Strip Poker 388

iTrader: (6)

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articfriends

iTrader: (1)

I tried to do exactly what you mention,make a air inlet tube to feed supercharger. With my stainless marine high tq 3's the supercharger is literaly wedge against exaust manifold. I had to notch the air bonnet suction screen and saw 3/4 inch off supercharger casing just to get it to fit at all. So I'm pulling intake air directly off a exhaust manifold which doesn't help power at all. Heres a pic to show what I mean,Smitty

Strip Poker 388

iTrader: (6)

Bob how much difference does the barometer make on a SAE correction factor if they used 15% on a motor that dynoed at 750HP .I am trying to get a better understanding .

Thanks
Rob

SB

iTrader: (1)

Barometric pressure. What a beatch.

Two things influnce it:
Weather
Altitude

If your at sea level it's quite rare for the BP to go below 29" or above 30.5" or so.

Example for extreme sea level barometric conditions here:

rv

Rob,
Using the formula from RMBUILDER for an engine that dynoed at 667 and corrected to 750hp (12.4% increase), 2 of several possibilities are as follows:

Air temp at 77 then the BP would have to be 25.98.
BP at 29.235 then the Air Temp would be 133 deg.

It is impossible to tell without know at least one of the two variables.

Makes you wonder what they were.

Rick

SB

iTrader: (1)

Math was off. Sorry. Whoever saw this forget about it.

JCPERF

What are you trying to say youngster?

rmbuilder

Rob,
First let me point out that no matter what correction factor (SAE J607 or J1349) you are using in your test session, the SAE clearly states that if correction factors in excess of 7% occur during the testing, the results are invalid and the input criteria must be brought to within accepted parameters. Simply put, valid dynamometer testing is not conducted under any conditions cited above. Testing is never valid with 133º F ambient temps at the inlet, or in the eye of category 3 hurricanes. The only way this level of C/F can be arrived at is to source 133º ambient air, or externally heat the temp sensor and manually input grossly depressed baro/altitude readings. All creditable dyno facilities and R&D programs use control limits on these inputs to keep them within acceptable levels. When you alter the atmospheric inputs you cannot control the accuracy and repeatability of your pulls and your time, money, and “data” are wasted. I have been to facilities that are repeatable to within <1 HP

Bare in mind, a 15% C/F on an engine making 650 OBSERVED HP is 747.5 CORRECTED HP. That calcs to 97.5 additional HP due to “atmospheric conditions”. That is not a legitimate test.

This is a good example of the level of variance in correction factor due to prevailing conditions:

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.

It required significant input to raise the C/F 4%. Also if temp goes below 77 deg F and baro rises above 29.235” the result is a negative correction factor. (Example:55 deg/ 29.92”) Corrected HP will actually be less than observed HP. Easy to do, but hard to find.






Rick,
You can quantify and qualify the variables in these equations by checking the historical data archived on sites like weather underground. All conditions are listed for all regions in the USA by date, hour, and location. For example, if you need the atmospheric conditions for Atlanta, on 9/19/2005 at 3:09 PM, the site will provide you with data from multiple locations in the Atlanta area for that period. Plug that in, with your elevation, and your C/F will be accurate +/- 2 tenths of a %.

Attached is a program used for testing to evaluate and graph BSFC # vs. TQ, BMEP, etc. It also gives a peak into correction factors.

Bob

Michael1

If this is the case, you can throw out just about every dyno run as being too high. SAE J1349 states a standard barometric pressure should be 29.235, and standard temperature of 77 degrees. That's a mile from 29.92 pressure and 60 degrees. Add this to the fact that most of these tests are not run as installed in the boat (accessories missing, different intakes, different exhausts, different fuel, non-stablized tests run on the fly, etc), and the conclusion is, THESE AFTERMARKET DYNO TESTS ARE A JOKE.

Michael

rmbuilder

Michael,
These are two seperate industry standards:

STD J607
J607 standard 60°F / 0% humidity / barometric pressure of 29.92 in-Hg

SAE J1349
SAE J1349 standard of 77°F (25°C) / 0% humidity / barometric pressure of 29.234 in-Hg (99 KPa).

The J607 rates 4-5% higher than J1349 due to a compensation for motoring friction loss.
Bob