Mongo

I can't seem to specifically locate the drivetrain loss for automatic cars in our archives, but rather mostly manuals, which is around 15% give or take.

Anyone know the approximate percentage loss for autos?

SeanR

Some days it feels like 50% but I could be 1/2 asleep.

Ducman82

i think its the price of the car..... :-P

Rob Edwards

Depends.

500 chp = 383 whp
550 chp = 383 whp
600 chp = 383 whp.

Or, put more qualitatively:

Imo000

5% more.

FredR

If you calculate the square of the ratio of the top speeds of the auto and manual varaints and assume there is a 5 mph difference [165mph versus 170mph] it takes 6% more power to generate the additional 5 mph.

In practice it is not quite as simple as that given the automatic has higher ratios [2.54 in my case] compared to the manual [2.75] and in my experience the auto box bogs down somewhat in top gear and that may cloud the issue.

Imre's number looks good comparitively speaking.

Rgds

Fred

Ninespub

This is not going to end well with Roger............methinks he would argue to the contrary, as I do. You have to compare the comfort of a fully equipped 928 vs the do it yourself manual transmission; especially when you are on a highway parking lot moving seven feet at a time for almost two hours like I was in Chicago Friday afternoon. I wouldn't have had a 5-speed for all the money in the world after it took took nearly two hours to traverse 21 miles. I'll take the 15% loss in HP and still have a left calf muscle that I can use at nearly 70 years of age. Just say'in.

Ducman82

LA traffic does not bother my legs at all!!! (limps around)

77tony

yep, for the +/- 5% diff get what suits you. T

Speedtoys

The automatic uses all of the HP that a broken never to be repaired manual costs to replace.

GT6ixer

Some has to have had their engine dynoed or at least has a link to a thread where the results were posted. Then just need to compare to dyno results with an automatic equipped with a similar motor. Not exact but close enough. My guess is 18% loss.

jcorenman

Lots of engines have been dyno'ed, the problem is that there are significant differences between dynos and engines (and seasons, altitude, and also local fuel: see e.g. California).

What we need is a volunteer to dyno their car, then swap the 5-speed for an auto (or vice-versa) and test again, then swap back as a double-check. What do you think? Are you game? Cool!!

Speedtoys

Dynos altitude and weather are never variables, just use the SAE corrections.

You can get a more accurate number with more data points per second on the latest dynojet gear, compared to the 1 sample per drum revolution prev generation, and the latest gen lets you place a weather sensor -at- the air source as well...but the same car on the same gen dyno, will..as long as the multitude of variables inside and around the powerplant are stable...will generate the same results.

99.9% of the "your dyno sucks" we get at the track during competitor measurement, are cars that have fuel/timing maps that are sensitive, some very much so at multiple points, to coolant temperature, which is very hard (impossible in the field)to make stable on a standing dyno..you just cant cool things like you're moving 50mph.

jcorenman

Nonsense. Drive your car to Reno, tune it there, and run it on the dyno there with the altitude corrections. You will get a bigger number. The reason is that thin air (and less fuel) makes less power, but the lower pressure also burns more slowly-- which means you can add a bunch of timing and get back some of that loss. Which means a bigger dyno number, as long as you stay at Reno-- otherwise it knocks like crazy and the EZK pulls timing and spoils the fun.

The question was whether comparing one generic dyno number to another-- presumably a different dyno done elsewhere under different conditions-- would yield useful info. I say no, not without a controlling the variables.

Which is your point, and exactly correct: The same type of dyno under controlled conditions will give comparable results. Which is what you do at the race track: You are not going to take a dyno slip from a Mustang in BFE.

Speedtoys

You changed a variable. You changed the -car-.

I said run to run, weather can change, altitude can change and with SAE correction, you will have accurate results because of that correction.

You cant tune spec racer build cars, generally speaking, so it doesn't come into play when talking about "dynos are never accurate".

They always are, things change around them, but not in them or how SAE correction compensates.

But change variables in the car, you get a different result, but that's not the dyno's fault.