351w Fuel Economy And Improvements

AFZ · Mar 15, 2016

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?

jrichker · Mar 15, 2016

AFZ said:
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?

Carbs and lead in the right foot...

It is rare to see OEM sequential EFI on a 351W. Most of them are carb'd because it is simpler to do and it doesn't require the understanding of electrical circuits and computers. Who needs technology with all its sensors and actuators when you can have a gasoline toilet, just push the pedal and it flushes...1.6 gallons per flush... (of the toilet, of course).

stykthyn · Mar 16, 2016

You can't compare Windsor technology to an lsx or any other modern engine for that matter. But, I suppose if you did adapt sequential efi, an overdrive or even a double overdrive tranny, better heads, more efficient accessories, you could have a toilet that will get you low teens on the street and high teens on the highway.
Just flush away.

Gearbanger 101 · Mar 16, 2016

Keep the cams specs conservative. All the aftermarket builds I've seen for 302/351 based Windsors are choosing their cam profiles based around maximum horsepower.....which is what you get, but that's going to eat up mileage.

LS engine tend to be on the more conservative side as far as cam profile from the factory, which is one of the reasons they're able to attain decent mileage. Modern LS engines also benefit from variable valve timing....which you're not going to build into a Windsor no matter how hard you try.

even still, there's no reason a 393/408 stroked Windsor couldn't knock out low to mid-teens for fuel mileage....even with a stock T5 behind it.

A BIG set of heads, high flowing intake, conservative cam specs, good suporting intake and exhaust components and most importantly a conservative rear gear set. No need for 3.73-4.10 rear gears with 400ci on tap....especially if you aren't running a wild cam. That motor would make enough torque that the stock 2.73-3.27 rear gears would pull it along just nicely.

If you've got the cubes....make that torque curve work for you. Don't try to scream through it.

AFZ · Mar 16, 2016

stykthyn said:
You can't compare Windsor technology to an lsx or any other modern engine for that matter.

Why not?

In the case of a LSX, it's still a 4" bore OHV engine. Many are iron blocks, just like the 351W.

Heads flow better, I think 300cfm, but what do aftermarket 351w heads flow?

stykthyn · Mar 16, 2016

AFZ said:
Why not?

In the case of a LSX, it's still a 4" bore OHV engine. Many are iron blocks, just like the 351W.

Heads flow better, I think 300cfm, but what do aftermarket 351w heads flow?

Better crank to rod ratio, better heads, better cams, it's apples to oranges. The 351w is many things including long in the tooth. It can be a torque producing power house as well, but the simple fact is it will never produce the power levels and maintain street manners that an Lsx or coyote will.

Mustang5L5 · Mar 16, 2016

IMHO you are stepping over dollars to save pennies. Putting together a $10K motor to bump fuel economy doesn't make a whole lot of sense when you consider the primary reason you build such a motor is power. Yes, you may gain fuel economy as a result, but how much are you really going to save given the cost of assembling such a beast.

stykthyn said:
Better crank to rod ratio, better heads, better cams, it's apples to oranges.

Not to mention intake design, location of fuel injectors, rotating mass weight, etc etc. It's a totally different design that has taen 20+ years of engineering to master.

AFZ · Mar 16, 2016

Mustang5L5 said:
IMHO you are stepping over dollars to save pennies. Putting together a $10K motor to bump fuel economy doesn't make a whole lot of sense when you consider the primary reason you build such a motor is power.

I'm not seeing it. Building an engine is building an engine.

Mustang5L5 said:
Yes, you may gain fuel economy as a result, but how much are you really going to save given the cost of assembling such a beast.

How much am I going to lose?

If I'm building a 351W, I'm building a 351w.

Having had cars that get 10-13mpg I can affirm it totally sucks. Not even from a financial stand point, but just the endless nagging of refilling it several times a week. You also become acutely aware of the how much you are spending when it's like "oh, I want to go there this weekend..... And that will be $30 in gas"

There is a substantial difference between 12mpg and 18mpg. Mind you, the most fuel efficient car I've ever owned averaged only 24mpg. I'm not looking to build a 30mpg sipper.

I look at the LS engines and on paper they are damn close to being a modified 351W.

Going back to my original statement. All the 351W builds I've read about are big cube, carbureted beasts pushing Powerglides and AODs. That is something I'm not interested in.

Rather, I'd prefer to build a 351w with aftermarket al. heads, port injection al. intake manifold, verticals throttle body, knock sensors, distributor-less ignition, 02 sensors and a full stand alone ECU (MS2 or MS3). This then mates to an overdrive manual transmission, TKO, Built T5 or T56.

I look at Carbs and Dizzies the same way old guys look at coils and EFI.

I'm not worried that spending $xyz to build the above engine does not result in fuel savings compared to just running a stock OEM 351W with a slush box.

I'm curious if that build would put out respectable fuel economy 16-18 town, 20 highway, or if it would be a 8-12 mpg guzzler as well.

Mustang5L5 said:
Not to mention intake design, location of fuel injectors, rotating mass weight, etc etc. It's a totally different design that has taen 20+ years of engineering to master.

None of that sounds particularly relevant. Stock for Stock, sure. But that's not at all what I'm talking about.

AFZ · Mar 16, 2016

stykthyn said:
Better crank to rod ratio, better heads, better cams, it's apples to oranges. The 351w is many things including long in the tooth. It can be a torque producing power house as well, but the simple fact is it will never produce the power levels and maintain street manners that an Lsx or coyote will.

Better cams? Having held a LS1 cam, I don't recall their being anything magical about it.

Better heads? Stock for stock, sure. But are you saying there are no aftermarket 351W heads that can compare?

Didn't someone years ago mount LS1 heads to a 351W?

351W Rod ratio is 1.702:1
351W Stroker is 1.667:1
LS2 Rod ratio is 1.69

...soo, 351W stroker is better?

351W Rod is 5.956"
LS2 Rod is 6.125"
351W stroker rod is 6.2"

stykthyn · Mar 16, 2016

AFZ said:
Better cams? Having held a LS1 cam, I don't recall their being anything magical about it.

Better heads? Stock for stock, sure. But are you saying there are no aftermarket 351W heads that can compare?

Didn't someone years ago mount LS1 heads to a 351W?

351W Rod ratio is 1.702:1
351W Stroker is 1.667:1
LS2 Rod ratio is 1.69

...soo, 351W stroker is better?

351W Rod is 5.956"
LS2 Rod is 6.125"
351W stroker rod is 6.2"

Then build a gas sipper 351
Not sure what else to tell you other than that if it was as cheap/simple as you are wanting it to be there would be a lot more people running those combos. Instead we are seeing coyote swaps for the same price or less as a stroker that are getting mid 20s on the highway and in town manners that would let grandma drive it to church.

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