MPG with a 383.
Thread Starter
Melting Slicks
Joined: Oct 2013
Posts: 2,569
Likes: 126
From: San Diego California
MPG with a 383.
I'm asking those with one in their car if this is normal.
I just filled up after a drive of 134 miles. The tank took a hair less than 10.3 gallons of 91 octane. Of this mileage, I would estimate about 50 miles were highway and the rest city driving.
Is 13 mpg normal for this type driving, a high compression engine, a mild race/street cam, and a stock Q-jet carb? There's 1600 miles on it.
I have noticed that after 85 mph, if I attempt to accelerate from there, there isn't much left of pedal to floor. Some, but not much. It may be an accelerator cable mis-linkage perhaps, or the stock Q-jet that's the limiting factor. But it's nice to know that in the engine's present configuration, 15 mpg highway isn't unrealistic. Any thoughts?
Thanks.
Steve
I just filled up after a drive of 134 miles. The tank took a hair less than 10.3 gallons of 91 octane. Of this mileage, I would estimate about 50 miles were highway and the rest city driving.
Is 13 mpg normal for this type driving, a high compression engine, a mild race/street cam, and a stock Q-jet carb? There's 1600 miles on it.
I have noticed that after 85 mph, if I attempt to accelerate from there, there isn't much left of pedal to floor. Some, but not much. It may be an accelerator cable mis-linkage perhaps, or the stock Q-jet that's the limiting factor. But it's nice to know that in the engine's present configuration, 15 mpg highway isn't unrealistic. Any thoughts?
Thanks.
Steve
Pro
Joined: Dec 2014
Posts: 530
Likes: 10
And how heavy is the car, what kind of tires do you have on it...
FWIW, 13 or so is about what my 1980 C3 with completely stock L48 with Q-Jet, TH350, 3.07 gears and 255/60R15 tires gets in town driving. Curb weight is around 3300lbs without me in it. It will do maybe just a little better on the highway if you keep the speed down, but since the speed limit around here on most highways is 70-75, not so much.
FWIW, 13 or so is about what my 1980 C3 with completely stock L48 with Q-Jet, TH350, 3.07 gears and 255/60R15 tires gets in town driving. Curb weight is around 3300lbs without me in it. It will do maybe just a little better on the highway if you keep the speed down, but since the speed limit around here on most highways is 70-75, not so much.
Thread Starter
Melting Slicks
Joined: Oct 2013
Posts: 2,569
Likes: 126
From: San Diego California
Thanks for the response.
This '73 Coupe's 383 has a '76 C-10 block, original TH400 and 3.08 Rear End.
Tank Sticker print says: GVWR 4054 GAWR F 1927 R 2127
But this weight was with the original engine (Iron Heads & Intake Manifold, 2-Bolt L48 Block--which is still at the shop being checked out with no machining necessary so far). Also, PO installed a fiberglass Rear Leaf Spring/Band, and the original Leaf Assembly he did not have anymore. Back in Nebraska, I was never farther away than a mile from a grain elevator scale. Don't really have a handy way of weighing it here in the big city, but would say it's somewhat lighter than documented at birth.
P225/70R 15 Cooper Cobras were the closest I could find to the GR70-15 door sticker recommendation. They're performing great so far.
Steve
This '73 Coupe's 383 has a '76 C-10 block, original TH400 and 3.08 Rear End.
Tank Sticker print says: GVWR 4054 GAWR F 1927 R 2127
But this weight was with the original engine (Iron Heads & Intake Manifold, 2-Bolt L48 Block--which is still at the shop being checked out with no machining necessary so far). Also, PO installed a fiberglass Rear Leaf Spring/Band, and the original Leaf Assembly he did not have anymore. Back in Nebraska, I was never farther away than a mile from a grain elevator scale. Don't really have a handy way of weighing it here in the big city, but would say it's somewhat lighter than documented at birth.
P225/70R 15 Cooper Cobras were the closest I could find to the GR70-15 door sticker recommendation. They're performing great so far.
Steve
Last edited by Cavu2u; Jun 8, 2015 at 07:18 PM.
Drifting
Joined: Jan 2012
Posts: 1,908
Likes: 129
From: Fredericksburg Virginia
I have a high compression, high horsepower, highly cammed, not bored, not stroked 350. 4-speed with 3.36 gears.
I would be pleased with 10 MPG.
I will say that you should be able to do something more than 85. I suspect you have some issue where your throttle cable isn't fully "pulling through" the carb. With even a mildly built 383, you should be able to get somewhere quickly enough. I know I shut mine down at 100 (haven't ever gone faster...just plain scary in my car), but I get to 100, like, RIGHT NOW and there is still lots and lots of motor left.
In short, I wouldn't be displeased with 13 MPG at all and I also believe you have something going on that is limiting your full throttle application.
I would be pleased with 10 MPG.
I will say that you should be able to do something more than 85. I suspect you have some issue where your throttle cable isn't fully "pulling through" the carb. With even a mildly built 383, you should be able to get somewhere quickly enough. I know I shut mine down at 100 (haven't ever gone faster...just plain scary in my car), but I get to 100, like, RIGHT NOW and there is still lots and lots of motor left.
In short, I wouldn't be displeased with 13 MPG at all and I also believe you have something going on that is limiting your full throttle application.
Pro
Joined: Dec 2014
Posts: 530
Likes: 10
Thanks for the response.
This '73 Coupe's 383 has a '76 C-10 block, original TH400 and 3.08 Rear End.
Tank Sticker print says: GVWR 4054 GAWR F 1927 R 2127
But this weight was with the original engine (Iron Heads & Intake Manifold, 2-Bolt L48 Block--which is still at the shop being checked out with no machining necessary so far). Also, PO installed a fiberglass Rear Leaf Spring/Band, and the original Leaf Assembly he did not have anymore. Back in Nebraska, I was never farther away than a mile from a grain elevator scale. Don't really have a handy way of weighing it here in the big city, but would say it's somewhat lighter than documented at birth.
P225/70R 15 Cooper Cobras were the closest I could find to the GR70-15 door sticker recommendation. They're performing great so far.
Steve
This '73 Coupe's 383 has a '76 C-10 block, original TH400 and 3.08 Rear End.
Tank Sticker print says: GVWR 4054 GAWR F 1927 R 2127
But this weight was with the original engine (Iron Heads & Intake Manifold, 2-Bolt L48 Block--which is still at the shop being checked out with no machining necessary so far). Also, PO installed a fiberglass Rear Leaf Spring/Band, and the original Leaf Assembly he did not have anymore. Back in Nebraska, I was never farther away than a mile from a grain elevator scale. Don't really have a handy way of weighing it here in the big city, but would say it's somewhat lighter than documented at birth.
P225/70R 15 Cooper Cobras were the closest I could find to the GR70-15 door sticker recommendation. They're performing great so far.
Steve
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Le Mans Master
Joined: May 2002
Posts: 5,086
Likes: 786
From: Lebanon Township New Jersey
GVWR stands for gross vehicle weight rating. Its the weight of the car plus the recommended limit of passengers and cargo. My 77 with iron heads, no AC and no spare actually weighs 3400. Yours might be a little heavier.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
I'm asking those with one in their car if this is normal.
I just filled up after a drive of 134 miles. The tank took a hair less than 10.3 gallons of 91 octane. Of this mileage, I would estimate about 50 miles were highway and the rest city driving.
Is 13 mpg normal for this type driving, a high compression engine, a mild race/street cam, and a stock Q-jet carb? There's 1600 miles on it.
I have noticed that after 85 mph, if I attempt to accelerate from there, there isn't much left of pedal to floor. Some, but not much. It may be an accelerator cable mis-linkage perhaps, or the stock Q-jet that's the limiting factor. But it's nice to know that in the engine's present configuration, 15 mpg highway isn't unrealistic. Any thoughts?
Thanks.
Steve
I just filled up after a drive of 134 miles. The tank took a hair less than 10.3 gallons of 91 octane. Of this mileage, I would estimate about 50 miles were highway and the rest city driving.
Is 13 mpg normal for this type driving, a high compression engine, a mild race/street cam, and a stock Q-jet carb? There's 1600 miles on it.
I have noticed that after 85 mph, if I attempt to accelerate from there, there isn't much left of pedal to floor. Some, but not much. It may be an accelerator cable mis-linkage perhaps, or the stock Q-jet that's the limiting factor. But it's nice to know that in the engine's present configuration, 15 mpg highway isn't unrealistic. Any thoughts?
Thanks.
Steve
Almost sounds like your secondaries are not opening properly or the air flaps for them.
You should be able to put a brick or board or whatever to fully depress the throttle when the engine is off and see if the throttle blades are opening all the way, primary and secondary both. This does not eliminate the mis-functioning of the air flaps on the secondaries though.
There is a lock-out lever on the right side of the carb that moves out of the way when the engine is fully up to temp that allows the opening of the secondaries. Check it's function to see that it moves when up to temp and the high idle has gone down to normal idle.
You can see it in this photo. the pin on the shaft engages the lever to it's right. That lever has to move so that the shaft can rotate. Secondary blades are on that shaft.
Last edited by REELAV8R; Jun 8, 2015 at 09:18 PM.
Thread Starter
Melting Slicks
Joined: Oct 2013
Posts: 2,569
Likes: 126
From: San Diego California
I thought GVWR might mean "maximum allowed", not actual empty vehicle weight, but threw it out there anyway. I'll check those secondary throats next.
Will prop the pedal down all the way, then see if any more can be opened at the carb by the cable. Will also investigate the possibility of a wrong carb gasket hindering the Secondary butterflies from opening. Any thing's possible with me.
Thanks to all.
Steve
Will prop the pedal down all the way, then see if any more can be opened at the carb by the cable. Will also investigate the possibility of a wrong carb gasket hindering the Secondary butterflies from opening. Any thing's possible with me.
Thanks to all.
Steve
Thread Starter
Melting Slicks
Joined: Oct 2013
Posts: 2,569
Likes: 126
From: San Diego California
Photo with pedal propped to the floor. Note cable's casing.
I can pull the linkage more and open the secondary's by hand, with the limiting factor being that cable casing.
Steve
I can pull the linkage more and open the secondary's by hand, with the limiting factor being that cable casing.
Steve
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
Ok looks to be a throttle cable/pedal problem. That casing can be cut back if it is restricting full movement. Looking down the throat of front and back barrels are the blades vertical when you push it all the way open?
Pro
Joined: Dec 2014
Posts: 530
Likes: 10
From that picture it doesn't look like the rear throttle cable and bracket are quite right. When you force the throttle all the way open look how much bend there is in the throttle. That should be straight. You may need to re-orrient the bracket so it holds it further back and maybe even flip the part of the cable shield that hooks to the bracket over so it is underneath if you can.
Pro
Joined: Apr 2012
Posts: 517
Likes: 2
From: Orlando FL
It would be safe to say that your engine was broke in pretty gently. I would say you do have a cable issue or maybe a bracket issue. If it was me I wouldn't be able to keep my foot out of it. Keep us posted on what you figure out and how it is after you fix it.
Melting Slicks
Joined: Jan 2014
Posts: 2,478
Likes: 254
From: Downers Grove Illinois
Looks like you can move the cable back in that clamp and get more throttle travel. It also looks like the bracket could/should be sitting back a bit farther. That second hole in the bracket is ahead of the hole beneath it. Your missing a lot of fun right now!