A good post I found for you AOD guys out there.

W

Wayne Ing

Guest
A good post I found for you AOD guys out there.

One thing I want to add first is that a stock converter and a shift kit are a bad combination and it is very hard on the clutches and bands. Why is this the stock converters are heavy and very grippy at low RPMs. If your shifts are set on firm it's just like powershifting at every gear change. With a stall, even a mild one the converter does much to soften part throttle shifts by absorbing the torque peak. I have recently put a shift kit into my daily driver's C4 and set it on street/strip. It was always barking at every gear with a 2.73 open rear, annoying but no big deal. Then I swapped in a 8.8 with 3.27 traction-lok and holy crap this tranny beats the hell out of the car and me. I just ordered a 2000 RPM 10" converter from Summit to add accelleration and soften up normal motoring.

I've put this up before so if you've already read my AOD/converter post don't go any further.

Here's my speech on AOD's and torque converters:

Read carefully: The lightest (smallest) non lock-up converter you can get that will still stall as low as you need it to. This is the absolute most significant performance increase you can make to the AOD without question. I say this in all correctness to help but I'm not going to spend a week defending it so if you don't agree I don't care, my cars run fast and I don’t apologize for it. I will share my hard found or thought up secrets from time to time but tire of arguments quickly.

Here are some reasons and or things to consider when thinking about it:

Crankshaft weight:

You gain 2.2-2.5 HP per pound removed off the crank at the wheels. Stock AOD converters even without fluid are much heavier than stock clutch & flywheel assemblies. I put a 15 lb. aluminum flywheel in my SC and use the factory 35 lb. unit for a grinder stand base. I gained 1/2 a second on this mod alone. This is with a wide ratio 5-speed, which is not good for the drags; the most gear I can run with this tranny is 3.27’s. 20 pounds less times 2.5 HP per pound = 50 HP that was used for spinning a big hunk of iron through every gear now getting to the rear wheels.

Rotating weight absorbs more power when accelerating than static weight on a car. I can't remember the exact figure but it seem to recall the SAE says that every pound of rotating weight on a car is equivalent to 1.7 pounds of static weight. This is why we always reduce rotating weight first: wheels, pulleys, drive shaft, etc.

But consider for a moment that the crank assembly is not only a rotating weight but also the only one that is accelerated multiple times in the timed quarter mile. This weight must be accelerated through each gear while the wheels and drive shaft only achieve max speed once, at the finish line. All static weight is the same start to finish. So the most significant place to remove weight to increase performance is the crankshaft assembly. Easily 3 to 4 times more than the drive shaft yet more people think of the drive shaft first. Easily 10 times more than equivalent static weight removed from the car yet most remove static weight second.

Ask any under drive pulley manufacturer where the power comes from, rotating weight reduction or slower accessory speeds. The vast majority of the gain is from the weight reduction.

Lower crank weights make it easier to modulate wheel spin, especially with performance gearing and or street tires. Comes in handy in the rain also.

With light crank weights drive train life is improved due to lower peak torque loads at shifts. This also improves drivability when cruising.

With a lighter converter higher stall speeds can be used without putting the tires up in smoke.


Torque multiplication and the non-lock converter:

Converters multiply torque, some over 2 times as much below stall speed. This means that below stall the converter causes the input shaft of the tranny to see more torque than the engine is producing. This is the only reason automatics run worth a crap at all. Think about it they have really steep gears (2.4, 1.47, and 1) and without torque multiplication they would be slugs of the line and take forever to get up to speed. The AOD is designed to bypass all of this in 3rd and 4th gear by taking power directly from the converter body, which is bolted to the crank. A non-lock unit eliminates this and gives converter function in every gear.

In a quarter mile run with a three-speed auto you will spend most of your time in the lower half of third gear. Even with 4.10 gears a naturally aspirated motor can fall below torque peak going into third. With a lock up converter you loose any torque multiplication you would get at this crucial point, the bottom of third gear. Friends of mine who took my advice on this with the AOD came back saying things like "WOW, third gear rocks now!” With a lock-up unit all potential torque multiplication is lost and the converter becomes dead weight, in the worse place possible. You’re going to be spending time here anyways so spend it applying the most power to the input shaft as possible and you’ll get into the higher torques of third faster and your car will again be quicker.

Lock-up converters are good for mileage and drivability with a stall but if you want to get an AOD into the twelve’s they will make it harder. Mileage with a non-lock is still acceptable though, it doesn’t go away just because of 150 more RPM at 80.

Gear Ratios:
The stock close ratio set is the best for performance, particularly with performance axle gearing. Use the converter stall speed and stall rate to ride out your launch and then stay in first gear as long as possible. You accelerate greatest amount in first gear, the longer you stay in first the quicker your car will be. If you install the wide ratio set you will find it much harder to launch hard and maintain traction and once you hook up your almost done with first gear. Same goes for second, you still end up in third so get there at the highest engine speed. The wide ratio set is good for highway gears or towing, not the best for max acceleration with a properly chosen converter taking care of the launch and race gears in the rear end.


I'm not a converter salesman or do I know anyone who is but Precision Industries makes a superior product in my opinion (the Stallion converter). They machine billet aluminum turbines and shells to get more precise control over stall characteristic while using a MUCH smaller and therefore light weight converter. It's like having your cake and eating it too, a 9.5” aluminum converter that will still stall street-able. They are expensive but hey you want to put an AOD into the twelve’s so put up or shut up. Just to prove I'm unbiased I'll tell you there are copies out there now that are cheaper but I like Precision because they were smart enough to do it first. If you want the best start to get that car into the twelve’s call and talk to their tech telling him exactly what you have and what you want to do. Converter selection is a science and catalog converters don't cut it. Here's Precision's web site: www.converter.com


In conclusion:
A Lentech valve body should be your next step after this type of converter. Then your AOD will be as optimized as reasonably possible and you can reduce weight, add traction, stiffness and HP to achieve your target times without worrying if your tyranny’s still holding you back. Put the battery in the back and add all the traction you can afford to the rear suspension, Nittos are only half the battle. Get load transfer onto the rear end under launch by making the suspension push the wheels down; the stock geometry pulls them up. Run one of those big a$$ rear sway bars. Things like this will allow you to come off the tree at WOT and just leave as in gone. The launch is the most important factor in ET not the tranny, I just told you how to optimize the AOD not how to make your car run twelve’s with it alone. Actually an AOD in this tune will work as well as a stick with a standard weight flywheel and gear spread. One more thing, since the gear spread on an auto is so wide yet with only three gears you should build an auto tranny engine to work with it. You ever wonder why camshaft recommendations for autos spec smaller than the manual? With four close gears the engine can be built to make max power at the engines highest effective RPM, but with the auto gear spread you will spend more of your time in the mid ranges. So the trick is to maximize the torque produced in the mid range. The torque multiplication of the converter in the bottom end works even better with more torque here than the same torque achieved after stall, more power in more out. This holds true to intakes, heads, and exhaust systems also. All should be selected for max mid range torque. Leave the peaky parts for the stick shift guys, as they are the only ones who can take advantage of the constant peak RPM’s during acceleration.

Stall speed:
On stall speed the converter will slip and multiply torque up to the point when it can hold against the engines torque. The more torque made and the heavier the car the more stall will be generated and vise versa, lower the power and or weight will allow the converter to "grab" sooner. Gearing comes into play here also. This is why you shouldn't use catalog converters for serious performance, there are to many variables to be considered for a one size fits all.

In order to maximize acceleration your converter should stall (grab) at the engines torque peak, this ensures that the converter will always boost torque until the engine can take over. The greater the rate of slip is means more torque multiplication. By the time you’re approaching stall the engine is taking over so this should be the actual torque peak of the engine. Just make sure you have enough gear in the rear end that the converter will be fully stalled by the time you reach normal highway cruise speed. This way it's not slipping as you drive along, this slippage generates a bunch of heat and will burn up the tranny if allowed to slip constantly. A big, really big tranny cooler is always a must for stall converters and streetcars.

Vernon


__________________
Fixer of all things broken, breaker of all things fixed. Somewhere along the way they end up twice as fast.
 
Wayne this was not intended at you but I just saw it and figured I would add accurate information. Please take no offense.

Wayne Ing said:
I'm not a converter salesman or do I know anyone who is but Precision Industries makes a superior product in my opinion (the Stallion converter). They machine billet aluminum turbines and shells to get more precise control over stall characteristic while using a MUCH smaller and therefore light weight converter. It's like having your cake and eating it too, a 9.5” aluminum converter that will still stall street-able. They are expensive but hey you want to put an AOD into the twelve’s so put up or shut up. Just to prove I'm unbiased I'll tell you there are copies out there now that are cheaper but I like Precision because they were smart enough to do it first. If you want the best start to get that car into the twelve’s call and talk to their tech telling him exactly what you have and what you want to do. Converter selection is a science and catalog converters don't cut it. Here's Precision's web site: www.converter.com

I agree with most of what you have said except this statement here. Aluminum converter...it doesn't exist in these applications. Let me explain how PI builds a converter. I am not saying it is all bad I am saying your statement is wrong here. The pump or impeller of a PI converter is nothing more than a stock fwd GM converter impeller that has been furnace brazed and a ballooning plate and hub welded to it. The Turbine is a stamped steel stock FWD GM turbine that has pieces of metal added to the tips and furnace brazed. Then it gets a new turbine hub added to it to adapt it to our applications. The stator... The stator is aluminum all of them were it's nothing special Pi made it's a stock fwd gm component which to all honestly is not the best part in the world. It has 8 roller elements that are .372 wide giving it an overall holding capacity of about 400ftlbs in a very high stall converter. The machining of this part is known to the industry as butchery if this is the part you are bragging on PI for machining you need to research it some more.. Yes they machine them and they work like crap when they do. Good thing they don't machine all of them.
Alan
 
Damn....that's an old post. 04-11-2002, 11:04 AM I don't even remember posting that.
Never owned a PI convertor, I went with a Lentech AOD and convertor.
 
Last edited:
Now that we're on the subject...

Alan, I believe Wayne didn't write that article, Vernon did... he was just reposting what was written for us "AOD" guys. There is lots of useful info in what Vernon wrote (minus the PI stuff, which you have corrected for us).

However, now that this subject has been brought up... :D ...

I recently had my AOD rebuilt with the following:

- 4R70W wide-ratio gearset (complete assembly)
- 2" OD band and appropriate drum
- Lentech "street terminator" valvebody (with 4R70W calibration and correct tailshaft governor)

Also, I upgraded the rear end from 3.27:1 to 3.55:1 gears.

The last piece to be replaced is the torque converter (I still run the OEM 12" direct-drive in there). I want to upgrade this to a non-locking piece but retain the 2-shaft configuration. Len has indicated to me he offers a custom TQ that utilizes both shafts for power transfer in 3rd gear, thus spreading the load between the 2 shafts (which I like... I don't want to spend more money on a single shaft or a hardened inner shaft).

My question is, what should I be looking for with regard to size and stall speed?

A bit of further needed info:

Motor is mildly modified; ~250 RWHP.
Car is my daily-driver; fuel economy is always a concern.
I have an external transmission oil cooler in series with the radiator cooler (and I have Mike38sc's aluminum rad, which already has an improved trans cooler).

I'm thinking modest reduction in size (10"~11"), and a modest increase in the stall speed (2000 ~ 2100 RPM). Does this sound like the right way to go in your opinion?

Any thoughts or advice would be appreciated...
 
Back
Top