lowvette

Since installing a 383 in my 1977 Corvette, I've been getting 8 to 10 MPG. At today's Premium Unleaded prices, in Charleston, SC, this is the same as 4 to 5 miles per dollar.

The engine has 2.02" / 1.60" valves, ported heads, 280H competition cam, 9.5:1 C.R., 600 CFM Holley and a stock exhaust system (minus cat conv). The transmission is a Turbo 350 and the rear gears are 3.08. I plan to install headers dumping into dual 2.5" exhaust with a crossover and a 700R4 tranny.

At 70 MPH, the engine is turning about 3000 rpm. In park and neutral, the engine idles at a very fast 1500 rpm. In gear, idle is 600-700 rpm. I'm using the stock torque converter.

For those with modified engines, what's your fuel economy like?

What should I be looking for to improve my Corvette's fuel economy? (e.g., vacuum leaks?)

Anyone ready to swap their engine for a stock V6? :jester

Much of this is a :_dupe: from my OT post. I'm hoping to get more visibility by posting it again here.

gdh

I am close to 20 mph if I average 60 mph with my 383 stroker. I also have a 3.55 rear, Muncie M22. When I ran a Holley 4779 I was at 18 mpg, the Edel Performer gave much better bottom end (big top end loss) but even better mileage. Have you checked you plugs (and gap), what tolerances was your deck machined to, condition of your carb. I am sure many others will chime in and ask you more specific questions about your setup (intake etc).

thevetterisbetter

I have a 383 in my 74, 268H cam, Q-Jet. I am getting pretty poor gas mileage as well, but I have timing issues. The carb is not running rich as I checked my CO levels at idle. HCs were off the chart though. Who cares? When I step on it, the whole back end slips and slides all over the road. When this happens, fuel economy goes out the window

:cheers: :lol: :lol: :lol:

SanDiegoPaul

My engine is very similar to yours, save for a .060 overbore, giving I think about 388 or so CID. I'm running Stainless 1.94 valves and 1.6 roller rockers on a flat tappet hydraulic Crane cam.

There is an Accel Super coil (HEI Style) and Accel shorty plugs gapped at .045 and the engine has approx 10K miles since it was built. 10:1 compression and cast iron heads.

Carb is a 790CFM Edlebrock Quadrajet #1903 and I have Hedman Headers too. Exhaust is true dual 2.5" with no cat, and a crossover behind the Turbo 350 trans. Gears same as yours.

Mileage same as yours. :smash: Runs great though!! :cheers: :cool: :cheers:



[Modified by SanDiegoPaul, 9:42 AM 5/19/2004]

Fevre

Are you running vac adv?

thevetterisbetter

As far as Fevre question, I am finding this to be important as the current VA can has no effect on my engine, need something that comes in earlier because of my poor vacuum.

Matt Gruber

u need a cam with a wide LSA, ~114-120.

markdtn

I would have thought that was reasonable for a 383 without overdrive and with 3.55 rears. "Speed costs money, how fast do you want to go?" ****I re-read your post and see you have 3.08 gears. Maybe 10 mpg is a bit low. Still, a hot engine is gonna need more fuel than stock. And the better it feels, the heaver the right foot becomes.......


[Modified by markdtn, 2:26 PM 5/19/2004]

Fevre

Check out Lars' list of cans:

https://www.corvetteforum.com/techti...=115&TopicID=3

flynhi

I'm no expert on this but,it seems to me that you have a mismatch in cam and carb. My experience has been that a street driven 383 responds to a 268 or 272 cam for a bit more low end to get you going from stop lights.
If your carb is dialed in, check your timing.

mrvette

I have demonstrated this 3 times to myself, on two differant cars, and on a boat even, boat being open loop run with a stock GM computer on a freaking VOLVO engine.....

DPFI, closed loop will about double your fuel economy, one car was an old analogue system from years ago, and it picked up 3-4 mpg right here, not closed loop even, but it was DPFI,,......same damn car....my '72 vette here with original/rebuilt stock numbers match engine, mild L48 cam, doubled it fuel economy over the stock Qjet.....in lotsa ways wich I"d have left it alone and just put in a then non existant 5-6 speed overdrive stick....

GENE

thevetterisbetter

Timing is definitely my problem. I pick up the new B28 can today and it will go in this evening so hopefully I will be able to drive it tonight and suck up lots of gas.

carl a

i was gettin 6to7 mpg with my 383 and the old th400 but once i put in the 700r-4 i went up to 18 mpg and im runnin 373 rear gears- als had 3200 rpm at 70 mph now it droped to 2300 rps at -75 mph-----get the overdrive-----

flynhi

If you want to improve fuel economy and performance, install a tpi from a C4. I have one on my LPE 383 and it's great!

The Money Pit

My 406 gets 14mpg last check and I think my speedo gears are off a bit,so mileage may be even better.I run a dual plane intake(RPM) and took great pains to set my advance curve in the distributer,and carb mixture.The XE-284 cam shakes the fenders at idle in gear but does so at 650 rpms.I think you have a vacuum leak or are running a single plane intake(huge vacuum leak).Your mileage with 3.08's should be way better than 8mpgs. :yesnod:

AlwaysWave

Make sure of the basics. Timing, carb adjust (1500 idle is too high), and vacuum leakage. Later on, you'll want to improve your exhaust system.

lowvette

The problem I run into with carb adjustments is that to lower the out-of-gear idle, the in-gear idle drops so low the engine stalls. I've heard that high-stall speed torque converters can help improve idle speed on engines with big cams. If this is needed, it will be after I switch to a 700R4 tranny.

lowvette

Valve overlap with a big cam will draw additional unburned HC out the exhaust. I tested HC before and after the engine swap and saw the same thing.

lowvette

Plugs and gaps are OK. Deck was machined to tight tolerances (very flat). The carb is in good working order, but out-of-gear idle is 1500 rpm.

Tominator

I was running a holly 650 DP and was getting poor milage, and switched to a Edelbrock and the gas milage improved into the teens.

The Holley had been rebuilt by a professional.