Better Fuel Economy

[san]

Hi

I have a Ford 302 (188 hp Mercruiser 888) in a boat that weighs about 2 ton. The fuel economy isn't that good although engine seems to be running smooth. A friend of mine have a 350 Magnum (275 hp) in his boat and much better fuel economy. His boat is almost the size of mine, but with less weight.

The 888 has 2-barrel intake manifold and if I replace it with a 4-barrel and another carb I will get 225 hp, i.e Mercruiser 225 (at least what I read in another thread). The Mercruiser 888 is with 351 heads.

I dont care about the extra power, the only thing I want is better fuel economy. Will the fuel economy be better with more horse power at same running speed (engine doesn't need to work as hard) or am I just fooling myself? Or is it the torque that matters?

Stefan

[woodboat]

Fuel economy is a delicate balance. Reducing weight will help. A lower engine load will help. So make sure your engine can twist like 4200 RPM WOT. With that said the biggest room usually is in carburator tuning. Most carbs add way too much fuel for a given RPM. Boats load the engine much more so the "power" circuit tends to kick in due to lower vacuum. I more than doubled my fuel economy in my 26 FT Trojan by replacing the power valve spring in my Qjet/small block chevy combo. So I would consider the 4 barrel conversion as you are likely to get better control of the fuel on a light cruise giving you a range that gets better mileage while retaining top end speed.

[san]

Thanks, Woodboat

[gonzo]

The two barrel carb reduces the maximum HP. A four barrel won't be more efficient at the same power. As you said, your friend's boat is lighter. That gives him more speed and economy. Also, to get more power you will need a camshaft to match the extra flow capability of the four barrels.

[woodboat]

A four barrel won't be more efficient at the same power

that's not true. A spread bore carb will have smaller primaries which increases velocity, thus the mix and greater efficiency while giving higher HP when fully opened.
The camshaft thing is a complete guess on your part. Often the only difference is the carb. I remenber looking at specs on my old 429 ford. Specifically The ONLY difference was the carb. So it may have a different cam OR it may not.

[san]

Just for information. I checked on Doug Russell and with 4 bbl instead of 2 bbl I will get 215 hp instead of current 188 hp. If I swap camshafts as well I will get 225 hp.

Thanks

I feel for you Ford. My 2002 Chaparal 18.5' merc 5.0L ( 305 ) 220 hp 2bbl carb can drain gas so fast I am always sniffing the bilge on start up. Ideled it goosed it tied it between 2 docks @ 2500 rpm neutral & in gear. No change in plug color - coco brown. No sheen on water for hundreds of feet out. Oh, boatweighs 3500 lbs/ me. does 48 mph @ 4800 rpm . I now accept draining a tank in 4 hours of raising hell in a empty cove. 38 gal. gone. One day I went back to gas up @ 1000 rpm for 15 minutes- killed me to go so slow so long. Rich

I just picked up what you said about you engine having 351 cylinder heads. Of course you will burn more fuel, The engine is hopped up already. Probably has a different cam also. Forget MPG, Your PEAK WOT hp is probably 250 to 300 hp. If you have exhaust pipes above the water and they are not getting black. Don't worry, you drive like me and are cheap at the gas pump, like me. Loosen up and enjoy the speed and freedom. Rich.