Mercruiser 898 WOT RPM?

[novausc]

I recently purchased a 1982 Chris Craft Scorpion 182 BR(My First) and found out that the engine had melted and been rebuilt with JB wield. I had the engine rebuilt and the mechanic has given me some conflicting info regarding the RPMs the boat should be turning. He told me that the boat was running about 5400rpm and that he changed the prop for a larger one and got it down to where it needed to be, which is 3200rpm. The boat runs well and I get a top speed of about 33mph WOT(verified with GPS) but I'm finding placeson the internet thatstate the WOT RPM for this engine is anywhere from 3800 to 4800 rpm. Can anyone shed any light? Should I change out for a smaller prop to bring the rpms up? Also, I fear the mechanic may have taken me for a ride since he told me he changed it from a 17 to 19 which should should have only affected the rpms by 300 to 400, not from 5400 to 3200. Any help?

 

[jtmarten]

Re: Mercruiser 898 WOT RPM?

Download the manual from the adults only section. WOT should be 44-4600rpm if I remember correctly. 3200rpm is too low, not even in the range the engine makes good power.
Did you get an itemized receipt? How does the mechanic know it was previously turning @ 5400?
33mph is really slow for a V8 18 footer. My '79 Glastron/Carlson CVX-18 would run upper 50's to 60 with a 19p ss prop, at 4600ft elevation. PO had replaced the 305 with a 350 due to non-winterizing the block, but the 305 should get you into the 50s easily. Sounds like it was either not rebuilt properly, not broken in properly, or not setup properly (carb, timing, etc).
A 19p prop should work well, at least a good starting point.

 

[greg82255]

Re: Mercruiser 898 WOT RPM?

WOT should be 44-4600rpm if I remember correctly. 3200rpm is too low, not even in the range the engine makes good power.

I second JTMarten... I had the exact same engine you do, a 1982 Merc 898. Mine is in a 19 ft checkmate. I upgraded the intake and carb this year, but last year with the Rocehster 2 barrel, my WOT RPM was 4400 and I was running a 21 pitch prop and getting 54 MPH. 3200 RPM and 32 mph is way too low for that engine.

BTW - Ive heard running that engine at over 5000 can cause serious damage, let alone 5400..

 

[QC]

Re: Mercruiser 898 WOT RPM?

Good info above, but confirm the tach is right . . .

 

[novausc]

Re: Mercruiser 898 WOT RPM?

Did you get an itemized receipt? How does the mechanic know it was previously turning @ 5400?

I did get an itemized receipt but true to classic mechanic(or Doctor) fashion it's practically illegible. He said it was turning 5400 rpms after rebuilding it and that the prop that was on it was causing it not to "settle down" properly and turn to many rpms so he swapped the prop from the 17 to a 19.

I guess the first thing I need to do is verify the tach is correct though like QC suggested. Once I have that verified I will have a better understanding of where the problem lies.

 

[QC]

Re: Mercruiser 898 WOT RPM?

Yeah, a 2 inch change would not pull it from 5400 to 3200, Maybe down 400 RPM, but something ain't right here. Not that anybody has done anythign wrong, but the math isn't working . . .

 

[novausc]

Re: Mercruiser 898 WOT RPM?

Something just dawned on me, I feel kinda stupid for not realizing this before. If I remove the tach reading from the equation(assume tach is wrong) anyone know what the mph difference would be on a boat like this with only one person versus the same boat with full load of 6 adults? I don't think I performed any trial tests with just myself in the boat. I think it was always full. Could that account for only hitting 33mph, especially with two people in the bow?

 

[QC]

Re: Mercruiser 898 WOT RPM?

Load will definitely slow you down, but there is no fixed number for that. 3200 is too low of RPM regardless of speed, so need that number. I would guess 5 MPH for an 18 footer, but still should be above 40 MPH for an 898 . . .

 

[jtmarten]

Re: Mercruiser 898 WOT RPM?

Shouldn't be drastically slower, may take longer to get up on plane though. Two in the bow will help get you planed quicker than if all six are in the cockpit.
I'd take it out solo for a quick run, just see how much difference there is in rpm.