wot rpm?

[Dauer33]

I see where mercruiser has a rpm they don't want your to go over. Is that for engine safety or they outdrive don't like it? My bayliner has a 4.3 that I built and would withstand 6500+rpm all day!!!

 

[wca_tim]

Re: wot rpm?

:facepalm:

 

[Bondo]

Re: wot rpm?

I see where mercruiser has a rpm they don't want your to go over. Is that for engine safety or they outdrive don't like it? My bayliner has a 4.3 that I built and would withstand 6500+rpm all day!!!

Ayuh,.... 'n why would ya wanta do that,..??

 

[Scott Danforth]

Re: wot rpm?

stock automotive engines are not precision pieces, however mass produced assemblies. hence the below 5000 RPM limit. So, because the motor is not meant to spin that fast, the drives generally have a similar limit.

all the precision machining, oil system revision, balancing of rotating assemblies you did to your motor to get it to turn those RPMs are wasted unless you do the same to the outdrive now.

 

[series60]

Re: wot rpm?

I see where mercruiser has a rpm they don't want your to go over. Is that for engine safety or they outdrive don't like it? My bayliner has a 4.3 that I built and would withstand 6500+rpm all day!!!

But your outdrive won't!

 

[spdracr39]

Re: wot rpm?

Unless you are racing the boat and/or own oil company stock there is no practical reason to do that. All your power needs come at low end for torque. High speed operation will only make up a very small percentage (< 5%) of your time on the water.

 

[Flysfloatsor]

Re: wot rpm?

"My 4.3 v6 is a BEAST!" Says, no one...

 

[Dauer33]

Re: wot rpm?

Not saying I want to boat to run at that rpm, just don't want to ruin the out drive doing so. I build race engines part time so doing all the work to this one was no problem just my time at the machine shop. For the smart***** sorry for the question.............

 

[Oshkosh1]

Re: wot rpm?

You need to understand that there's all sorts of people who post here who "build" supposedly Hi-Po engines expecting that to translate into out of this world "on the water" performance...only to find out that in a MARINE enviroment, things aren't as black and white as the similar street/stip application. More variables/weak links...such as the drive/prop/prop slip/cavtitation/ventilation/hull/load/sea state...so on and so forth. As the previous poster mentioned pushing more HP/RPM will not necessarily mean equivilent performance gains.

At the very least you can expect more heat on the drive so you may look into a "shower" to help cool the exposed upper while underway.

 

[Silvertip]

Re: wot rpm?

Not saying I want to boat to run at that rpm, just don't want to ruin the out drive doing so. I build race engines part time so doing all the work to this one was no problem just my time at the machine shop. For the smart***** sorry for the question.............

Don't need to be sorry for the question. Depending on the cam you put in the motor you are leaving yourself open to reversion -- and if that term means nothing to you, it will when the cylinders load up with water due to too much cam overlap. And a 6500 rpm engine just has the power curve moved up in the rpm band. Terrible for hole shot but it will scoot on top end if the drive holds up.

 

[Scott Danforth]

Re: wot rpm?

building a race engine is great if your on land. for a great marine engine, think tow truck engine (power from idle to about 5500 rpm). as Silvertip pointed out regarding the cam. generally, between 268 and 270 degrees is as much as you want with thru drive exhaust to prevent reversion.

 

[Dauer33]

Re: wot rpm?

Motor is built for torque. I had everything but the cam to build a good solid motor is the only reason its built the way it is. Cam is a custom ground erson roller cam for marine use. Im still debating on changing the 2barrel intake for a 4barrell as I hate to give up the response from the 2 barrel but the stock intake is so restrictive. Im looking at getting a turning point hustler 14x19 or 14x21.

 

[Mischief Managed]

Re: wot rpm?

Piston speed and the strength/weight of the reciprocating assembly determine the maximum safe operating RPM of the bottom end of a motor. Drives don't have a reciprocating assembly and are limited to a specific amount of torque by the strength of the gears and the ability to shed heat. I can't imagine why the drive would have a problem with 6500 RPM coming from a low-torque engine like a 4.3 when Mercruiser routinely powers them with 5.7 liter engines that make 33% more torque in stock form.

 

[Flysfloatsor]

Re: wot rpm?

I was just joking around with you! I apologize.
But seriously, if you want to run it to 6500, do it. Get a drive shower to keep the lower cool.
There are plenty of guys turning alphas behind hopped up sbc's to 6k+.
How big is the boat? I think youll need to go down in prop pitch to see 6k though.

 

[Bondo]

Re: wot rpm?

I was just joking around with you! I apologize.
But seriously, if you want to run it to 6500, do it. Get a drive shower to keep the lower cool.
There are plenty of guys turning alphas behind hopped up sbc's to 6k+.
How big is the boat? I think youll need to go down in prop pitch to see 6k though.

Ayuh,.... Even though the 4.3l is the little sister of the SBCs,...

With the off-set grind of the 4.3l crank, it's known as bein' less than Tough,...
'n I've never seen a forged crank for 1,....

I have to think, the 4.3l, run at 6500 rpms, would be a short lived motor....

just sayin'...

 

[Dauer33]

Re: wot rpm?

I don't want to run the motor at 6500 all day, just wanted to make sure the out drive would be ok if I went screaming across the lake. After giving up racing after 10yrs I still like to go fast once in awhile just not in circles lol

 

[Dauer33]

Re: wot rpm?

Oh and boat is small 17ft 6 passenger capri. Out drive ratio is 1.84, that's why I asked about rpm. Best speed at wot is 39mph w/ gps at around 4600rpm.

 

[thumpar]

Re: wot rpm?

I think what people are saying is that you should have built a motor to make power at 5-5.5k rpm. RPM is the not key to boats.

 

[Dauer33]

Re: wot rpm?

Motor is built to build power from idle to 5500. So really anything over 6000 im out of my power range. That's if I change carb and intake. I don't think the stock intake would turn 6500 if it had too lol. Only reason I asks the max rpm is I didnt want to hurt the out drive!!!!!

 

[Scott Danforth]

Re: wot rpm?

^+1, then sell the 1.84 drive for a 1.62