Importance of engine running at max RPMs

sebas54

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Why is it important to run your engine at the rated max RPMs? I know that it will lug your engine but what does that mean. What are the cons and ill effects of running under the rated max rpms? Will running under rated max rpms cause damage??
 

JJJ

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Re: Importance of engine running at max RPMs

Well there are a lot of pluses running closer to max rpms.
A. Usually better fuel economy
B. Usually better overall performance

An engine lugging all the time can start to run warm and if close to deto, it can start to have detonation - which can be fatal.

If you are way over propped, it often times makes it harder to plane out or stay on plane.

Many people think - lower rpms, better fuel economy, but this isn't always true due to running at a lower manifold vacuum ie. more throttle opening.
 

kenmyfam

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Re: Importance of engine running at max RPMs

It can be likened to driving uphill in overdrive. Works the rear end out of your vehicle and put's undue strain on everything. No gears on a boat so you want to have the ability to get to the WOT rpm range and have your motor in it's "powerband"
2 strokes especially thrive on this.
 

steelespike

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Re: Importance of engine running at max RPMs

There is no need to operate your motor at max rpm as long as it is propped the attain max rpm at wot(wide open throttle).
 

MikDee

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Re: Importance of engine running at max RPMs

Getting the right prop on your boat, is like having the right gear to keep your engine in the sweet spot of it's maximum rpm range. This keeps it running the cleanest, and most efficient, throughout it's full rpm range. Delivering best power, & fuel economy, keeping nasty combustion deposits at a minimum, and preventing excess stress on all it's parts. Basically, It takes the load off your engine, prolonging it's lifespan, no matter what throttle setting it's at ;)
 

Span 24

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Re: Importance of engine running at max RPMs

How much 'give' is available here?

I purchased my boat (20' Starcraft Aurora 2000 w/Volvo-Penta 4.3GL) with an OEM 15x17 prop. Volvo WOT specs for this engine is 4200-4800 RPM. When I test drove the boat we had 4 adults aboard and it ran nicely maxing out at about 4400 RPM.

I recently purchased a new (duplicate) prop, another OEM 15x17, because the old prop had two dime sized nicks (one each on the leading edge of two of the blades). I figured to retain the old one as a spare.

After installing the new prop the boat now runs (WOT) at about 3900 with 4 adults and two children. Now I am just guessing but I think that the boat is only carrying about 300lbs more than it did when testing. This includes the two additional children, differences in adult weights, and additional fuel (about 20 gallons).

Is the difference in RPMs due to the nicks in the old prop causing it to slip?

Do these numbers sound right?

Does the engine have a 'recommended' prop or are they matched to the hull? I cannot find anywhere what prop was shipped with the boat from the manufacturer.

Is the 300 RPM (slower than WOT minimum) enough to cause the kind of problems listed in the posts above? Should I try a different prop?

Michael
 

Bondo

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Re: Importance of engine running at max RPMs

Should I try a different prop?

Ayuh,... I'd go Down 2" in pitch,... You're lugging the motor at 3900rpms...
 

Tail_Gunner

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Re: Importance of engine running at max RPMs

Here is great explanation as to getting a engine to it max rpm range... What i have gleaned from the whole subject was the fact that the whole power train works as one unit.

Bob Bergeron

Editor's Note: Bob Bergeron is an expert in his field as a high performance specialist at Land and Sea.

powerboat_getting_it_on_the_curve_fig-1.gif
In past issues I have explained some of the methods used by outboard hot-rodders to increase the horsepower of their production powerplants. Often, however, even after modifications have been carefully planned and correctly executed, the engine's performance when reinstalled on the boat is quite disappointing. All too frequently after several attempts at tuning or trying to diagnose a non‑existent internal engine problem, the owner quits in disgust. He sells or scraps his high dollar powerhead, convinced that modified engines have no low speed power and not much more top speed than a near stock engine.
Fortunately, neither of those conclusions is correct! Assuming that an engine has been properly modified, it will be quite capable of delivering both a substantial increase in top speed and acceleration over its stock form. The ?secret? to achieving maximum performance lies in accurately placing the operating power/rpm curve of the modified engine in synch with the power requirements of the hull! Of course, this is not a problem exclusive to modified outboards, it's a concern for anyone trying to get optimum performance from a boat powered by an internal combustion engine.
To illustrate the importance of getting the engine operating on its power curve, let?s look at a typical example. We can take an average combination like a stock V‑6 outboard installed on an 18' V‑bottom. We?ll assume the owner (I?ll call him Joe) has already done extensive testing of his stock rig and has attained best performance driving a 14 x 28 Chopper prop at 5,800 rpm with a 1.78:1 ratio lower unit. Top speed from Joe?s combination is now 84 mph with good acceleration. Still, Joe wants more, so he decides to go the route on the powerhead by getting it modified.
powerboat_getting_it_on_the_curve_fig-2.gif
Before we go on, let?s examine Joe?s present horsepower vs boat speed curve. First, we need a wide open throttle boat speed vs rpm curve (Fig. 1). Joe could get this with his present tachometer and speedometer.
Next, we need to examine the stock engine?s horsepower curve. (Fig. 2) Most boaters won't have access to a powerhead dynamometer, but conveniently the power curves of identical brand, model, and year stock engines are typically very close. It is only necessary to find someone who has already done a powerhead dyno test on a stock engine identical to your own. If you can not get a power curve from the engine?s manufacturer, try one of the outboard race engine builders. They should be happy to supply you with dyno curves for their modified engines vs stock engines in the interest of getting your business.
Next, using the information from the two previous tests, we can plot the horsepower available vs boat speed in 10 mph increments (Fig. 3).
Now Joe sends his motor off for porting, installs bigger carbs, high compression heads, etc. He also has the engine dyno tested to generate a new power curve (Fig. 4).
powerboat_getting_it_on_the_curve_fig-3.gif
The results look impressive and Joe can hardly wait to get his boat on the water. Unfortunately, when Joe installs the modified powerhead he, like many hot-rodders, gets a rather shocking disappointment! The boat is slower accelerating and has less top speed (only 70 mph) then when the engine was stock. Why!? Take a look at the speed vs horsepower curve for the modified engine (Fig. 5) compared to the curve for the stock engine in Figure 3.
At every speed increment the available power is less than it was with the stock engine! At 5,000 rpm and 70 mph the boat?s load exceeds the power available from the modified engine. So Joe has a loss of 14 mph compared to the stock powerhead. Even acceleration. is reduced because the engine has less power surplus at each speed level. All that high rpm power waiting and Joe can?t even use it. Sad Joe, he needs to find a long down hill section of a lake to get his motor up to speed!
powerboat_getting_it_on_the_curve_fig-4.gif

Well, Joe may not find any navigable ?down hill areas? on the water, but he could regear the lower unit and realize the full performance potential of the modified powerhead. A change of the gear ratio from 1.78:1 to 2.2:1 would raise the engine rpm by about 24 percent at any boat speed. Take another look at the horsepower vs boat speed curve for the stock motor (Fig. 3) compared to the regeared modified engine (Fig. 6).
Now Joe's happy! Not only is there increased power available at every boat speed for considerably stronger acceleration, but top speed has moved up as well. Actually, if top speed were Joe?s only concern he could use a bit less gear to make the 270 hp peak intersect with the boat?s speed/power requirements at about 95 mph. (Power requirements increase approximately as the square of the speed increases.)
powerboat_getting_it_on_the_curve_fig-5.gif
Many boaters will wonder why the same effect couldn?t have been accomplished with a simple prop pitch reduction, saving the hassle of changing lower unit gears. First, realize that the most efficient performance props (in the types and sizes typically found in 60‑100+ mph outboards) all have a pitch/diameter ratio close to 2:1. Go too far up or down from this ?optimum? 2:1 pitch/diameter ratio and you will need more horsepower just to make up for the prop's reduced efficiency.
Actually if the operating rpm range of Joe?s engine had only been slightly off its power curve, then a prop change would have worked fine. But a prop change equivalent to the gear ratio change used in his case would have required dropping to a 22" pitch. Joe?s original prop was a 14 x 28. The only 22" pitch prop available in the same style also has a 14" diameter. So, while the original prop had a pitch/ diameter ratio of exactly 2:1, a 14 x 22 prop has, a pitch/diameter ratio of only 1.5:1 and therefore would waste valuable power due to its lower efficiency.
powerboat_getting_it_on_the_curve_fig-6.gif
Even if Joe was able to obtain an 11 x 22 prop of the same type, to retain the 2:1 pitch diameter relationship, he would still have problems. The blade area of such an 11 " diameter prop would not displace the required volume of water (at prop shaft speeds in the 1,500‑4,000 rpm range) to efficiently handle Joe?s power. Which is why outboard manufacturers use different gear ratios on various models rather than attempting to cover all applications with different pitch props only!
Try to follow this procedure whenever replacing a powerhead with one having a different power curve. First, insist that your engine builder give you a power curve for your modified motor. Pass this information, together with your stock combinations speed, power and rpm data, along to your prop man. Have him help you select a gear ratio to put your new engine on its power curve with a prop in the 2;1 pitch diameter ratio range. Then you can do your engine and propeller fine tuning from that point. Warning: if your prop guy isn't concerned with such ?details? maybe it's time to find someone who works with a calculator as well as a hammer.
 

QC

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Re: Importance of engine running at max RPMs

Love that post TG :cool:

Many people think - lower rpms, better fuel economy, but this isn't always true due to running at a lower manifold vacuum ie. more throttle opening.
You are correct, lower RPM, same load, does translate into a more open throttle . . . Buuuuutttt that is actually the reason that fuel economy improves. Messes with a lot of people's heads, but the main reason that spark ignited engines are less fuel efficient than diesels is a throttle, funny enough it is with it's closing that efficiency goes to carp. However, with the load of a typical marine application's propeller curve, and all of the other reasons posted above, gear-fast/run-slow (truck and car plan for efficiency) just doesn't work.
 

JJJ

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Re: Importance of engine running at max RPMs

"You are correct, lower RPM, same load, does translate into a more open throttle . . . Buuuuutttt that is actually the reason that fuel economy improves."

Are you saying this is for a boat, due to the way a prop loads an engine OR engines in general?
 

QC

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Re: Importance of engine running at max RPMs

Are you saying this is for a boat, due to the way a prop loads an engine OR engines in general?
Throttled (Otto cycle) engines in general. Hull efficiency throws a lot out the window, but all throttled, spark ignited engines are more efficient the more the throttle is opened. This does not necessarily mean max RPM, it means at whatever point of operation, throttled engines are most efficient with the throttle wide open.

Again, very hard to wrap your head around as running around at WOT whether it be a car or a boat would give very poor fuel economy, but you'd be going 117 MPH in your car for example, but the aerodynamics and other factors kill you.

Try to imagine a V10 with active cylinder management, so that you could turn off any number of cylinders you wanted. And lets say that the best compromise for combustion and parasitic losses was around 2500 RPM (made that up). So we set the engine up to run a fixed 2500 RPM all the time and for 10% power we run one cylinder with the throttle wide open and for 20% power we ran two cylinders at WOT etc. all of the way up to 10 cylinders WOT at 2500 RPM for max output. Never mind that we couldn't sell this vibrating POS :eek: But theoretically, this would be more efficient than the way we do it now by manipulating the throttle for 10% air to all cylinders for 10% power.

What you really need are 10 little engines and you turn one on for 10% power, and then two engines for . . . ahhh . . . never mind, let's just throttle one big one instead. Easier.
 

CharlieB

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Re: Importance of engine running at max RPMs

Great explanation of why my bored out SS396 can get 22 mpg 4200RPM 80 MPH with the throttle just touching but not opening the secondaries on an 850 Holley Spreadbore.
 

TilliamWe

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Re: Importance of engine running at max RPMs

Will running under rated max rpms cause damage??


Absolutely it will cause damage. Specifically to the valves first. Since "lugging" causes excessive combustion chamber heat, the valves get too hot and deform. This is called "tulipping" the valves. This will cause them to not seat properly, and you will lose comprssion on the affected cylinders. Loss of compression will lead to the affected cylinders to not fire properly.
 

MikDee

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Re: Importance of engine running at max RPMs

Getting the right prop on your boat, is like having the right gear to keep your engine in the sweet spot of it's maximum rpm range. This keeps it running the cleanest, and most efficient, throughout it's full rpm range. Delivering best power, & fuel economy, keeping nasty combustion deposits at a minimum, and preventing excess stress on all it's parts. Basically, It takes the load off your engine, prolonging it's lifespan, no matter what throttle setting it's at ;)

And I thought I pretty much summed it up Clear, & Concise :rolleyes:
 

kenmyfam

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Re: Importance of engine running at max RPMs

Yep I think some earlier posts pretty much summed it up !!!! in a nutshell.
 

JJJ

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Re: Importance of engine running at max RPMs

QC, yes that is correct in theory and regarding "engine" efficiency. Only because you are eliminating the work needed to fight the low pressure on the top of the pistons during the filling stroke.
 

QC

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Re: Importance of engine running at max RPMs

Yes ^^^ It is significant though, and usually referred to as pumping losses. I guess ultimately it is why people chase the opportunity to cut RPM down for cruise. The fact is that the load factor would be so high you end up with Tilliam's Tulips :eek:
 

sebas54

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Re: Importance of engine running at max RPMs

Thank you all for the great information!!

I was running a 14.5x18.5 prop and it was under revving my engine about 400-600 rpms, running at 4000rpms. I have a 4.3l merc. alpha 1. I ran with this prop for an hour or two. The next day the engine or outdrive had more vibrations and hummed a little. Could I have already caused damage?

I was thinking of going to a 15x17 but I not sure if the rated max rpms will be achieved. I was using a 15x15 last season and was running at 5000rpms. Does the diameter size change the rpms like the pitch?
 

TilliamWe

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Re: Importance of engine running at max RPMs

Diameter has a much less effect. Not like the pitch at all. And most of the time you don't get to choose the diameter. If you had a 17p prop to borrow, I would certainly try it.

You should not have done any real damage in 2 hours. Tilliam's Tulips (I love the term, QC, thanks!) took a little while to show up on my engine. Certainly not 2 hours.
 
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