Big Block vs Small Block ref: fuel economy

BonBini

BonBini

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Sep 28, 2009
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Great Lakes
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This seems to come up a lot when a member is looking to purchase. Technically speaking I’d like to hear your thoughts on the matter.

My thoughts: When it comes to producing prop horse power, the bigger displacement engine will typically do so at a lower rpm. Therefore two comparable hulls crusing at similar speeds with big block vs small block power I suspect would be very similar in fuel economy!
 
The two comparable hulls crusing at similar speeds with big block vs small block power I suspect would be very similar in fuel economy![/QUOTE]

I believe your thoughts are correct. That is AFTER reaching plane. Fully loaded boats being equal the small block will be left floundering while the big block is GONE.

Two lightly loaded boats when all things are the same will probably be close in fuel consumption while cruising.

Loading up the boat with the small block is the game breaker.

IMO
 
I concur with the above comments. The larger engines tend to be geared and propped differently so your speed is at a lower RPM on the larger engines resulting in similar fuel economy to the smaller engine running at higher RPM.
 
While both are big blocks, my 8.1s do much better on fuel than my 454s did on a much lighter boat. The 37 weighed around 14,000 pounds and the Tiara is around 19,000. The 454s produced around a half mile per gallon or slightly better. The 8.1s get about .9 MPG.
 
sbw1 said:
While both are big blocks, my 8.1s do much better on fuel than my 454s did on a much lighter boat. The 37 weighed around 14,000 pounds and the Tiara is around 19,000. The 454s produced around a half mile per gallon or slightly better. The 8.1s get about .9 MPG.
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Interesting point. Perhaps the old school 454 wasn't as efficient in the combustion chamber as the 8.1 (502??). I personally never really looked at a 8.1 up close. Maybe has a better designed intake? Are your 8.1s carburated? If carburated, are they running something other than a q-jet? Now my curiosity is up.
 
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.
 
BonBini said:
Interesting point. Perhaps the old school 454 wasn't as efficient in the combustion chamber as the 8.1 (502??). I personally never really looked at a 8.1 up close. Maybe has a better designed intake? Are your 8.1s carburated? If carburated, are they running something other than a q-jet? Now my curiosity is up.
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The 8.1s are MPI and make a lot more horsepower than the 454s. 385HP vs. 330HP. Not sure how they make the extra 55HP per engine, but I believe the efficiency is due to the Tiara's more efficient hull and the extra HP of the 8.1s. They move the boat without effort, very quick to plane and RPMs can be backed off while cruising without much loss of speed.
 
BonBini said:
This seems to come up a lot when a member is looking to purchase. Technically speaking I’d like to hear your thoughts on the matter.

My thoughts: When it comes to producing prop horse power, the bigger displacement engine will typically do so at a lower rpm. Therefore two comparable hulls crusing at similar speeds with big block vs small block power I suspect would be very similar in fuel economy!
Click to expand...

My thoughts are the same.

While having 320 with small blocks (5.7L 350MAGs MPI) I kept an eye and talked to the owners of similar models (320s and 340s) with larger engines. In the end we're all chasing the same magical number (~0.8MPG). At the begining of the season with clean bottom boats can achieve .9mpg, but later in the season a lot of boats might start seeing this number go down. Even though my bottom is always clean, the slight layer of slime slows me to the avg of .8mpg.

Having said that I think it's always nice to have spare HPs, but it's really not necessary to chase the bigger engines as long us the smaller ones are good match for a model. In case of my 320, after 4yrs of ownership I think that those engines are a great match.
 
My take is.....Bigger always uses more! My two friends have a 270 Bayliner with a 350 OMC and the other is a Sea Ray 268 with a 454 Alpha1....the 454 uses more fuel on all trips we take. That's just a fact with these two carbed engines! Another example....a 350 in a 23ft 3,000lb boat will always get better mileage than a 454. If you cruise both engines at 2800rpms, the 454 will always use more fuel. The 454 will go about 10mph faster with the additional 70hp but always use more fuel even covering 10 more miles per hour. Even at idle the big block uses more fuel!....It's just physics. If you increase the boat length & weight to 27ft/7,000lbs, the 350 will need full power to perform well and the 454 may only need 3/4 power but will still use more gas. The extra 70hp just uses more fuel with the additional weight of the engine and bore size. My 270/454 B1 performs best at 3800rpms and I know I'm into the 4bbl a bit. The difference in that vs running at 1400rpms is huge in fuel use. A Bravo III does help get more mpg than a Bravo 1 due to more contact surface with the water....better grip. Fuel injection is also more efficient than a carb.....kick in the 4bbl and cut your mileage in half! My friends 2005 260 Sundancer FI 350mag BIII gets more mpg than my 1990 270 Sundancer 454 4Bbl B1. I may run a bit faster but the 4 Barrel is a killer on mpg when I'm into it. We are being forced to sell ethanol gas this summer at our gas dock so I'm sure the boats will use more of that crappy gas than the good old stuff....my vehicles have all lost 2mpg with ethanol crap....another federal govt blunder at our expense, Mike.
 
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I have a friend with a '99 260DA with a 454. I have the 5.7 EFI. He and I cruise at the same speed(right around 25 knots). I'm running around 3500 RPMs and he's around 3000. He burns 13-14 gallons an hour at this speed and I'm between 8-9.
 
Nehalennia said:
I have a friend with a '99 260DA with a 454. I have the 5.7 EFI. He and I cruise at the same speed(right around 25 knots). I'm running around 3500 RPMs and he's around 3000. He burns 13-14 gallons an hour at this speed and I'm between 8-9.
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This is probably the best example of this thread, so far. If you compare big vs small block applications, it needs to be an apples-to apples comparison. In this example, the big block motor is not working as hard, but is probably capable of a lot more speed and HP than the smaller engine. Obviously, the big block motor has more torque and is able to spin a larger prop (diameter and/or pitch), and more likely has a steeper gear in the outdrive. Bottom line; there is no free lunch. Every application has its compromise. In this case the small block cruises in tis ideal power band and operates more efficiently than the big block. The rat motor can power the boat faster because if its reserve horsepower, but there's a cost that goes with it.

In this same sceanario, if you added 2,500 pounds to both boats, the big block motor might cruise more efficiently.

Don
 
I think everyone nailed it in this thread, though some comparisons weren't perfect. If you are going to use the power of the big block - get it. If you aren't, then don't get it. No one has mentioned the extra weight you're carrying around with a big block - in the aft of the boat - not fun. You have the power to overcome the weight, but you pay for it in gas.

The only time that a big block will be more efficient is in similar conditions with the small block overworked (think heavy, high speed). Another HUGE consideration is engine life. If you're going to be slammin' the throttle to the firewall with the small block for 30 seconds to get up on plane and running it hard to get where you're going, the money saved in fuel will go to your mechanic and local Mercruiser dealer.

I considered all of these thoughts 14 years ago when I ordered my '97 250 from the factory. We had 7 engine choices - they were all wrong. We had them install an 8th choice - 5.7 EFI LX. Didn't want to carry the weight. Had to be EFI. Wanted the extra power when needed. Wanted the efficiency the rest of the time. It was perfect. 2.5 mpg.

If you ever run into a '97 250 with that engine, I owned it for 10 years.
 
Before anyone get's too hung up on HP ratings, the real story is told with torque numbers at specific RPM's. I know even with the 8.1's the HO 425HP version only gets it's higher rating through higher RPM WOT (more agressive cam and different computer mapping). If you look at the torque numbers between 3000 and 3500 RPM, I recall that the "higher output" engine actually produced a bit less torque at the lower end of the RPM scale than the base engine.

If your boat is heavier, or the weight is more aft than most, or you have tunnels, or inappropriate gearing, (and on and on...) you may actually need that big block just to properly power the boat without constantly going past the 80% WOT line and chewing up your engines.

The only true way to answer the big/small block question is to have two identical boats tested with the exception of the power choices. There's just too many variables.
 
The extra 70hp just uses more fuel with the additional weight of the engine and bore size.

I thought I said that? Just busting on you hampton, Mike.
 
CV-23 said:
The extra 70hp just uses more fuel with the additional weight of the engine and bore size.

I thought I said that? Just busting on you hampton, Mike.
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I guess you did I guess you did. (I did say everyone nailed it).
 
Nehalennia said:
I have a friend with a '99 260DA with a 454. I have the 5.7 EFI. He and I cruise at the same speed(right around 25 knots). I'm running around 3500 RPMs and he's around 3000. He burns 13-14 gallons an hour at this speed and I'm between 8-9.
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But when the time comes I'll beat you to the anchorage and get the best spot. :grin:
 
You get about 3.625 MPG. OK, who's towing you?
 
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