BBC torque convertor help

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prairiehotrodder
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BBC torque convertor help

Post by prairiehotrodder » Wed Jun 03, 2015 2:54 pm

So i went to the track this weekend and made some runs with my new 540 BBC. Its in an 81 malibu with a turbo 350, 3.50 gears and a TCI 4000 stall. The car weighs 3300 ready to race. I ran 10.65 at 127. This translates to about 550 RWHP. I'm missing 100 hp or more. Starting to have doubts about my convertor. Talked to a guy at TCS and he said i'm definitly "blowing through" my convertor. Not sure what that means or how to tell if that was happening. I wish i had paid more attention to my RPM at the track through the lights but i honestly didn't notice what it was. The guy i talked to was Rusty at TCS and he said my car would be much quicker with a 3500 stall. He said the 3.50 gears are what is causing me to blow through the convertor. Does this sound reasonable? I pulled the dipstick on the tranny and it smells burnt but its bright red and clean and does not look burnt.
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Re: BBC torque convertor help

Post by prairiehotrodder » Fri Jun 05, 2015 7:27 pm

No response on this. Well i went ahead and installed a different convertor. My first one was rated at 3800-4000 and my new one is 2900-3300. Next time i go to the track i'll know if it was worth it!
Brian
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Re: BBC torque convertor help

Post by F-BIRD'88 » Fri Jun 05, 2015 7:53 pm

Blowing thou the converter means the converter stalled so high behind the motor on launch that it not only stalled higher than peak torque output but stalled higher than peak HP rpm point. This happens when the engine is way bigger CID and or supercharged than the intended engine was for the converters design.

Like running a 540cid motor with a "4000 stall" intended for a 350-400 CID engine.
Or running a big big shot of nitrous etc.

Geberally when a converter designed and intended to create a "4000 stall" when behind a 350cid engine is bolted up to a 450cid engine the observed stall on launch will be about 500rpm higher.
The "4000 stall" becomes a 4500 stall when behind the larger CID 450 CID engine.

Without knowing more about your converter and how it was rated (behind what CID motor)
cannot say much more.

The low lame rear gear ratio will cause the car to stall on the "stall mode" too far down the track when you run. "stall mode" is when the stater in the converter is held stationary and
multiplies torque on launch. When the driven element and the drive elements catch up to each other on the run the stater spins with them and the converter "locked up"

The lack of rear gear ratio will make the converter seem mushier than it should seem, on the run. On a 1/4 mile pass you want the motor to reach about 4%-8% higher rpm than what rpm peak HP is created at, when passing the traps.
So if peak Horsepower occurs at 6000 rpm you want to trap at or near 6300 rpm.

If you could watch-video or data log the tach rpm during a pass you would see on launch
the rpm flashes to the stall speed, then stays there for a brief time ("stall mode")
The stator is held and the converter multiplies torque on launch.
The drivne element is turning slower than the drive element in the converter.
When the two elements catch up as the car accelerates the converter switches from stall mode to lock up mod the two elements are at near the same rpm and the engine rpm rises
as the car accelerates.
If the gear ratio is too lame the car is in stall mode for too much time/distance on the pass
Its "on the converter" for too much of the pass.
More gear fixes this.

Blowing right thru the converter is different and more extreme. The engine is so powerfull that the converter stalls way higher than intended and can actually Cavitate inside.
The converter looses hydraulic coupling and or cavitates. The enigne rpm flairs up and it never locks up. The cure is a different (usually larger diameter) torque converter.

If you say install a 7" 8" full race super stock race converter intended for a 283 with a 6000+++ stall speed (behind a 283 cid engine) and install it behind a 540cid motor. say 600+ hp and a ton of torque YUP its going to "blow right thru the converter"
its a big mismatch.

Your car needs way more gear. The converter may need looking at.
If it is a "off the shelf" catalog converter with a typical advertized stall speed of such converters
It is rated for a 350cid motor not a 540cid engine.
You probabily need a 10" custom built race converter to get it right.
Built for the power of a 540cid engine.
With a actual converter stall speed at near peak engine torque or a bit higher.
More engine torque output will cause that converter to show a higher stall,
Less torque='s shows a lower real stall speed.

A real engine dyno test readout will help the converter tech dial in the ( custom built) torque converter for you.

That 'Off the shelf" rated torque converter is going to see about 1200-1600 rpm more real stall on launch, when behind a big 540cid engine.

Get some gear for that car.
Last edited by F-BIRD'88 on Fri Jun 05, 2015 7:57 pm, edited 1 time in total.

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Re: BBC torque convertor help

Post by F-BIRD'88 » Fri Jun 05, 2015 7:54 pm

This does not address the fact that the engine may not be making as much real horsepower in the car now than what it did on the engine dyno. The details matter.
Whats different now.

What was the complete data from time slips? What rear tires diameter?

On a time trial drag pass the reaction time does not effect the ET or MPH. So to see what the torque converter is actually doing as you launch the car just let the light turn green and watch the tach as the car first launches and accelerates thru first gear. Watch the tach. It will "flash" to the converter stall. It is in stall mode.
Te tach will briefly stay at about the same rpm as the car first accelerates. When the tach then starts to rise rpm as the car accelerates to the shift point this is the "lock up mode"... A lack of rear gear ratio makes this "stall mode" lock up Mode switch point happen too late- too far down the track on the pass. The pass is less efficient than it could be.
You want a quick efficient launch and the converter in stall mode only briefly, thru first gear.
If the real stall is too high and or if the gear ratio is mismatched and or you shift it too soon (shift rpm) the converter can fall back into stall mode on the shift. This is also inefficient and will slow the car slightly. (its also hard on the stator)
You ideally want the shift point (shift rpm) high enough so the converter stays "locked" on the shift.
The converter never "locks up 100% 1 to 1. a 90 to 97% lock up is typical. If the gear ratio is lame the lock up ratio %
will be higher than it could be if the car had the right gear in it.
Again you want to trap at or near 4% to 8% higher than peak HP rpm occurs at.

The stall-lock up point and the amount of converter torque multiplication (typically 2:1 to 3:1 stall torque ratio) and the "top end" lock up efficiency is a function
of the converter diameter+ the drive and driven elements fin angles+ the clearance between the two elements + the design shape and angles and widow area of the fins of the stator element. They all work together to get the job done.

A custom built converter fine tunes all these performance effecting elements.

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Re: BBC torque convertor help

Post by F-BIRD'88 » Fri Jun 05, 2015 8:26 pm

Rear tire diameter?

peak torque rpm and peak Horsepower rpm point from engine dyno tests?

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Re: BBC torque convertor help

Post by prairiehotrodder » Sat Jun 06, 2015 1:31 am

Holy smokes f-bird. thankyou for all that explanation. I'm going to have to read it a few times to try and understand it all. I appreciate the work you put into that. To answer some of your questions:
My car has small 28 x 9 ET drags. Trans is a turbo 350. Engine has never been on a dyno. When i said "its missing 100 hp" thats just a guess based on the amount of hp i figured it should have based on the parts that i built it with. I figure it should make at least 700 hp. Its easily possible that its not making near that much hp and that there is no problem with the convertor and that i'm chasing the wrong problem. If thats the case i will keep plugging away with my tuning to try and go faster.
There are a few reasons i have the 3.50 gear. I know alot of guys won't like this but i street race fairly often. There is a group of buddies in my area and we have a remote spot thats fairly safe and out of the way. Anyways as everyone who street races knows that traction is everything. I've found that with a taller gear i can reach the stripe easier without being redlined too soon when the launch is poor with lots of wheel spin. Also the 3.50 gear works good on the bottle. Also i street drive it quite a bit.

The motor is brand new and the day i had it at the track i only was able to make 4 passes. My first pass was a tire smoking 11.3 at 127. I then jetted up a little and the car slowed to 11.4 at 125 still spinning hard. Then i went back to my original jetting but pulled the timing up from 33 to 37. I also found i had a unaccurate tire gage, my tires were at 17 psi. I set them back to about 11 psi and ran 10.65 at 127.5. Then i noticed my valvetrain sounded pretty noisy so i adjusted the valves. They had loosened up quite a bit from when i first built the engine. I reset them to .014 with the engine warm. I ran 10.67 at 127.5 again so no gain. I still had lots of stuff to try but no more time. I would have liked to try going leaner because i think my carb was way to fat. Also i would have liked to drop the exhaust.
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Re: BBC torque convertor help

Post by F-BIRD'88 » Sun Jun 07, 2015 2:28 am

Use the camera in your smartphone to video a view of the cars gauges/tach while making a drag pass.

Now you can collect data on what the converter is doing, stall speed , shift rpm, rpm through traps etc.
Recording the sound can be revealing too. Make up a stable mount say on the roll bar to hold the phone so
it sees-records the tach etc.

Are you sure the car + driver etc is actually #3300.

I always find that you need to set the valve lash tighter than cam card. I prefer to set them Cold.
About .006" to .010" less than cam card. Especially if aluminum heads. BBC's seem to expand a lot.

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Re: BBC torque convertor help

Post by chevy art » Sun Jun 07, 2015 1:03 pm

when f-bird speaks about any subject on this forum, i always listen to every word he says. he is a great man to have on our forum. art

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