20% L99 drivetrain HP loss question!

[jbrown13]

Is the 20% drivetrain HP loss in the Camaro L99 SS linear? That is if someone has 700 crank HP are they going to lose 140 HP through the drivetrain, or is it some reduced percentage as the crank HP rises? Inversely, if someone has 500 RWHP (Rhino79 for instance), is his crank HP 500 divided by 0.80, or 625 HP? Or, does the drivetrain lose approx. 80 HP regardless of the crank HP?

Thanks,
Jeff

[GretchenGotGrowl]

Everything I've seen is it is a constant proportion. If it was 20% stock it will be 20% when modded unless you change something about the drivetrain. A more effecient tranny or differential (lighter components) can reduce the percentage.

[Unreal]

1st of all, 20% is just a number thrown out. It could be 10, it could be 20. Unless you measure who knows.

I don't think it is linear. If it takes 10hp to spin the gears, 10hp to spin the driveshaft, etc why would those numbers go up as you make more power? A lot of people say it is a fixed number like the drivetrain sucks up 50hp. Some stuff goes up, some doesn't.

[jbrown13]

Two replies, two different answers, and I can see both sides of the coin. I was hoping someone might be privy to engine dyno numbers for a modded engine, and chassis dyno numbers for that same engine installed in a car. That info is probably in the hands of a vendor on here, but I can see why they might not want to share it. Just thought I'd shake the trees.

[Pro Stock John]

I'm not exactly sure what it is either. We used to say up to 20% drivetrain loss with an auto trans.

My L99 cranked off an amazing 315rwhp stock in July heat, seemed low to me but the car did go 13.1@106 stock and the 106mph seemed decent to me.

I did 368rwhp with exhaust, cai, tune, ud, went up to 112mph, so it would appear I make over 400 crank hp to propel a 4170 raceweight car down the track.

[SSDan]

I find this issue intriguing. The only way to get a solid answer is to pull your engine and put it on an engine dyno. Who's going to spend the money to do that? Not me!

So, here is my take. My LS3 M6's stock baseline done for my cam/bolt-on package was 378 RWHP and the finished product two days later on the same dyno was 461 RWHP which equals a 22% increase in RWHP. By my logic the engine crank HP must have went up by the same amount - right? So then my engine crank HP went up 94 HP (426 + 22% more). Thus it is about 520 HP. If I use the conventional 15% driveline loss for a manual tranny my HP would be 540 HP (461/.85).

Which is right? Who knows? Like most things in life the answer probably lies somewhere in the middle.

[Doc]

The loss of efficiency between the crank and the rear wheels is due to several things so it's not just a random number somebody pulls out of thin air. The hp number the factory gives you as the rated engine horsepower is the power at the crank with the engine sitting on a test bench. If you install that engine in a car and then put the car on a dyno, the hp the dyno is showing is the power that's actually making it to the rear wheels. It will be less than the rated factory engine power.

Everything in the drivetrain that's between the engine and the rear wheels absorbs a certain amount of power. The first thing attached to the engine is the flywheel and it's weight takes power to get it moving. The next thing is the clutch and it's weight takes power to get moving. It also has a friction surface that has to grip the spinning flywheel so there's a small amount of lost power there due to friction converting it to heat.

Then you have the transmission. All the gears in the trans have weight and resistance, as well as they're sitting in a heavy oil that also has resistance. It takes power to move all that and get it spinning.

Next you have the driveshaft which is connected to the end of the trans by a coupling joint and another one at the other end connecting it to the differential. In the case of our cars there is also a third joint in the middle. Each joint has a certain amount of resistance which takes power to overcome. The spinning balance issue of two tubes connected by 3 multi-link coupling joints in the driveshaft also comes into play and results in a resistance to being spun. The weight of the driveshaft is also something that has to be overcome to get it spinning.

Then you have the differential which is transferring a gear spinning in one direction to another set of gears spinning in a competely different direction. There's loss due to friction and weight there too. Inside the differential, like the transmission, the gears are spinning through a pool of thick oil which creates resistance. Have you ever run down to the water at the beach and the moment you hit the water your forward motion becomes slower and more difficult because the water is resisting your movements? That's the effect thick oil has on gearing inside a trans and diff. It takes power to push through that. On both sides of the differential are driveshafts which have their own coupling joints that have a certain amount of resistance that has to be overcome with power. At the end of the driveshafts they sit inside a bearing housing that is packed with thick grease and all that has resistance that needs to be overcome.

Finally after all that you get whatever power made it through to the rear wheels.

All that resistance in the drivetrain absorbs a certain amount of power and that's rated as a loss percentage. It's not a fixed amount; it is in fact a percentage. On a stock 2010 SS with LS3 it was rated at 426 hp at the crank. On a dyno an SS was typically getting about 370 rwhp give or take a couple of hp. That's a loss of 56 hp or a loss percentage of roughly 13% (13.145%). These engines are SAE certified which means they have to be within 5 hp of the rated amount, so any change in the rwhp means the resistance in the drivetrain was changed in some way.

If you increase the power the loss percentage might actually get worse due to additional factors such as metal distortion and the issues caused by heat. The factory components are chosen to perform safely within the amount of power the stock engine produces. However they typically have an extra 20-25% of safety margin so things aren't working right at the edge of breaking. This is why you can make some increases in power and still be okay. However the more you increase power and use up that safety margin, the more components begin to experience distortion due to friction and heat, which can result in a loss of performance efficiency and actually increase the resistance to the point where things can start to malfunction and break.

When you're slamming hundreds of pounds of torque through the drivetrain the metals are actually twisting and distorting to various degrees. At a certain point the assemblies begin to lose efficiency and performance actually starts to decrease and if you keep pushing it, they'll fail. This is why you'll see a dyno chart where power and torque go up as rpm's go up until they reach a certain point and even though the rpm's continue to go up, the power and torque begin to go down. Once you cross over the peak power point, the loss percentage is starting to increase. The red line value for rpm's is the maximum you can safely spin that engine crankshaft assembly before damage and/or failure is about to happen.

The thing to remember is a car is a system; not just an engine. If you increase the performance of the engine you need to look at improving the efficiency and performance of everything connected to it as well. If you add 80 hp and torque at the engine, you're still losing 13% to the rear wheels and that may increase if the metals and assemblies are distorting beyond what they were designed to handle because of the increased stress. So going from 426 crank hp to 500 crank hp means you're gonna get about 435 rwhp and are losing at least 65 hp.

You can improve efficiency by reducing resistance. Lighter flywheels, lighter and more efficient clutches, better trans fluids, better driveshafts (1-piece instead of a 2-piece), better diff fluids. If you can reduce resistances by even a small percentage with better fluids and the loss percentage goes from 13 to 12%, that will increase rwhp without ever touching the engine.

[jbrown13]

Doc, you're spot on, but haven't really offered any insight towards an answer. As a retired ME I certainly understand the principles of physics, etc., but thought someone might have some data that they would share that might help develop a curved/straight line relationship between crank HP and wheel HP.

Please don't take me wrong, I'm not trying to be a smart a$$. I just have found over my 40+ years of working that very few relationships are a straight line, but then again, my work experience was not in the automotive field. Just trying to better understand (if possible) my new toy.

[Doc]

Originally Posted by jbrown13

Doc, you're spot on, but haven't really offered any insight towards an answer. As a retired ME I certainly understand the principles of physics, etc., but thought someone might have some data that they would share that might help develop a curved/straight line relationship between crank HP and wheel HP.

Please don't take me wrong, I'm not trying to be a smart a$$. I just have found over my 40+ years of working that very few relationships are a straight line, but then again, my work experience was not in the automotive field. Just trying to better understand (if possible) my new toy.

I don't think there really can be a standard curve/straight line because there are too many variables. You've got torsional elasticity of all the various materials, frictional resistances which are a combination of efficiency of design, component tolerances, assembly tolerances, fluid viscosity, and then temperatures messing with all of that.

I've always heard that as a ball-park average you can generally assume around a 15-20% loss percentage but I've noticed in the last few years as computerized processes and synthetics have improved things that percentage has dropped to around 12-15%. For a Camaro assume 12-13% with the drivetrain completely stock.

[SSDan]

Originally Posted by Doc

I don't think there really can be a standard curve/straight line because there are too many variables. You've got torsional elasticity of all the various materials, frictional resistances which are a combination of efficiency of design, component tolerances, assembly tolerances, fluid viscosity, and then temperatures messing with all of that.

I've always heard that as a ball-park average you can generally assume around a 15-20% loss percentage but I've noticed in the last few years as computerized processes and synthetics have improved things that percentage has dropped to around 12-15%. For a Camaro assume 12-13% with the drivetrain completely stock.

Interesting. In my personal example above if you take the 22% increase of power as the right new crank HP number of 520 then the drivetrain loss is 520 -461= 59 which equates to a percentage of 12.8%. Again this is for a manual tranny. I would think a torque converter style auto would be a little higher.

[1989GTA]

I actually did this on my 1989 Trans Am. I built up a 369" SBC motor and put it on an engine dyno and it made around 530hp. Installed the motor in the car and put it on a chasis dyno and it made 420rwhp. Now you have to keep in mind these are different dyno's and the calibration's are most likely not the same but in this instance it tends to support the around 20% loss. This was with a 4L60E transmission and a Yank SS3600 converter.

[jbrown13]

Originally Posted by 1989GTA

I actually did this on my 1989 Trans Am. I built up a 369" SBC motor and put it on an engine dyno and it made around 530hp. Installed the motor in the car and put it on a chasis dyno and it made 420rwhp. Now you have to keep in mind these are different dyno's and the calibration's are most likely not the same but in this instance it tends to support the around 20% loss. This was with a 4L60E transmission and a Yank SS3600 converter.

That's the kind of data that I was hoping some people would share. With enough data points like this, one could come to a reasonable conclusion.

[Snidely Whiplash]

Keep in mind the tranny that can take the torque from a 700 HP motor, is gonna be beefed up to take that power. Which means more power from the motor is needed to turn it.

There's LOTS of big block Super Stock cars running turbo 350 trannys...but they use a few of them during a season. If you're gonna really push a standard A6, be prepared to have it freshened now and then.

It bums me out that if:
You lunch an auto tranny, the converter is also trashed.
If you lunch the converter, the tranny is also trashed.