Will a Merc 650 handle the same RPMS the 650XS did?

[outboardnut]

Anyone know if the max rated RPMS on the 3 cyl Merc 650's
was conservative?

My book says operating range is 4800-5300. Would proping for 6300 be to much
to ask out of this old girl?

It's a 71' model, 49.8 CI. I know they made a 650XS that had
a range of 6000-7000....in later years. I pulled up some parts
books and am showing they used the same crankshaft and pistons.
My book shows the 650XS as only 70HP but I've read they put out closer to 90.

The carbs and intake were obviously different on the two motors and I am showing a different block.

Also would running the P tube without the choker nipple help it cool better? I think they put those on there
for warmer operating temp to help on idle.

The other optionS I have for this motor is to hunt down some 70 HP carbs or rejet these.

 

[DJ]

Re: Will a Merc 650 handle the same RPMS the 650XS did?

I would shoot for 5300 with an average to HEAVY load.

 

[Chinewalker]

Re: Will a Merc 650 handle the same RPMS the 650XS did?

You may be able to spin it to 6300, but you'll be making less horsepower. Horsepower is a curve and they generally rate them at the peak of the curve. Beyond that, RPM may go up, but horsepower will likely go down. The differences between the 650 and 650XS are what make the XS able to peak at a higher RPM point...

That said, your motor isn't a 1971 - first year of the 3-holer was 1973... The 1971 model was a 4-holer...

 

[outboardnut]

Re: Will a Merc 650 handle the same RPMS the 650XS did?

Hi Chinewalker. I was thinking of you when I wrote this post, "I bet chinewalker would know", ha. Nice to hear from you again.

Well I messed up what I wrote down on my years. Sharp eyes caught it. The motor is actually a 74'. I stand corrected. My Seloc(please don't slap me in the back of the head for saying that word) shows that 72' was actually the first year of production for the 49.8 3 cyl merc 650, starting at serial number 3293184.

I think I am hitting 5300 with just me in the boat, maybe going down one in the prop size will get me what I am wanting out of this rig.

I've got a GPS now so I can get the MPH and RPM.

I was thinking if I dropped a pitch or two and not afraid of the extra WOT rpms I would get more out of the hole and not sacrifice the top end.

I don't know where the heck my head is though in reality I'm thinking there is no real world gain by the RPM increase unless the engine was modded and I'm getting more RPMS with this same prop. If I'm up over the peak HP then it's all going to be fairly relevant.

Sound better?