Have had a couple of people notice that my rpm's seem to be high for the speed I am going.  Full throttle, my rpm's read approx 4500 and my speed is approx 35.  What should my rpm's be?  ....and if this is high...what may be the cause? Thanks for any help you can give me.

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Dean Owens
1998 268cr "Better When Wet"

What engine and drive do you have?  But you are correct, at 35, 4500RPM's does seem a little high.  One thing to check is your tachometer--there is a switch on the back for 4, 6, or 8 cylinder--make sure its on 8.  Also, 35 seems a little low for top speed for a 268.  What prop pitch are you running?

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Un-Civil-ized 2002 230BR, MX6.2 BIII on a Prestige Trailer. Moonstone/Amethyst with silver diamond graphics

Is the boat kept in the water? Growth on the hull will limit your RPM's as well as your top speed. As Randy indicated, engine, outdrive ratio and prop pitch would be useful.

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Phil Belcher
2003 266CCR
496 MAG, Bravo 3

thanks guys for the responses. the engine is 5.7 and the prop is the bravo ll with a19 pitch... Also, yes it does sit in the water and has a cover of the wonderful green hairy stuff!!!!  Hope this info helps.

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Dean Owens
1998 268cr "Better When Wet"

I'd get the hull cleaned and apply bottom paint--otherwise true performance will be hindered by the growth on the bottom.  I bet you will pick up some speed then.  Otherwise your RPM"s at max might be a little low for wide open throttle.  Max rpm's for that motor should be in the range of 4600 to 5000 rpms.

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Un-Civil-ized 2002 230BR, MX6.2 BIII on a Prestige Trailer. Moonstone/Amethyst with silver diamond graphics

With a max RPM's of 4500 - 5000...what should my max speed be?   And thanks Randy for your reply!  Ever get up to the Chain?

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Dean Owens
1998 268cr "Better When Wet"

With the weight of the 268 and a 350 engine, I would bet your max speed would be low 40's. 

I have been to the chain once, but it's a haul for me.  Ron Dulle is up there.  I mostly go to the Illinois River since its much closer.  Was just there yesterday for the day.   

 

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Un-Civil-ized 2002 230BR, MX6.2 BIII on a Prestige Trailer. Moonstone/Amethyst with silver diamond graphics

 There are lots of variables that will affect this including weight, wind and water conditions, tilt/trim and tab position.  Also, are you sure that your speedo is reporting an accurate speed?  The pilot hole on mercruiser drives often get fouled or clogged making the speedo inaccurate. Try comparing to a GPS if possible.

I have a 1998 268CR with a 7.4L carb'd B2 drive.   The maximum recommended RPM range as indicated on my carb cover is 4200-4600 RPM. Make sure you know what your recommended RPM range is. Ideally, you should have a prop that does not allow you to exceed the recommended RPM but still gives you the best all around performance. For cruisers, generally hole shot and planing performance is far more important that top end. 

Remember that a 268CR weighs 6500LBS dry. Add fuel, water, gear and passengers and you are quickly pushing 4 tons of mass through the water. Adjust your performance expectations accordingly.  I typically cruise at 26-30 MPH at 34-3600 RPMs.  I almost never run at WOT (except when planing) due to fuel burn. I can coax 42-45 out of mine but rarely choose to push her this hard.  

 Is 4500 RPMS too high for 35 MPH?  While perhaps a bit high, In my opinion you are in the ball park for a small block, especially if carbureted. Its You could possible increase your prop pitch one size to decrease your RPM's by 3-400 but you will sacrifice some measure of low end performance.

 

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1998 268CR 7.4L B2
"La Vida Loca"