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Big block vs small block fuel consumption

Started by _Bomber Watson_ , Nov 12 2012 09:03 PM

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14 replies to this topic

#1 _Bomber Watson_

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Posted 12 November 2012 - 09:03 PM

In a curious mood tonight, and this ones been bugging me for a while.

Everyone, as soon as you mention the word "big block", instantly states the fuel consumption will be absolutely terrible.

But upon pondering this in my mind for quiet some time now, I cant quiet understand why.

im sure in standard guise, straight from the 70's, theres a marked diffference between a big block and a small block, but i cant understand why a big block will, apparently, always chew more fuel.

Lets set up a few nice little scenario's.

I quickly selected two engines, based on basically nothing bar both being Chevrolet one being a 350ci chev and the other being a 427 chev and the fact that they both make near on the same peak HP and torque.

Here are the engines:

350:
http://www.summitrac...45910/overview/

427:
http://www.summitrac.../make/chevrolet

Sadly these engines i picked dont list the rpm at which peak HP and torque are achieved, i was trying to find two which did list these but i couldnt find any in the same kinda hp/torque ballpark.

But basically we can assume roughly that the Big block being 22% larger in capacity will make its peak torque 22% lower than the 350, i say roughly and assume, remember.

Now lets throw a few constants in. Both engines are tuned for good efficiency within reason, exhaust system is designed around promoting the area around max torque etc.

The car will weigh 2000kg with the small block. Obviously the big block car will weigh a little more, About 2050kg. I couldn't find a reliable weight figure for the two engines but it seems a BBC is about 50kg heavier than a SBC... I think the actual type of car is irrelevant for this. Lets assume its teh same car in both situations so same frontal area same drag coefficient etc etc.

Both cars will run the same 5 speed manual. Both cars will have the final drive ratio tailored to suit the particular engines characteristics.

Everything else on the two cars will be identical.

So, explain to me, please, why the big block powered car will have so much worse fuel economy?

I can see a little bit of difference, Mainly extra friction due to the larger bore, more ring contacting = more friction, larger bearings, and the extra weight.

I also think it may have something to do with the size of the inlet ports, BBC's seem to run larger valves and inlet port volumes than SBC's of comparable HP. This would effect efficency by reducing air speed i think.

But why the massive difference? The way people rant on you would assume that the BBC would consume up to 10 litres per 100km more fuel, but i'd be surprised to see, in this instance more than 1lt per 100km difference?

Around town? Bit more, on the highway, negligible?

Now for fun lest stick a 2000kg trailer on the back of our sample car, how would this affect the difference?

I think the BBC would always last longer, being a bit less highly strung, assuming the same consideration was put into the engine building.....

Im just curious about this, please, discuss.

Cheers.

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#2 TerrA LX

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Posted 12 November 2012 - 09:07 PM

Topgear tested a Toyota prius against a BMW M5 on the track, the Toyota was driven flat out around the track and the BMW was paced behind it, guess who used less fuel?

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#3 _Bomber Watson_

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Posted 12 November 2012 - 09:10 PM

Yep, the M5 used less. This is my train of thought in this case to.

To expand upon that slightly, i just noticed that the SBC in this case uses 1.94 in. intake, while the big block uses 2.190 in intake. Considering they both make the same HP and torque i think the larger inlet valves, and generally fair bit larger inlet port, and inlet manifold runners, would be a factor, especially around town?

Cheers.

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#4 _Quagmire_

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Posted 12 November 2012 - 09:11 PM

if you where driving like miss daisy you'd get close to the same fuel figures
as you wouldn't be reving the big block as much for the same performance
i know i could get better economy with my 253 powered hj than my 202 powered ones if i tried
towing would be a no brainer....

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#5 TerrA LX

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Posted 12 November 2012 - 09:15 PM

More torque at lower RPM would require less throttle opening.

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#6 dattoman

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Posted 12 November 2012 - 09:22 PM

Theres no way my 500 cube Caddy will use less fuel than a comparable small block
But thats just the way I like it

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#7 _Bomber Watson_

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Posted 12 November 2012 - 09:25 PM

But that engine is still ala 70's technology, do you think that if you performed a few select modifications in your particular vehicle you could get the same performance and mileage as a suitably worked small block in your particular vehicle Neil??

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#8 _Shaune_

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Posted 12 November 2012 - 09:35 PM

I guess it comes down to how much fuel the larger capacity engine sucks when you hit the accelerator. On highway it is fine, but in the city different story.

I guess that’s why its called average fuel consumption – over a variety of driving conditions.

When I accelerate in my VZ Monaro, the fuel consumption goes through the roof, although on the highway it’s not too bad. A Barina on the other hand, accelerating doesn’t consume as much a 6L engine constantly accelerating, however on the highway its reving like no tomorrow….

but again we're not talking about similar enagines and talk as Bomber outlined....

torque even..

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#9 _Bomber Watson_

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Posted 12 November 2012 - 09:37 PM

I drive like miss daisy around town, Usually have pensioners overtaking me.

Suppose i should have added driving style as a constant in the above thing to.

Cheers.

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#10 _Shaune_

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Posted 12 November 2012 - 09:40 PM

As long as they're not over taking you driving their zimmer frames...

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#11 _Viper_

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Posted 12 November 2012 - 10:59 PM

Theres no way my 500 cube Caddy will use less fuel than a comparable small block
But thats just the way I like it

If you had a 350 Chev in your caddy would it not be under a lot of load and need to be pushed a lot harder to pull the weight compared to the big block doing it with relative ease?

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#12 _oldjohnno_

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Posted 13 November 2012 - 05:03 AM

The biggest factor in fuel usage is combustion efficiency - the port and chamber design will have a much greater influence than anything else. A long time ago I had an oval port 454, dual plane, 3310, 3.55s in a 1750kg car. It'd give about 18mpg cruising at 70 mph. Not bad but a mate had a 455 Buick in a Riviera that did mid 20s mpg. The Buick engines were miles ahead of anything else at the time chamber wise, and needed much less advance than the rat. Much lighter than the bbc too, a very overlooked engine. There's no reason why a modern big block couldn't give comparable mileage to a smallblock, I think though that some of those old bigblocks (like the closed chamber rectangular port BBCs, among others) gave them a reputation for being thirsty.

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#13 _Bomber Watson_

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Posted 14 November 2012 - 08:12 PM

I like Buicks

So do you think the comparitively larger inlet port and larger inlet valve, even with the two engines listed making the same HP/torque would be a factor?

After all the smaller SBC valve can obviously move enough air to make the power....But then i suppose its opening and closing more frequently than the BB one....

Cheers.

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#14 _oldjohnno_

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Posted 14 November 2012 - 08:28 PM

Provided you use appropriate heads I don't think it would matter whether big or small block. Personally I'd always go for the little one if it would do the job, unless it was in something like a truck with a huge engine bay. The little ones are just much nicer to work on, you have more room for headers, not so many dramas fitting them around the steering shaft etc and you can change the plugs comfortably etc. Plus they're a bit lighter.

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#15 dattoman

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Posted 14 November 2012 - 08:47 PM

Plenty of room in my engine bay

And if I swapped out the cast inlet manifold for alloy and put alloy heads on it the SBC is allegedly heavier

500 cubes for max torque
Very few stock engines (comparable petrol) can compare to a Caddy donk