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  #1 (permalink)  
Old 06-11-2008, 09:37 PM
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Default How bad is your gas mileage?

I have a 428 fe with 428cid
425 hp 475 tq
800 cfm holley double pumper
msd mechanical advance
edelbrock rpm heads, cam, intake
plugs look great, not rich
getting 5-6 in town, 8-9 hwy

Does this sound right?
Anything to improve this without killing performance?
I knew this thing wasnt a prius.
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  #2 (permalink)  
Old 06-11-2008, 09:59 PM
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What's your diff ratio? Tranny ratios? Made a big difference for me going from a 3.54 rear to a 3.00 when my Richmond had a 1:1 final gear. I was starting in 2nd and getting to 4th often in town with 5th just to quiet it down.

I ended up with about 10-11 miles per gallon. It will probably be much better on the road once I do a road trip.
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Old 06-11-2008, 10:21 PM
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Superformance
3.43 rear end
tko 600 with .64 5th
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Old 06-11-2008, 10:42 PM
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CSX 4250 with an all-alum 427 stroked to 482ci. close to 11:0 comp. 850 Holley with NASCAR mods. Doesn't like anything less than 100 ocatane ($8.50/gal here), but really likes 104 ($14/gal). I get around 5-6mpg.
So, I rode it for 109 miles on Sunday and that cost me......
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Old 06-11-2008, 11:26 PM
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588 cu/in + 650hp + 15 gallon tank + 7mpg + $4.20 gal (93oct) + 110 miles per tank = who gives a crap, everyone's still looking at me flying down the road
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Old 06-12-2008, 12:42 AM
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well I paid 1400 to have my Superformance MkIII transported from Ross Ohio to SJ CA. (2400mi.)

based on my local gas station at 4.67 per gallon for super, that would buy 299 gallons.

299 Gal/2400 miles is 8.06MPG and it doesnt have a motor in it yet.

Steve
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Old 06-12-2008, 12:59 AM
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I think the PRIMARY cause of poor mpg relates to the cam profile and rear gear. Of course a myriad of other details come into play, but cam and gearing are the big two.

At cruise rpm the cam profile needs to be within a reasonable operating range. If your cam profile is 2500 to 6200 rpm range, let's say, you need to be at or above 2500 rpm engine speed minimum. Thats where the final gear ratio and chosen cruise rpm/mph speed come into play.

I used to get 5 or 6 mpg with my old side oiler, radical solid roller cam with a range of 3000 to 7000. BIG dual carbs that ran rich as well, not to mention 12.5 to 1 compression, what a beast THAT motor was! I changed the carbs to smaller vacuum secondary Holleys and leaned out the jetting. Got me a 'solid 6 mpg', up from 5! DANG! Had to change the cam profile (solid flat tappet, 2000 to 6200 rpm range), along with new custom pistons which lowered the compression so I could run more advanced timing. But the BIG factor was moving the pistons UP in the cylinder bore to zero deck height to gain 'quench' in the combustion chamber (which also allows for more advance and lower octane). End result: 11 to 12 mpg. Top loader with a 3:31 rear gear. About 3000 rpm at 70 mph.

Advancing the timing is a HUGE factor in improving the mpg, but it must be done carefully and wisely to avoid knock or ping.

Now if I threw an overdrive in there with the 3:31 rear gear the rpm might drop TO LOW at 70 mph and actually hurt the mpg. Guess I would just have to drive faster...

Realize I'm in HAWAII, it's not common to be able to maintain 70 mph around here for any distance to speak off. With the old motor and the high rpm cam I would often run THIRD gear on the freeway (50-55) just to keep the freakin' rpm within a reasonable range to keep from fouling the plugs! Close ratio top loader, 1st to 65 mph, 2nd to 85, 3rd to 115, 4th to wonderland, still pulling like a freight train. Approximate mph per gear.

Last edited by Excaliber; 06-12-2008 at 01:14 AM..
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Old 06-12-2008, 03:24 AM
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The accelerator pump circuit uses a LOT of gas, so your right foot has a dramatic impact on mileage. If you had cruise control, your hi-way mileage would increase.

A carb with vacuum secondaries rather than the double pumper and a vacuum advance distributor would be the first two things I would change to increase MPG.
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Old 06-12-2008, 03:47 AM
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I can hit 10 in the city
On a good day 17 hwy.....
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Old 06-12-2008, 04:41 AM
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I had my car down the street to www.422motorsports.com These guys are true experts and fanatics. I wanted to get the carb and distributor fine tuned, with the extra benefit of maybe squeezing a couple more MPG's out of the car. Rich at 422, told me to get rid of the double pumper with mechanical secondaries. He said to get a basic Holley w vacuum secondaries and he could tune it so the car makes even more power and get 4-5 more MPG's
I still have the double pumper on there for now and it is fine tuned and tricked out, but, if I can get significantly more MPG's with a carb change and not even loose any fun............
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Old 06-12-2008, 05:45 AM
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If you want acceleration get a big engine and short gears. If you want mileage get a smaller engine and tall gears. Rolling down the highway my SPF (Windsor power) turns 1,900 RPM at 60 MPH (3.73 & 0.68 5th gear Tremec). I can get into the lower 20 MPG range.

Bob
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Old 06-12-2008, 05:57 AM
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Default How great is my gas mileage

I just checked mine the other day. I was pleasantly surprised. My stock stroked 427 was worse.

482cu in FE, 11:1 compression ratio, 2x4s 1200cfm, 1:1 4th gear top loader, 3.31:1 gear, 26.5" tall tire
cruisin' 3000rpm @70mph with short passing burst.

12 mpg

city; don't know

I only checked to see what the range is on a tank full for trips. Pump gas of course.
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Last edited by Naumoff; 06-12-2008 at 06:08 AM..
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Old 06-12-2008, 06:13 AM
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466 ci, dual quads, 275/60 -15 rear tires.... 3.73's and .68 5th gear.. 17 mpg highway

changed to 3.27's and .82 5th gear .. 15 mpg...highway, although much better drive ability.
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Old 06-12-2008, 06:21 AM
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428 with 750 cfm double pumper mechanical secondary carb and a pretty stout cam (.607/.622 lift, 245/252 duration). About 10.25 compression, 2.88 gears but 1:1 5th gear. I get around 10 mpg around town and 11-14 on the highway.
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Old 06-12-2008, 06:21 AM
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Quote:
Originally Posted by Bob In Ct View Post
If you want acceleration get a big engine and short gears. If you want mileage get a smaller engine and tall gears. Rolling down the highway my SPF (Windsor power) turns 1,900 RPM at 60 MPH (3.73 & 0.68 5th gear Tremec). I can get into the lower 20 MPG range.
Bob
Ditto for me. I can get about 24 MPG on the highway. Around town? Who cares ... never checked it ... I'm having too much fun but its probably in the 12 to 15 MPG range.
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Old 06-12-2008, 06:22 AM
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7 in town. 10 highway.

2x4's (600's) on Tunnel Wedge intake
12.0 to 1 compression
252/262 solid roller
.637/.667 lift

4-speed Toploader
3.70 rear

David
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Old 06-12-2008, 06:37 AM
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Your Plugs may look ok, but you are way to rich! You need to get your car tuned with a wideband and get the car leaned out on the idle circuits and in your cruise range.
There is a very good articel on this in one of the Chevy magazines; google wideband and tuning a car for fuel mileage. With 3:73 gears a 750 MD carb combined with a 540 hp / 406, they got in the low 20s.

Justin
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Old 06-12-2008, 07:04 AM
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Sideoiler 427, tunnel-port heads, 2-4X4's and 3.77:1 gear with toploader = 9.5 mph when I'm peddling it. My Carbon Footpring is bigger than yours.
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Last edited by speed220mph; 06-12-2008 at 08:33 AM..
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Old 06-12-2008, 07:06 AM
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Smile gas mileage

last year trip to the London Cobra Show I got 20.8 mpg for the 1354 mile trip. We drove 70 miles in the rain on the way home and still had a great time.

Small block, fuel injected, 3:55 and a TKO 600

2006 I was hitting 101 mph on on the Main Street of London.

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Old 06-12-2008, 07:09 AM
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390 FE, intake, cam, headers (duh), 735 carb, 9:1 cr, 425hp. My car runs on 87 and gets 10-11 around town and 12-13 highway. I recently changed out the metering blocks, so I still have to retune it all. Once done it should get about 15 mpg highway.
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