engine dyno vs chassis dyno

[Marv D]

I don't put much stock in the things printed is a multi-page book of paid advertisings,, (HotRod, Carcraft, Super CHevy etc) but a friend sent me this and it was quite interesting...
Sorry for the long and unformatted cut/paste read, but worth it just for something to think about...

19. ENGINE DYNO HP vs CHASSIS DYNO HP

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Don’t put too much stock in Chassis Dyno data. Here’s why:
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From time to time, folks try to determine what their engine HP is by back calculating from the Rear Wheel HP data they obtained from a Chassis Dyno. Let’s take a look at what it takes to make sense of that.
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First we need to look at the 3, count ‘em 3, different correction factors in use.
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1. SAE J607 also called SAE STD, which is the classic Hotrod and Racing Engine correction factor used by most folks on an engine dyno. So, if you plan to compare Hotrod or Racing Engine dyno figures between various engines across the nation, you must use this correction factor in order to be on the same page with most everyone else. It is corrected to 60*F, zero % humidity, and 29.92” hg. This one gives GROSS HP, and excludes the use of accessories, full exhaust system, full air cleaner, or any emissions equipment. Since this correction factor has the most favorable correction conditions, it will of course provide the highest numbers of all the correction factors shown here.
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2. SAE J1995 also called SAE GROSS, was used by the OEM’s through ’71. It is corrected to 77*F, zero % humidity, and 29.234” hg. This one gives GROSS HP, and excludes the use of accessories, full exhaust system, full air cleaner or any emissions equipment. The results using this one, are usually somewhere around 20% higher than SAE NET HP figures.

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3. SAE J1349 also called SAE NET, has been used by the OEM’s since ’72. It is also corrected to 77*F, zero % humidity, and 29.234” hg. But this one gives NET HP, and DOES include the use of accessories, full exhaust system, full air cleaner, and any emissions equipment.
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Since each correction factor will provide different HP results, when it comes to trying to compare and/or calculate one way or the other, between engine dyno numbers and chassis dyno numbers, you MUST use the SAME correction factor for both the engine dyno and the chassis dyno. This will keep things an apples to apples comparison and/or calculation.

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If you don’t use the same correction factors, you end up with, at best, an apples to oranges comparison, or at worst, an apples to elephants comparison. Neither one of these is much good for back calculating engine HP from rear wheel HP.
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When Engine and Chassis Dyno numbers are compared properly, by using the same correction factor, the most widely accepted drive train loss figures for non-IRS cars have typically been around 12 to 15% for stick cars, and around 25 to 30% for automatics.

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So, let’s look at some real world Engine Dyno vs Chassis Dyno tests, to see how things typically shake out:
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‘70 Chevy Nova
555ci BBC
675 HP from Engine Dyno using SAE J607/SAE STD correction factor
9” rearend (not IRS)
Powerglide automatic

1. On a Dyno Dynamics Eddy Current chassis dyno with 2 smallish 12” diameter rollers, using SAE J1995/SAE GROSS correction factor. This is an apples to oranges comparison because of incompatible correction factors, and Rear wheel HP = 487, for a 28% drive train loss. The loss was so high here, not only because of the incompatible correction factors, but also because under load, the 2 smallish dyno rollers caused MAJOR out of shape, HP robbing, tire distortion.
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2. On a Dynojet Inertia chassis dyno with 1 large 24” diameter roller, using SAE J1349/SAE NET correction factor. This is an apples to elephants comparison because of REALLY incompatible correction factors, and Rear wheel HP = 564, for a 16% drive train loss. The larger dyno roller did not cause any HP robbing tire distortion here.
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The results between these two chassis dyno’s varied by a whopping 77 Rear wheel HP, or 12%. This leaves you with absolutely no chance of back calculating engine HP with any degree of accuracy. Even if you threw out number 1 with all the tire distortion, and looked only at number 2, you still couldn’t accurately back calculate engine HP. Because that 16% loss is considered to be more in line with stick drive train losses, when compatible correction factors are used, and NOT automatic drive train losses when significantly incompatible correction factors are used.

The results here just leave you scratching your head. So, you can’t accurately back calculate engine HP and you can’t even feel confident about how much HP you are actually putting to the ground either. This certainly questions the value of using a Chassis dyno at all.

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There is a lot more research and cars tested on various brands of chassis dyno starting around article 19 down in the bottom of this page. Seems Mustang and Dynapack brands of chassis dynos are amongst the worst out there .

In the end,, a Dyno (engine or chassis) is just a tool to get the motor in tune,, and if you expect any reliable calculated results at the drag strip (in anything but a $100,000 prostock chassis) Your kinda fooling yourself anyways. But this sure makes for an interesting read.

https://540ratblog.wordpress.com/201...-test-ranking/

The whole page is worth a read if you have the time (especially the oil section)