pewter82

What is the consensus on the % loss on automatics? Is there a difference in the 4.5 to the 5.4 automatics? What I'm seeing here is roughly 15% but some people are saying 20% !!


Jason

93 GTS

mspiegle

i personally use 15% for manual and 20% for auto.

There really is no way to tell though unless you pull the motor, put it on an engine dyno, then put the motor back in and put it on a regular dyno.

Jim bailey - 928 International

Numbers for drive line losses are pretty arbitrary and probably are not linear but you need to use some factor. Might as well be 15 and 20 % that gives higher "crank" numbers

Carl Fausett

I've taken a LOT of different 928s to the Dyno, and when the engine is in good condition - 15% added to RWHP dyno numbers matches Porsche published CHP numbers very, very well on manual transmission cars, so I think 15% loss thru our 5 spd transaxle is a good number.

The automatics are tougher.... the Mustang owners say they loose 20% in their slushboxes - but my experience with the 928 automatic suggests that ours is more efficient than that. Once again, I do before-and-after dyno testing before-and after superchargers are installed. On an automatic, the RWHP number is just 18% off of the CHP number that Porsche publishes - so I like to use 18% for auto trans loss instead of 20%.

But - like Jim Bailey says: Many who sell supercharger kits or other HP gadgets (cams, throttle bodies, what-have-you) will prefer the higher 20% number - it makes their performance claims look better.

GoRideSno

You have to consider that with a 5 speed transmission the loss will pretty much remain consistent wether it's stock HP or, let's say 50% more HP that stock. Because if the clutch is working properly, the engine delivers all it's power to the transmission.
On the other hand, since an automatic transmission always has a bit of slip because of the torque converter. The the greater amount of HP an auto car has, the greater the amount of slip it has. Therefore the % loss on a stock automatic 928 may be 18% but with for example 50% more HP, the drive train loss could be much higher.

Andy K

Fastest928

I have dyno stroker engines with motec on engine dyno and chassis dyno....15-18 percent loss with manual, depending on engine oil temp and gear oil...yup. it matters. Only did one test on the same engine. BUt have over 10,000 dyno runs on a dynoJet and now beginning acquiring DynaPak tests!

Increasing TQ increases friction on the gears, thus increasing loss. The lower the tq, the lower the losses ... Increasing sweep speed also increases loss....think of gears "swimming" tough to do fast....

When building chevys, whose trans use ATF, the loss is less than 12%!

pewter82

so a stock GTS engine puts 285HP to the ground? That sounds a bit light to me, I was thinking closer to 295-298 at the rear wheels.

Fastest928

BEST stock GTS was at 300 rwhp, most are at 285 ish, sutos less.

Marc

mark kibort

Good points

Having done coast down tests, i can also say that as far as just rolling friction, the losses were between 10hp and 20hp. (20hp at the top speeds, and 10hp at the lower speeds in 4th gear)
also, depends on the gear. in 3rd, we lost 7hp with a mirror image graph, but shifted down.
when you look then at the losses beyond the rolling friction, you get a clearer picture of the total losses. if it is 15% as Marc has the most experience , will say, it probably is 10% from 60-100mph and then 15% from 100 to 150mph in 4th gear.

mk

GlenL

Carl and all,

Gotta wonder if we're all going apple-to-apples with these numbers. The Porsche specs are all DIN horsepower, if you read the fine print. That's just a few percent from the SAE horsepower.

Haven't looked up the differences, but the correction factors are done differently.

mark kibort

I think i remember a DIN number for the US 85-86 cars to be 292 vs 288hp. pretty close
i think the main thing is that the porsche numbers is probably a min value too.
based on lots of dynos from this list, i think 15% seems to be a good number to work with, as long as we all use it. heck, its only for comparison anyway!

MK

GlenL

Pulled out a dyno chart from last Summer. Conditions were 92F in the garage that day.

247 uncorrected. (What???)
255 SAE (That's OK)
260 DIN (Sounding good)
267 STD (Yes! But what's that?)

Using the 15% _loss_ factor (divide by 0.85) that's:
290 uncorrected.
300 SAE
306 DIN
314 STD

If you calculate it as a 15% _gain_ (multiply by 1.15) over the chassis dyno:
284 uncorrected
293 SAE
299 260 DIN
307 STD

So ya see, one guys 314 could be another guys 284 and they've got the same power. Switch SAE for DIN and do the math wrong and I could be worried about a missing 7hp instead of a 6hp advantage.

david928

Another data point:

Two weeks ago I tested my stock '87 S4 automatic at the PacNW Dyno Day in Portland. It measured 267 hp and 278 ft/lb (identical numbers on two different pulls) on a Dynojet 248X.

The owner's manual specifies 316 hp/317 ft/lb. So the ration is 316/267 = 1.18

IcemanG17

David
267 for a stock 87 auto is a very good dyno number! So figure your crank numbers are slightly better than stock...say 320ish HP with higher torque.

pewter82

267 RWHP is 16% loss, which seems consistant with the 15% idea.