Big Block vs Small Block ref: fuel economy

Those big blocks are big for a reason. They make a lot more HP and torque than a small block.
Granted as you fellows have pointed out, a lot of the time you don't need it.

For those instances when you do, big blocks are priceless.

Put on an aluminum intake and heads and where did the weight differance go? Oh, and you picked up about another 75 HP to boot with a good carb. :thumbsup:

I have the Big block Ford down pat, now it's time to look into the 454 to see what we can do without killing the Bravo 2. You can make HP and be thrifty with fuel at the same time with the right balance of parts.

I'll vote big block, "just in case".
Great discussion
 
Thank you all for the input. My thoughts at the beginning of this thread were geared as a diesel tech thinks "turn slow and make torque". The big block is indeed capable of this over the small block. The increasing internal friction loss as rpms go up would seem to balance the rat/mouse engines when it comes to fuel efficiency.

An interesting point surfaced partway through the thread about the small block making more efficient hp "a better bsfc", even when comparing two carbed engines. I suspect this would be due to the smaller intake path, increased air speed, turbulence and mix.
 
I'll throw this into the mix...
Engines are big air pumps basically. For fun let's compare a 5.7 GM 350 to a 7.4 GM 454. We see the 350 pass 354 ft2 per minute at 3500 rpm. The 454 does the same at about 2700 rpm. Assuming the A/F ratio is the same on both engines and the volumetric efficiency is basically the same... the 454 isn't making as much power as the 350 at the 800 rpm disadvantage... or should I say with a level playing field. In this case the 350 wins.
I realize every installation is different but check this article out...
 
Last edited:
Interesting reading but,
the 454 should have had a bigger prop for this test to make any sense at all.
This tested a 454 spinning a 350's gearing and prop...
Lets try a 350 spinning a 454's gear and prop and see the results then...
I dont think the test has any point. I could be wrong.
 
Air O'Nautical said:
Interesting reading but,
the 454 should have had a bigger prop for this test to make any sense at all.
This tested a 454 spinning a 350's gearing and prop...
Lets try a 350 spinning a 454's gear and prop and see the results then...
I dont think the test has any point. I could be wrong.
Click to expand...

I would also add that the extra 1,000 hrs of life you get out of the big blocks running at 2,700 rpm versus 3,500 rpm out of the small blocks. Our practical experience running with same type boats with small blocks is that to run at good pace and keep up raises the rpm's and will get into the secondaries which burns more fuel yet and shortens life expectancy.
 
I just had a funny thought. Whom in their right mind would buy a boat like these then think about fuel economy?
Sorta goes against the grain. :eek:)
I agree with the posters that point out that there are many times when a small block is stressed that the big block is
not.
A big block with a problem will still probably get you up on plane and make it home. The small block,
probably won't make plane.
This is assuming a miss on one or two cylinders.
JMO
 
Here another funny thought for you to ponder .I've got a 390 with 7.4's I was going to buy a trawler,but didn't like the lines and I like a v hull boat so I ended up with a 88 390 I run the boat at hull speed I am very happy with it 'nice slow,heavy,stable ride (9mph @ 1350 RPM. My question is ,being that that the engines are not under a real load why could I re power with 350 EFI or 302 EFI small blocks,and save fuel? I don't see why it wouldn't work and work well. Get the right bell housings and bolt up to trans. that are in the boat,would have to re prop,but look at the weight I would lose an extra room in the eng room,and yes I'am in my right mind.Next year at this time there is going to be lots of people out there out of there minds over fuel prices, believe me I can afford fuel ,but I can't see giving it to the rag heads and the fools in Washington if I don't have to.Two new Mercs should help the people in the US that build them.Do you think that there would be any fuel saving to amount to anything? Yea or Na.....Rip
 
No - not compared the lost value in the boat. Are these motors ok? If so, leave well enough alone. Need to save gas - great, go slow. Need to dodge a storm? PUSH IT UP! Need to sell the boat? You're set for that too.
 
OldSkool said:
I believe it comes down to weight, power and gear options. Stroker small blocks make great low end power (torque) and do relatively well and high RPM. Stock small blocks make great power (torque)at higher RPMs. Big blocks make great torque at low RPM and sustain it through the RPM range to there Max rpm. This is channeled throught the gear which should be matched to the motor torque power band and loaded boat weight.

Wheel has much to do with this also so just looking at the engine is really like asking how long a piece of rope is. You need to do the math on all the aspect of the application.

Just my opinion but it's worked every time I've applied it.
Click to expand...


Ruh Roah...Not only engineering...Logic as well.

IMO, it's a function of all the variables.

Saint Max (6.2L Horizons) burns ~22GPH at 22knots fully loaded. I now run 3450 RPMs. Before the tab modification, I would suspect she burned close to 28 GPH at the same speed at 3800 RPMs.

The biggest issue with newer small blocks is the human learning curve. The small block stroker engine is built and meant to put out a lot of power at a higher RPM than a heavier older big block.

The one HUGE factor for my running at the RPM I do is NOISE. The difference in noise between 3450 and 3800 is deafening. I wish I had flow meters. I would be interested to see what the MPG was at 3800 with the new tabs. I know the speed is upwards of 28 knots.

If the lottery numbers and the stars fall into alignment, there is a fellow in R.I. getting 420 HP from a NA GM 6.2L stroker. He has put them in 330's with great results as they are a bolt in for the 6.2 which is a bolt in for the 454.

They are not cheap mind us. He was looking at bringing them to my bilge for roughly 13K per side using my engines as a trade, installed. Not bad really. The power curve I studied brought 300HP to the shafts at roughly 2500 RPM.

To review the bidding....well over a 30 knot cruise at 2500 RPM (less noise) based on my current configuration and God knows what is on the top end.

OH...did I mention they are 400 pounds lighter per side?!
 
Strokers would make sense but at a big price. Why not use the big block with aluminum heads and intake? Remember the stroker exhaust plays a huge role in the HP curve too. If you make a big small block you lost your fuel milage anyway as per the flow rate again.
Oh and one other thing, if the manufacturers put in only big blocks, you might ask yourself why?
My jet boat has a big block would the small block work better and survive? NOT.
JMO
 
Last edited:
Keep in mind too that higher rpm does not mean reduced engine life. The opposite holds as much truth. An engine running 'happily' at 3500 rpm will way outlast an engine turning under 3000 that's lugging. This fact shows us how important prop selection is. An engine can only accomplish what a prop will allow it to.

Riptide makes a good point that I've seen put into use with great success. If you don't plan on cruising past the hull speed... installing a smaller engine works very well. No, the engine won't achieve it's wot number, however, the engine is not overworked at hull speeds. The boat glides through the water easily so you don't need big cubic inches to do that. You can use an engines dispacement to compare fuel numbers. In it's most simplified form; a 305 is about 67% of a 454... 33% less. There's your reduction in consumption (roughly). It may be a bit more since smaller engines are more efficient at lower hp levels. I think, because of fuel prices of course, we'll see a LOT more of this type of install in the coming years. Take it a step further... if you only need 150 hp to hit displacement speed of the 340... 2 75 hp diesels will knock your fuel down to a gallon an hour... maybe that's getting a little too far ahead but I don't see fuel prices ever going down.
 
Ok here is some food for thought.. 280DA 496 MAG, 280DA twin 4.3's Alpha drives, 300DA 5.0's Bravo 2, 300DA 5.7 Bravo III's we all took a trip to Cincy ( from Chas. WV) and at most fuel stops we were all using about the same amount of fuel. How come ????
 
Magster, I had given that some thought,but was concern about a need for power,say coming in a inlet. I had a chance to buy a 32 bayliner that had been changed over to 2- 75 hp diesels. It was a nice set up,there was one guy ahead of me that had put up a deposit and he got the boat .Granted that is a smaller boat,but a couple of 100hp should do the job if the 75s didn't . Doing that is going to involve trans,shafts cutlass bearings ,maybe not shafts and cutlass bearings .I think that if the engs worked out it would increase the value of the boat Not that I would want to sell it I like the old style hull and deck design and I would have the set up that I want.Like I said before this fuel thing is going to be a big thing next year about this time,It's not a game changer for me ,but it doesn't hurt to think ahead....Rip
 
wvsundancer said:
Ok here is some food for thought.. 280DA 496 MAG, 280DA twin 4.3's Alpha drives, 300DA 5.0's Bravo 2, 300DA 5.7 Bravo III's we all took a trip to Cincy ( from Chas. WV) and at most fuel stops we were all using about the same amount of fuel. How come ????
Click to expand...
I would think the lone big block would be the obvious choice... Who’s carbed and who’s fuel injected?
 
wvsundancer said:
Ok here is some food for thought.. 280DA 496 MAG, 280DA twin 4.3's Alpha drives, 300DA 5.0's Bravo 2, 300DA 5.7 Bravo III's we all took a trip to Cincy ( from Chas. WV) and at most fuel stops we were all using about the same amount of fuel. How come ????
Click to expand...

280 with a Single Gas Hog.
280 with very efficient engines, but 2 - more weight, more drag, less work for each.
300 with V-8's - should use a little more gas, but, depends on injection systems...
300 with bigger V-8's, but the most efficient drives. Should use a little less gas than the 300 above.

I can see them using about the same gas.
 
You must log in or register to reply here.

Forum statistics

Threads
113,241
Messages
1,429,111
Members
61,122
Latest member
DddAae
Back
Top