Steady state fuel economy...carbed V8

[travis]

One of my recent ponderings on my new 91 mile a day round trip commute...

If you have 2 engines built appropriately for good fuel economy at a low cruise rpm (1700 rpm@75 mph in this case...OD and 2.73 gears), one a 302 and the other a 393w, would you expect to see a significant difference in steady state fuel economy? My thoughts are that the low torque 302, besides being smaller and lighter, would have lower internal friction and would require more throttle input to maintain steady speed which would reduce pumping losses. The 393 on the other hand would make significantly more low rpm torque and require less throttle to maintain speed. Thoughts?

[Tom68]

What you said pretty much.

The pumping loss is the vacuum though, not so much the friction.

If they both averaged 12'' of vac 393 ci is a bigger capacity so it'll burn more fuel just to create the same vac. Of course similarly specced it will be fighting even more vacuum to do the same work.

[Ken_Parkman]

Also at a higher throttle and less vacuum the cylinder pressure will be higher which increases effective compression and efficiency. The true "dynamic" compression ratio.

[Geoff2]

Every engine has a 'sweetspot' where the sum of the parts come together for best efficiency. Gear ratio needs to be factored in also. So a larger engine may well give better steady-state economy.

[mag2555]

In theory being that the 393 is some 30% bigger in cid then a 302 then you should be able to spin the 393 to 30% of that 1700 rpm and then have matching fuel economy with the 302 @ the same road speed.

I don't think the 10 lb difference in engine weight is even a factor to consider here.

[travis]

The weight difference is considerably more than 10 pounds...off the top of my head I think it's more like 100-150 pounds but it still may be insignificant for steady state cruise. 8.2" deck vs 9.5" deck.

That "sweet spot" that Geoff mentioned...that's a big one I was wondering about. Most anything that I've paid any attention to in the last quite a few years usually has a cruise rpm in the 2000-2200 rpm range at highway speed, whether it's a V6 or V8, small car or full sized SUV. There must be something to that.

My daily commute is 90-95% 70-75mph highway.

[PRH]

Displacement matters.

Which is why many manufacturers have gone to MDS/DOD systems for better fuel economy....... and why smaller displacement turbo charged engines are becoming the norm.

[mag2555]

Sorry, I was unclear.
I was talking in terms of rotating weight.

[PackardV8]

Also at a higher throttle and less vacuum the cylinder pressure will be higher which increases effective compression and efficiency. The true "dynamic" compression ratio.

The caveat is our OP said "carbed V8" and therein lies the problem. Carburetors and distributors are stone ax technology and do not work well at high throttle, less vacuum. It was only after computer control of EFI, ignition and AT became standard that OEMs could force the engines to lug down to the low cruise RPM we have today.

FWIW, those of us old enough to be there in the day lived with 2500 - 3000 RPM cruise, but to today's young ears which grew up with double overdrive six and eight speed automatics, that sounds disasterously loud and thrashy. The difficulty comes when a customer wants a BigMuthaThumper fairgrounds lope at idle and then wants to cruise in overdrive at barely above that idle. The poor cam and carb are fighting each other all the way.

[jeff swisher]

I have been there with a 302" It would out run my friends 3/4 ton 351 W powered 1984 Van
My 302 had ported heads 260H cam 2V intake and 1984 E150 Van but mine was heavier.

My MPG was 17.6 most of the time his was under 15 and we had the same Trans AOD.

I could not use OD and had 3.50 gears and 29" tall tires most of the time.
OD would lug the engine and more throttle opening just hurt MPG.

Low RPM hurt MPG as that vehicle is a brick and no power to be made down low so as was stated carbs in that situation liked 2700 RPM or more for best MPG in that brick.

I tested many carbs Q jet 390 economaster AFB carbs of many sizes ..many 2 barrel motorcraft carbs.

Once all carbs were tuned in there was only 1/2 MPG difference between all of them with the win going to an Edelbrock 1407.

Took more RPM to get into the mains on that carb.

Engine heat made more difference than carb type or size.
195+ temps turned in best MPG.

I vote smaller engine for the win but if you work it too hard to get 75MPH you will not win with the smaller engine.

I have seen weak engines 5.3 LS in a 2013 truck not get the MPG that my Van with 302 would.
Buddies 2018 Limited ecoboost V6 3.5 twin turbo truck gets 12.1 pulling my Pontoon boat.
24 MPG without it.
My 57 chevy with 350" engine and 3.08 gear NO OD and 750 carb and 280H comp cam got 11.5 with AC on pulling the same Pontoon boat.

Had a 3000 stall converter in mine..So room for improvement.

You can't just lay down engine size and figure MPG in my book.
You have to drive the miles and find out what works and what does not .. just speculation otherwise.

We tried many vehicles pulling my 15 foot bayliner and see which one got best MPG.
Isuzu Rodeo V6 FI 11 mpg.
4 cylinder 22R in a truck FI about burned the thing up went back home abandoned test.

1982 Ford 200" six in a mustang 3.45 gear, C5 transmission recurved distributor and tweaked carb.
27 MPG empty and 24 pulling the Bayliner.
1947 dodge with 350" chevy 3.25 gear TH 350 268H On 112 lSA 102 intake centerline 225 PSI cranking pressure 1407 carb 18 MPG with bayliner in tow.
18 MPG empty also.

Yes I drive a lot of miles and usually had a boat in town on weekends.

[77cruiser]

In my areo brick 77 Monte, the biggest engine got the best MPG by about 1 to 1.5 MPG over the smaller ones.
305 18 MPG
385 18 MPG
421 19+ MPG

[Tom68]

One of my recent ponderings on my new 91 mile a day round trip commute...

If you have 2 engines built appropriately for good fuel economy at a low cruise rpm (1700 rpm@75 mph in this case...OD and 2.73 gears), one a 302 and the other a 393w, would you expect to see a significant difference in steady state fuel economy? My thoughts are that the low torque 302, besides being smaller and lighter, would have lower internal friction and would require more throttle input to maintain steady speed which would reduce pumping losses. The 393 on the other hand would make significantly more low rpm torque and require less throttle to maintain speed. Thoughts?

So looking at all the good answers now it seems for a carb'd application this will come down to the size and weight of the vehicle being used as to which engine will win.

[Truckedup]

Low rpm at speed you risk the vacuum operated enrichment system opening on slight grades with smaller displacement engines...Yes it can be adjusted...but drivability can suffer...

[dannobee]

I see no mention of egr. I do know that a non-functioning egr system will drop the mileage by 1-1.5 mpg in most cars. Like Tom said, get the intake vacuum as close to atmospheric as possible to avoid pumping losses. EGR will certainly help with that.

[David Redszus]

Fuel consumption is the result of two factors; engine efficiency and chassis characteristics.

As far as the engine, the following can be used to estimate engine fuel consumption (lbs/min).

rpm
displacement
air density
fuel stoich
fuel specific gravity
air/fuel ratio

A more accurate evaluation would be dyno testing for BSFC to find the efficiency sweet spot.

But then aero drag, tire size, gearing, rolling resistance and vehicle weight must be factored into
somebody's equation.