350 mercurys installation

bobfioresi · Post by **bobfioresi** » May 4th, '10, 13:51

I am installing a pair of 350 merc's in my 31 express. they have 185 hrs on them and are fresh water cooled they are 260 hp and much lighter than the 44o's coming out. I should get better fuel economy and the same speed at 3100 rpm. my question ? I now have 18x17 props on the 440's can anyone give me an idea what size props you are using if you have this combo. the trannys are 1.5 to 1 thanx bob

Bertramp · Post by **Bertramp** » May 4th, '10, 14:51

I there documentaion/research that has shown a comparable speed at cruise between the big and small block ga engines due to the weight reduction ? Out of curiousity, what do you estimate the weight savings to be ?

cmccool · Post by **cmccool** » May 4th, '10, 22:30

Hi Bob

If you do a search on Bertram31.com for "Ryan 16x16" you will find a 3 year old thread about 350s in the 31s. Ryan has a pair of 5000 rpm engines turning 16x16 props.

Most of the B28s with 260hp Mercruisers came with 17x17 props turning to 4400 rpm. I would be tempted to re-pitch yours to 18x16 and see what happens.

Cliff

Leigh · Post by **Leigh** » May 5th, '10, 05:35

Bob: I have the 260HP carbed 350's in my 1985 B28. Standard props were 17x17. I dropped the pitch to 16 so I could turn an easy 4300 RPM when loaded. The engines seem to run nice and easy at 3000-3100 and I get about 21-22 knots. Leigh

Bruce · Post by **Bruce** » May 5th, '10, 08:50

Minus the exhaust manifolds, the weight of a small block with cast iron intake is about 575lbs.

A big block weights about 685lbs.

Small block exhaust manifolds will be approx 40lbs a pair lighter than big block on average for cast iron.

So about 300lbs total weight savings.

But you also lose 140hp, rating the small block at 260 and the big block at 330.

I would tend to prop them on the high side of the rpm limit, not the low.

Also remember HP ratings aren't reached until your running WOT.

If you find them not enough push, you can always change out gears for a
1.88:1 or a 2.10:1 and swing a bigger prop as long as your running 1-3/8 shafts or bigger.

bobfioresi · Post by **bobfioresi** » May 7th, '10, 11:53

thank you all for the information.i will post information when we run some sea trials. i have flo scans and a gps and digital tachs, so i think the info should be good thanx again

bertram31.com