Horsepower with mileage options

[toolmanmike]

A lot will depend on the type of driving this is intendend for. A lot of idling time and stop and go is different than improving mpg on the county roads and interstates. Some will also depend on the climate. Since emissions are not a condideration, carb and timing is fine.

I think the 318 is very decent base. In stock form, using a timing curve from 67 or 68 non-CAP, should pull 21-22 mpg highway with the current drivetrain and provide good pep.
Reducing weight is always a plus for both goals. (feather duster for factory example amed at milage)
A four barrel carb will help power in the rpm range. As others have said it should not hurt mpg. The extra two barrels allow it to operate with more efficency over a wider rpm range. If trying to improve 1/4 mile times and mph - this is what is needed. Horsepower is all in the top rpm - its not the same as pep.
Small tube headers will help the upper rpm and possible the entire range. These will need to be custom for what you want to do.

Changing intake to aftermarket is sometimes better for mid and top power (2500 - 6000 rpm) and often worse for idle to mid. A lot has to do with fuel distribution during part throttle. With good vacuum this is (part thrtottle/low throttle distribution) less of a problem but it still can be worse than many stock intakes. Losing weight is good of course.
Aluminum heads. Losing weight is good. Others have already commented on other impacts. This change will require some time working on part throttle timing and fueling.

Agree there. @Penstarpurist , what kind of mileage are you guys building for? What numbers would be good for you? If you're thinking low 20's you are really close with the combo you already have. Mid to high 20's might be a tall order without a O/D and then if you throw much performance in the mix, your mileage will go away. You can build a more efficient 318 with better than 2 barrel performance but there is a line of diminishing returns when more performance starts killing mileage. Small camshafts are good for mileage. Thermoquad carbs are good for mileage. Small tube headers and smaller dual exhaust are efficient. Heck even stock manifolds with a nice 2 1/4" duals will increase mileage. A properly tuned distributor and more initial timing can increase mileage and performance. I think aluminum heads are a waste for your goals. Bottom line is, up to a point in performance, you can have it both ways. Too much performance, mileage goes away. This will be a interesting discussion.

 

[Mattax]

Agree there. @Penstarpurist , what kind of mileage are you guys building for? What numbers would be good for you? If you're thinking low 20's you are really close with the combo you already have. Mid to high 20's might be a tall order without a O/D and then if you throw much performance in the mix, your mileage will go away. You can build a more efficient 318 with better than 2 barrel performance but there is a line of diminishing returns when more performance starts killing mileage. Small camshafts are good for mileage. Thermoquad carbs are good for mileage. Small tube headers and smaller dual exhaust are efficient. Heck even stock manifolds with a nice 2 1/4" duals will increase mileage. A properly tuned distributor and more initial timing can increase mileage and performance. I think aluminum heads are a waste for your goals. Bottom line is, up to a point in performance, you can have it both ways. Too much performance, mileage goes away. This will be a interesting discussion.

yes. And an aluminum manual 4 speed with OD for the 4th is most likely easiest fit for running with minimal drivetrain losses and lower rpm on the highway.

 

[ir3333]

Lots of good info but just read Daves' mileage build on Moparts.
Everything he did is provided with the result obtained.
It is not as simple as "adding this or using that". I haven't read through it in a long time and it is a long read but if i remember correctly the best cam was a 60's 273 piece!

 

[Penstarpurist]

He is looking to get into the low 20's mpg, 95% of the driving is done in town driving currently. He doesn't have concerns on 1/4 mile timeslips, just good light to light power. Well and being able to beat my other son's powerstroke diesel, which is no easy feat.

 

[Mattax]

He is looking to get into the low 20's mpg, 95% of the driving is done in town driving currently. He doesn't have concerns on 1/4 mile timeslips, just good light to light power. Well and being able to beat my other son's powerstroke diesel, which is no easy feat.

City milage is tough. Smaller engines have the natural advantage when idling.

 

[toolmanmike]

City milage is tough. Smaller engines have the natural advantage when idling.

Agreed. City mileage shouldn't be considered especially if "performance" is in the picture. A 2+ hour trip on the interstate @ 2000 rpm or less will get your best mileage (depending on your combination of course)

 

[Penstarpurist]

Lots of good info but just read Daves' mileage build on Moparts.
Everything he did is provided with the result obtained.
It is not as simple as "adding this or using that". I haven't read through it in a long time and it is a long read but if i remember correctly the best cam was a 60's 273 piece!

I gave it a read, definitely long but like you said great amount of info there. I got my son to see that going with a 4bbl is a more viable option, for both milege and wanted performance. Currently and sadly he is only getting around 14mpg, which our one ton diesel gets better (If my son leaves the tuner stock).

 

[toolmanmike]

I gave it a read, definitely long but like you said great amount of info there. I got my son to see that going with a 4bbl is a more viable option, for both milege and wanted performance. Currently and sadly he is only getting around 14mpg, which our one ton diesel gets better (If my son leaves the tuner stock).

It's fun to compare but realistically a unfair comparison.

 

[Penstarpurist]

I would agree not a realistic comparison, as the valiant is near 50 years old and the truck is 14 years old and computer managed with injection. I think if he could get into a solid 20/22mpg and still be able to roast the tires he would be happy as a clam at high tide.

 

[273]

You guys must do a lot of highway driving?
I could go my whole life without ever taking the highway again, the odd time I do I'm on it 15-20 minutes max.

 

[Penstarpurist]

No highway driving in the valiant other than a few miles at a time.

 

[273]

I would agree not a realistic comparison, as the valiant is near 50 years old and the truck is 14 years old and computer managed with injection. I think if he could get into a solid 20/22mpg and still be able to roast the tires he would be happy as a clam at high tide.

You talking highway or intown ? Big difference.

 

[ir3333]

some check their mileage when they take a trip and then tell everyone what their mileage is.But that is only 2 or 3 % of the time for most of us.An od seems to be the logical choice,and for highway driving with a cam that is most efficient at the od cruise rpm this is probably the case.However it may be terrible for some engine combos for lower speed applications..I have a friend who installed an od in his 3.91 geared 408 truck and his mileage went down!

 

[273]

So 14 intown. Hard to gain on intown mileage. Modern cars are still in the teens with intown mileage.
Best you could hope for is to gain 100-200 hp without a drastic loss of fuel mileage. You might be able to increase the 14 a few mpg's with moderate hp gains 0-50 hp.

 

[Penstarpurist]

Our truck gets better mpg in town and on the highway than does the valiant. I think an od transmission is a great idea, I know we picked up a 3.91 headset for it initialy, but when he decided milege was more important to what he was looking for he started thinking the 3.55 rearend would be agreeable with the end goal. Obviously not as good as the 2.76 gets for mileage, but I think with a 4th gear it would be a good compromise.

 

[273]

Our truck gets better mpg in town and on the highway than does the valiant. I think an od transmission is a great idea, I know we picked up a 3.91 headset for it initialy, but when he decided milege was more important to what he was looking for he started thinking the 3.55 rearend would be agreeable with the end goal. Obviously not as good as the 2.76 gets for mileage, but I think with a 4th gear it would be a good compromise.

I imagine the truck is diesel ?

If you don't do highway OD waste of money, and if looking mileage only (no performance improvements) don't spend much its cheaper to put it into the tank.

Don't know if 3.91 would hurt to much over 3.55. Easier to get the car moving where a lot of mileage is lost.

 

[Mattax]

For in-town and local roads (which even that varies alot) you could experiment with non-CAP vs. CAP timing and fueling a big difference between the two is at idle. CAP runs leaner but with retarded timing and higher idle speed. Non-CAP probably has a slight edge in fuel consumption. Should have an edge in throttle response too. I'd need to see exactly what pollution controls were used in '71 to have a stronger opinion on that. The Orifice spark delay I think is not until later - that's a real a downer.

You guys remember when GM thought they had a system worked out to drop cylinders at idle?

I think the best you can do in this situation is keep the fuel milage about where it is, while getting a bit more torque and Hp.

 

[Penstarpurist]

Yeah the truck is diesel. Freeway driving is minimal with the valiant. What gear ratio would be an even trade off of get up and go vs Mileage? As I'm sure with most of us on this forum, I've never built an engine for mileage. It's why I daily drive a Kia soul and leave the hot rods for car shows and drag strips mostly. But my son from the first time he drove our old 68 fury has been in love with classic muscle cars and hair bands. Lol. My boys drive our cars when their tanks are low in fuel, which is more than they like. Hence the reason he started thinking about getting more mileage out of the valiant. He lives to drive that thing, which he does to school daily, but wants to be able to drive it to go out with friends and what not without having to put more fuel in it. It's the new generation thing, style and mileage, he just doesn't want to drive a ricer if he can get the mpg he is looking for in a classic.

 

[273]

I did a quick calculation. At my gas price up here in Canada which is ridiculous. If i put 5000 miles a season at 14 mpg would cost me about $1600.00 now if I increased my fuel mileage by 25% which is pretty optimistic it would be at 17.5 mpg now costing about $1250.00, saving $350 a year. If you spent a $1000 it would take 3 years to break even but the things we're talking about could easily cost $3000-6000 and 9-18 years to pay back.

 

[AJ/FormS]

Oh hey! There is always a super charger! A TorqStorm is reasonable in cost and your only looking at two other expenses to make it work. A boost reference fuel pump and MSD ignition with retarding the timing. This in addition to basic breakfast or on parts and a well thought out cam would yield excellent results.

So it's ok to talk about supers.......but not about a 360 core, When the man has already mentioned aluminum heads and an overdrive?

 

[Penstarpurist]

I did a quick calculation. At my gas price up here in Canada which is ridiculous. If i put 5000 miles a season at 14 mpg would cost me about $1600.00 now if I increased my fuel mileage to 25% which is pretty optimistic i would 17.5 mpg now costing about $1250.00 saving $350 a year. If you spent a $1000 take 3 year to break even but the things we're talking about could easily cost $3000-6000 9-18 years to pay back.

I did not even think about it like that! Got so caught up in, "can it be done vs cost to do it" that the long term didn't even cross my mind. So, really then we should be thinking about how to get more power out without sacrificing existing mpg, lol. Gas prices in my area are on average $2.71 per gallon. And I would guess they put about 5000 miles a year like you said. So, yeah $1000.00 in fuel for paying off 3k in parts to make better mpg. The things we do to see if we can.

 

[BigBlockMopar]

Calculations like those, while down to earth and hit the mark, will usually take out the fun in driving a car and upgrading it as you go along.

I had (still have) set similar goals with my own '73 Dart.
Being a daily driver for me, every upgrade I did over the years, made for a more fun to drive car in the process.
Unfortunatly my car has showed me it doesn't like to 'mileage' well at all and improvements if any have been very small, compared to the money spent, but the car is sure much more fun to drive than when I first bought it.

 

[Penstarpurist]

Calculations like those, while down to earth and hit the mark, will usually take out the fun in driving a car and upgrading it as you go along.

I had (still have) set similar goals with my own '73 Dart.
Being a daily driver for me, every upgrade I did over the years, made for a more fun to drive car in the process.
Unfortunatly my car has showed me it doesn't like to 'mileage' well at all and improvements if any have been very small, compared to the money spent, but the car is sure much more fun to drive than when I first bought it.

It's the nature of evolution in a car guy. How many if not all of us have put more time and money into a car than we get back in a monetary exchange. It's all really more smiles per gallon than the miles.

 

[yellow rose]

It's the nature of evolution in a car guy. How many if not all of us have put more time and money into a car than we get back in a monetary exchange. It's all really more smiles per gallon than the miles.

Which is exactly why I don't like building for fuel mileage. Doesn't mean it has to be a fuel burning pig (and that definition has wiggle room in it) but the more compromises you put into the build, the fewer smiles you get.

You need to set reasonable goals for power and mileage and go have fun.

 

[rumblefish360]

So it's ok to talk about supers.......but not about a 360 core, When the man has already mentioned aluminum heads and an overdrive?

I’m just sticking with the displacement that was asked about.
Further down the road, a super charger could be added. That’s there call.
They are a awesome power adder and if you actually keep your foot out of it, you’ll gain mileage.