Recommended max rpm for a Mercruiser 470

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zlman1970

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• Aug 11, 2012

• #1

I'm running a 21 pitch prop which I was told would be good for pulling tubers and skiers. When I drop the hammer I was running 4800-4900 rpm, speedo said I was about 40 mph. I wasn't pulling anything at the time but did have about 600 lbs worth of passengers aboard. Is that too high, whats redline on this??

 

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stonyloam

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• Aug 11, 2012

• #2

Re: Recommended max rpm for a Mercruiser 470

4200-4600 for the 2 bbl, 4400-4800 for the 4 bbl Get another tach and check it against your boat tach to make sure it is correct.

 

Don S

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• Aug 11, 2012

• #3

Re: Recommended max rpm for a Mercruiser 470

You should be between 3800 and 4200 rpm at WOT for your 79 470.
With the rpm you have, either your tach is way off, or you may have the wrong gear ratio in your drive. Like a 1.98 ratio for a 3.0L.

 

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zlman1970

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• Aug 11, 2012

• #4

Re: Recommended max rpm for a Mercruiser 470

Thanks gentleman, I'll have to do some more investigating.

 

nola mike

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• Aug 12, 2012

• #5

Re: Recommended max rpm for a Mercruiser 470

Don S said:

You should be between 3800 and 4200 rpm at WOT for your 79 470.
With the rpm you have, either your tach is way off, or you may have the wrong gear ratio in your drive. Like a 1.98 ratio for a 3.0L.

Click to expand...

Why would that range be so much lower than the newer engines (or most other mercruisers)?

 

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stonyloam

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• Aug 12, 2012

• #6

Re: Recommended max rpm for a Mercruiser 470

I was wondering the same thing. The 4 barrel version is the same for the pre 83 engines (44-4800) it is just lower on the 2 barrel version (38-4200)

 

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daydreamer1252

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• Aug 12, 2012

• #7

Re: Recommended max rpm for a Mercruiser 470

Only a guess.....Mercruiser engineers probably found that there was nothing to be gained at higher revs....no more torque or horsepower...this would go along with the lower RPM on a 2 Bbl. unit as is flows less air/fuel than a 4 Bbl....probably part of the low fuel usage per horsepower reputation....just my thoughts

 

Don S

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• Aug 12, 2012

• #8

Re: Recommended max rpm for a Mercruiser 470

I have no idea why, just in service manual 3 it says the rpms are lower. I checked and there was no service bulletins I could find to give a reason for the higher rpm rating for the newer engines.
The #3 manual was last updated in 88, so that was well after manual #5 came out. And that information didn't change.

 

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1977 SILVERLINE

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• Aug 12, 2012

• #9

Re: Recommended max rpm for a Mercruiser 470

I have a 77 470 with a 21p stainless prop and it runs in that 4800-4900 range at wot also.

 

Don S

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• Aug 12, 2012

• #10

Re: Recommended max rpm for a Mercruiser 470

1977 SILVERLINE said:

I have a 77 470 with a 21p stainless prop and it runs in that 4800-4900 range at wot also.

Click to expand...

You may have the same thing wrong.

 

thumpar

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• Aug 12, 2012

• #11

Re: Recommended max rpm for a Mercruiser 470

Check the drive ratio.

 

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bigdaddypt

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• Aug 12, 2012

• #12

Re: Recommended max rpm for a Mercruiser 470

Out of curiosity, what does it idle at?

 

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zlman1970

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• Aug 12, 2012

• #13

Re: Recommended max rpm for a Mercruiser 470

bigdaddypt said:

Out of curiosity, what does it idle at?

Click to expand...

700-800ish

 

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bigdaddypt

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• Aug 12, 2012

• #14

Re: Recommended max rpm for a Mercruiser 470

zlman1970 said:

700-800ish

Click to expand...

Your problem might be the need to calibrate the tachometer. It's supposed to idle at 650-700 rpms. So if you're off by 50-150 rpms at idle, that might explain why you're WOT is off. As someone else mentioned, try a different tach to check. Best of luck.

 

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lselmes

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• Aug 12, 2012

• #15

Re: Recommended max rpm for a Mercruiser 470

don't mean to hijack this thread but I've been having issues with my 1981 470. It is mated with a 1.65 ratio Merc 165 drive and an 18P prop. I can't get over 3800 RPM and the engine runs nice at 3000, but anything over that and things go awry. Any input would be much appreciated.

 

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bigdaddypt

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• Aug 12, 2012

• #16

Re: Recommended max rpm for a Mercruiser 470

lselmes said:

don't mean to hijack this thread but I've been having issues with my 1981 470. It is mated with a 1.65 ratio Merc 165 drive and an 18P prop. I can't get over 3800 RPM and the engine runs nice at 3000, but anything over that and things go awry. Any input would be much appreciated.

Click to expand...

You completely hijacked his thread. Start your own.

 

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lselmes

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• Aug 12, 2012

• #17

Re: Recommended max rpm for a Mercruiser 470

I did. and excuse me for asking something that might be very relevant to his 470 issue and wrong gearing, was only trying to help.

 

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zlman1970

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• Aug 13, 2012

• #18

Re: Recommended max rpm for a Mercruiser 470

Had the boat out again this afternoon. Nice calm waters, WOT settled in around 4200. So I guess all is well. Probably where it was in my original post but it was choppy and I wasn't looking as closely as I should of.
Thanks for the replies!