Gas Mileage query Holley 318

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Jason Maitland

Jason Maitland

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Hi all... Just did a trip in my 68 Dart coupe, very mild 318, .030 over rebuild with around 13000 miles, headers, Fresh complete overhauled Holley 650 DP, Auto w 3.23 rear - both near new rebuilds.
Sitting on 55/60 mph, highway/country driving mostly, and getting approx. 13-14 miles per gallon (Exactly 17L / 100KLM in our language!)...
Interested to know if this is normal or high, thanks...
 
Sounds a little low but only a few mph. Would think18 could be done i would change carb To vacuum secondary
 
Seems it should do a little better.... 17 mpg would be an average with maybe a little more potential IF it's tuned properly and the engine is as it should be.
 
13-14 is a little low I think. Do you have a tach on the car? 2000 rpm or less at cruise speed is optimum. My 2 barrel will get 18 at 55 but 65-70 and I am down around 15-16 with a 3.23 gear.
 
What do you folks down there have for alky in the gas? Alky reduces mileage because it takes more of it to properly burn. What do you have for a camshaft? HUGE difference

Any idea on your head/ piston combo? Very low compression ratio is not going to help.

And with others, play with timing and jetting. Look at the plugs. "If you are serious" you might consider a wide band O2 setup which last I knew were around $200 US

"Back in the day" a stock B body 318 single exhaust say, 67--70 ish year could pull 18-20 mpg if driven carefully

I swapped a 71 (hi compression) stock 340 into my 70 RR around ?73? and that thing got 17 with headers, 3.54 Dana, 4 speed
 
That's about right. When I removed the stock single exhaust on my Van and installed headers and dual exhaust, my milage went into the can. I haven't seen a 318 stock ever get more than 17-18 mpg, and I have had a few. I don't have a heavy foot most of the time.
 
I agree with everybody else that your gas milage appears a little low, and there is probably a little room for improvement. But here is the deal. It is very difficult to build an engine for performance AND at the same time get good gas milage. Here are some thoughts:
1. You said it was a 'mild' 318. One person's idea of mild is another person's idea of moderate. We know you have headers and a dual pumper carb. We don't know what you have for heads, intake, cam, or compression ratio. So the less 'mild' your engine truly is, the more power it has, and the more fuel it will use.
2. That is an unusual carb for a 'mild' 318. A 650 double pumper is definitely more performance oriented. how is it jetted? If you jetted the primaries down, you could cruise using less fuel, but still have power available at WOT. Every time you step down on the gas pedal past a certain point (maybe just to pass or zip through a yellow light) the accelerator pumps in the secondaries may be squirting in extra gas. A jetted down 650 vacuum secondary carb would actually give better fuel milage and still have plenty of power when you put your foot in it. You might even consider a smaller 4 barrel considering how mild your engine really is, and what your performance/milage goals are.
3. Although 3.23s are higher than 3.55s or 3.91s, they are still considered to be a performance gear ratio. I think it is a very good ratio, but 2.76s would be better for milage.
4. Lastly, we do not know how you drive. A heavy foot will yield far less fuel milage than a lighter foot.

I can only imagine how expensive gas must be in Australia. But overall, if you drive a modified classic car with a V8 and a performance gear ratio, I don't think you should be worrying about gas milage too much. I have a 69 Barracuda with a 340 that is around 375HP, a 4 speed and 3.55 gears. I don't measure my gas milage because I don't care. I measure Smiles/gallon, Thumbs Up/gallon and Waves/gallon. Don't get me wrong; I am NOT belittling you for wanting better gas milage, I just think that if you are going to drive a classic car with a modified V8 engine and a performance gear ratio, you should not be worrying about fuel milage too much.
 
Well the 1970 Dart with a slant 6 engine was rated at 14.1 l/100 km average (0-100 kmh mph in 12.1 seconds!) so 17 l/100 km doesn't sound bad.
 
I drove from Ca. back tp Missouri a few decadea go in a 67 Cuda, rebult 383 with 10,000 mi;, 4 speed, 2.94 gears, and a good thermoquad. Interstate mostly at 70, it averged 18 mpg. Longest trip ever I ever made in an old Mopar and only time I ever checked mileage!!!! LOL Usually I just don't care!!
I bet you can do better with a Carter!!!
 
Thanks everyone for your feedback...
I am using 98 octane fuel (equivalent to US 95 I believe), and engine/heads are from around 73 vintage (I was told by seller) Cam is just a bit spec up from stock specs. Those figures are from me driving sedately at highway speeds around 50/55 mph, being approx 2100 - 2400 rpm. I once had a 68 Dart with a 383 running a Carter 600 which I think was MUCH better on highway trips ; I plan to replace the Holley with a TQ for the record!
 
dp are not mpg carbs
if you are into the secondarys you are lucky to get that
 
Put a thermoquad on it and watch the mileage explode.
 
An O2 sensor and a air/fuel gauge is the bees knees. I could get over 20 with my 68 dart on road trips after I bought one and started fiddling with the bbd. I had 3.23 and an a 42rh behind a stock 318 from a big RV.
I wouldn’t think a dp Holley would have any disadvantage to any other Holley pertaining to fuel efficiency unless you are into the secondary’s much, but any carter type carb tuned close would probably make an improvement. In my experience the carter type is easier to fine tune.
 
Aren't double pumpers all 4 all of the time? Mechanical, no vav secondary.
 
I bought an AFR meter years ago and always weld a bung just behind the header for an O2 sensor. You can really see how horrible carbs are for economy. They are crude mechanisms and you need to keep the AFR relatively rich. Yeah a modern carb that you can get a metering rod, spring and jet kit would be good if you plan on tuning. How old are Thermoquads now? 45-50 years?
 
CRUZE 418 said:
Aren't double pumpers all 4 all of the time? Mechanical, no vav secondary.
Click to expand...
I believe the secondary's are operated mechanically (whether you need the fuel or not)....could be wrong as I have always had vacuum secondary carbs.
 
I can't get the AFR sensors to last very long. I don't think they like accelerator pumps.
 
At 2k-2400 rpm you are still running on the idle circuit and haven't fully transitioned to the primary jets. Pin the IFR size and see what size they are. Reduce the IFR in .001 increments until it stumbles, then go back up .001. If the carb is 4 corner idle do both metering blocks. Vacuum advance is also important if you want gas mileage.
 
My Duster with a mild cam, Edelbrock Performer 318/360 intake, 750 DP with a stock 78 318 bottom end running through an A500 OD auto and a 4.10 geared 8-3/4 rear with 28" tires got me 20.6 mpg after tuning it for mileage. It maybe could have got a hair better but I quit messing with it. That was at steady 65-70 mph, 25-2600 rpm on a 300 mile round trip.

Cley
 
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