Starting a rebuild of a 78 440 out of a D100.

With optimum miles per gallon using regular gas as the end goal, what would be the best compression ratio to go with? Block is bored .030.

Thanks for your inputs!

452 heads, pocket ported.

Planning on using a Cam = Int .458 / Exh .470 --- 208 at .050

I was leaning toward 9.5:1 compression

Engine will be going in my 84 D350 Crew Cab Dually. I average 150 to 200 miles per week.

In about a year, I intend on adding a MSD Atomic EFI Throttle Body (2900) and an overdrive automatic trans.

You will be juggling several parameters if max mpg and regular gas is your goal. Have you selected a cam yet? That will play a huge part, as well what cylinder heads are you running.

You want the highest DCR that'll keep you just under the pinging point with regular gas octane with room to max out the dist curve. & you never want the SCR high enough that you have to compromise the curve. I would definitely achieve quench & with open chambered heads you'd need to mill the OC areas to square them up & go with KB plateau pistons. To ans your Q I'd say ~9.5 max SCR with quench but wait for more input from others

to use 87 octane fuel there aren't any good options.

6 pack piston zero decked and a closed chamber head with about .035-.040 piston to head clearance and you could run close to 10-1 with a smallish cam (closer to 11-1 with an aluminum head). Like everyone else said run as much as you can get away with and a tight quench. You will not need near as much advance setting up the engine this way. Personally I would run another full point of compression and run premium for more power and MPG, the trade off in cost per mile (almost none) is worth it. I would run a cam around 210@.050 and 110 LSA and as much lift as you can get in that amount of duration. When pushing the boundaries of cylinder pressure you will want an intake with good fuel distribution and no heat crossover like an eddy RPM.

Im betting this wont be driven enough miles to make premium gas costs matter. Im with dave.

The cool thing about owning a muscle car that's paid for is you can blow gas through it at 8 to 10 MPG and it's still way cheaper than making payments and filling up a 35 mpg egg beater grocery getter!

Amen on that!!!

An 84 Crew Cab dually is not going to get good mpg, especially with a 440. I can understand the desire to get all you can. I hope it has the 3.54 gears instead of the 4.10 set.

Dave went down this road with his LA build years ago. I'd lean on his practical experience.

11:1 440 with a 210 cam and pump gas? I don't see that working out well. Even with quench and aluminum heads his DCR will be too high. With a cam that small, even 10:1 with alu heads and quench would be difficult on pump premium.

I made that type of combo work, but I also ran it at 6000-10,000' worth of altitude too, so it lived just fine with the rarified atmosphere. Sea level to 1000' would be a whole 'nuther ball game though.

A member just went through this, his handle on here is Dunnuk Racing, he recently did some cam articles for Mopar Muscle.
He built a towing 440 for his late 70's 1 ton reg cab for towing. I cant remember the exact details but wasn't he getting close to 20 mpg empty? Well over 400 hp and 500 lb-ft torque. I had it saved on my watched topics on the old forum but that all went bye bye on the switch over. Did a search but I can't find it.
I agree with most on here, get rid of the open chamber heads and build some quench in it. I'd stick with under 10:1 if you want 87 octane, if u step up to 91 you could probably get away with 10:1 ish. Also the small cam with as much lift as possible is also good (Hughes engines etc) I honestly think you could do better than 20 with FI and an OD

I ran 10-1 with an iron head 318 in an extended cab long bed 98 ram with a stock cam and very tight quench, even pulling a 65 dart on an open trailer and a bed full of parts out of LA on the big pass it never pinged on 87 octane california crap gas.

440 cubic inches is never going to get "good" fuel economy. If you get everything right you might get 12-14 mpg.

440's are for shredding tires and pushing you back in the seat.

Yes folks, I totally agree that I'm not going to get the MPG of a Prius, so all I'm saying is that if I'm going to get 8 mpg, I want to make sure that I did everything as best I could to get that 8 mpg. That's better than 7 mpg!

Thanks, I appreciate all the comments to date!!

*Sigh* - I have the 4.10's...

I really wish I could find Dunnuk's article, he got ridiculous mileage with his 440, I'm sure it was 20 when empty

You have gotten some good advice already in above posts.

To answer your original question,
the "pure" engineering advice would be to
set the static compression ratio to 14,
which is the static compression ratio that has been found in prior experiments to be the practical maximum for Otto cycle efficiency. Above 14 "negative work" of compressing the air/fuel charge exceeds efficiency improvement.

Note that your expansion ratio is 14,
and the dynamic expansion ratio will be only slightly less than 14.

Now, how do you keep this from detonating?

Keep the air/fuel ratio either richer than 8 or leaner than 20
( detonation is worst near 15 to 1)

Obviously for fuel economy you would go leaner than 20 to 1.

Keep coolant temperature as low as 140 F.
( this low increases bore wall wear rates)

Use sodium filled exhaust valves.

Use irridium tipped sparkplugs, large gaps, and a high energy ignition system.

Use a tri-y exhaust header system to reduce internal exhaust gas recirculation, which can heat up the intake charge.

Use a cam with intake valve closing at least near 65 degrees past bottom center, and maybe as late as 80 degrees.

Mazda has used some of these items in their current SkyActiv engines,
but could not use the very lean air fuel ratios, because they still had to use a three way catalytic converter for NOx.

It is a whole nuther story,
but have you considered building a compressed natural gas engine,
which will give lowest fuel cost per mile,
and methane has a Octane of around 120.

I was consistently getting high teens with the 440 in my '72 crew cab truck; 440, 4bbl, 727, 4:10, 33" tall tires. I was running open chamber heads with KB pistons to get exact quench distances. Its definitely possible to get a big block truck out of the single digits, even pulling a load.

Here is a compression ratio calculator I've used in the past to help see where dynamic ratios fall relative to static ratio and cam events.

https://www.rbracing-rsr.com/comprAdvHD.htm

It has more to do with your cam selection. If you want the BEST mileage possible a Six Pack would be your best option. My 440-6 Charger avg'd about 15mpg and that's with 11:1 compression, a 588 roller cam auto w/ 3.91's.