black_knight

iTrader: (1)

Big cubes for better gas mileage?

Don't laugh!

Okay, this one's going to need some explanation...

I've been spending a lot of time thinking about daily, and-I-do-mean-daily drivers. One element of what constitutes "streetablility" that I hadn't really thought about is gas mileage. If you're getting less than 14 MPG, for instance, then that can get expensive. It can also make driving across country troublesome, since constantly stopping for gas will eat up more time than you might think.

(as an example of this, I drove to LA from Phoenix last summer in the Grand Cherokee. On the way there, I went ~70-75 mph, which is the legal speed limit. I got ~19-20 MPG and had to stop for gas about 3/4 of the way. On the way back, I set the cruise control to 65 and kept it there. Got ~28 MPG and didn't have to stop for gas at all. I made much better time going back, even though I was going slower, because I didn't have to stop for gas.)

So this got me thinking about modding my 'Bird, and what kind of cubes I might want to put in there. Crap, I thought, I'm already getting 16-18 MPG with my 347, and that's with mostly highway and just about no stomping on it. If I go for a 402-427, how much gas will it eat, and will it really still be daily (and-I-do-mean-daily) drivable at that point?

But the thought occurs to me:

Once you go max effort with a cam, gears, etc you really start to suffer in terms of gas MPG.

So here's the big question: By going with bigger cubes, is it possible to actually average better fuel consumption since you can use a milder cam/tune?

I mean, hey, the '06 ZO6 has a 427 and it's rated at 16 city 26 highway, if that is to be believed...

(P.S. yes, I am aware of a thing called the turbocharger. Let's leave that out of the equation for now)

spy2520

iTrader: (3)

i've been wondering the same thing, a high rev, or alteast decently modded 346 will suck gas. but the general assumption is that bigger cubes get worse mileage. i always thought gas mileage had something to do with power to weight, like the z06 has 500hp and only weighs 3000lbs.

i wonder though if it is also based on the effort of the motor, like a high effort smaller cube motor will probably have better gas mileage than a comparably equipped (yet obviously more powerful) larger cube motor, but maybe a mild setup on a larger cube motor will net more power and better mileage than a real high strung small cube motor...i have no clue though...

black_knight

iTrader: (1)

Sure, sure. Weight, drag coefficient, rolling resistance, gearing, and lots of other fun stuff. But for this question I mean all other things being equal.

v8bug

from what I hve heard (which may be a bunch a BS) A larger will have to work less to maove the same amount of mass than a smaller engine and will get alittle better milage. A friend of mine is looking to put a 440 in place of his 318 in his 84 2wd ram pickup 4spd, the 318 is getting about 8-9mpg, probibly does not help that it is carbed though.

11 Bravo

iTrader: (3)

I drove my car, with the engine brand new and not broken in, on a 508 mile trip from NC to NJ cruising at 1500-1800 rpm in 6th gear most of the way (not recommended, took forever for the rings to finally seat). Had to fill up the tank once, and had 1/4 of a tank left. I think the average was 24-25mpg.

06 SS

It depends. If the VE at the cruising RPM of a bigger engine is substantially better than the VE (again, at cruising RPM) of a smaller engine then, that might offset the increased fuel use of the bigger engine (ingests more air and the O2's are trying hard to maintain the same a/f ratio at cruise). And even if that is the case, it would likely be only at a very narrow RPM band and under certain road conditions. Short answer is that it is theoretically possible, real World answer is not bloody likely.

JL ws-6

iTrader: (50)

can it be done, yes. But, look into the "lean cruise" that the aussie's have in their computers, I don't know if a computer change would be nessassary or if it'[s a programming thing, but what it does is if the engine is at a steady speed with X% of load for an amount of time, it will kick into lean cruise and get about 3 to 4 mpg better economy, but the us gov. doesn't like it because it up's the nox emmissions..... but if you're building a bigger CI motor you probably have no concer/cares for that like most of us on here.

I'd look into that, that, along with a stockish gearset like 3.42's with a 6 speed, and I bet a big CI motor could do pretty good on gas.

treyZ28

RPM and piston velocity are real killers for mpg. your friction really kills some gas.

My 6.0 gets better gas milage than my 4.8. why? its a heavy truck and the 4.8 was always downshifting. the 6.0 doesn't even unlock the converter. the 4.8 would need 4.10s- at least- to cruise the way the 6.0 does.


BUT- with bigger engines come higher...well lots of things
bigger bores are notorous for sucking down fuel for a number of reasons. I wont get into that.
bigger cubes = more pumping loss, bigger valves, etc etc.
bigger stroke = higher piston velocity for a given rpm

Another huge factor is load. Higher load = higher efficiency. a 750hp 10 liter engine isn't going to be under much load coasting at 55mph.

best fuel use = high load, low rpm
big engines lend themselves to low rpm and low load
small engines lend themselves to high rpm and high load.

Like life, it's all in a balance

LostCauseZ06

iTrader: (4)

i still get 31 MPG in 6th gear lugging it on the freeway with my 408 and monster cam...

black_knight

iTrader: (1)

Cool. What speed were you going? That's.... 70ish?

black_knight

iTrader: (1)

I hear that the Fbody computers won't do lean cruise, so that might be needed. Not sure how possible that is; to swap computers.

Naturally.

Yes, stock gears are staying put.

black_knight

iTrader: (1)

Huh. High load increases fuel efficiency. That's sort of counter-intuitive. Any idea how that works?

black_knight

iTrader: (1)

That's encouraging, but then again 'vettes do have a better drag coefficient. What speed? Also, is that measured by trip computer or calculated by the number of gallons you put in the tank?

black_knight

iTrader: (1)

I assume that VE (volumetric efficiency) is a function of airflow (i.e. the heads, intake, good exhaust/headers, etc) and cruising RPM airflow means you would need heads set up for "streetablility;" I.E. what the AFR 205's are to the 346 (not too big of runners, good velocity, etc). Anyone, feel free to correct or expand on this...

Tony Mamo @ AFR

iTrader: (13)

Guys,

Black Knight PM's me because of a comment he saw my post in another thread....asked me to chime in here if I had the time.

The quick answer is that all things being equal, a big cube engine will get slightly worse economy (on average) for a few reasons....they have bigger pistons and larger rings which will create slightly more internal drag and friction cruising down the freeway at the same RPM. Also, additional stroke cause more angularity of the rod creating slightly more sideloading of the piston also increasing friction a small amount. The other factor is that if your driving a larger cubed engine you may be tempted to use it more where it will obviously have an appetite for more fuel.

BUT....the differences here wouldnt be dramatic if both combinations were driven extremely conservatively....with the larger engine you could also install taller gearing in the car to bring down cruise RPM and still have enough grunt to propel the car without having to kick it down a gear, but once again, the same combo (gearing) with the smaller engine cruising across the desert (HWY run) would still be more fuel efficient.

I say screw the fuel economy....build the bigger engine....its not like the 1-2 MPG difference will change your life anyway.

black_knight

iTrader: (1)

Thanks Tony.

If it's 1-2 MPG loss then eff it. Go big.

-----------------

One point I was thinking of in the big vs small thing is that right now, with the 347 and a cam, my idle is set pretty high. (~1000, up to 1200 if I neutral it while moving) Tuner said it had to be, if I didn't want to foul the O2 sensors. I figure a big engine with a mild cam wouldn't get this problem. I'm under the impression an idle like that eats a lot of gas. Is that correct?

spy2520

iTrader: (3)

so the size of the boat does contribute to the motion in the ocean lol...

treyZ28

efficiency- not consumption.

think of it as buying in bulk

gun5l1ng3r

I still don't understand...

The only way I can imagine it is that at high load, your throttle blade will be open further, creating less of a restriction and increasing VE(just a guess).
But you also said that you need high load at LOW rpm's.

So if you could have a car at full throttle at 1800RPMS in 6th gear you could gain mileage as compared to a 'regular' engine where the throttle is only open part way at 1800RPMs.

In another thread where Solenoid activated valves were being discussed someone mentioned how beneficial it would be to have an infinitly variable valvetrain, thus allowing the throttle blade to be eliminated and the engine could theoretically be at 'full' throttle all the time.

I could be totally wrong though.

spy2520

iTrader: (3)

the more open the throttle is, the more air the engine is ingesting and therefore more fuel...when the engine is not under load i.e. at idle, you are technically getting zero mpg, that would be my guess...