QUOTE ]
17:1 ? Is it a diesel? Sorry , that might be a dumb questionn, but I´d really like to know? http://www.jalopyjournal.com/ubbthreads/images/graemlins/confused.gif

[willowbilly3]

It also looks like you had a milder cam in the first engine. And I would have to wonder how accurate the milage numbers were on the first engine. Like if you only checked it once and the angle was different at the pumps it could have been erroneous due to the tank being "full" at a different level. A true accurate fuel consumtion requires taking an average of several consecutive fill ups.Just checking one tank here or there can be way off.

[Ayers Garage]

[ QUOTE ]
[ QUOTE ]


Hell, my daily driver has 17:1 compression and gets 20 mpg. Definitely no where near apples to apples comparison though.

[/ QUOTE ]
17:1 ? Is it a diesel? Sorry , that might be a dumb questionn, but I´d really like to know? http://www.jalopyjournal.com/ubbthreads/images/graemlins/confused.gif

[/ QUOTE ]

Yes, it is a diesel. 17:1 compression and 20 lbs of turbo boost.

I made sure to say it was definitely not apples to apples, as diesel has way different characteristics than gasoline does. Amount of available heat energy per gallon and such.

But, in my experience with gasoline engines, higher compression does make a profound effect on fuel economy.

[Fat Hack]

Compression ratios DO affect mileage, for sure...but so do other aspects of the engine/drivetrain combo. It all has to work as a balanced equation for maximum efficiency AND performance.

I had a Dodge wagon with a 318 that I over-cammed the shit out of and ran around with a Torker 340 intake on it topped with a 600 double pumper! Driving the car in second gear on the freeway (automatic) actually UPPED fuel economy...by putting the engine into the RPM range where the intake and cam worked best. Leaving it in "Drive" made it surge and lug...killing mileage...even at 60-70mph! (2.73 gear!!!).

[Fat Hack]

[ QUOTE ]
BTW Hack, I forgot to ask...how'd this motor sound when I left Jays the other night?

[/ QUOTE ]

Not too shabby...but not as ball-rattling as the old mill!! http://www.jalopyjournal.com/ubbthreads/images/graemlins/wink.gif http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif http://www.jalopyjournal.com/ubbthreads/images/graemlins/cool.gif

[C9]

[ QUOTE ]
Holley dbl Pumper

[/ QUOTE ]

I believe the double pumper is the main culprit here.

I'll go along with the Hack-Man and agree that the lower C/R enters into it as well.

Slightly off the subject, but right now I'm running the first tank of fuel through a box-stock 750 Edelbrock street model carb on my 462", mildly cammed BBB engine. (BBB = Big Block Buick fwiw.)
Previous carb was a 750 Carter competition model leaned down one step. (According to the Edelbrock carb site recommendations/chart.)

The only other change to the car was moving from about 350' altitude in Central California to 3400' here in N/W Arizona.

I'll pass the info along after I get a few tanks of fuel through the carb and perhaps make some rod/jet changes for altitude compensation.

In the meantime, you may want to look at the mileage discussion on the V8 Buick board.
They're talking mostly 455's and 462's, but it's interesting.

YMMV (http://www.v8buick.com/showthread.php?t=54315)

[Hot Rod To Hell]

The thing is guys, the Demon is ALSO a dbl pumper!?!

The fuel economy figures I listed are on the Highway, cruisin about 3200-3300 RPM. It was actually the same trip with both mills, so elevation, hills, etc were the same.

I guess my thinking was that with bigger carb, bigger intake, bigger heads, and bigger cam, the old motor could move more air, and if the A/F ratio is the same that would also mean more FUEL.

Now that I read that, I guess at 3200 RPM, the old motor wouldn't even have to work to pump the air, where as the current one may be working harder (more flow restriction) to move the same amount of air. I suppose that's where that whole "Volumetric efficiency" comes into play, huh? http://www.jalopyjournal.com/ubbthreads/images/graemlins/crazy.gif

Thanks for all the ideas so far though guys... http://www.jalopyjournal.com/ubbthreads/images/graemlins/smile.gif

[squirrel]

If you checked the mileage on just one trip with each engine, that could be a big part of it.

But if the numbers are real, then I vote for the mismatched setup you have now being the problem...the "before" combo seemed to all work together, radical as it was.

[Hot Rod To Hell]

Well, I'm not sure about this combo being mismatched Jim. I think that this $1500 motor runs damn near as good (from a power and acceleration standpoint) as the $8500 motor! http://www.jalopyjournal.com/ubbthreads/images/graemlins/crazy.gif (of course the "real" motor never ran as good as it "should" http://www.jalopyjournal.com/ubbthreads/images/graemlins/confused.gif)

Oh, I also did about 20 long (200-300 mi. round trip) trips over the last couple years with the old mill and it worked out to 12.5-13 every time (as long as I kept my foot out of it http://www.jalopyjournal.com/ubbthreads/images/graemlins/blush.gif), and I've done about 6 with the current mill, and it's 9 everytime (again, as long as I keep my foot out of it!)

[fuel pump]

Steve I'll bet it's the Holley dbl pumper. I had a "right outta the box" dbl pumper on a HOSVO 351 (I know I know) and it ran like a slug. One of the Holley guys gave me a "tuned" dbl pumper off the holley NHRA truck and you wouldn't believe the difference. Mileage and performance improved a ton. Demon carbs are MUCH better otta the box than Holley's are. IMHO

[squirrel]

Yeah, blame the Holley double pumper...I have two of them on my blower motor and got 12.0 mpg over a 4500 mile trip this summer. Set up mostly "out of the box", tad richer on the secondaries, tad leaner on the primary on the "main" carb.

By mismatched, I'm talking more about matching the driveline to the cam....

[fab32]

I don't suppose you guys are even interested in the 21+ MPG I used to get with my old 283 2bbl, 3spd overdrive, '63 Biscayne 2dr. are you. Now this was driving VERY conservatively and on long trips. I played with the jetting and timing but everything else was factory.
Don't you just love it when someone posts something totally unrelated to the subject at hand? http://www.jalopyjournal.com/ubbthreads/images/graemlins/blush.gif http://www.jalopyjournal.com/ubbthreads/images/graemlins/smile.gif http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif

Frank