mercruiser 165 2bbl to 4bbl

[rcig]

I am looking to increase top end on 24 cigarette with twin 165's. would changing to 4bbl's work and increase top end. is it too much fuel for those engines? suggestions please.

[Paul aka watertaxi]

Is this an inline 4? I've heard you can get 10 to 20 hp more from going to a 4 barrel on those but no direct experience doing this on those.

On an unrelated note, how about posting a few pictures of your boat -- those 24s were beauties!

[Jango]

I believe the 165's could be had with a 4 Barrel - 180HP. This results in about a 10% increase in Power.
Since power change to speed is expondential, you would probably gain a little less than 5% increase in speed. In order to get any kind of increase, you would need to do a little more. I believe your engines are basically half of the 429 cu in Ford V8. If you could find some 429 CJ Heads ( Higher compression and Larger Valves and Ports) you could see up to a 25 - 30% increase in speed.

[rcig]

the mercruiser 165 is a GM 250 cubic inch inline 6 cylinder. I'll get some pictures up. not a well known model (1972). what size (cfm) should I use???

[Jango]

Your Engines are most likely the 250 cu in Chevy Truck adapted for marine use.

If you're satisfied with only a 2 or 3 mph increase (less then 5%), 450 - 500 cfm carbs would probably work. Also increasing the compression ratio (milling the Head) would accomplish about the same. Just make sure of adequate piston to valve clearence.

Probably get a little more, 10 - 15% HP increase by going to an Alum, High Compression, Big Valve Head.

Bottom line with "Full Hop -up", cam etc. probably max speed increase of about 25% and still have reasonable low speed operation.

[Jango]

By the way, I was thinking your 165 was the 4 cyl marine motor (470) Built by Mercury in the late 70,s early 80.s. This was an aluminum block motor exclusive for marine use and not from the automotive transplants. Utilized a Ford Big Block Head and had real power. Pushed my friends 18', 2000# fiberglass boat to near 50mph. Sorry for the confusion

[powerabout]

Merc had an inline 6 with a quadrajet it was called a 200 model, came with an early II drive.
As you can see there is an adapter under the carb to go from the old 4bbl to the 2 you have on the 165 so easy job

plenty of inline 6 stuff around... i.e.
http://www.cliffordperformance.net/
http://www.sissellsautomotive.com/
some things to make a 165 last in a boat when you want to go WOT all day;

larger sump with splash screen- install oil temp gauges - keep the oil away from the crank
balance the engine
screw in rocker studs
roller rockers or just tips
floating wrist pins
5000 rpm no problem...

[rcig]

thank you for that info. too bad you are not local, you seem to know GM 6 cyl engines.