Learn me...Dyno! "at the wheels" vs "at the flywheel"

So Saturday was our annual SCCA Dyno Day at the local shop owned by a member and racer. Two years ago the Abarth had about 400 miles on it and I didn't trust it on a dyno just yet, last year I kept letting newer members go ahead of me and ending up running out of time. This year I finally got to run it.

Stock is listed at 160hp and 170ft# torque. It posted 177hp and 210ft# torque pretty consistently over 3 runs, with a cold air intake and Uni Chip with a new map. I was mostly curious to see what the chip might have done.

Afterwards I had a few people telling me that the 177hp at the wheels would mean with FWD and 10% it would be more like 197hp at the flywheel, and now 37hp over stock. I thought the OEM rating was at the wheels as well?

I have a feeling this has been covered before but I was hoping some could shed a little light on the subject for me. (I'm happy with the 177hp!)

OEM Hp ratings are almost always at the flywheel. I think it is mainly a sales tactic. "300hp engine" just sounds better than "260hp at the wheels".

Also, if you are only losing 10% through the drive-line, that is a very efficient setup. My tuner always says 15-30% driveline loss. 15% for FWD (being the most efficient) and up to 30% for AWD/4WD (being the least efficient).

In reply to NordicSaab:

Someone that I trust very much for car knowledge did say he thought it was more like 15%.

Only 10% is very efficient. Most usually use 15% roughly, although I've seen up to 25% drivetrain loss. Also, keep in mind, OEMs test with a set standard that you may not be replicating. Plus, correction factors on dynos can cause some wild inconsistencies (A few taps of the keyboard can make a 200hp make 1000hp at the tires). As a whole, chassis dynos are less useful for measuring actual output and more useful for measuring the effects of changes.

FWIW, the power is done via a set procedure anymore- and it witnessed so that it's "real". The engine is fully dressed (full FEAD, full exhaust, correct intake) and is run at a specific temperature for both the air and engine coolant.

Dyno is directly connected to flywheel.

This way, the measurement is consistent with Brake power and not net power (which is used to be 40 years ago).

I don't understand why it should be a percentage of power lost to driveline gremlins. If I bolt a 200 hp motor to the same transmission as I pair with a 1,000 hp mill, how does it use up 5X on the big motor?

I'm not even sure I'm asking this right..... it just feels like a weak motor would suffer a greater percentage of loss to driveline friction than the more powerful engine.

There's also the Dynojet thing... They tend to read a good bit higher than a Mustang dyno or other load bearing dyno (Dynojets are inertia dynos). As a result of this, reductions in rotating weight will show as a power gain on a Dynojet if you don't correct for them, while they'll show far less on a Mustang dyno.

rslifkin wrote: There's also the Dynojet thing... They tend to read a good bit higher than a Mustang dyno or other load bearing dyno (Dynojets are inertia dynos). As a result of this, reductions in rotating weight will show as a power gain on a Dynojet if you don't correct for them, while they'll show far less on a Mustang dyno.

As well as the dual-roller vs single-roller debate. The setups with dual rollers were the wheel sits in between them have been known to rob power because of the tire deflection.

All I know is, a 150hp 90s car only makes about 120hp on a chassis dyno, while a 150hp modern makes 150hp on the same chassis dyno. Horses are bigger these days.

KyAllroad wrote: I don't understand why it should be a percentage of power lost to driveline gremlins. If I bolt a 200 hp motor to the same transmission as I pair with a 1,000 hp mill, how does it use up 5X on the big motor? I'm not even sure I'm asking this right..... it just feels like a weak motor would suffer a greater percentage of loss to driveline friction than the more powerful engine.

It's just an estimate. Doesn't really meany anything, as it barely even matter except for people who care about what the SAE rated power of the engine is.

What you and I feel is the power delivered via the wheels, and that's all that really matters. The rest is advertising and bragging.

Unless you really want to increase the efficiency of your total powertrain.

If nothing else I got to hear the exhaust at full tilt from outside the car.

just drain the oil out of your manual transmission. efficiency will go up for a short while before the thing grenades.

Robbie wrote: just drain the oil out of your manual transmission. efficiency will go up for a short while before the thing grenades.

Reminds me of back when there was that heads-up crate motor class (Early 2000s, not the current crop like Coyote Stock) I read an in-depth article on the class leader's Fast Burn 383 crate that they had massaged. One of their tricks was only 2.5 qts of oil in the 7 quart pan to reduce weight and windage. They said by the end of the quarter mile pass the pan would be completely empty and the oil pressure warning light would be on.

The OEs are required to use a certain measurement standard, just like they are for fuel economy. It's the aftermarket that's dodgy. Interestingly, GM quotes SAE numbers for their crate engines that are found in production cars (such as the LS3 crate) but something more optimistic for the crate engines that are not (such as the LS3 crate engine with the Hot Cam).

KyAllroad wrote: I don't understand why it should be a percentage of power lost to driveline gremlins. If I bolt a 200 hp motor to the same transmission as I pair with a 1,000 hp mill, how does it use up 5X on the big motor? I'm not even sure I'm asking this right..... it just feels like a weak motor would suffer a greater percentage of loss to driveline friction than the more powerful engine.

There are two aspects to driveline loss: frictional and inertial. Frictional losses are pretty easy to understand and are fairly consistent. Figure the driveline (and rollers, etc) will suck up a fixed amount of power. So a 100 hp and a 200 hp version of the same car will have similar frictional losses.

Inertial are more interesting. They're based on the rate of acceleration. A dyno that measures power (torque, actually) at set rpm points with the engine at a constant speed will have zero inertial losses. One that runs through a sweep quickly will show much higher inertial losses than one that runs through the sweep slowly. On a sweep, the inertial losses are closer to a percentage - that 100 hp car will have half the inertial losses of a 200 hp car.

Fun dyno chart. This is the same car on the same dyno, but we took 15 seconds for the full (red) vs 25 seconds (blue). The difference is inertial losses.
https://www.flyinmiata.com/tech/dyno_runs/NC_sweep_times.pdf

If you're a tuner trying to exaggerate the gains on your dyno, you'll always use a percentage. On a 1.6 Miata, you see about 25 hp between the published flywheel specs and the measurement at the wheels on something that reads like a single-roller Dynojet. That's about 25%. So when you ram that engine full of boost and pull 300 hp at the wheels, you'll add 25% to claim 375.

If you're looking for a pessimistic number ("we know it makes at LEAST this much"), you assume those are frictional losses so you add 25 hp to claim 325. This is what we tend to do if someone really really really wants a flywheel number. We prefer to quote wheel hp.

I believe the Brits do a coast-down test to basically measure the inertial and frictional losses, then add them back in to come up with a flywheel number. Of course, if the time of the coast-down is different than the pull, you'll get incorrect inertial numbers. In reality, it's a mix of both.

Note that drivetrains are getting more efficient in an attempt to improve fuel economy figures. Either Mazda is totally sandbagging on the ND Miata engine or that's one really efficient trans, as they're pulling numbers at the wheels that are higher than expected. I'm voting for the latter.

Also, every aftermarket dyno reads lower than every other dyno.

Like the Forte, Same car, same dyno 2 different days were almost 4hp different for the baseline. That's why I always think of the numbers the dyno spits out are just numbers with little meaning to them. Using the dyno to tune it, that's one thing. Using a Dyno for numbers is somethign else. Just my $.02

Doing a before / after on the same dyno to get an idea of how much power you gained from mods isn't useless, although you could definitely end up a few hp off in either direction.

rslifkin wrote: Doing a before / after on the same dyno to get an idea of how much power you gained from mods isn't useless, although you could definitely end up a few hp off in either direction.

aGREED. I'm talking about the single run getting a number. To get it as close as possible, you'd want to do those runs after modifications on the same day. Conditions will change giving you diffrent numbers.

Dyno's are a tool, but like any tool they can be used in the wrong manner to get incorrect data.

a GREAT read - http://www.superstreetonline.com/how-to/engine/modp-1005-drivetrain-power-loss/

Bobzilla wrote: Like the Forte, Same car, same dyno 2 different days were almost 4hp different for the baseline. That's why I always think of the numbers the dyno spits out are just numbers with little meaning to them. Using the dyno to tune it, that's one thing. Using a Dyno for numbers is somethign else. Just my $.02

It takes a lot of effort to get repeatable numbers. It can be done, especially if you're a specialty shop that uses their dyno only for in-house R&D instead of a shop that does the "three pulls and NEXT!" style of dyno testing. We've got ours pretty consistent.

In reply to Keith Tanner:

The shop that tuned the Forte was usually pretty consistent, but the temperature differences between the two days was about 25*. That alone will cause some variances that can't really be helped. Chicago in early April... definitely not conducive to consistency!

NickD wrote:

rslifkin wrote: There's also the Dynojet thing... They tend to read a good bit higher than a Mustang dyno or other load bearing dyno (Dynojets are inertia dynos). As a result of this, reductions in rotating weight will show as a power gain on a Dynojet if you don't correct for them, while they'll show far less on a Mustang dyno.

As well as the dual-roller vs single-roller debate. The setups with dual rollers were the wheel sits in between them have been known to rob power because of the tire deflection.

Everything I have ever heard about & seen firsthand is that Mustang dyno's seem to always read higher than dyno jets and dyno dynamics piles of crap always read low.

Granted, with any type of dyno, indeed a few clicks on the keyboard can magically yield whatever power figures you want.

Jerry, what dyno type was that?

I think a very interesting article could be written about reducing driveline loses

Now a "what if" scenario. What about roller dynos vs. dynos that mount to the hub? Are one more accurate than the other? I've seen wheel spin on roller dynos on some real high power cars. Don't believe you can factor that back in.

Bobzilla wrote: Like the Forte, Same car, same dyno 2 different days were almost 4hp different for the baseline. That's why I always think of the numbers the dyno spits out are just numbers with little meaning to them. Using the dyno to tune it, that's one thing. Using a Dyno for numbers is somethign else. Just my $.02

It depends on the dyno and it depends on technique. A dynojet, for example, is an inertial dyno that works by measuring how long it takes to accelerate a drum of a known mass. The mass doesn't change and the measurements depend only on time and drum speed, two things that are easy to measure very accurately. The biggest variation in dynojet numbers is if the car is strapped down slightly sideways so that the tires are scrubbing, and that's where the technique comes in. Dynojets are consistent enough that NASA uses them for power-to-weight class rules in road racing.

The fancier dynos that use eddy current generators or hydraulic pumps and load cells require calibration, and tend to be a lot less consistent from unit to unit. OTOH, they can be quite consistent with themselves over a short period of time, so they're preferable for tuning.