Interpreting horsepower in our cars.

[Cabbage Fumes]

20 hours ago, uai said:

My dyno sheets show 78% =22% loss

How is your car set up? Is it a race prepared car? If it is similar to mine, I could use your loss percentage as a baseline to my results. 

[Preyupy]

I have taken BMW m10 based race engines that made 225 hp @8000 rpm on a DTS engine dyno and put them in race cars and run them on our Dynojet drum dyno.  In a standard 2002 with a 5 speed transmission (running in the 1:1 gear) and 4.11 final drive with just enough air in the tires to keep traction throughout the run we saw 185-192 hp (tire pressure and engine oil temp seem to make the most difference ).  The same engine in a sports racer using a Hewland transaxle at somewhere close to the same overall gear ratio ( tire size and gears = roughly the same speed/rpm) the numbers were slightly higher 188-196hp. Obviously these tests we not done on the same day so weather conditions did change but this is what I usually use to compare OUR Dynojet to the DTS dyno I usually use.
 

 I never split hairs on HP/Torque numbers between different dyno,   I have seen some dynos that are obviously calibrated in Clydesdales and others in Shetland Ponies.  

[uai]

55 minutes ago, Furry Camel said:

How is your car set up? Is it a race prepared car? If it is similar to mine, I could use your loss percentage as a baseline to my results. 

Probably different as I run a ti/sa 5 Speed and a 3.64 longneck diff but I'd say between 20 and 27% loss is a good estimate on these cars. Measurements were on 185/70-13 Avon Cr6 zz

Edited April 6 by uai

[Cabbage Fumes]

27 minutes ago, uai said:

Probably different as I run a ti/sa 5 Speed and a 3.64 longneck diff but I'd say between 20 and 27% loss is a good estimate on these cars. Measurements were on 185/70-13 Avon Cr6 zz

So the answer going forward would seem to be to assume at least 20%, and possibly as much as 27%. The next time I go do a dyno session, I'll ask to have the number set to 20. In my world, the actual number coming from the crank really isn't that important to know, it's just bar stool conversation. Gaining confidence in my own abilities as well as the car's, and having fun is all I'm really after (and not breaking stuff as often as I used to, of course). I want to drive my slow car fast!

This is me trying to get me AFRs sorted, and a final go-go-go.

 

[jimk]

10 hours ago, uai said:

When you press the clutch at a known speed / angular velocity the grade of the dyno slowing down gives you the figures. It is done twice with a little braking force on the second run. With the difference between these runs the inertia moment of the car components and the drivetrain loss can be calculated- I

Is the power loss thru the hypoid gearset the same with reverse driving where the ring gear drives the pinion as ithe loss in the forward direction where the pinion drives the ring gear?  Need to have some of those details before I can accept the concept.

 

In the forward direction, the gearset is a speed reducer but in the reverse, the gearset is a speed increaser.

Edited April 6 by jimk

[uai]

34 minutes ago, jimk said:

Is the power loss thru the hypoid gearset the same with reverse driving where the ring gear drives the pinion as ithe loss in the forward direction where the pinion drives the ring gear?  Need to have some of those details before I can accept the concept.

 

In the forward direction, the gearset is a speed reducer but in the reverse, the gearset is a speed increaser.

Let's put it like that - you're at least more in the ballpark as with a rule of thumb. As mentioned in another thread I know a guy who was a dyno engineer @Mercedes F1 team and they calibrated the climate and barometric controlled dyno sometimes more than once a day.
On the other hand in the linked AMS article they have a spread of less than 1% among 3 dynos.
If you look closely at the pics of the dyno sheeets in the linked Sport Auto/AMS Article the Schleppleistung differs significantly between the runs although the (calculated)power of the car is (within 1%) the same. This may relate a lot to different temperatures of gears etc. Modern oe ECU for turbo engines (for BMW and Porsche) compensate for ambient influence and just put the car in the Nm range where thy shall be.

Edited April 6 by uai

[d.hitchcock]

My engine makes 125-130 whp (runs on different dynos give different results). I assumed a parasitic loss of 15% because that's the number I've always heard tossed around. Totally unscientific.

 

[visionaut]

On the other end of the scale — I’ve been using a phone app dyno for the last 6/7 years called PerfExpert. I use it on all my cars. Very effective tool.
 

It uses the GPS and sensors in your phone (or external Vbox GPS). You supply it a bunch of technical data (displacement, gearing, weight, fuel load, suspension pitch, etc), then lay down a single-gear WOT run on a flat surface (avg 3 runs). Includes drag and weather condition effects as well..

 

It can calculate crank or wheel power value (estimating drivetrain losses). It’s tested to be pretty accurate for absolute values ( and I’ve cross-referenced its results to my BMW OBDII Drive-Analyzer). But most importantly it serves my purpose nicely for repeatable relative data (making comparisons between changes)…

 

https://www.perfexpert-app.com/

 

[visionaut]

Nice article on drivetrain losses, though I don’t necessarily like the conclusion: don’t try to convert.  Lol

 

https://www.motortrend.com/how-to/modp-1005-drivetrain-power-loss/

[TobyB]

I'll say it baldly- it's tire losses. 

Transmissions don't heat much at all,

and the diff loses maybe 2% at most*. 

Otherwise, it would melt.  And they don't.

 

I measure my RWHP by lap times.  it's totally not scientific.

If I want to try to see if a mechanical change has made a difference,

I'll look more carefully at GPS data and sector times, and try to find laps

where I came out onto the front straight at the same speed and direction,

and overlay those.  I have to say, 87.6% of the time, I don't have to do that-

I can tell you that 'yep, it's pulling harder from 4k to 6k, but after that, it's falling

on its face' or 'there's a bump at 5k, but it's odd' and most of the time it's just

'yeah' or 'no' and not all that often:

'imma look at the data, because it's different, but is the good better than the bad?'

Same thing's true of suspension changes, too. 

Tires?  I always look at the data.  What feels good is often really, really bad.

 

t

*figuring that, if I make 165 hp, and the oil in the diff stabilizes at 200f with air temps at 100+f,

there just CAN'T be that much heat coming out of that tiny little ring and pinion.  It's an iron case.

With not much surface area.

[uai]

On 4/5/2024 at 4:58 AM, TobyB said:

I've never seen trans temps above water temp, and the small- case diff in good shape seems to sit around 100c 

most of the time.

I had Diff temp of 137°C (280°F) on that drive
 

 

Edited April 26 by uai

[TobyB]

Limited slip diff?

Later finned cover?

 

You might have more power than I do...

 

and I do use welded diffs.

 

When mine get over 250f consistently, I figure they're getting towards the end of their life.

t

 

[Daily02]

23 hours ago, uai said:

I had Diff temp of 137°C (280°F) on that drive
 

 

Great video: )

[uai]

On 4/27/2024 at 4:30 AM, TobyB said:

Limited slip diff?

  Yes

On 4/27/2024 at 4:30 AM, TobyB said:

Later finned cover?

  Until I fitted the diff cooler yes -
Then back to the old cover
The fuse of the diff cooling pump failed

On 4/27/2024 at 4:30 AM, TobyB said:

and I do use welded diffs.

That removes the friction and explains the low temperatures

power ~175 but more to come