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Author Topic:   Comparing old HP ratings to current rating
tomstin posted 04-22-2010 11:10 AM ET (US)   Profile for tomstin   Send Email to tomstin  
I am restoring a family heirloom (70 Delquay Dory) and looking to replace the 1973 Johnson 40 HP two stroke. With the change in HP calculations from the head to the prop, how does the old 40 HP compare to today? I'm a little concerned about the weight of the new 4 strokes. Would a new 30 HP performance roughly compare to the older 40s?
dgoodhue posted 04-22-2010 11:42 AM ET (US)     Profile for dgoodhue  Send Email to dgoodhue     
In the 80's when the change happened, the difference was claimed to be 10%
Buckda posted 04-22-2010 12:02 PM ET (US)     Profile for Buckda  Send Email to Buckda yesterday's 40 HP motor is really a 36 HP motor.

Plus, with the age of your motor, you've likely lost some performance - but I'd still replace your current 40 with a new 40 - not downsize to the 30....

Tohsgib posted 04-22-2010 02:05 PM ET (US)     Profile for Tohsgib  Send Email to Tohsgib     
With that 1973 40 you have, I think modern 30 would be even faster. That 40 has the old style lower unit and shear-pin prop, they are not a great performer. Head to foot ratings are usually about 10-15% like Buckda stated. My friend had a 13' with a 1973 Evinrude 40 on it and my 1982 35hp would beat him no problem. A modern 30 is better than my old 35 which was still powerhead rated.
pcrussell50 posted 04-22-2010 04:45 PM ET (US)     Profile for pcrussell50  Send Email to pcrussell50     
I'm guessing your old boat is not going to be used for luring other unsuspected boaters into a race, and the surprising them, by beating them. If my guess is correct, why not continue to use the old motor? ... unless it's got expensive problems like a bad lower unit or worn rings? Those are about the only things that justifiably take those old OMC twins out of service.

I have a 1959 35hp Super Seahorse that is as reliable as the sunrise, affixed to a 1960 classic fiberglass runabout. It runs too deep in the water and still motors along quite happily at 29mph or so. Now, it _does_ require "tuneups", just like cars from the 60': points and condenser, change of a fuel filter, every 100hours or so. EASY DIY stuff, and parts are cheap. Now, I'm not sure I'd recommend it for heavy, daily, commercial use, (only because I've never tried it).

But you're talking about a restored classic, right? How often and how hard do you plan to run it? I use my classic on nice days and for a few miles here or there of the local harbor. In a year and a half, I haven't put anywhere _nearly_ 100 hours on it, so it's not even out of tune yet.

Keep your Whaler for the serious stuff, and keep your old classic an... old classic.

My $0.02


Moby Dick posted 04-23-2010 08:17 PM ET (US)     Profile for Moby Dick  Send Email to Moby Dick     
My Montauk has a 1979 100HP Johnson and while it was fairly close, it would outrun a newer 90HP Yamaha. So I would say maybe not quite 10% less.
Backfire posted 04-23-2010 10:52 PM ET (US)     Profile for Backfire  Send Email to Backfire     
In 1984 the outboard [manufacturers] started to rate engines at the propeller shaft instead of at the power head. A power head running open exhaust or not, variable conditions, not a strong set of rules and specifications, not a good comparison between brands. The propeller shaft rating is closer to as-manufactured, and some engines revised their decals downward to show the change.

Peter, your 1969 35-HP was one of the best during its time. Most of the heavily used 1969 motors finally gave up when the wrist pin needle bearings came out of the cage, caught in the ports, stuck in the head, or the top of the piston. Just a guess on my part, give or take 1,000 hours. As good as that engine proved to be, the worst power head they built was the 1960 40-HP. OMC slightly improved the 40 in 1961, and totally new engine, excellent, in 1962.

jimh posted 04-25-2010 11:20 AM ET (US)     Profile for jimh  Send Email to jimh     
Many thanks to Backfire for his mention of some of the variables that could have affected horsepower rating at the power head compared to rating at the propeller shaft. In most prior discussions of this topic, the general assumption has been that the difference in horsepower between those two points was primarily due to power lost in the mechanical inefficiencies of the gear case and drive train.

The figure of ten-percent loss from power head rating to propeller rating is often tossed around in these discussions, and it seems to be generally accepted. If making a real-world comparison of a current engine with one made prior to 1980, you should also consider the effect of thirty years of running. An engine 30 or more years old may no longer make its original rated horsepower.

Today horsepower rating seems to be much more accurate because just about every engine must be certified for its compliance with EPA exhaust gas emission. The level of exhaust gas emission is determined as a rate of emission per unit of power of the engine. Consider an engine which is rated as 100-HP. As a 100-HP engine it must produce not more than a certain amount of exhaust gas emission in order to get under the bar of the EPA limit. If we suppose the engine is having a problem meeting the limit, one way to help it pass would be to overstate its power. If we call the engine a 115-HP engine, it will be permitted to produce more emission gas output. In this way an engine that was not really compliant could be shown to be compliant. Of course, I am sure that the cadre of mechanical engineers in Ann Arbor, Michigan at the EPA testing laboratory are smart enough to see this possibility. So, accordingly, they insist on the engine horsepower being accurately measured. This prevents a manufacturer from dialing up a favorable rating for emission by just cheating on the horsepower. Rating an engine higher than its actual horsepower makes it easier to certify as EPA compliant.

This same problem applies to the other direction of horsepower rating. We often hear talk about how such and such a manufacturer--almost always Mercury is mentioned--makes engines that are really much more powerful than their rated horsepower. Considering this proposal in light of emission regulations, the effect of this would be to require that the engine produced even fewer emissions. Going back to our 100-HP engine, suppose the manufacturer tried to declare it a 90-HP engine. As a 90-HP engine it would be allowed less emission output than a 100-HP engine. So if the engine were really a 100-HP engine its combustion would have to be even cleaner than normally required to meet the emission certification, because its allowance of exhaust gas emission would be reduced to correspond to a 90-HP engine. Rating an engine lower than its actual horsepower makes it more difficult to certify as EPA compliant.

Now we have examined the two trends of horsepower rating with regard to EPA certification. In the first case, rating higher than actual power would help the manufacturer to comply. It is in the self-interest of the EPA to prevent this. In the second case, rating lower than actual power would hinder the manufacturer to comply. It is in the self-interest of the manufacturer to avoid this.

Now we see that in regard to emission certification testing it is not in the self-interest of the EPA or the manufacturer to use power ratings that are higher or lower than the actual engine horsepower. From this it is reasonable to conclude that horsepower ratings used today are quite accurate.

Although it is fashionable and still popular to suggest that certain engine manufacturers--and again this is almost always attributed to Mercury--produce engines whose actual power output is higher than the rated power, it appears that the influence of the EPA emission certification works to prevent this from happening.

tomstin posted 04-28-2010 09:06 AM ET (US)     Profile for tomstin  Send Email to tomstin     
"I'm guessing your old boat is not going to be used for luring other unsuspected boaters into a race, and the surprising them, by beating them. If my guess is correct, why not continue to use the old motor? ... unless it's got expensive problems like a bad lower unit or worn rings? Those are about the only things that justifiably take those old OMC twins out of service."

Peter, you are correct, no plans to race this thing, never was very quick. I think the Dorys are a bit heavier than the Whalers anyway.

The motor is in good shape. I had the head off a few years ago after a spark plug stripped and the cross hatch was still visible in the cylinders. I would guess this motor has maybe 50 - 60 hours on it total. It's been stored properly over the years.

My motivation to considering replacing is the new engines are more environmentally friendly and fuel efficiency. Also, if I restore the gel coat, I may as well take care of the 15 inch transom at the same time.

Thanks for all the input. I looking at all my options before I take on this project. Did it half way a few years ago, and if I go at it again, I want to do it right and finish it.

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