350 mercruiser fuel economy

[THEFERMANATOR]

A set of vortec heads and a cam swap could really help out your economy. Pretty sure marine engines in 92 used swirl port truck heads. They're not that great for economy, and IMO have way to low of a powerband to them for marine use. Just be carefull, Alpha outdrives do NOT take kindly to alot of horsepower in heavier boats. A drive shower is a good investment to keep it cool, and run the Mercury synthetic drive lube in it.

[Porscheguy]

For what you’d spend to find a good 4.3 w/vortec heads you could install vortec heads, new cam, intake, and 4 barrel on your engine and push the power to 300-325. The only caveat is vortec heads were designed to run with a roller cam. Your engine probably has the bosses in the valley for the lifter retainer. But it’s unlikely they’ve been drilled and tapped. That would require a trip to the machine shop. Still, you won’t be the first to have this issue and I’m sure there are plenty of flat lifter cams that can work with those heads.

You’re the second person who has mentioned this oddball engine setup. I’m curious to know what gear ratio you have in your outdrive. If it’s a 1.5 that’s perfect for a vortec and 4 barrel. The mods may not improve your economy in terms of your fuel burn rate per hour. What 100 extra horses and a good prop will do is increase your cruise speed significantly so you won’t have to run it as long to get from point a to point b.

[THEFERMANATOR]

Originally Posted by Porscheguy

For what you’d spend to find a good 4.3 w/vortec heads you could install vortec heads, new cam, intake, and 4 barrel on your engine and push the power to 300-325. The only caveat is vortec heads were designed to run with a roller cam. Your engine probably has the bosses in the valley for the lifter retainer. But it’s unlikely they’ve been drilled and tapped. That would require a trip to the machine shop. Still, you won’t be the first to have this issue and I’m sure there are plenty of flat lifter cams that can work with those heads.

You’re the second person who has mentioned this oddball engine setup. I’m curious to know what gear ratio you have in your outdrive. If it’s a 1.5 that’s perfect for a vortec and 4 barrel. The mods may not improve your economy in terms of your fuel burn rate per hour. What 100 extra horses and a good prop will do is increase your cruise speed significantly so you won’t have to run it as long to get from point a to point b.

The heads don't care what cam you run. GM ran a roller cam because they were trying to get max flow from minumum lift which worked great with the LT type runner of the vortec heads since they flow nearly 95% of there available flow at .400 lift, so more lift wasn't needed(hence the puny .418/.422 stock lift cam they used with them), a roller cam reduced parasitic drag, and also allowed them to be able to run low zddp oils without the worry of wiping out the cam lobes. Vortec heads work just fine with flat tappet cams, or you can use retrofit hydraulic roller lifters in any 350 block. The big drawback to vortec heads is they are limited to .450 valve lift if you want to run valve stem oil seals. To run more lift you must have the heads modified(good news is most of the aftermarket vortec heads are already modified). Remember if you decide to flirt with 300HP, you better invest in a driveshower for the alpha outdrive, and prop it to run high in the rom range to keep the torque load on the outdrive to a minumum. Otherwise you will be kissing the alpha outdrive goodbye in short order if you let it get hot.

[fixit]

i get 2.5mpg loaded up 2.7mpg light


best ive ever seens was 2.9

smart craft numbers, not estimated
thats with a 300hp mpi with bravo 1

[Dr. Hook]

Well I got my info wrong, upon actually looking at the motor and working on it a bit I see it has a Rochester quadrajet 4 barrel carb.

This is my first season on the boat and time is limited so I'm still getting things sorted out. my fuel economy has been horrible, like 2 mpg at 30 mph.

I just cleaned the flame arrestor today so that may help. Picking away at other tune up stuff.

However I think the prop is the issue, it has a 14.25x21P aluminum prop with a big chip out of it and its in general rough shape. I can only get 4000 rpm so I think its too much prop for the boat.

Can you guys recommend a better prop? I want to go 4 blade aluminum, can't afford stainless right now. 4 blade because I like lift, control, and lower planing speeds, I couldn't care less about top speed.

[THEFERMANATOR]

You need to know exactly which engine you have. 4000 to an early engine is dead on, some turned 4400, and late model vortecs go up to 5k-5200. Also check your outdrive gearing. It sounds like your gearing may be off for a 350 as I would expect a 17 or maybe a 19 pitch prop on that boat, but not a 21 if it has the 1.5:1 lower unit.

[Dr. Hook]

The cover over the flame arrestor says WOT range 4200-4600, engine serial number OD 391290

Is there a number on the block too?