351W fuel economy and improvements

[A&F]

Maybe this is a noob question and I should know better, however -

So the LSX and most modern V8s are know for making both power and retuning solid fuel economy numbers. My understanding is this phenomenon is due to great improvements in head efficiencies and the addition of modern overdrive transmissions, both manual and automatic.

In many 351W builds I see owners reporting single and low teen digits on their fuel economy. All of these engines are pushing old slush boxes with carbs (controlled fuel leaks).

Is it unreasonable to think a 351W with +12:1 compression, aluminum heads and something like a T56 could make both good power and get good fuel economy? I would be looking at running a Victor Jr style intake, port injection and MS2 ECU.

If not, what is the primary barrier to getting modern style fuel economy?

 

[96blak54]

I like you post! Good question! Ill give it awirl.

When all this fuel/air rushes into the chamber space within the cylinder walls causes this mixture to act different ways. Such as that 351w with a 4" bore. When the fuel ignites off creating heat expansion, timing of the heat to spread out to 4" and then push the piston down takes a long time compared to a small bore like 3.5". Nearly all engines have stepped down to a piston diameter under 3.8" to aide in combustion. Longer stroke makes up for the cubes lost to a smaller bore only adding more benefits. The long throw of the crank keeps the piston at the top for more crankshaft degrees aiding in more time the piston sits while the heat expands...dwell time. Long rods also benefit the dwell time and vastly improves the torque output of the engine. Couple all these benefits with the giant camshaft lobes, more efficiency is gained!

Remember that 4" bore? The incoming rush to a 4" bore becomes more lazy than a smaller bore. In other words....keeping the space very small helps all that heat stay together to push the piston down equating to no heat loss. Then we get into head design, injector pattern, direct injection and so on. Designing the chamber to be detonation resistant is key because while maintaining a constant engine speed, those knock sensors tell the engine how much more over stoich it can go with out melting it all down. At low rpms, the engine can handle alot more intense air fuel ration. Heat doesnt become a problem untill rpms are up. And then knock sensors really help!

 

[96blak54]

My suggestion. Long intake runners, longest possible rod, mist pattern injectors. Bumping compression up is a good way to excite the combustion, but comes with disadvantages because of the big cylinder bore. Add the whole in cylinder motion the head offer.... Im not saying improved fuel economy is not possible. ..this is all bench talk to give ideas.

 

[g36 monkey]

Lol you're a freaking genius dude

 

[A&F]

An LS3 is 4.065 X 3.622

I don't think any of the domestic V8s are Direct Injection.

 

[96blak54]

Knock sensors paired with proper computer goes along way! You could score a computer, harness, and sensors from a 02-up crownvic. They are known for 35mpg on the interstate

 

[A&F]

Knock sensors paired with proper computer goes along way! You could score a computer, harness, and sensors from a 02-up crownvic. They are known for 35mpg on the interstate

I like your thinking.

 

[bmcgc]

Ive never seen a CV getting anywhere near that kind of mileage.

its a new ballgame today. My misses 2014 3.7 V6 is rated at 305hp and gets 27-28mpg, over 30mpg for the coupes.

I would love to stick a 3.5 ecoboost in a 65 Mustang. 380 hp 460 lb torque.

The 2.7 and 3.5 EB are showing up in salvage yards for $2500-3500. That's my next project.

 

[white95]

This probably doesn't help much but my 410w showed 13.2 mpg (118 miles, 8.934 gallons) but that was without a functioning TPS. It didn't help that I was driving the piss out of it either.