2.4-2.5 rev to 10 k can anything be done to 90-115 to get them over 6k?

[derekfl]

i didn't say merc in the title but I think kinda obvious. from the looks of it, the inline 3 and 4 is 1/2 or 2/3 of a 2.5. same bore essentially same porting. longer stroke and longer piston skirt. is the extra stroke and piston weight so much that they can't rev? and is older 3-3/8 bore motor any tougher or weaker than the 94-up 3-1/2 bore motor? or is it just "go fish"?

[rock]

There are fishing motors and there are go fast motors. 3 cyls are not racing motors. My best guess is reciprocating weight. The inline sixes would not turn 10,000 either at least to my knowledge.

Rock

[skialot2]

If I remember correctly, the inline 3 and 4 cylinder motors from the late 80's and 90's got some kind of hormonic vibration if they were revved over 5800RPM and could break the crankshaft.

[David - WI]

Neither will a stock 260, 280 or 300 Drag. :cheers:
What is the actual running life expectancy of a 10k 300 Drag... 90 seconds of WOT between rebuilds?

[powerabout]

If you want a 3 cyl that revs go OMC

[derekfl]

ok, apparently there is a harmonic vibration at 5850. and if you can push thru that to 6k-6200 it is ok. I have a couple of 4 cyl and one 3 cyl power head. and they are easy to find and cheap. the 2 blown ones I got both had cyl liners that slid down the bore until the top ring hung up and destroyed the piston. both on the top cyl. I guess heat made the block expand until the liner let go. is there any way to pin the liners in place? and do liners ever migrate in the v6's?

[derekfl]

AND do these merc ignition controllers have rev limiters?

[rock]

I once had a pinned bridgeport. He used button head screws screwed into the sleeve low in the sleeve in an area not contacted by the piston skirt. The sleeve was then bored. How necessary it was, I don't know but the sleeves stayed in place.

Rock

[David - WI]

AND do these merc ignition controllers have rev limiters?

The 300 Drag says 11,500 rpm here: https://www.buckshotracing77.com/post/mercury-racing-v6-v8-spec-sheets?srsltid=AfmBOopG2KNMiKsZNyc-csbOn7zny-YsRlpPh0dxRq-sbUHdXh8GtOww

[derekfl]

I doubt the ignition system in a 115 4 banger has any similarity to that motor's setup... I have a switch box set and a cdi set.

[David]

Didn’t Merc make a racing 3 cyl that was 1/2 of a V6?

[Glastron1987]

Those 4 cylinder 115's or 125's I think you are talking about are the 2+2 engines. Their max RPM is somewhere around 5250, which is really low. I believe they have a wacky balance because of the positing of the pistons on the cranks and a heavy flywheel. Seems to work fine, when adjusted properly, for those low rpms, but it will be a difficult problem to solve to get more out of them. Good idea though to try to get those relatively light engines to pack some more punch. If you could do that there would be a lot of benefit for many 16-17 foot boats.

[merccrazy]

Didn’t Merc make a racing 3 cyl that was 1/2 of a V6?
Pretty sure your thinking of the v6 that ran on three cylinders to compete with a specific racing class. I don't know of any Mercury I3 or 4 that would turn a lot of rpm. The 650xs and 850xs didn't turn crazy rpm from what I remember.

[derekfl]

I can't tell this flywheel from a 2.5 flywheel. the crank throws are even 90 degrees apart. .28 more stroke than a 2.5. but you add 1/4 inch of stroke to a chebby small block it doesn't come apart at 6k. the 2+2 is just no idle circuits in the bottom 2 carbs. I have 4 sets of carbs so I will be playing with that. this is going on a 675 lb Allison 18-1/2 foot bass tri hull. center console conversion so a bit more sail area. the outer keels never touch the water on plane so it is essentially a pad deep V. the outer hulls act like a pair of tunnels supplying lift.

[108]

The cranks break.

[derekfl]

I suppose there is no aftermarket for cranks-pistons-rods for these motors? other than custom built or a few stray single wiseco pistons?