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)

[cableguy0]

Dyno numbers are great for stuff to brag about. Every single dyno seems to kick out different numbers for the same engine even with the same correction factor. Im honestly shocked that in 2015 you still cant find an accurate dyno. It doesn't make any sense.

[Hollow65]

I thought hp was figured by dividing peak torque @ rpm by 5252 or something like that. Dynos don't mean anything IMHO. Track tested is where it's at. I ran 12.2-4 @ 118mph with a 3800# truck. That's around 450-540hp depending on which calc you use. If I went off just the engine dyno hp I should run 10.1-3 @ 130mph or so. So going of dyno numbers I can tell people I have a 10 second truck. When in reality I have a 12 second slip and slide machine. I'm sure a 400hp car that hooks can hand me my ass from a stop. From a roll, not happenin..

[Super73]

I have a lot of time on chassis dynos and can tell you, one must read the fine print which most don't want to share.

[BR3W CITY]

I've only used an engine dyno 2-3 times, and I always liked the "no BS" results. I know that there are still correction factors for temp and load, but not being able to argue things like parasitic loss and other less-quantifiable factors is nice.

I will say that the differences in result with DynoJet vs Mustang dyno's alone causes arguments and butthurtness in reputation. Mustangs tending to be "stingy" but lately I feel like they've been the most "Accurate" to expecations (if that makes ANY sense). Dynojets have always given me more favorable #'s, but with much less repeat accuracy.

Have you guys ever tried those axle/hub-mount dynos? The ones where you bolt the rear hub up to a torque tube on an engine-dyno looking machine. They don't have the tire distortion or slip issues, but still get the rear wheel numbers. I haven't seen any big (8-900+) power cars run on one.

If you consider those trailer-dyno's in the mix, they might as well be throwing numbers at a dart board.

I will say that the excerpt kinda ignored the fact that those dyno's aren't locked to those correction factors.

[Marv D]

just IMO here but the ONLY important factor is getting a tune and monitoring the motor for ANY hiccup. If there is ANYTHING froggy it will show up on a good dyno measuring EGT in all 8, O2 in both banks, fuel flow for BSFC, monitoring oil temp/psi, water temp, air intake temp mass flow, bla bla bla.. and any bumps or dips in the torque curve through the rpm range. The actual number is somewhat meaningless unless your using the same dyno to compare changes in the motor / build.

Just like flow benches,, the Superflow computer controlled dyno / flow bench (like the 901+ series that controls the engines RPM sweep per sec) seems to be one of those that appears to be repeatable dyno to dyno. I'd like to think they will be consistent like the SuperFlo flow bench is. I had a set of heads flowed on Bischoffs flow bench in MI, then flowed in Tucson on another SuperFlo 600 series bench and was within 3cfm (@ 370cfm)
Now that SuperFlow is getting into the chassis dyno it will be interesting to see how they compare to others out there for consistency.

Flow numbers aren't everything just like data from the dyno. It's just one more 'tool' at our disposal to see if things are moving in the right or wrong direction

[Hollow65]

When was the last time someone here has been on a dyno? I haven't been since 99-00. Back then 10 seconds was real fast. Now we have 7 second street cars. If I made 642 then on an old school dyno, I'm sure my numbers are different today. Super73 spent a lot of time on a dyno trying different camshafts. I'm curious which one made more power and which went through the rpms faster? I've heard it's better to be able to rip through the rpms range than to be pumped about a peak number. The torque curve is where it's at.

[71Dragtruck]

Like Marv said my opinion is the same, dynos are for tuning and break in only, using as a tool to try different combos and settings, the numbers are for comparison on that machine only.

It's funny how I know of trucks that on the engine dyno their motor made 100hp more than mine yet on motor they run high 11's and on motor I run low 11's, weight likely about the same, I have a lighter front end they don't have a cage, both on slicks. Now I know setup has lots to do with that, but that's a large difference in time when I'm suppose to be making 100 less hp. Just comparing engine specs I know their numbers are bull****, but hey if that's what the engine builder told them and there's no way you can convince them otherwise they can quote numbers all day but in the end timeslips at the same track tell the truth.

[BR3W CITY]

Originally Posted by Hollow65

When was the last time someone here has been on a dyno? I haven't been since 99-00.

I haven't had mine on a dyno in like a year and a half. I just did heads/headers/mounts/massive tune change so its needing some dyno time. You can do so much stuff now with the eddy current (sp?) and roadforce dyno's to make false load and all that, that things have become soooo convoluted.

I've heard the conversation is even worse with diesel guys. They have been using them to put huge amounts of load onto the drivetrain so they can come up on huge boost #'s really early (without having to load up their own brakes). You can swing TQ #'s significantly by doing so.

[Marv D]

BR3 you think the diesel guys are bad.. And you REALLLy want to have fun.. let a owner of a C-5 /C-6 Vette tell you how the HP and Torque lines 'cross' at a lower RPM than our 'old school ' carburated cars. (rolling eyes)

I just smile and say "OooooKay then" (and think to myself... I should ask how much HP that new $2000 carbon fiber hood added)

[Hollow65]

Isn't the dude that wrote that on chevelle tech? I read the whole thing, kinda overwhelming. Had some interesting thoughts on cam lsa vs overlap. Also the compression notes were a good read. Marv what did your engine put down? Ever test it?

[Marv D]

Umm which one?
Of the latest I have races

The 434 small block in the nova 797HP Runs 9.teens @ 145 in good air

The 434 SBC I just took out of the truck, 666HP and that possessed SOB just came out and on an engine stand to learn some respect! Took the 3550 pound truck to 10.4's in 3500' air

The old 383 I took out struggles to make 587HP

The sand rail made 460HP

LOL,, If you couldn't guess I spend a day tuning on the engine dyno getting the motor RIGHT,, then it's just small adjustments once it's in the vehicle.

[BR3W CITY]

Originally Posted by Marv D

BR3 you think the diesel guys are bad.. And you REALLLy want to have fun.. let a owner of a C-5 /C-6 Vette tell you how the HP and Torque lines 'cross' at a lower RPM than our 'old school ' carburated cars. (rolling eyes)

I just smile and say "OooooKay then" (and think to myself... I should ask how much HP that new $2000 carbon fiber hood added)

Hahah hey, I make good money off c5 and c6 vette owners. We've adopted a "write checks until your happy" shop policy.
...and besides...I haven't had a carb in this thing in years

[hotrod 80]

I try to get on a chassis dyno once a year . I don't care what the number is I just try to inprove it . I am more about getting the A/F ratio correct then anything else . I look at it like a controlled pass down the drag strip . I get a printout of whats going on and I don't have to drive it . Most of the time I can get 3 or 4 passes in a test and tune night . I usually have no problem getting 10 pulls in on an afternoon dyno session with jet/timing changes between pulls

[vin63]

With so many correction factors available by the dyne makers to skew power figures, I tend to also keep an eye out for a particular metric. I usually request for no correction factors to be used, and I tend to be more interested in the brake specific numbers through the entire RPM range of the pull.

[moggey01]

The fans in the room and how they get the air effects engine dynos. A trick that needs to be looked at are acceleration rates of the engine. In other words 2 engines can make the same power. The 1 that accelerates through the pull faster will be better. It's not just a h.p.measurement tool. You can make a lot of power with camshaft that are too big ,carbs to big manifolds and intake ports. Shows numbers and won't accelerate.