I understand the general idea that larger dimaeter converters have lower stall speeds and are more efficient. What I am wondering (and I hope is "advanced" enough) is why this is so?

Rich Krause

 

It has alot to do with the efficiency of the fin blades in the fan. It takes more force to push the car forward at 1800rpms than it does at 4800rpms, so there are more fins required (and at a steeper angle). Since it takes less force at higher stalls, less fins are needed, and a smaller converter case can be used.

This also explains the fact that there can be a 10" and an 8" 'verter with the same stall speed...the 8" has steeper fan blades (but less of them) and can push enough fluid to get the trans spinning at the same rate that the 10" (with more blades at a lower angle).

 

My $.02.

TC's are fluid couplings first and torque converters second. For a fluid coupling to be efficient at transmitting power, it is sized for the input power expected. Especially before Torqe Converter Clutches (TCC) this was very important for reasonable fuel economy. A few% slippage at cruise can be achieved. Of course it must slip some to transmit power without a TCC. The larger TC has more fluid volume and more fin area to transmit the power, so it slips less and is more efficient. Size matters.

Stall speed is related to input power, so increasing your engine's output at 1800-2500 should make your stock converter stall higher. That's why a stock 10 inch Vega converter with an 1800 or stall behind a 4-cyl Vega engine had maybe a 2500+ stall behind a big block. Of course, at high rpm that 10 inch Vega TC slipped a ton when facing 500-600 hp at 6000+. So for high stalls, the smaller TC gets there easier. Size matters.

Obviously the stator is what makes a coupling into a TC. Prior to TCCs GM used a variable-pitch stator in some applications so at cruise it was a very tight coupling and when torque multiplication was needed, the stator vanes switched to a different pitch. With all those moving parts reliability and warranty concerns earned it the moniker "Switch Pitch Bitch" at some levels within GM. The concept has merit, IMO. It might be used in our aftermarket high-stall converters, if someone was willing and able to develop and tool it.

Putting in a high-pitch fixed stator can increase the stall, but might prove a bit of a hinderance to efficient coupling at high rpm.
Some of the things you can do to TC fins to increase stall also make it a less efficient fluid coupling. No free lunches (except maybe with drug company detail people).

Small high-stall TC's also generate lots of internal pressure, and heat a smaller volume of oil to a higher temp than does a larger converter. Of course the smaller TC has less inertia, so that's a plus. If pressure gets high enough, the TC may balloon enough to injure the engine (thrust bearing). I'd guess that really good small diameter/high stall/high hp TC's would need to be specifically produced with metal thickness greater than OEM 8 in. TC's. That alone should make them costly.

As much as anything in a powertrain, TC's are a compromise.

 

Thanks for increasing my understanding of TCs. Given the above, I wonder if I am on the right track in picking a converter for my combo (I am pretty certain to be going with a built TH400). It seems to me I need a large, relatively low stall convertor given that my combo makes high torque at low rpm. This avoids the power loss of a smaller converter?

Am I missing something?

Sorry to seem so ignorant, but I have never had or worked on an automatic.

Rich Krause

 

Sir, you are about as far from "ignorant" as one can get. Not blowin' smoke here. I've read a great deal of what you've written, and I'm continually impressed.

How 'bout a Switch Pitch? The 400 was the trans that used it. It even helps keep the car from creeping with higher than normal idle speed. You could make it manually selectable. I'm not so sure that one of the aftermarket trans houses doesn't offer it. Launch in high pitch and as boost comes in switch to low pitch. Might be worth a tenth.

FWIW, did you see that newest GM autos will have 2-plate TCC's for higher output applications?

 

Something like that would appeal to me. As would a high performance lock up converter. But so far I haven't found one. I talked to Griner Engineering and they indicated that they used to make one, but that since it can't be used in sanctioned racing they stopped R&D on it. I will keep looking into this for an "unfair advantage".

As far as my level of ignorance goes, thanks! Everyone needs a little positive feedback now and then.

Rich Krause

 

There are lockup TH400s out there....but they are pricy

i know that www.sound-performance.com makes them

What are you revving the motor too again rich and how much power? If i remember correctly i'd say someting in the 10" 3000-3200 range would work great for you

You just have to have the verter company know that you are a boosted car, that way they can change the stator so it doesn't hit the tires so hard since the boost will be doing that.

 

So far no luck with finding a lockup unit suited for high hp. Everything I buy seems pricey, so why should this be different .

This year I will be using a 62-6,300rpm redline and figure on about 850-900rwhp (was mid 700's last year). I was thinking about something in the 3,000rpm stall range as well.

Thanks.

Rich Krause

 

http://www.api-racing.com/350VR4.htm

I have not seen the 350 version - but have carried around a VR4

Almost bought it on trade for my 4 converters and 2 trans brake 400's plus cash - then reality began to set in...

VR4 is a work of art - if you're chassis can handle the torque multiplication of a converter. The trans did not work in the application I was witness to - blown alcohol on 10.5W tire - for that reason.

MAT

 

Lots of good info here.

What many do not know is that the Lock-up TC is a dirivitive of the Switch pitch th400. Many of these did not use movable fins in the stators like a Flight Pitch does. What they did was just add or relieve pressure to gain or loose clearance between the input fan and the Stator.

This is the same technoligy used today for Lock up. Even in the After market TH400 units

Lockup is the natural state the Converter wants to be in. Pressure is applied behind the plate to "Float" it off the front cover. When pressure is released the natural force inside the TC push the clutch plate against the front cover giving you lock up. In muliti clutch applications more Clutch are used(more on that in a moment). Turbulance inside the Converter can also cause lock-up to come off or buffet under heavy loads

Now the main reason of late to go to More Lock-up clutches is the use of Lock-up in a Pulse modulated fashion. As many may or may not know all 98 and newer cars(some back to 89) have this type of lock-up. The sooner you can bring on Lock-up and the softer the better your MPG and the factorys C.A.F.E. rating.

Many singal disk clutches are not holding up to this type of duty so the next step has been more disks. This has helped in many ways and failed misserably in others(4.0 A4LD comes to mind).

Most of the Converters we buy are made from used parts of once produced cars. Yank and few others have started buying straight from GM and Ford so the parts are at least new. One thing everyone should remember is that 99% of what we get in the after market is mear left overs from the Big three

Now I am not and Engineer but I do have a little under standing of this stuff and thought I would share a little more then has been posted to help

BTW the TH350C was a production unit GM built till 86

Good luck all!

 

For reference I have a 10" A-1 converter in my glide, rated stall is 2800. Thats where it was naturally aspirated anyways. With 7 pounds of boost it would hit around 3500-3800 on the transbrake and with 14psi it would go to 4800rpm. My engine with this amount of boost puts right at 570 to the wheels and with no boost it made 380.

I'm wondering why noone here has ever tried one of these:
http://www.lentechautomatics.com/

They have several cars running 8s with an AOD. They can bolt up a JW ultra bell for a Chevy as easy as a Ford. Their lockup transmission is rated to 800hp to with a 1 year unconditional warranty. They are spendy but apparently they hold up.