What Is Good Gas Mileage?

[1969VADart]

I was wondering what kind of gas mileage people are getting from performance modified 318 engines? Particularly anything with 318 block but 340/360 heads as well as cam upgrades and 4 barrel carbs? Have not had a chance to inspect more closely the engine of my 69 Dart, but the guy I bought it from said the mileage was not the best with his engine combination (318 block, 2.02 heads from a 360, Mopar purple cam, 4 barrel carb). Makes me wonder if all of the components were properly matched for peak performance. Any info on gas mileage would be appreciated.

 

[Mopar to ya]

40 mpg is good, but out of reach for us Mopar guys. I have a 408 built to 500hp and 11.8:1 compression. I jetted the primary side down a bit to 72 and left the secondary side at 86. I don't drive it nicely very often. I check my mileage all the time out of curiosity more than anything else. When I am tuned for 93 octane I average 11.5 mpg and have gotten as high as 15 mpg when I drove it nice the whole tank. When I tune it up for race gas I obviously drive it harder and get around 8-9 mpg.

 

[rumblefish360]

A lot depends on the engine & drivetrain set up. Out of a basicly stock engine with bolt on parts and a mild cam, getting better than 17 is good.

 

[inkjunkie]

40 mpg is good, but out of reach for us Mopar guys. I have a 408 built to 500hp and 11.8:1 compression. I jetted the primary side down a bit to 72 and left the secondary side at 86. I don't drive it nicely very often. I check my mileage all the time out of curiosity more than anything else. When I am tuned for 93 octane I average 11.5 mpg and have gotten as high as 15 mpg when I drove it nice the whole tank. When I tune it up for race gas I obviously drive it harder and get around 8-9 mpg.

You have driven it gently on a whole tank?? Were you feeling well???

There is a member here that has a big cube HEMI with efi....Made 725 hp . Tweaked the EFI for mileage and got 14 mpg......

http://fastmanefi.com/TheMPGPage.htm

 

[jimmyray]

My 408 stroked Duster with a TQ and 3.73 Dana, and a TKO600 5 speed gets about 17.8 mpg @ 70 mpg. No better at slower speed. Tuned to 15:1 A/F ratio at cruise.

 

[moparlover]

dude the 318 was probably fine with the original heads. by plopping on the 360 heads you probably dropped the compression ratio down to 7:1. that right there killed the mileage. then adding a 4bbl hit it some more. then a duration cam (pumping extra gas/air into the 7:1 compression lol) did it even worse.

ideally for mileage:
1. light car = better
2. highest streetable compression. shoot for 10:1 for ezmode or if you are extremely good at tuning you might get away with it at 11:1 (i wouldnt advise that)
3. moderate performance cam
4. moderate performance carb
5. lockup converter (requires major mods) or get a 4speed. old school 727 with torq converter lose a couple mpg vs manual trans

 

[Princess Valiant]

my old mopars are my only transportation as I don't have a newer car....my newest car is a 72 ....so in my world anything better than 16 MPG is a real bonus.

my big question is why are you worried about MPGs .....even if an old car does not do very good gas milage ...its still worth the extra money in gas to drive what you want and not be a zombie in a Toyota.

 

[MRL Performance]

I have a customer in Texas that we built a Magnum 408 for. Its in a 2000 Ramcharger and here is his truck and last time slip, he is LEFT lane. He is getting 20-22mpg out of this big pig.

 

[rumblefish360]

LOL at Rani. Well true that one! I also like that I pay $110 a year for full coverage on a drive anywhere, any time except to work policy.

Jimmy ray, your OD is the big help here! Nice!

MoPar lover... A split duration cam of modest increase duration at best will help. Same with the carb. Honestly I can/have tune the crap out of carbs to there best mpg giving state I could. Not that I'm a genius at it. Trial and error lead the way.

I have found no matter what carb I have on top it is the state of tune that makes or breaks the mileage. I have found an edge in a TQ. Just an edge though. Nothing to scream about just yet. The small primary is a great plus.

A lock up converter is good for mileage. It is just a tiny bit, a minor edge. But if you keep adding all the little edge parts up, all the tiny gains, you get somewhere with them. So pile them on.

Know it all Tony made a good point about the distributor. I suggest adding a MSD or equal ignition on top. Open up the plug gaps as wide as possible until miss fire and the. Close it up a bit.

Other items?

Open air cleaner
Small primary carb
Light weight and high flow intake
Well prep'd heads w/a well prep'd valve job
Small valves help (an edge)
Headers
Lock up converter 904
O.D. Trans a plus for meage and performance
Light weight drive shaft a plus but pricey
Thin tires
Low body stance/height
Rhoads lifters are a nice help for a slightly larger cam than needed. They make some noise. It never bothered me personally.

And when it comes to a cam, a split duration should be used. I would suggest that the comp cams line designed with exhaust manifolds in mind be looked at. The exhaust duration is much longer than a typical split cam. Not for nothin either, a custom cam is t expensive to get done. Call a grinder or two and ask a out it.

You can still increase the rpm band without major I'll effects but they'll be there non the less. I know you want more power. But as you go up in power ( and cam size) your mileage will drop. No way around it. Modern cars use the computer to help out with this. With that in mind, it is no wonder people are doing HEMI swaps. The modern Chargers and Challengers do well for mileage and even better when you consider there in a vehicle probably a 1,000 heavier than yours.

Food for thought.......

MRL- interesting addition on top of the intake. Could you comment on it a d the basic engine set up of the Furango please?

 

[1969VADart]

dude the 318 was probably fine with the original heads. by plopping on the 360 heads you probably dropped the compression ratio down to 7:1. that right there killed the mileage. then adding a 4bbl hit it some more. then a duration cam (pumping extra gas/air into the 7:1 compression lol) did it even worse.

Just to be clear, I did not put this set up on the car. The previous owner had all that work done. I have not even been able to start the car yet because it needs a new starter. Once I get it running, then I can get a feel for how the car runs and proceed from there.

 

[rumblefish360]

Do you know what it has for a cam? Saying it has a "Purple cam" is extremely broad since there are probably a good 50+ cams under that name.

As much detail a out the car and your intended target goal would be good.

 

[SirDan]

dude the 318 was probably fine with the original heads. by plopping on the 360 heads you probably dropped the compression ratio down to 7:1. that right there killed the mileage. then adding a 4bbl hit it some more. then a duration cam (pumping extra gas/air into the 7:1 compression lol) did it even worse.

I've got a 318 with stock 360 small valve heads, 340 cam, headers,torker340 intake and eddy 600 cfm. It gets pretty good gas mileage.

The low compression motor will want more timing than a higher compression motor.

 

[1969VADart]

Do you know what it has for a cam? Saying it has a "Purple cam" is extremely broad since there are probably a good 50+ cams under that name.

As much detail a out the car and your intended target goal would be good.

Going to talk to the guy tonight and see if I can get some more info on the specs. Probably pulled the trigger on this a little too quick without doing a little more research on the motor. But the car as a whole was in too good a shape to pass up.

 

[CAMOPARS]

On a '71 92k 318 with only the following mods (Weiand 8007 intake, Edelbrock 1406, Holley open element air cleaner,Pertronix conversion and dual exhaust w/stock manifolds), I can get between 17-19mpg if driving sanely.

 

[1969VADart]

Talked to the guy last night. Got a little more information out of him. New pistons were installed in the bottom end (could not tell me if flat tops or not). Only could say that work was supposedly done to the heads (shaved, ported and polised). Did not have cam specs. Edelbrock intake. I think he said Carter 4 barrel carb. New headers. Thought that the compression was something on the order of 10.5:1. I guess I will just have to get the car started and then evaluate backwards from that point.

 

[66plyValiant]

if you get better then 20 MPG your doing excellent, some of the /6's claim 25-28 but I don't know the truth in that, any slant I had was just as hard on fuel with alot less power