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Donzigo
01-06-2008, 06:24 AM
I replaced the 7.4 454's last July with new 496HO's and "X" outdrives.

I suspect that the new packagre is getting about twice the fuel economy.

The new engines seem to be lighter as well.

It's curious that the boat takes longer and is harder to get on plane now, as well. I suspect that has something to do with the weight and the way the new outdrives interact with the water.

FYI - I kept the same propellors - (23 inch pitch)

Lenny
01-06-2008, 09:54 PM
Top speed better or worse ?

WOT rpms and GPS speed vs/pitch. What is the slip now and what was it then?

This could be interesting. :yes:

RedDog
01-06-2008, 10:01 PM
The 496s are fuel injected. Were your old 454s carbed? That is the fuel mileage difference.

Not sure why the difference on time to plane though...

You say new outdrives - what is the difference? That is likely the planning factor. Different X-dimension?

yeller
01-06-2008, 11:22 PM
The new engines seem to be lighter as well.
It's curious that the boat takes longer and is harder to get on plane now, as well. I suspect that has something to do with the weight and the way the new outdrives interact with the water.
This one confuses me. How could less weight in the rear cause the boat to take longer to plane?

osur866
01-07-2008, 04:20 AM
This one confuses me. How could less weight in the rear cause the boat to take longer to plane?
I'm thinking the drives aren't the same length.

harbormaster
01-07-2008, 05:28 AM
Silly Rabbit...

Bravo 1, 1x, 1xz, & 1xr drives are all the same length...

The only time a Bravo drive is shorter is when you put a short sportmaster or IMCO shortie lower.

MOP
01-07-2008, 06:18 AM
There is not that much difference in fuel economy between carbed and FI engines, the biggest difference is when you advance the throttle to accelerate. FI meters the fuel where as a carbed engine gets a blast from the accelerator pump, once at a set speed they burn the same amount. If they were to run leaner they would very quickly destroy themselves. The stop and start crowd will see the biggest savings but the longer range cruisers see very little. Being a boat broker and a wrench head I have had the opportunity to ask many owners and clients over the years. You can argue the point but the the fuel to air ratio must be matched to the load or the engine will either burn its pistons or load up that is just a simple fact.

Phil

BUIZILLA
01-07-2008, 06:24 AM
FYI - I kept the same propellors - (23 inch pitch) :confused: didn't you gain like 100hp per side ?

blackhawk
01-08-2008, 08:02 PM
:confused: didn't you gain like 100hp per side ?

Yes but he also now has motors than run at a higher rpm.

RedDog
01-08-2008, 09:49 PM
There is not that much difference in fuel economy between carbed and FI engines, the biggest difference is when you advance the throttle to accelerate. FI meters the fuel where as a carbed engine gets a blast from the accelerator pump, once at a set speed they burn the same amount. If they were to run leaner they would very quickly destroy themselves. The stop and start crowd will see the biggest savings but the longer range cruisers see very little. Being a boat broker and a wrench head I have had the opportunity to ask many owners and clients over the years. You can argue the point but the the fuel to air ratio must be matched to the load or the engine will either burn its pistons or load up that is just a simple fact.
Phil

Your argument seems to make much sense, but why is Donzigo now getting twice the mileage?

gcarter
01-09-2008, 04:45 AM
EFI engines can and do get much better mileage as a rule.
W/electronic controls and lean burn technology, F/A ratios can exceed 20:1 in some cases.
As an example, the article about 496 build-ups Poodle posted a few months ago in the performance spread sheets showed a fuel burn rate of .39 lb/hp/hr......that's in Diesel territory. I don't think you can get a carbed engine under .55 lb/hp/hr.

VetteLT193
01-09-2008, 08:14 AM
I replaced the 7.4 454's last July with new 496HO's and "X" outdrives.
I suspect that the new packagre is getting about twice the fuel economy.
The new engines seem to be lighter as well.
It's curious that the boat takes longer and is harder to get on plane now, as well. I suspect that has something to do with the weight and the way the new outdrives interact with the water.
FYI - I kept the same propellors - (23 inch pitch)

I suspect your stern is not dragging into the water as much anymore because of the loss of weight... this caused the boat to be at a more aggressive attitude when initially planing which means the entire bottom of the boat acted like a big trim tab and helped to lift the rear out of the water as speed increased.

Now you are running more level so there is less stern lift at slow speeds. Have you always planed with tabs down or up? If up, try them down and I bet you'll pop up on plane really quick.

Donzigo
01-09-2008, 08:01 PM
Thanks for all the replies.

The boat goes about 10 oe 12 MPH faster. I will put a GPS to it so I can be exact.

I always have the tabs down when planing.

The old engines were naturally asperated......carbs, Holly 600 CFI.

The new engines are sooooooooooooo sweet...........