Converter thoughts !

Transmission to Rear-end

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SBC-68-FIREBIRD
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Converter thoughts !

Postby SBC-68-FIREBIRD » Sat Sep 24, 2016 2:05 am

Hi Guys,

I have a 68 Firebird, full weight street car.
Turbo 400 - 3.73 gears.
Have just fitted a 421SBC into it. Dyno'd as follows -
4100RPM - 478tq - 373hp
4500RPM - 502tq - 430hp
5000RPM - 545tq - 519hp
5200RPM - 547tq - 541hp - maximum torque.
5500RPM - 538tq - 563hp
6000RPM - 507tq - 580hp - maximum horsepower.
6200RPM - 490tq - 577hp.

I currently have a 9.5" x 3800-4000 stall in the car but feel that this stall speed is holding it back a bit.

What would you guys recommend to get the best out of it keeping in mind this car is 99% street & only has a few days at the track each year.

Thanks.
Scott.

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Re: Converter thoughts !

Postby Brian P » Sat Sep 24, 2016 5:53 pm

I hate slippy torque converters in street driving. You've got no lock-up clutch to cover up that behaviour, either.

In theory a little higher stall speed (say 5000-ish) will have the engine at max torque through a dragstrip launch instead of being a bit below, but if this is 99% a street car, is it worth doing? Your call. If it were my car, I wouldn't ... but if it were my car, it would have a row-your-own transmission with three pedals and at least one overdrive gear.

You're spinning 2800 - 2900 rpm at 60 mph in top gear (no overdrive) plus torque converter slippage, which will be quite a bit, and would be even worse with a higher-stall torque converter. Heat, mileage, throttle response in street driving, low speed driveability will all get worse if you have more torque converter slippage.

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Re: Converter thoughts !

Postby F-BIRD'88 » Mon Sep 26, 2016 9:51 am

Go to the track a lot more.

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Re: Converter thoughts !

Postby Bob Hollinshead » Fri Sep 30, 2016 7:46 am

Brian P wrote:I hate slippy torque converters in street driving. You've got no lock-up clutch to cover up that behaviour, either.

In theory a little higher stall speed (say 5000-ish) will have the engine at max torque through a dragstrip launch instead of being a bit below, but if this is 99% a street car, is it worth doing? Your call. If it were my car, I wouldn't ... but if it were my car, it would have a row-your-own transmission with three pedals and at least one overdrive gear.

You're spinning 2800 - 2900 rpm at 60 mph in top gear (no overdrive) plus torque converter slippage, which will be quite a bit, and would be even worse with a higher-stall torque converter. Heat, mileage, throttle response in street driving, low speed driveability will all get worse if you have more torque converter slippage.


Is it possible to run an adjustable clutch, maybe an iron disc setup and adjust it for street and strip? Often thought about doing a setup like this with possibly a clutch turbo400 with a GV overdrive. If the launch can be good and it could live at the track plus be adjusted for the street and shifted with the clutch and with no slip for OD use it seems like it could be the best of all worlds.
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Re: Converter thoughts !

Postby Brian P » Fri Sep 30, 2016 1:39 pm

How do you propose that this clutch be arranged - like a normal torque converter lock-up clutch so that when disengaged, the torque converter operates, and when engaged, it drives mechanically? And what do you mean by "adjustment"?

Lock-up clutches are standard practice on all late model automatic transmissions. Most of them nowadays have pulse-width-modulation of the valves that control it and electronic monitoring of the amount of slip (the ECU obviously knows the engine speed, and it also has the transmission input shaft speed as another input, so that it can establish the amount of slip and control it via PWM). The lock-up clutch isn't just "on" or "off". This way, the torque converter lock-up doesn't cause driveability complaints like they often did in the early days, because it can engage and disengage "softly". And, nowadays, generally the gearbox has a lot more than 3 speeds and a lot more range between 1st and top, so that it no longer relies on the torque converter's torque multiplication to get a good launch off the line. It is no longer necessary to have a torque converter with a "slippy-feeling" high stall speed.

A lot of the old transmissions did not have enough torque capacity in the lock-up clutch for full engine torque. That's why they unlocked if you breathed on the accelerator pedal. The new ones will allow lock-up right down to just above idle speed. They normally have vibration dampers built into them (similar to what's on the clutch disk of a manual transmission) to allow this to be done without juddering. The automatic in my van - which is a nothing-special Chrysler 62TE - will hold lock-up in 6th down to just above 1000 rpm. If you're climbing a hill in 6th, it will go right to full engine torque in 6th with lockup engaged - and then downshift to 5th and usually lock it up after the downshift. The controls are a lot smarter now than they were in the old days.

Retrofitting this new tech to an old tech vehicle pretty much means ... replace the old school transmission and its controls with a new one.

I don't know if the GM THM400 was ever available with a lock-up clutch at all. I know that the basics of that transmission morphed into the 4L80E, which did, but I have no idea how compatible the new parts are with the old stuff. Again, if you want a 4L80E with overdrive and lock-up ... you replace the THM400 with a 4L80E.

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Re: Converter thoughts !

Postby SBC-68-FIREBIRD » Fri Sep 30, 2016 3:26 pm

Thanks for the replies.

I have a few mates that are running around with 8" x 5000-5500 in their street cars that reckon they drive real nice but none of them do a whole lot of driving.

I spoke to the guys who built my converter & they reckon they can get about 5000 out of my 9.5" but not sure if trying to get that sort of stall out of 9.5" converter will make it in-efficient. At 5000 stall is an 8" going to work much better than a 9.5" ?

I can get mine changed for about 1/3 of the price of a new 8" but might be better off to leave this one alone & buy an 8" & give it a try that way can always go back.

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Re: Converter thoughts !

Postby BigBlocksOnTop2 » Fri Sep 30, 2016 4:13 pm

SBC-68-FIREBIRD wrote:Thanks for the replies.

I have a few mates that are running around with 8" x 5000-5500 in their street cars that reckon they drive real nice but none of them do a whole lot of driving.

I spoke to the guys who built my converter & they reckon they can get about 5000 out of my 9.5" but not sure if trying to get that sort of stall out of 9.5" converter will make it in-efficient. At 5000 stall is an 8" going to work much better than a 9.5" ?

I can get mine changed for about 1/3 of the price of a new 8" but might be better off to leave this one alone & buy an 8" & give it a try that way can always go back.

I have an ATI Tree Master. Its old and about 35- 40 1/8 mile runs on it. It's in great shape, just older than the hills. I purchased it I think in 1984. The car was around 2400 lbs. with a 351 C. It's not junk piece, just old.

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Re: Converter thoughts !

Postby AMXstocker1 » Sat Mar 25, 2017 1:06 pm

excuse me for asking but the fact that you make peak torque 800 rpm before peak hp kinda makes me think you have something choking this combo like a big cam with a dual plane and 600cfm carb or something. but regardless picking a converter for this i would look for something in 8inch and around 4500 rpm stall i know people who drive around with 4-5000 stall converters alot, if you look at dragweek competitors theres alot of 5000 plus stall converters making that 1500 mile drive with a trailer just make sure you put a really good transcooler in it.

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Re: Converter thoughts !

Postby Seanh832 » Sun Mar 26, 2017 3:20 am

8" 5000. Have one built for your combination and you will be happy. I've got a Hughes Pro IV custom 8" 5000 behind a 700hp 434 with a big solid roller 3.90 gears with a 28" tall tire and it drives around at town practically like it has no stall at all. Until i get on it! \:D/ \:D/
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Re: Converter thoughts !

Postby quickd100 » Sun Mar 26, 2017 8:13 am

I've always tried to get them to stall at torque peak rpm. I've run 6200rpm stall convertors on the street with no problems. I generally put a couple thousand miles on it a year. Dave
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Re: Converter thoughts !

Postby dfarr67 » Thu Apr 27, 2017 4:09 pm

Switch Pitch?

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Re: Converter thoughts !

Postby dfarr67 » Thu Apr 27, 2017 4:11 pm

Copied


by Speedbump » Fri Feb 24, 2017 10:15 am

I just went to a bunch of trouble to change my switch pitch TH-400 with DNE overdrive (my first idea) to a 4L80E built by Joel on Joy with a HD lock up converter and a TCI controller. The difference is astounding. So I completely disagree with F-Bird based on experience. Mine is a 505 CID BBC with a tunnel ram and a 240's @ .050 cam. The truck is SO much more fun to drive and the freeway cruise is great due to the lock-up converter. TCI gives you lots of options with the touch of a button on the console. It did cost a bunch for top notch stuff but I don't regret anything other than I wish I had thought of the 4L80 before the expensive TH-400 experiment. By the way, I have a killer switch/pitch chevy 400 with an A-1 converter and a DNE overdrive/custom shifter for sale if you want.

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Re: Converter thoughts !

Postby steve cowan » Sun Apr 30, 2017 3:37 pm

hi scott,
sounds like a real fun street ride
i think you can run a 8 inch converter no problem,i have done it plenty of times in a true 10 second streeter
but we have to look at what is the expectation and what do we want to achieve
my current ride is a 520 hp 383 sbc
turbo 400 full reverse manual
9'' 4500 stall
9'' rear 4.1:1
255/60/15 drag radial,they come in around 27.5 inches
my one and only local track is 2 hrs drive each way,highway driving
so to do approx 55 mph i am on 3000-3200rpm
with your 3.7 rear gear should be nicer,i dont know your tyre height
but here is the trade off
you want to go through the traps at 500-800rpm above peak power
will you do that and are you prepared to do that,up to 7000rpm of course is not a problem for that engine i suspect
but will it rpm that high through the lights???
maybe with a 8'' 5000rpm converter you may have to much slip and it may rev higher but run slower because of slippage
i agree a big stall and low rear gear is no big deal idling around the street but on the highway for a decent amount of time driving and your girlfriend trying to yell at you
over the exhaust note can be draining
at the best of times
( i actually enjoy the exhaust note on my car twin 3 inch dumpers with aerochamber mufflers)
any way something to consider i guess but no mater what you do its all about the fun
steve c


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