Automatics On The Dyno

[WazR32GTSt]

I would like to insert a quote from High Performance Inports (HPI) issue no. 73, page 100

Article titled "dyno myths: want to know more about chassis dyno's?"

Quote:

"Dyno's and auto's.

Comparisons between auto and maual packages on the dyno will never make much sense. Where the manual transmission has direct drive through the clutch (provided it is not slipping) to the rear wheels, the torque converter by nature of its design is continually slipping. hence it scrubs off indicated rear-wheel power in the process.

There isn't much great auto control software around for imports that lets you measure this slip amount, nor are there many control systems that will let you lock the torque converter clutch for the dyno run (hence drive like a manual transmission). This kind of software exists for fords and holdens, but the best you will be able to do with an auto skyline or supra is to put a manual electronic switch on the torque converter clutch, which allows you to lock the solenoid to the 'on' position.

With most factory automatics having what's known in the trade as a 'tight converter' (not a great deal of slip, hence a low stall speed), the losses generalyl aren't huge (maybe 10-20rwkw on a stock converter auto skyline), but add an aftermarket high-stall into the equation and all of a sudden the loss effect becomes far more dramatic.

The general rule of thumb is that the 'bigger' and more race-orientated the stall converter fitted to an auto vehicle, the bigger the rise in low-rpm torque on the dyno, and the bigger the loss in indicated power. It's not unusual to see 10sec drag cars dyno as little as 260rwkw through a race-style high stall, which quite obviously makes any manual versus auto comparison void."

So i read this as a summary from the article:

- automatics slip on the dyno

- this cannot be measured by the dyno, thus power shown is actually less than what there is

- higher the stall converter, the greater the losses

- comparing auto and maual figures is inaccurate

is it saying that this slip is a dyno thing, but an auto and manual car will actually have almost the same rwkw but its just that it can't be measured as easily?

and, is there a way to lock the torque converter as said in an auto skyline that anyone knows of?

can anyone experienced shed some more info on the matter?

i've heard people question HPI before, but want to know how exactly how true this is...

this obviously interests me as i run a 2800rpm torque converter, but never considered that my losses might be greater than 20 or 30rwkw compared to a manual transmission... its good to know so as to compare how one's engine is going with regards to power per level of mods.

And before anyone says it, yes i know dyno figures dont mean much, rather using the same dyno can give you good figures as a camparitor between "before" and "after" ragardless of the total figure.

cheers,

Warren

PS any comments like "auto's suck" or "get a manual" will be reported to a moderator. This is a serious thread about a topic that not many know about and could be a valuable learing resource. Thankyou.

[MerlinTheHapyPig]

i've been told another issue for auto's is the kickdown switch, you can't have your foot fully flat to the floor unless you disable it.... And also even with hold on, it will still change up a gear when the revs get over a certain amount.

I helped run a dyno day, and we had a couple of auto fords and holden's, their dyno readings were around what we expected. But no way of telling how much power they lost.

if the clutch/convertor is slipping on the dyno and giving a lower power reading, it's fair to say it will be slipping on the street also. so the power and torque readings it gives will be a more realistic than if you figured out a way to lock up the converter.....

[2rismo]

Don't know if it helps or not but Mike from MV Automatics in SA supplied Shane's gearbox and converter which has a push button lock up on it.

edit: 1jZ Cressida though....

[TurboX]

"Is it saying that this slip is a dyno thing, but an auto and manual car will actually have almost the same rwkw but its just that it can't be measured as easily?"

I read it as the the Manual & Auto can have similar ENGINE kw but the rwkw will be considerably different because of the Auto slip.

AS I also have a beefed auto with 2800RPM stall I will be happy to see the RWKw around 200 mark when ROM finally tuned to perfection by Dr. Drift.

[2rismo]

I read it as the the Manual & Auto can have similar ENGINE kw but the rwkw will be considerably different because of the Auto slip.

I agree with all bar the "considerably" comment. It will vary from very different to not different at all.

[SECURITY]

i remember reading about a loom near the fuse box that you pull out and will lock the gearbox into 3rd position. (starts in 3rd, doesnt kick down or gear up).

thoughts?

[MS-75]

Have a look at this guys.

I run a toyota A340 4 speed auto (supra auto), modified, manual shift.

I also run a lock-up converter with increased clutch area. The converter is relatively small as the motor I'm running is 4.5L, meaning it has good bottom end torque.

The below graph shows 2 power pulls, done back to back, with the second one done with the lock-up in the converter activated.

It pulled an extra 10kw, but if you run a bigger converter, that number will be bigger.

Also, consider that in std trim the clutches are not meant to hold a motor at full power, so increasing clutch area and line pressure is adviseable.

Ultimately I plan to try the lock-up out in the deep end of the track for extra MPH.

Cheers

Sean

[WazR32GTSt]

interesting...

a while back, HPI also had an autoamtic rb20 that they converted: they ran it o the dyno, changed the gearbox to a manual, and ran it again. it make 20rwkw more with the manual...

i don't know what issue that was, maybe like 2-3 years ago...

[TurboX]

I remember that story and I was thinking of converting mine but for 20 extra Kw I willl turn the boost up a bit to compensate!

Rebuilt the Auto with kevlar clutches, drilled oil ways, extra bearings, mod plate, 2800RPM Converter etc etc. instead to handle planned power increases to 200awkw.

Besides I like the Auto as a daily driver in traffic and a Skyline is not a real sports car any way.

Waiting.....

[MS-75]

The two graphs on my dyno sheet tell the story when it comes to acceleration.

Peak hp does not win races, average hp, or area under the power curve, does.

Have a look at the two graphs.

Although the locked run has higher peak hp, the open converter run wins hands down for average hp over the power pull.

[Adriano]

As the graph shows, another thiing to remember is that at low rpm, the auto "slip" causes torque mutiplication, and the engine revs dont correspond to road speed, hence the car making double the road torque at low speed, as with the locked converter, it drags the rpm down to match the road speed, so the motor is making less power and it isnt being multiplied.The difference at the top of the graph is the actual power loss through the converter when it isnt locked. However standard converters only lockup at light throttle in top gear. Usually when dynoing an auto, you cant give them full throttle at low revs anyway, as it causes kickdown, so you give it half throttle until 3-3500 rpm and then give it full throttle after that during a power run. An auto full power kicking down on a dyno is very dangerous and will often result in broken straps and coming off.