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Explain to me why people like after-market boost but hate over-propping

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Explain to me why people like after-market boost but hate over-propping

Old 01-09-2020, 12:47 PM
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Default Explain to me why people like after-market boost but hate over-propping

I was on another thread about the Nizpro, and it just made me realize I do not understand the general acceptance that overpropping is bad, with the simultaneous perspective by a lot of people (apparently, at least) that you can add a blower and reliably get untapped potential.

Make it a 300 Yamaha, for simplicity's sake, since Nizpro is available, but the argument applied for any purpose-built, lightweight NA engine.

So, if I run a 300 Yamaha and prop it so it only turns 4000 rpms at WOT, everyone will tell me I need to get a better prop because I am overloading the motor. I agree... at any given rpm each rotation is doing about 1.5 times the work, and seeing about 1.5 times the force, compared to the properly-propped engine.

But if I say I am going to fix that by adding boost, what have I fixed? Say I can now reach 6000 rpms with that same engine. At any specific cruising speed, nothing has changed physically, so I am putting exactly the same overload on the internals. Even worse, the extra power means that when accelerating or running above the old 4000 rpms cap, I can put even more stress on the internals than in the case of the over-propped rig. All I have done by adding a super-charger (whether belt or turbo driven) is make the combustion even more powerful, loading the parts even more than before, so essentially making it like over-propping but worse.

IF the motor has the capacity to be run reliably boosted to 450 hp, then it was unnecessarily overbuilt, adding unnecessary weight, and means you could over-prop it if you like the trade-offs of over-propping for greater economy.

IF the motor should not be over-propped if you want reliable long-term service, then adding a supercharger should be even worse than over-propping.


If I were an engineer tasked with making a light, reliable outboard rated for 300hp, then it seems to me that it will not be reliable boosted to 450hp, because this would indicate wasted weight and materials in an engine designed for 300hp. (As an added curiosity, I would be interested to know how much of the internals of the MErc 450 are shared with the 300. Given how light the NA 300 is, I would be shocked if pistons, crank, bearings, etc. are the same.)

Last edited by Texas 17; 01-12-2020 at 10:36 PM.
Old 01-09-2020, 01:00 PM
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Boosted engines must be built for the subjected loads and heat, if you want them to last. Forged pistons, stout rods, Stainless or Titanium valves and other performance goodies. Then, they can take the harsh pounding m the high cylinder pressures.
Lugged or over-propped engines can see very high cylinder pressures without the flow to properly cool components. If they aren't built to withstand the loads and heat, parts will suffer.
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Old 01-09-2020, 01:07 PM
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Originally Posted by Texas 17 View Post
So, if I run a 300 Yamaha and prop it so it only turns 4000 rpms at WOT, everyone will tell me I need to get a better prop because I am overloading the motor. I agree... at any given rpm each rotation is doing about 1.5 times the work, and seeing about 1.5 times the force, compared to the properly-propped engine.
Not sure where you get that the engine is doing more work if it's overpropped. It's doing less work and it's working harder to do the work you are trying to ask from it (which it can't) so combustion chambers get hotter and you start getting preignition, detonation and other issues caused by the heat buildup.

Forced induction allows the motor to do more work, which in turn allows it to turn a bigger prop. Which is fine until it blows up components that weren't designed to handle that much power output.

You're trying to compare two things that aren't comparable.

Last edited by acme54321; 01-09-2020 at 01:31 PM.
Old 01-09-2020, 01:20 PM
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Originally Posted by Texas 17 View Post

If I were an engineer....
We'd be fu..ed

The danger of over propping/over loading any motor is from lugging it and the resultant pinging/dieseling/detonation. Is unimaginably hard on the wrist pins, temp starts climbing and at some point she will say no mas.

Gonna overgeneralize this a bunch - getting more power from an existing platform (without touching hard components) means tweaking fuel or ignition curves to increase combustion pressure. Next step up is changing hard components for more compression, more aggressive valve timing, forced induction, etc all in the name of, once again, increasing combustion pressure. Now virtually any motor you can buy will be robust enough to tolerate maybe an 8% - 10% increase in "tuning" without substantially impacting it's expected service life. As any old hotrodder will tell you there is point in say a 350 Chevy build where forged pistons and crank, roller rockers, double roller timing chain etc etc become recommended or even mandatory but, and it's a big, grain of salt but - any basic piston engine will have a "sweet spot" of power production where maximum service life is realized and any increase beyond this will negatively affect service life and the bigger the increase the bigger and more immediate effect.


Still onboard? Bumping an otherwise unaltered 4.2L block that is successful as 225/250/300 HP unit with forced induction to 450 HP = time bomb.

Reference to Verados is out of context - they are purpose built with internals that can handle the heat of the kitchen.

Last edited by HTJ; 01-09-2020 at 01:26 PM.
Old 01-09-2020, 01:24 PM
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A motor that is designed to be naturally aspirated and produce 300 HP at 6000 RPM will have a typical service life of X if operated continuously at that power level and RPM.

Take that same motor and increase its HP with the use of forced induction and it will not have the same life expectancy. The anticipated life will be reduced.

Take that same motor with forced induction and increase its RPM to create even more power and it will not have the same life expectancy. The anticipated life will be reduced.

The internal stresses will be increased. Either due to an increase in manifold pressure from the forced induction and/or the increased inertial loads from the increased RPM's.
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Old 01-09-2020, 01:28 PM
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Originally Posted by Texas 17 View Post
So, if I run a 300 Yamaha and prop it so it only turns 4000 rpms at WOT, everyone will tell me I need to get a better prop because I am overloading the motor. I agree... at any given rpm each rotation is doing about 1.5 times the work, and seeing about 1.5 times the force, compared to the properly-propped engine.
Nope. It will be doing less work.

Less power will be created. Less power being available equates to less work being done.
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Old 01-09-2020, 02:10 PM
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Originally Posted by acme54321 View Post
Not sure where you get that the engine is doing more work if it's overpropped. It's doing less work and it's working harder to do the work you are trying to ask from it (which it can't) so combustion chambers get hotter and you start getting preignition, detonation and other issues caused by the heat buildup.

Forced induction allows the motor to do more work, which in turn allows it to turn a bigger prop. Which is fine until it blows up components that weren't designed to handle that much power output.

You're trying to compare two things that aren't comparable.
Semantics... I was mixing regular language with the physics language. More force, less distance, same work, gotcha, wrong word.
Old 01-09-2020, 02:12 PM
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Originally Posted by alloyboy View Post
Nope. It will be doing less work.

Less power will be created. Less power being available equates to less work being done.
Same work, I used the wrong word. Same cruise speed, same force produced, more force per rotation, less rotational distance.

Back to the engineering question, forgive that one choice of words.
Old 01-09-2020, 02:14 PM
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Originally Posted by Texas 17 View Post
Same work, I used the wrong word. Same cruise speed, same force produced, more force per rotation, less rotational distance.

Back to the engineering question, forgive that one choice of words.
Not a chance we're stringing you up
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Old 01-09-2020, 02:18 PM
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Originally Posted by HTJ View Post
We'd be fu..ed

The danger of over propping/over loading any motor is from lugging it and the resultant pinging/dieseling/detonation. Is unimaginably hard on the wrist pins, temp starts climbing and at some point she will say no mas.

Gonna overgeneralize this a bunch - getting more power from an existing platform (without touching hard components) means tweaking fuel or ignition curves to increase combustion pressure. Next step up is changing hard components for more compression, more aggressive valve timing, forced induction, etc all in the name of, once again, increasing combustion pressure. Now virtually any motor you can buy will be robust enough to tolerate maybe an 8% - 10% increase in "tuning" without substantially impacting it's expected service life. As any old hotrodder will tell you there is point in say a 350 Chevy build where forged pistons and crank, roller rockers, double roller timing chain etc etc become recommended or even mandatory but, and it's a big, grain of salt but - any basic piston engine will have a "sweet spot" of power production where maximum service life is realized and any increase beyond this will negatively affect service life and the bigger the increase the bigger and more immediate effect.


Still onboard? Bumping an otherwise unaltered 4.2L block that is successful as 225/250/300 HP unit with forced induction to 450 HP = time bomb.

Reference to Verados is out of context - they are purpose built with internals that can handle the heat of the kitchen.

It seems like you are agreeing with me, with the addition that the overpropping is not only overloading but also causing detonation issues. I am not sure how that works internally versus the timing issues with extra air pumped in there, but I believe you. My point, the whole point, is that it seems crazy to me that people seem to be fine with boosting an engine, but complain about overpropping. I think the NizPro makes a time bomb, and there is no way I would stick one on my boat.

The Verado is not out of context... a boosted motor made to be boosted to 275 (originals) does not imply it can be safely boosted to 425. I get this is a purpose built block, but 425 was not the purpose on the originals, and yet people are happily getting an aftermarket chip to go there and expecting it will be fine.
Old 01-09-2020, 02:18 PM
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Originally Posted by HTJ View Post
Not a chance we're stringing you up
Already done. Dear lord I should have edited for scientific precision.
Old 01-09-2020, 02:24 PM
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OP an over propped motor is being lugged, doesn't matter if it's naturally aspirated or not. A motor boosted beyond the strength of its internals could certainly be doomed but correctly propped it isn't experiencing the same/similar stress as being over propped.

My skiff has a 25 HP 3 banger, it willingly revs to redline and runs like the proverbial turpentined dog, if I were to replace it with a correctly propped 9.9 it would be all kinds of slower butthe motors would experience the same effective stress
Old 01-09-2020, 02:25 PM
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A motor will have a max amount of power it can make. Over propping it does not increase the power a motor can make, it decreases it. The problems associated with overpropping do not come from the motor making more power, comes from possible pressures being made when the motor is not ready to handle them IE. early fuel detonation.

Boosting a motor will make it more powerful, but should not cause the pressures to build when are not suppose to. Some motors are way overbuilt, and this does not always mean they have added or wasted weight. A motor can be over boosted and parts fail, how much boost a motor can take is only found out by one method.
Old 01-09-2020, 02:27 PM
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Well hell, I wasn't even wrong. lol. Notice I say per rotation. Work per rotation is higher in an overpropped motor. Same force to push the boat, more distance pushed, same work.
Old 01-09-2020, 02:28 PM
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You're missing something very simple. Tuning. What happens during and over-propped scenario is that the motor begins to detonate/ping as a result of the high load. Detonation is the gas/air mixture exploding independently of the spark/burn. Over propped causes high cylinder pressures because of the load vs available energy equation. The inconsistent cylinder burn acts as a striking force on your rotating assembly. This is brutal on wrist pins (connect your cylinder rod to the piston) as well as every oil based bearing. There is more damage, and more science, but we're on THT here...

The reason a supercharged or turbo'd motor survives, is because the fuel curve, and timing have been adjusted to deal with the necessary cylinder pressures among other concerns(in theory). in addition, compressed air means more fuel, more fuel helps cool the combustion chambers.

These are a few simple rules and in no way indicative of all the variables involved in either scenario. And unequivocally, under propping has been proven to damage motors. I've seen rod failures resulting in block failures on both Yamaha and Mercury 150's precisely because they could not turn rated rpm. The mercury 4-stroke was turning approximately 4400 WOT. Windowed the block around #3. Somewhere I still have half that rod since I used to keep it on my desk specifically to show people why you DON'T over prop.

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Old 01-09-2020, 02:33 PM
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Originally Posted by Jeepman View Post
A motor will have a max amount of power it can make. Over propping it does not increase the power a motor can make, it decreases it. The problems associated with overpropping do not come from the motor making more power, comes from possible pressures being made when the motor is not ready to handle them IE. early fuel detonation.

Boosting a motor will make it more powerful, but should not cause the pressures to build when are not suppose to. Some motors are way overbuilt, and this does not always mean they have added or wasted weight. A motor can be over boosted and parts fail, how much boost a motor can take is only found out by one method.
AT X rpms at stable speed, overpropping absolutely increases the amount of power being produced compared to the properly propped boat at X rpms. If I prop for 4000 rpms, then the overpropped motor is producing more power at 3000 rpms than the properly propped motor at 3000 rpms, assuming neither are accelerating or decelerating.

To make it even on the detonation, let's say I overpropped to 4000 rpms and adjusted the octane and timing to prevent detonation.

Boost would also increase pressures in the cylinder, and on the components.


Honestly, I think you guys who are upset with me are the ones who agree with me.
Old 01-09-2020, 02:37 PM
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Originally Posted by Texas 17 View Post
Well hell, I wasn't even wrong. lol. Notice I say per rotation. Work per rotation is higher in an overpropped motor. Same force to push the boat, more distance pushed, same work.
Well when your operating premise is from the planet of what the f&%k did he just say being not wrong is one of them oxymoron things...yes I'm hacking on you, all in good fun.

"Work" is a result, think wattage. Two otherwise identical boats but with vastly different engine packages traveling at 30 MPH - both engines are producing the same work.

A 475 Detroit that is both supercharged and turbocharged will merrily drag an 80K load all over the country and do it for a million+ miles....but shift it early and lug it....the party will come to a quick close.
Old 01-09-2020, 02:38 PM
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Originally Posted by Texas 17 View Post
AT X rpms at stable speed, overpropping absolutely increases the amount of power being produced compared to the properly propped boat at X rpms. If I prop for 4000 rpms, then the overpropped motor is producing more power at 3000 rpms than the properly propped motor at 3000 rpms, assuming neither are accelerating or decelerating.

To make it even on the detonation, let's say I overpropped to 4000 rpms and adjusted the octane and timing to prevent detonation.

Boost would also increase pressures in the cylinder, and on the components.


Honestly, I think you guys who are upset with me are the ones who agree with me.

Sorry, but no. The motor is not making more power by overpropping it.

Old 01-09-2020, 02:40 PM
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Originally Posted by Jeepman View Post
Sorry, but no. The motor is not making more power by overpropping it.
Very correct
Old 01-09-2020, 02:44 PM
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think speed is being confused with power

Dozer slow but powerful, Corvette fast but much weaker then the dozer. Think this may come from the fact "horse power" is used and has the word power in it, when actually HP is a measurement of time. How many miles per hour you are going has no bearing on the power at 3000 rpms.

Last edited by Jeepman; 01-09-2020 at 03:02 PM.

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