I always see people talk about drive train loss etc., I see people posting HP numbers and wonder how some are so different than others?

I know there are some dyno tuners here, I'd like to hear some of them chime in with their experience on this.

For example a guy making 500chp makes 420whp?

Other guys making 600+chp makes 450whp?

I'm not following the math.

Is there something to the equation I am missing? I don't mean stick to auto, or types of dyno either.

 

Some people just want to have a big number, so they use a higher %.
IMO you shouldnt use a % to add up HP after doing mods. It is only useful to find a drive train loss % on a known/rated setup, say a C4 LT1 6spd stock. So 300bhp, loses about 40hp+/-, so people say around 12-15% for a standard. But it only applies to that setup, not all cars, and you can't extrapolate forward like a lot of people do by using the same % after doing a lot of mods. Let's say you mod the same car to 1000whp (only modding the engine, exhaust, etc.), a lot of people want to use the same 15%, and say the car makes 1150hp. So how does the required power to move the same parts go from 40 to 150hp? It doesn't, it is still the 40hp + probably some small increasing variable.

 

Some estimate to "net horsepower", and some to "gross horsepower" to make it sound good.

 

Same ol stuff man, tuning tool/measuring stick for yourself
Comparing it to the interwebs is futile

Had a dyno done on my mulletmobile many many yrs ago on a budget type engine it had at the time. Old Clayton waterbrake dyno it was used mainly for tuning (guy was a carb/dist curving god)

When he told me it made less than 200rwhp and just under 300 lb tq I about choked. He then commented how strong it ran thought he was patronizing me
Was young and ignorant.

Course I opened my mouth to my buddys before I went over there so had to take a LOT of flak cause there was "something wrong with your car, its a turd etc"

 

FYI, the amount of power required to turn the drivetrain is related to the amount of power being put in. The percentages stay close to the same. The 40 HP required on the LT1 at 300 HP should be close to 80 HP at 600 HP. The power loss is coming from friction between all the moving parts, the more power you put in the the greater the actual loss. (More load = more loss from friction) Think of it this way, the LT1 loss is 40 HP at 300 HP input. You stated that it would be the same at 600 HP. Then ask yourself why is the loss not 40 HP when you are cruising down the road at 60 MPH? The loss at 60 MPH is around 4 HP because the HP required to move the car at 60 MPH is less then 20 total. (You are not losing 40 HP at that speed, because the load on all of the components is less so the frictional loss is less)

I have dynoed bone stock 2015-16 Z06 vettes and they usually read between 540 and 550 RWHP. That is 100 hp+ loss from the engines rated 650 HP. That works out to 16% loss.

My dyno proven 555 BBC drag car made an honest 774 HP on the engine dyno, when it was installed in the drag car I ended up with 636 RWHP. That was a 18% loss and that was with a powerglide which is known to be very efficient. Over the years of working at the dyno shop I have seen many combinations of cars loss between 12% to 25%+ when comparing engine HP to rear wheel HP. It all depends on how accurate the engine dyno was and what the final installed combination of parts was. So a rough estimate of power loss that is used by the drivetrain is 15% for a stick and 20% for an auto. These are just estimates but they get you close.

Why are there such large variations between some dyno combinations:
-All chassis dyno's do not read the same, could be set-up different, could be calibrated different. The truth is the dyno is a tuning tool that can be used to maximize power and show changes when different combinations are tried.
-Not all engine dyno numbers are the same, some use different calibrations which will result in different numbers.
-How the engine is installed and tuned in the car will make a difference, not all installations are the same. Details make a difference.

 

True, this is coefficient of friction. By that theory the friction loss is a linear proportion of the total load. I would say, however, that there are a whole bunch of variables that mean the coefficient of friction isn't completely constant with differing power levels in a single car. For example, on a chassis dyno the rear suspension still reacts all the acceleration forces, which means with more power the bushings flex more and alignment changes, and the car squats more so the axle angles change, etc. Then consider that there are a ton more variations between different cars on the same dyno (different alignments, different bushings, etc), and it makes it impossible to have an exact number.

But I think the biggest between different cars and different locations is weather conditions. I don't know if most people are quoting their results after correction to standard conditions or not. But if they aren't, then any comparisons are completely meaningless.

PS - Another big difference would be driveline and tire inertias. A car with a lighter flywheel or lighter wheels/tires will show more power on an inertial wheel dyno. Lower gears magnify the differences in flywheel inertias, but higher gears magnify the wheel/tire inertias. At the typical 1:1 gear that is supposed to be used, I would guess the wheel/tire inertial differences are dwarfing the rest of the drivetrain's contribution to power loss.

 

-When you look at dyno numbers you always need to look at corrected numbers to have meaningful information (As you stated)

-When you run dyno's the best way to run the test is with a constant acceleration dyno so you can compare back to back runs at note the actual changes not the inertia differences. I will shoot for a 500 RPM change per second or a 300 RPM change per second. If you want to see the actual power at any RPM and ignore acceleration rates you can run the dyno at a fixed RPM and note the power output.

The dyno is a tool and you need to know how to use it to understand the actual information that it spits out.

 

On higher and higher output engines, its been shown that the % rules don't apply as well. That said, the drivetrains and suspensions of more modern cars like C7 or other hypercars are much more efficiently designed than ours were, and they don't get the same drivetrain losses that we would. I've seen Hot Rod or some other magazines do dyno testing on the same day and dyno with several cars, and they get crazy good efficiencies if you just use the % rule. To me, this is further evidence that the blind application of the 15-18% rules is problematic.

The coefficients of friction will have some variance based on fluid choices as well as the variables mentioned above. There are just too many variables to factor into the math to make a comparison here, so I totally agree that a dyno is just a tool for you to use on the day of the run and get a gauge on how the car runs and the tune. If I really wanted to compare two cars I'd look at the 1/4mi times on the same day.

 

But I don't like the drag race scenario, unless everything is the same with the 2 cars, same driver, gears, tires, track, day, time, etc., except engine power, which is probably never going to be the case. I think there are even more variables here, but if everything was the same, then I think the MPH is a good indication of overall power.

 

OP is talking about why people come up with different gross HP ratings for their modded engine. I agree you can come up with a rough estimate of around 15% manual and 20% auto on an SAE rated engine on a stock car, to find the loss, but not to add those % to modded cars to get the gross HP. Also I think these % will come down as drivetrains etc. become lighter and more efficient.

Example some will say their 400whp car makes 450HP, while others will say 500hp. (they are using a % factor they want to get the end result that sounds good to them)

Here is a good discussing on the crank to wheel losses.
https://www.corvetteforum.com/forums...s-hp-loss.html

"I have dynoed bone stock 2015-16 Z06 vettes and they usually read between 540 and 550 RWHP."

Ok, but there are also ones that did 572 to 585.
From the web:
"Vengeance posted the dyno numbers on the Corvette Forum and it shows the new super car putting out an amazing 585 hp at the rear wheels along with 617 lb-ft of torque. Considering a standard average drivetrain loss of 15%, that mean’s the Z06 was putting out around 688 hp and 725 lb-ft at the crank!

So was Chevy sandbagging with their official numbers of 650 hp and 650 lb-ft torque? "

This writer is doing the same thing a lot of owners do, to make the car's HP # sound better, and they are adding in the "standard" % taken from all cars, with all kinds of trans, gears, wheels, tires, etc. from years ago. Yeah, GM is going to underrate their premier car on HP, in the middle of HP wars...that should help sales. Also it is an SAE rated engine from the factory, so what is more likely, they underrated it(why?), or the drive train losses are less than the "average" 15%?

"Then ask yourself why is the loss not 40 HP when you are cruising down the road at 60 MPH? The loss at 60 MPH is around 4 HP because the HP required to move the car at 60 MPH is less then 20 total. (You are not losing 40 HP at that speed, because the load on all of the components is less so the frictional loss is less)"

Momentum. I agree to maintain a relatively low speed, with an aerodynamic car, and low rpm, it takes little HP, but to increase the speed it will take more. It is not having to do as much work, but the faster you spin it, it will require more, up to the max of 40HP. On a dyno you are not in 6th gear at 60 and 1300rpm. I do agree there are some friction and thermo losses as power/rpm increases. Iif you raise the redline, as one often does to get more power, there will be some additional losses, as I mentioned before, but I still do not think it is a constant, fixed %.

On the Z06 585whp car, all we know is it had a 65whp cost with that setup, and then you can obtain a %. For all we know if we put in a C4 LT1 300HP engine in the car, it might put down 235whp, because I think it would take the same amount of power to spin those components to the same RPM.

Anyway, all good discussions.
I just go with RWHP #s, while they can be manipulated, and can vary from dyno, I think they are the best indicator of power, and like others mentioned, a dyno is a tool to try to maximize power.

 

As I stated in my earlier posts these are only estimates to get you close, but the 15% to 20% even on higher HP cars does seem to apply. My 774 HP drag car lost 18% and the brand new 650 HP vette losses around 16% or more. I have seen some dyno number as low as 520 RWHP for the new Z06's.

 

Say what!? How? Are gears cut differently now than in the 90's? No, they're not. Are bearings different in a meaningful way? No, they're not.
*My stock LT1 '92 put down 279 RWHP and 276 RWHP on two different dynos on different days. The engine is rated at 300 chp. If the engine really makes it's rated 300 chp, then my drive train loss calculates to 7-8%.
*For yuk-yuk's, my '06 C6 (400 CHP) did 360 at the rear -a 9% drivetrain loss, and...
*Our '05 CTS-V (400 CHP) did about 335 at the rear, for a whopping 16% loss. Even though it has the same trans as the C6. Newer drivetrains are not meaningfully more efficient -if they're more efficient at all.

How did two 400 hp, 400 lb-ft cars make such dramtically different RWHP numbers? That is the question of this thread, and here is a nearly perfect illustration of just such a case. In this case, the 400 hp C6 vs. the 400 hp V, the nearly 30 RWHP difference is due to drivetrain mass. The V has a 40+ lb flywheel, a steel drive shaft, and heavy, 18" rims with heavy, 14.5" rear rotors. And there is your ~30rwhp difference on an inertia dyno.

The major reasons that people get numbers all over the place are these:
Different corrections/smoothing (dyno operator)
Different drive train mass (rotating inertia)
Auto vs stick
Tq Converter stall speed (slippage) and efficiency.

 

My point is on the dyno I use and tune on which is a Mustang dyno the numbers work out pretty well. I have no idea what the other dyno operators are doing and what kind of dyno they are using. As I listed with actual dyno proven numbers, not guesses our dyno showed a 138 HP loss on a 774 HP car and a 100+ hp loss on a 650 HP car. So when our customers ask for an engine HP guess I use the 15% for a manual and the 20% for an auto and I tell them it is just an estimate.

 

It will take more power to spin the drivetrain at higher RPM but in no way accounts for the high HP losses that we see on the dyno. To prove that jack your car up off the ground and put it in gear and run it at various speeds up to the speed you run on the dyno. What you will see it that it takes very little power to rotate drivetrain at any speed because there is no load on the drivetrain. FYI, I do this on occasion to trouble shoot drivetrain vibration issues and the throttle input to run at high speed is almost nothing.

 

 

Then we'll agree to disagree, I believe the transmissions and differentials and suspension tech to get the power to the wheel are more efficient because of tighter manufacturing standards and lighter/smaller parts that reduce the inertia, and special coatings to reduce the simple coefficients of friction between parts irrespective of fluid choices. I have seen numerous articles over the years discussing it.

 

In addition to all the info above, these factors affect power output:
barometric pressure
relative humidity
ambient air temperature
engine temperature
elevation (available oxygen)

The first three items normally make a small difference in power output.
Engine temperature shoule be an obvious factor -- most engines built in the last 20-25 years have ECMs with performance parameters that are largely decided by engine temperature, among other things. My engine will put out way more power at 180F than at 240F.
Higher elevations mean thinner air (less oxygen). This can seriously affect power output. That's why turbochargers and superchargers are so great at elevation.

Also I understand we are comparing power at the crank vs power to the ground/dyno. I mention these factors above because some people will just take the SAE power rating for their vehicle, go to the dyno, and gasp in horror when the best pull is just 60% of that SAE number. Other performance-limiting factors do add up and should be considered but often are not.

 

O.K. So then how do you explain the results that I just posted, where an older car was more efficient than two newer cars? I even posted the drivetrain make up of the Caddy...which is clearly much heavier than the C4 'Vette's drive train....the opposite of one of your points that you just tried to make. C7 has a steel drive shaft, DM flywheel, larger ring gear, steel axle shafts, and larger/heavier wheels and brake rotors, than a C4. How is that lighter? What say you?

Apparently, you "believe" that newer drive trains have less inertia and friction...but have shown no evidence of that. I "believe" that they're about the same, in general...but vary widely from car to car...new or old. The 400 hp/400tq C6 compared to the 400 hp/400tq CTS-V is proof of that. To back that position up, I've shown real, objective evidence showing the opposite of your claim -an older car producing more efficient results than two newer ones... (though I'm not claiming that "new cars are less efficient")




All of those factors SHOULD be accounted for during the set up of the dyno. That is what "corrected" means. My car (stock LT1) should only make about 260 CHP on a perfect day, at our elevation. Using the 7-8% loss I get from my actual results, that would be about 241 RWHP. But it didn't dyno 241 RWHP. In reality, it probably DID actually put down something like that, but since the operator put the temp, DA, alt, humidity, etc all into the dyno, the dyno computer corrected my terrible, high elevation results to show perfect day, sea level results.


Superchargers are just as affected by elevation as NA engines. You're right about turbos, however, and that is b/c of the waste gate boost control feature, that superchargers do not have.



I would certainly hope that someone would gasp in horror if their car only produced 60% of the SAE rating!

 

Chassis dynos are not all created equal. They vary unit to unit, and even more brand to brand. They should be considered a tool to load the engine for tuning and nothing else.

Engine dynos tend to be more consistent, but even then they are still a tuning aid.

It has been said drag strip performance is a more accurate means of calculating engine HP than a chassis dyno - I tend to agree.

I haven't had my C4 dyno'd (it's all stock - no compelling reason to do it). I did have my LS-swapped (stroked 6.0, L92 heads, big Erson cam) '57 Bel Air dyno tuned, once to get it running, then again a year later after further mods (intake manifold, switch from pump gas to E85). The tuner is a Corvette specialty shop. They said my car produced 415 RWHP the first time, and 435 RWHP the 2nd - that's with a 1.15 altitude correction factor included. They also noted that my "big stall" converter (stalls at about 4800 in the car - they are used to tighter converters in Vettes) was skewing the numbers downward. Using internet calculators to convert 1/4 mile ET & MPH to HP, I come up with 435 RWHP - except that's uncorrected for our altitude.

Bottom line: The dyno was useful to tune the engine. The car has run very well at the track. The RWHP numbers don't mean a lot. I'm not motivated to get it on an engine dyno to verify the "real" HP.

 

w/all of that.

Trap and weight tell the HP story, pretty darn well. No "data entry" required that can skew numbers. Car runs what it will run, and that's it.