I recall reading in one of Fred's posts that the drivetrain loss is a higher percentage in automatics as opposed to manual transmissions.

This led me to two main questions:

1. Is drivetrain loss an actual percentage, or is it a specific amount of an engine's horsepower? To say that an automatic has about 22% drivetrain loss sounds fishy to me. This means that a 500 Horsepower motor will put down 390 RWHP, where a 250 horsepower motor in the same car will put down 195 RWHP. Thats 110 HP loss to 55 HP loss, that's quite a margin.

2. Where does the extra loss from an automatic come from? The presence of a torque converter, the use of a planetary gear system rather than direct gears. Does clutch have any affect?

Thanx for any opinions or comments.

 

The difference is both a proportional (a percentage, based on friction primarily) and fixed (based on the inertia of the rotating components in the drivetrain).

Both a clutch and a converter have inertia, so they both contribute in that way. But a clutch (if it isn't slipping) has no frictional loss and a converter certainly does. Fred (Injuneer) has some engine and chassis dyno numbers for his combo that show a 21.5% loss with a TH400 and race (loose) converter compared to 12.6% with the T56. This is quite consistent with what I have seen. A tighter converter is likely more efficient and loses less, but some are likely eating up even more HP. I suspect mine does. Most lockup converters are setup to avoid lockup under WOT. The lockup mechanism isn't strong enough to stand up to the load.

I assume the different gearsets have different frictiuonal losses, but am pretty sure the difference is negligible in this context.

Rich Krause

 

We've gone through this before a few times, but let me simply say this - you've got both proportional and fixed losses. Some of the proportional losses are related to flywheel torque, some to driveline torque, some are related to engine speed, and some are related to vehicle speed. Inertial "losses", while not strictly considered as wasted energy, still need to be considered when looking at only the acceleration portion of a vehicle's velocity profile if we're talking about drag racing, and these losses are proportional to the acceleration of the engine and vehicle.

The big difference between the two main types of transmission are converter losses, and the power required to drive the pump in an automatic. While planetary gears may not be the most efficient type of gearing, they're not the main culprit.

Rear-end losses should not be neglected, and different rear ends have very different levels of efficiency.

The bottom line is that applying a fixed percentage is likely to work only for a narrow set of conditions, and should only be used when trying to compare vehicles composed of a limited number of variations in drivetrain components and a relatively small difference in flywheel horsepower.

This month's Sport Compact Car magazine has a editorial by their tech editor that covers this topic in very good detail for a mainstream mag; it's much better than anything I've seen in Hot Rod or Car Craft in the past 15 years.

 

its all about the torque with autos

 

Hate to bring a post back from the dead. . . But rather this than being lynched by everyone for starting an already talked about thread. .

Anyhow. . .

I'm curious to what losses are acquired by going to a 12 bolt rear or 9". I personally have a Street Twin, 3" Chromoly dirveshaft, and a Moser 12 bolt w/ 3.73s and stock size tires. I weighed the street twin w/ alum flywhl and came out to 38lbs, lost 2-3lbs from the stock setup, gained a 1lb in the chromoly shaft over the stocker, and god knows what kinda losses I acquired with the added 12 bolt. Anyone have any references or opinions on the added loss of an aftermarket rear end?

I've seen so many numbers regarding drivetrain loss % and even after reading this I'm still weirded out to whats a more "accurate" number. . .

Also, does going to a bigger numerical gear add to the drivetrain loss? I heard going from something like a 3.42 -> 4.10 will have a greater loss. Which makes sense since the drivetrain is turning more to achieve the same speed as a 3.42. IMO, I would think a steeper numerical gear would help off the line capabilities and deplete top end a bit. I've heard of people being able to turn 6th gear past 130mph with 4.10s but that doesn't mean they're accelerating any faster than 5th gear does on a 3.42 car. In my stock Z28 I hit 157 with the stock 3.42s in 5th, I went to 6th and it bogged down to around 130mph. I again attempted it when I switched to 3.73s (stock 10 bolt still) and only gained 2mph on the top end but still bogged way down to around 130mph in 6th. Anyone have any opinions on this?

PS: And please hold the comments on the "stupidity" of going above 100mph. I'm very aware of the risks I take at those speeds and I just would like to say I do it only if the enviroment provides it and no one else is at risk and the risk of myself is as minimal as possible. Its not like its an everyday thing, only done it 3 times. . .

 

12.6%? That's not much AT ALL!

I'm wondering how much power to the wheels (MPH) I'll pick up by going from a 4L60E with a 3200 stall 9.5" Vigilante to a T56

 

I dyno'd my car before and after with a 12 bolt install and showed 8RWHP loss

 

Funny thing is.... rear wheel HP isn't all its cracked up to be.... at least not when you look at the differences between an auto and a manual tranny for the same car/engine setup. Installing the TH400 lost me 50rwHP. The 635flywheel produced 555rwHP with the T56, and 505rwHP with the TH400. But performance didn't change that much.... except for the greater consistancy (my manual shifting was never very good). The rwHP loss is offset by getting the engine into the sweet spot of the torque curve right off the line.

I wouldn't worry all that much about the "fixed" component - inertia losses. I think Keith WS6 has done coast down dyno tests showing less than 20HP. He also saw only a couple HP going to aluminum and CF DS's. When I used my numbers to solve for fixed vs. variable losses, I also found that the "fixed" component is less than 20HP. So I wouldn't worry a whole lot about a couple pounds in the flywheel or DS.

My 12.3% at 635flywheel was run with a steel flywheel Street Twin, 3" chrome moly DS and Strange 12-bolt.

As far as selecting a rear axle assembly, the published figures for the various seteup ar 10% loss on a 9-inch, 7% loss on a 12-bolt and 5% loss on a Dana. Probably one of the biggest components of a manual trans drivetrain losses.