all discussions to date typically assume drivetrain to include all accessories...

15% is a good # for a stick car with all accessories in the 300-500 FLYWHEEL HP range with average weight wheels, tires and typical rear end gears (3.27-4.10)

stray away from those "standards" and the numbers change a bit...heavy wheels, super steep gears...etc.

I would expect a good stick setup to lose 40-60 HP from flywheel to tires.

An AOD with full trim is around 90HP average.

 

Why is drive train loss given as a percent? This implies that as HP goes up then drive train power loss goes up as well, but that doesn't make sense to me. If the drive train components stay the same and HP is added (Say thru Nitrous) then the power loss will remain the same HP level and actually decrease as a percentage of total power. Is this right or am I missing something.

 

Why is drive train loss given as a percent? This implies that as HP goes up then drive train power loss goes up as well, but that doesn't make sense to me. If the drive train components stay the same and HP is added (Say thru Nitrous) then the power loss will remain the same HP level and actually decrease as a percentage of total power. Is this right or am I missing something.

That's what I was thinking...

 

Why is drive train loss given as a percent? This implies that as HP goes up then drive train power loss goes up as well, but that doesn't make sense to me. If the drive train components stay the same and HP is added (Say thru Nitrous) then the power loss will remain the same HP level and actually decrease as a percentage of total power. Is this right or am I missing something.

You're missing something.

Drivetrain losses are largely frictional and inertial. The more torque you put through the gearsets, the more friction is generated, and more power is sapped. Furthermore the more torque you put through the system, the faster it accelerates, and that increase in acceleration costs more power to overcome the inertia. So a "fixed" percentage isn't totally accurate, but it's way more accurate than assuming drivetrain losses are a fixed value. And besides, nobody has a problem believing a stock 5.0 loses 34 hp through the drivetrain (15% of 225), but if it were a fixed value, you wouldn't be able to turn the drivetrain at all with anything less than 34 hp, and that is not the case. You can turn the whole thing by hand.

 

MFE, thanks for the response. I thought there had to be a good reason. I hadn't considered the inertial losses. That's why I asked.

 

excellent explaination MFE......... we have seen this debate over, and over.........
and over................. usually goes for 2-3 pages. hopefully MFE's description ends it.

 

Flywheel HP doesn't mean crap. Don't get caught up on it. As far as that is concerned, rwhp doesn't mean crap either, except as a bragging number. Chassis prep is where the real stars begin to shine, and chassis efficiency will more than make up for a pretty considerable HP difference.... for example, it's not uncommon to see a well-chassied car get outrun by 4-5mph or more yet out-ET a car of the same weight with no chassis work- there are plenty of guys on here that turn 109-112 or so that can't get out of the low 13s, yet there are guys on here that are in the low 101-105 that can get into the higher 12s. Get your chassis working first and then start to worry about HP, and with each increase in power make sure the car can still use all the newfound power to actually accelerate quicker instead of just creating a higher trap speed. Last time I checked, ET won heads up races, not MPH... Sorry if I was too far off topic to help. As a reference, 5.0/SF dynoed Robin Lawrence's Factory Stock motor at 380fwhp, and in the car with a T5, the car put down 330rwhp. That works out to 13.2% at the 380fwhp range with extremely light rotating mass. 15% should be close enough if you're really worried about it.