[BKB]

I have a 69 convertible that has the original 350 (47 heads) & th350 that was rebuilt by previous owner-typical no build sheet on motor with 3.90 possi. Has edelbrock performer manifold with 1406 carb.The motor had terrible gas mileage at that time.

I did a front disc conversion and found the cam that was installed wasn't providing enough vacuum for the brakes. I decided to install a new 204/214 cam and a 200r4 with 1800 stall to help with mileage & cruising on the highways. The motor has a strong 15+ inches of vacuum now and brakes work great.

The gas mileage hadn't improved to what I would expect from a basically stock motor. So I had the car dyno tuned, jetted the carb & installed a Dave's small body HEI. While the car runs excellent my gas mileage is barely getting 8-10 mpg city & highway combined. I don't have a heavy foot in case your wondering.

I've played with the timing to see if I can help things but no major improvements.

Right now I have it set at 15deg initial & 36 total all in at 2300rpm. I have a crane adjustable advance. The plugs are clean. WTH!! I am missing something. I would expect at least 12+mpg for this set up especially after reading what you guys are getting with bigger HP motors.

Brian

[Donovan]

What condition is the engine in? Have you done a compression check and leak down test? Steady vacuum reading? If you jack up the front and spin a tire does it freewheel at all or is it hard to rotate? Same for the back, with the parking brake off and in neutral is there drag other than normal? Have you checked for vacuum leaks?

What exhaust are you running? Can you get another carburetor to try?

As a point of reference, my 350 car got 12mpg city and about 15-16 HWY with 3.23s and a TH350

[fried boloney]

Get rid of the Edelbrock carb,get a good quadrajet. I've got a 69 lemans convert with 455,crower 60243 cam,shorty headers,7k3 heads,TH400 and 3.42 posi rear. When it had an Edelbrock carb it struggled to get 10 mpg downhill with a strong tailwind. Changed to a Cliff rebuilt quadrajet and improved fuel mileage to 13 -14(highway miles,city mileage still sucks)

[Murf'sDad]

The 3.90 gears are losing you a lot of mileage. Something in the 3.42 range would probably help a lot.

Stewart

[BKB]

I will do a compression test & see what I get.
The vacuum does bounce a little when idling but smooths out as I Increase rpm.
The wheels spin freely with normal resistance.
The exhaust is 2-1/2" dual with I believe Dynomax mufflers connected together.

I have been considering the quadrajet based on what I'm reading on this site. Just need to bite the expense of finding one and having it built to my set up. Will these bolt up to an edelbrock manifold without any issues?

[LIL-6T8]

My stock 350HO in a '68 LeMans with 3.23:1 gears delivered 15mpg on the highway at 60mph. I have a bit more rowdy of a build now with the same 350HO (395hp) and still get 13 on the highway if I can stay out of the secondaries.

I think that your main culprit is the carb (go with a QJ as suggested). Your gears are steep but the 2004r should neutralize any potential gas mileage issues at cruising speed. Do you have a loose converter (2400+) or fairly stock (1800-2000)? I would stay at 2400 or lower with that combo. Beyond that you should make certain that you have free flowing exhaust and your mileage should be in the mid-teens without any problems.

[BKB]

I'm definitely going to look into a QJet. Who are some good rebuilders near northern California?

My car runs 70mph @ 2400rpm.

[Formulas]

How was gas mileage figured ??
You have a hodge podge of parts have you calibrated your odo ?

or perhaps going by GPS ?

I got 16 mpg with a 350 Q`jet 2.73 gear 2801 cam and 500 over stock stall converter in a 3660 lb car.

work was 160 miles round trip and i would use right at 10 gallons a day give or take dpending on foot.

[BKB]

I clocked my speedo with my wife next to me on the freeway coming home from a car show and was within 2mph at 70 mph. So I think it's pretty close.

I just checked my compression & it's at 165 for 5, 164 for 1 & 160 for 2 cylinders.

I just don't get it. If I'm going to get this kind of mileage, I might as well build a performance motor & have a reasonfor this kindof mileage.

[Cliff R]

"I would expect at least 12+mpg for this set up especially after reading what you guys are getting with bigger HP motors."

Back when I had a mild 400 engine in the Ventura and 2.73 gears, it would easily get 14 mpg's, and as high as 17 on the highway.

We restored a 67 Impala SS, 327, 3.31 gears, 4L60 and it got 18-25 mpg's.

I haven't checked the milage on our Ventura with the new engine, but the last one would get 11-13mpg's the couple of times we checked it, combined city/highway driving with a 514hp 455 engine, and 3.42 gears.

The 350 engine with the small cam you are using should be capable of 13-15mpg's even with the crappy carb on it. I have customers running OD's with 455 engines and 3.23 to 3.73 gears that get as high as 17mpg's with them.

As much as I hate that line of carburetors, they are very easy to tune for part throttle A/F ratio. The primary metering rods have two sections, the upper section is the part that rides in the jets at cruise/part throttel/high vacuum. They can be changed without taking the carburetor apart.

Using the "tip-in" test from our book, install a set of metering rods that provide a very slight rpm increase when the choke flap is tipped in with the engine set at apprx 2000rpm's. This tells you that the part throttle A/F ratio is just on the edge of lean, and should provide efficient part throttle driving with decent fuel milage.

If your "tip-in" test shows no rpm increase, or more than about 200rpm's, it is too rich or too lean and hurting fuel economy.....Cliff

[TransAm525]

On top of the great advice already given, pay close attention to your vacuum. You stated the vacuum gauge would bounce at idle. A flickering vacuum gauge can be an indication of worn valve guides and/or bad piston rings. Both problems will lead to reduced fuel economy.

Perform a compression test and inspect the guides/valve seals for wear. Worn piston rings will often result in blue smoke out the exhaust pipe during acceleration. Worn valve guides will results in blue smoke out the exhaust pipe when you first start the car (especially when the vehicle has sat a while before starting).

[BKB]

I did a compression test yesterday and had compression from 160 to 165 on all cylinders. I have never seen any smoke come out the tail pipes, so I assume everything it good as far as rings & guides.
Cliff I just ordered your book & hope to have it today. I'll look into your suggestions. I'm sure you'll be hearing from me regarding a Qjet upgrade.

Thanks for all your suggestions.

[Half-Inch Stud]

The 3.90 gears are losing you a lot of mileage. MPG is DIRECTLY gear-locked on CFM/crank turn.

Skip the 3.42:1 and be thinking 0.67:1 OD. Our Combos NEED the equal of a final drive ration of 2.56:1 or even sicker with 2.21:1 being the practical limit. HIS

[Ben M.]

What no one is mentioning is that you are in California. The quality of gas here SUCKS. You lose 25% of your fuel economy compared to Arizona or other states. People can claim high teens for these engines, but my personal best is only 15MPG with a very tame engine and 2.41 gears. 11-12 is more the norm for anything remotely performance oriented.

[Half-Inch Stud]

My 68 LeMans 350 would get 18MPG only on the Highway, with Q-JET, HEI, TH400, with 2.73:1 gearing.

Gasolines with alcohol (1%-10) ought harm MPG.

[PunchT37]

Originally Posted by Ben M.

What no one is mentioning is that you are in California. The quality of gas here SUCKS. You lose 25% of your fuel economy compared to Arizona or other states. People can claim high teens for these engines, but my personal best is only 15MPG with a very tame engine and 2.41 gears. 11-12 is more the norm for anything remotely performance oriented.

I tend to agree with the gas situation. I`m here in Louisiana. I bought my truck new. At worst, it always went 285 miles to a tank. Almost like clockwork. Now that they are adding 10% ethanol, I can`t get this thing past 220-240 miles.

[Socrates]

Don't let the Q-jet cheerleaders delude you to believe you must give a pro rebuilder $600+ for Q-jet to get good mileage or street performance. Your E-brock is more then capable of stellar mpg, in fact I've had one on a 350 pontiac, 4k lbs, 2.73 rear gears, that would flirt with 20mpg highway, had one on a 400 pontiac with 2.9x gears, 3700lb car that would also get near 20mpg highway too (numbers averaged over many looong trips). Oh, before the cheerleaders cry foul, I run a Q-jet, Holley 950HP and an Ebrock on my various cars, and have built, tuned and raced plenty'O q-jets, though I still stand firm that the 600cfm Ebrock is a great street carb, especially for economy use, dead solid reliable and uber simple.

I think your vacuum reading at idle should be higher then 15. I'd say 18-20 would be more like it. Keep tuning! Idle screws, vacuum advance, timing curve, check for vacuum leaks, etc.

Speaking of vacuum, install a vacuum gauge atop of your column. Watch the needle, adjust your driving habits to maintain the highest possible vacuum. I just installed a vac gauge on my DD 4spd truck, and it has been an enlightening experience, changing my driving habits a good bit for the better.

Also, you guys are missing the 200r4 trans, 3.90 gears are not driven 1:1, so don't get fixated there.

Lastly, how do you figure mpg? Using your fuel gauge? IF so, you're figures are wrong. Factory fuel gauges are horrible measuring tools. Long trips and using tank fill ups, measuring fuel use at the pump, ideally the same pump, and with many measurements taken to eliminate outliers is the way to get a good economy estimate. One miracle road trip with perfect air, a tail wind and all down hill driving, which many use for bragging rights, doesn't tell the true mpg story. Getting your actual mpg figured out is not easy or quick.

[carbking]

When you called this morning asking about a carb and mileage; I didn't realize you had 3.90 gears.

Regardless of carburetor setup, mileage in town is going to be BAD!

Mileage at a constant 60 MPH with the 200r4 may not be too bad, don't know.

Jon.

[Cliff R]

Sorry, I'm not a "cheerleader", dated one once....but I do work on my share of carburetors for a living. I also test everything, street, dyno and at the track. The Edelbrock AFB clones are simply OK, dirt cheap and relatively easy to tune. Since they fall into the "dirt cheap" catagory, folks gobble them up and get the intake as well for less than the cost of a decent rebuilding of their OEM carburetor.

In actual use, they are the worst of all the units we have ever tested here. They lack any adjustment for the opening rate of the secondary airflap, so if it stumbles, and many will, especially on high HP to weight ratio applications, nothing you can do about it.

They also have several SERIOUS design flaws, which have also been outlined by myself and at least one other carburetor rebuilder, who happens to be the most knowledgable person I've ever known on the AFB's. We both concur on one thing, we will NOT allow any of those units in our shops!

BTW, our prices have never been $600, so the sarcastic comments must be directed at someone else. Anyhow, it amazes me these days that folks on this particular website can't just post usable/accurate information as they know it to be, without all the drama. Maybe it's the economy, maybe just the hot outside temperatures, but in any case, it takes enough time as it is to read the responses, without having to work your way thru all the NONESENSE.....FWIW......Cliff

[BKB]

Originally Posted by Socrates

Don't let the Q-jet cheerleaders delude you to believe you must give a pro rebuilder $600+ for Q-jet to get good mileage or street performance. Your E-brock is more then capable of stellar mpg, in fact I've had one on a 350 pontiac, 4k lbs, 2.73 rear gears, that would flirt with 20mpg highway, had one on a 400 pontiac with 2.9x gears, 3700lb car that would also get near 20mpg highway too (numbers averaged over many looong trips). Oh, before the cheerleaders cry foul, I run a Q-jet, Holley 950HP and an Ebrock on my various cars, and have built, tuned and raced plenty'O q-jets, though I still stand firm that the 600cfm Ebrock is a great street carb, especially for economy use, dead solid reliable and uber simple.

I think your vacuum reading at idle should be higher then 15. I'd say 18-20 would be more like it. Keep tuning! Idle screws, vacuum advance, timing curve, check for vacuum leaks, etc.

Speaking of vacuum, install a vacuum gauge atop of your column. Watch the needle, adjust your driving habits to maintain the highest possible vacuum. I just installed a vac gauge on my DD 4spd truck, and it has been an enlightening experience, changing my driving habits a good bit for the better.

Also, you guys are missing the 200r4 trans, 3.90 gears are not driven 1:1, so don't get fixated there.

Lastly, how do you figure mpg? Using your fuel gauge? IF so, you're figures are wrong. Factory fuel gauges are horrible measuring tools. Long trips and using tank fill ups, measuring fuel use at the pump, ideally the same pump, and with many measurements taken to eliminate outliers is the way to get a good economy estimate. One miracle road trip with perfect air, a tail wind and all down hill driving, which many use for bragging rights, doesn't tell the true mpg story. Getting your actual mpg figured out is not easy or quick.

I checked my vacuum last night while doing the compression test. It's closer to 17" but fluctuates a little so 17 isn't exact(I don't know if this is normal). When I give it some gas it smooths out. I also checked the ported side and it sits at 0 until I give it some gas and then it climbs fast & steady to 20+ inches.
I do check my mileage at the pump, dividing the gallons into the mileage. I rarely go over a hundred miles to the tank.