Engine Redlining? '88 OMC 4.3L

[PITBoat]

Hi,

I'm new to the site and new to boating. I just bought an '88 Renken 2000 Classic 19' ski boat. One of my big questions concerns how hard to run the engine. 4,500rpm is the top of the yellow(?) range on the tach, and I believe it redlines at 5,000. It will cruise at about 28mph at 4,500rpm, but it's got a lot more left if I push the throttle up. The power really comes on (in my new-to-it-all estimation) and it will do over 40mph, but the rpms are up well past 5,000.

Is it ok to run past the tach redline indication, or should I consider that a limitation? I've pushed it up a time or two, but I haven't run in that range for very long. I just hate to leave all that on the table if the motor is really made to run there. The prop is a 19; the PO said that was middle of the road. I don't know the diameter. I think the HP is 175.

Thanks for any info. I'm sure I'll have more questions as I continue into this.

 

[Scott Danforth]

welcome aboard

verify your tach to make sure that it is reading correctly. my guess is the tach is switched to 4 cylinder vs 6 cylinder

 

[PITBoat]

Thank you.

Is that simple to do? I think the tach and the motor are original to the boat.

 

[HT32BSX115]

Hi,

I'm new to the site and new to boating. I just bought an '88 Renken 2000 Classic 19' ski boat. One of my big questions concerns how hard to run the engine. 4,500rpm is the top of the yellow(?) range on the tach, and I believe it redlines at 5,000. It will cruise at about 28mph at 4,500rpm, but it's got a lot more left if I push the throttle up. The power really comes on (in my new-to-it-all estimation) and it will do over 40mph, but the rpms are up well past 5,000.

Is it ok to run past the tach redline indication, or should I consider that a limitation? I've pushed it up a time or two, but I haven't run in that range for very long. I just hate to leave all that on the table if the motor is really made to run there. The prop is a 19; the PO said that was middle of the road. I don't know the diameter. I think the HP is 175.

Thanks for any info. I'm sure I'll have more questions as I continue into this.

Howdy,

Welcome aboard!

Since you have an "unknown" boat, it's also possible that someone has changed the drive to one with a lower than correct ratio. I.E you should have a 1.68:1 ratio drive for a 4.3L engine.

That means that for 1.68 engine revolutions, the prop turns exactly 1 turn. If you want to be exact, 1.68 means 1, 360 degree turn + another 245 degrees. will cause the prop to turn 1 full 360 degree turn.

It's important because if someone changed the drive for one that was originally meant to be behind a 2.3 or 3.0L 4cyl engine, the engine will turn much faster with the same prop.

Below is an OMC Cobra ratio chart that might help.

Also, Scott is right, your tach might be set wrong or it may even be defective......many are....

Ultimately, you need to know if your speedometer and tach are correct and know the drive ratio to select the correct prop for your boat. You can use the GPS in a smart phone for speed.

Just about all marine engine manufacturers recommend selecting a prop that will give you MAX rated RPM (or in the max range) when operating at WOT with the boat loaded at the weight you'll operate at regularly and (the drive) trimmed for max speed.

It gives you "rule of thumb" to start at. but you must know the ratio, prop pitch, speed and RPM to do this correctly.

Regards,

Rick

 

[PITBoat]

Thanks much for the info. The boat has a depth finder with GPS that I'm getting the speed info. from (the original speedo is inop.). So I have speed, rpm (maybe), and prop pitch, but don't know the ratio. I just got my Clymer engine book, and I guess I need to order a Stern drive book also, for more info. related to that.

I can do the gear ratio on a car (turn a rear wheel once and count the driveshaft revolutions; or better, just look up the RPO code...). I suppose I would just pull the plugs and turn the crank on the boat motor while someone counts one turn on the prop? I do think the stuff is original, but I can't swear to it. Does the tach have a switch on the back or somewhere that would be used to set the number of cylinders?

I have an old GMC truck with a 4.3L that winds out at about 4,500, no matter what gear you're in...

Thanks...

 

[HT32BSX115]

I suppose I would just pull the plugs and turn the crank on the boat motor while someone counts one turn on the prop?

That's actually the easiest way to do it if you're not sure.

The boat is fully 30 years old. That drive could have been changed many times in that time and if bought used, there's no way of knowing what it is unless you check.

 

[Scott Danforth]

clymer manuals are not worth the paper they are printed on.

simply go to autozone or advance autos and borrow a tachometer to check out yours.

 

[PITBoat]

Alright, thanks for the replies. - Mike

 

[PITBoat]

It's looks like I'm going to be out of service for a while anyway. I've been perusing the must have technical links on the site, and find here, http://www.hastings.org/~stuart/cobra/index.html , that I may have bigger problems, with the shift linkage adjustment. Mine is quite stiff, and I don't want to scrap the gears or anything out because of that. It may take me a little while to get through the suggested parts replacements and adjustment(s) there.