thanks for the education, it helps me understand some of the things the dyno guy is telling me. He did not build the motors, so I think he is shooting straight. This information helped me know enough to ask far more intelligent questions.

So now I can sound like I know what I am talking about when I pester him

Good stuff rmbuilder!!!

Personally,I would like more dyno information on the printed report.The typical report starts at 3500 to 4000.

I would like to see simulation numbers starting at free idle,loaded idle and then on up through the power band to red line....That information may provide a bit of insight for props and drive gearing.

In fact,I'm still of the opinion,it starts with dock manners and Idle Torque.
Further,in my opinion,the cat and V hull may not need the same torque curve,from start to finish.

Bob,keep up the good work!

I am kinda surprised no one mentions the exhaust used on the dyno...or maybe I missed it. Let's say you dyno a 572 n/a with a dominator that shows perhaps 700 hp at 6000 with "dyno" headers that are dry, maybe 2.250 dia and equal length runners maybe at 34" or so. What happens when the customer slaps on his Gills or the CMI's that are on a Merc 525 and sets his gearing/prop to run at close to 6000? Roughly how much of a change in hp will there be? I realize we can only do an educated guess as it aslo depends on other variables but I would be interested in hearing some opinions.

Deduct 30-40 HP...

Along these same lines, what about accessories? You read all this stuff about changing pulley's and the like to pick up 12 HP each by slowing down the accessory. What does a waterpump, power steering, raw water pump, etc. take from final HP output, when added, if they were not on the engine during Dyno testing?

How many builders dyno with the customer's exhaust? I have had KE's folks run my motors a few times and they always requested that the motors be set up with my actual exhaust, alt. water pump etc. Is this the norm? If not, why?

My strong feeling is this - if an engine shop specializes and/or markets in performance marine engine building they should have their dyno set up for wet exhausts.

It can be a pain to set up this way, but what heck. A dyno should take out less guessing, not make more guesses.

I here an engine builder posting remarks in this forum and other forums to deduct 50-75hp-100hp. This is on many different engine builds.

One motor is not another, further more, exhaust physics class #101 says, a Gill is not a KE, is not a CMI, is not a Stellings is not a Lightning, is not a Merc, is not a Revolution. Furthermore, most companies make atleast a few different styles/sizes that will directly impact power #'s.

I personally don't think it is acceptable to throw #'s out in the wind. Sorry if I stepped on toes for this, but, heh, it's just one guys opinion.

If the customer does not have his wet exhausts sent to engine builder, then there is not much you can do about it.

If you are going to have estimated #'s + tune after the dyno, then why even bother ?

I had my boat prop shaft dyno'd, engine hatch closed, and all accessories. It came back lower than i thought, but i do believe the number.

Here is a real world example of how correction factors would be utilized in testing an example 100 HP engine @ varying temp and baro. As you can see a little correction factor goes a long way. If you stay within your baseline (only correcting for the variance between sessions) you can make useful and productive changes that you can both qualify and quantify. Once you lose your baseline, all bets are off.

Remember, tune for the application (boat/car etc), not for the number. You pay $400-$700 per day for dyno time to run on the water, not on paper.

Ideally you want to control and stabilize the conditions as much as possible from session to session. Failing that, it is imperative you keep the correction factors as accurate as possible relative to baseline to track incremental changes in your tune or component setup.

Pressure, Temperature, and SAE J1349
The Society of Automotive Engineers (SAE) has established a test standard that helps standardized engine horsepower testing and results so that the variable effects of barometric pressure, altitude, and intake air temperatures do not bias the test results. The SAE J1349 test procedure includes an engine horsepower correction factor so that, for example, dyno readings taken at 3500 feet on a 40 degree day can be compared with dyno readings taken at sea level on a 77 degree day.

This correction factor is used for Normally Aspirated Engines, not forced induction engines. Once the correction factor is determined, it can be applied to the actual dyno readings so they can be adjusted back to simulate a test conducted at sea level, on a 77 degree day, with 1% humidity.

The SAE correction factor can be approximated using this equation:

CF = 1.18 * (29.235 / Bdo) * ((square root (To + 460) / 537) - 0.153)

where CF = the correction factor, Bdo = the dry ambient barometric pressure in inches of mercury (in/Hg), and To = the intake air temperature in degrees Fahrenheit.

Test 1: The Baseline Test
Let's test this equation with a hypothetical engine that dynos at 100HP. We test this engine on a 77 degree day, at sea level. So, we set Bdo = to 29.235 in/Hg and To = to 77F. When we solve the equation for CF, the correction factor equals 1. That means according to SAE, our dyno reading does not require a correction factor for temperature or barometric pressure. It is a true 100HP engine.

Test 2: Temperature = 87 degrees F, Pressure = 29.235 in/Hg
What happens when the temperature climbs by 10 degrees, but pressure stays constant? Plugging in 87 for To and 29.235 for Bdo, we can calculate the value of CF. CF = 1.0104. Working our correction factor equation backwards, we take:
100hp / 1.0104 = 98.97hp.
So, according to the SAE correction factor, a 10 degree increase in temp should result in a loss of 1.03% of rated horsepower, or 1hp on our engine.

Test 3: Temperature = 77 degrees F, Pressure = 28.235 in/Hg
What happens when the pressure drops by 1.0 in/Hg, but temperature stays constant? Plugging in 77 for To and 28.235 for Bdo, we calculate CF and find it equals 1.042.
100hp / 1.042 = 95.96hp.
So, according to the SAE correction factor, a 1 in/Hg drop in air pressure should result in a loss of 4.04% of rated horsepower, or 4hp on our engine.

I hope this leaves many with a better understanding of how to best utilize a dyno for maximum performance.
Bob

Good stuff !!! Glad this was brought to the public eye.

Tomcat and other centrifugal guys - with the supercharger usually right over the exhaust, this must make it extremely difficult to get correct correction factor #'s.

I've thought about this quite a bit (haven't dyno'd centrifugal motors - just N/A 'd) and have come up with no clear answer.

Possibly do it off of inlet temp's after intercooler ? Air temp entering the motor is the most important air temp right ? to Or the one I keep coming back to - forget about correction factors and do it straight out.

Lastly - spend good amt of time to make unrestrictive inlet tube to frt of supercharger to stave off the header heat. I think this may be the best bet. PITA I know, but with all the testing you seem to do I think it would/could give more accurate data for you ?

Intake air temp is a huge factor in correction factor and thus has to be kept in check.