I've recently decided that I'd like to drive my car more on the street than at the track, but I'm getting about 3 mpg. It runs slightly rich at cruise at 12:1 afr, but I don't think this is the main issue. If I take the car out for a 10 minute burn around the block, I eat a 1/4 tank of gas. Its a 383 with a 110 lsa cam and about 8 or 9 psi of boost. I believe the cam is bleeding off a lot of pressure and wasting a lot of fuel. I've been told flames shoot out my tail pipes under WOT. Cool but a little expensive. The trans is a th350 with a 3000ish stall and no lockup, 3.50 gears. I've been debating throwing a t56 in and ditching the blower and maybe going turbo with a wider lsa cam...

No overdrive + no Lockup....On top of a blown 383?

Just taking care of your tranny & gears could double your mpg!

Granted, 6 mpg still wouldn't be GOOD, but......

I think it really needs to be close to 10 to be a reasonable street car. The blower is always blowing, at cruise it sees 4 psi, all going out the exhaust, along with a 3000 rpm cruise. I didnt think it would be anywhere near where it is.

The blower is also being overdriven quite a bit, that could really be hurting mileage too. 3 mpg just blows my mind, and thats only maybe getting into the throttle 5 or 6 times.

i figger your not useing a by pass valve?? I'd think you shouldn't be seeing any boost just cruisen

Yea I am. It does work and has been the same on both motors, any setup. I also have the gauge mounted above the carb, so it doesn't see manifold boost/vacuum. The engine might not be seeing what I think it does. I'm going to change that today, plus try out the new meth injection. Burn another 10 gallons of gas.

what do you expect from a blown 383? its not a street car and never will be. its a street racer plain and simple.
want a nice street car, buy a 2nd engine and drop that in and do the change when you want to race.

It shouldn't by burning 3mpg be it a race car or street car with just a 383ci and 9psi of boost. It needs a better CBV if it is building boost during cruise. That cam is not a 3mpg cam. More like a 15mpg cam or better.
It sounds like it needs to be tuned from the fuel description you wrote. 12:1 AFR cruise is way too rich. I would tune the fuel and spark for cruise and WOT. You should be in the 15mpg or better range.

EDIT: With the 3000 RPM stall and 3.50 gears I would guess it should be around 10mpg once tuned.

Batass, can you post pic's of your setup

If you are using too much fuel you need to go to the source of the problem: the carb (tune). As said before, 12:1 is not slightly rich for cruise and I can't imagine how much fuel you are adding at WOT to be shooting flames. Definately jet it down everywhere. Also, what does your timing look like? The tranny, FI, and cam change won't do much if the carb is still dumping fuel into the engine.

I'll post some pictures later today when I get a hold of a quality camera. The carb does need tuning, although I think I ran into the real possible issue. The MSD box seems to be erratic.

I have an LM-1 wideband O2. I couldn't get enough fuel into the motor at WOT. So far I'm at 96 secondary jets and the LM-1 is reading 14:1 WOT. Yikes. So as a result of that I've only been running 24* of timing. Yesterday we got bored and hooked up the alky injection, but couldn't find any meth so we used E85. Car picked up ALOT and WOT is now 12:1. I'm not sure how much the overlap of the cam affects the O2 reading, but this is when the box started acting goofy, so I need to get that straightened out first. The car wouldn't restart and had no spark. Eventually it fired. I'll post some pics, sort the box, lean the cruise, and post results.

Timing is locked at 32* and I was pulling 1* per psi, but last night ran all 32* squirting E85. The car was actually scary and hard to keep on the road last night. WOT pulls might have to be done at the track or on the dyno from now on.

the year is 2009 and just dont understand why some ppl are still using a carb today.Get EFI and be done with it.

my combo puts out 20 mpg, I can't see why your can't at least get into double digits.

first of all, you need overdrive
a th350 is meant to be used in one of two cases:
1) a track only car that never goes farther than 1/4 mile at a time.
2) a car with 2.73 rear gears and a lot of low end torque.

if you arent one of the above cases, you dont want a th350...
as for the no lockup, well thats just dumb IMO.
it wont lock up at WOT anyway, so no matter what it wont hurt your 1/4 times or performance.
all it does it lower your cruising rpms, which is good.

as for getting a t-56, if you have the cash, you are crazy for not doing it.
lower your cruise rpms from 3000 to 1500 and double your mileage.
you shouldnt be running any boost at cruise no matter what.
i dont care what your combo is, if you show boost at cruise speeds, you f-ed up somewhere.

my suggestion:
check your cam.
blowers dont like or even need overlap, so having overlap only blows gas out the exhaust, like you said... get a custom cam or put in a stock/rv cam. your power wont really be effected but your mileage will see a huge boost.
check your transmission.
either get some kind of lockup overdrive trans, or get a t-56.
you will be more happy with the t-56, but its expensive (but so is 3 mpg so take your pick)
check your timing.
if you have timing or spark issues, you are leaving power and mileage on the table.
your AFR could be 2:1 and you wouldnt get 3 mpg... you have other problems as well.

A carb is cheaper, easier to upgrade for power increases, and cools the intake charge by about 40 degrees. If the car was a DD it would be efi.

My engine is making 700+hp with more to come. Its rare to see any auto OD trans handle that power. I can still cruise down the road at 50mph and 1900 rpms. But generally when I take the car out, its out for 15 minutes balls to the wall.

The 110 lsa certainly hurts fuel efficiency, but many blower cams are 112. I dont think this is the real problem.

A t56 will have to be strengthened substantially to hold up at the track, but will be more fun on the street. It'll go in stock and won't see the track until its beefed.

I believe its just a combination of a rich cruise, possible faulty ignition, cam overlap, and the high stall which requires a good bit of throttle to get moving. Last night I drove the car for a hard 15 miles and burned a quarter tank.

As far as the boost under cruise, I have the gauge hooked up above the carb, so the what the engine is actually getting could a good bit lower. Its also a cheap blowoff valve. Though I'm only making 8-9 psi, the blower is overdriven, maxxed out.

Heres a crap pic, camera was lent out.

A BOV is not a compressor bypass valve (CBV). You said you put a BOV on it and not a CBV. That is why you are seeing boost at cruise. This makes for poor fuel economy.

You mentioned driving it hard for 15 miles. I would agree you are going to get less than 5 miles to the gallon driving it hard. Under normal driving / easy throttle I would expect 15+ mpg with the carb & timing tuned for cruise.

Its referenced to manifold vacuum and does open up quite a bit under cruise, but might be a little small. To me a bypass valve is one that returns air to the intake side of the compressor.

I dont see any point to taking the car out and easing the throttle and I dont want it to be a drag only car any more. I think the blower is coming off this winter and a T56 going in. Maybe a single 76mm or something.

On an upside the car was yanking the front tires on the street yesterday. Almost worth 3mpg....

I guess I misunderstood what you were asking in your original post. I thought you wanted to increase your mpg. It now sounds like you are justifying the 3mpg. Yes, if you drive it like a race car on the street then it will burn fuel like a race car.

You would be better off keeping the TH350 if you plan on going with a turbo. You could change the gears to around 3.08 or something like that though. Carb will need tuning either way. Tuning carb with turbo will be more difficult than with a supercharger.

FWIW my combo yields me about 130-150 miles per tank of gas, the math (with safety) points to: 8.5 to 9.7 MPG. I'm happy driving it on the street because if I need to go further than 150 miles I either am prepared to fill her up again or am going to trailer it wherever.

AC u SURE about that lmao