Big cubes for better gas mileage?
Big cubes for better gas mileage? 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)
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)
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...
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...
Originally Posted by spy2520
i always thought gas mileage had something to do with power to weight, like the z06 has 500hp and only weighs 3000lbs.
On The Tree
Joined: Mar 2006
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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.
Originally Posted by black_knight
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?
TECH Fanatic
Joined: Jan 2007
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From: LITH, IL.
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.
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.
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.
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From: Dallas, North Mexico
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
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
Last edited by treyZ28; Feb 5, 2007 at 02:18 PM.
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Originally Posted by 11 Bravo
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.
Originally Posted by JL ws-6
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
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.
Originally Posted by treyZ28
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.
Originally Posted by LostCauseZ06
i still get 31 MPG in 6th gear lugging it on the freeway with my 408 and monster cam...
Originally Posted by 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
Last edited by black_knight; Feb 5, 2007 at 11:41 PM.
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 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.
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?
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?
TECH Fanatic
Joined: Oct 2002
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From: Dallas, North Mexico
Originally Posted by black_knight
Huh. High load increases fuel efficiency. That's sort of counter-intuitive. Any idea how that works?
think of it as buying in bulk
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Joined: Mar 2004
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From: Laguna Niguel, CA
Originally Posted by treyZ28
efficiency- not consumption.
think of it as buying in bulk
think of it as buying in bulk
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.
Originally Posted by gun5l1ng3r
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.