Road Bike, Cycling Forums banner
1 - 15 of 15 Posts

·
Registered
Joined
·
3,557 Posts
Discussion Starter · #1 ·
I need a new automatic transaxle in my VW Vanagon. The rebuilder suggests that I go with a new torque convertor with a 200 RPM lower stall speed since I run a mildly bored / stroked engine.

The only justification they are giving is that "it gets power to the ground faster".

If that's such a great idea, why didn't the factory do it? In fact, VW increased the stall speed over the production run.

So, do any of you know why a lower stall speed would be beneficial? I asked this question on my VW forum but apparently torque convertors are some sort of dark magic.
 

·
Registered
Joined
·
3,394 Posts
It has to due with emissions. The engine below a certain RPM threshold is very inneficient, and will pass unburnt fuel to the exhaust system, fouling up the CATS in addition to spewing out tons of emissions that exceed regulations.
 

·
Registered
Joined
·
5,374 Posts
Do you feel you need to get the power to the ground faster... in a VANAGON?
If this is near the idle speed, you are going to have a hell of a time keeping it running.
 

·
Registered
Joined
·
3,557 Posts
Discussion Starter · #4 ·
Do you feel you need to get the power to the ground faster... in a VANAGON?
Ha, definitely NOT! It would stall at about 1000 RPM higher than idle but I'm just not seeing the point of this particular "upgrade".

I am going with a limited slip differential and aftermarket trans cooler though.
 

·
Registered
Joined
·
3,394 Posts
The upgrade is great for drag racing with cars that have a lot of low end torque. My C63 for example makes great use of it, usually combine with CAT delete (can't really do that in states with emissions tests). The M156 engine in the C63 has a ton of low end torque across the entire RPM range. Cars with turbos, then to make the most power in the upper RPM range, so have a lower stall speed will have little to no effect.
 

·
Registered
Joined
·
8,224 Posts
A torque converter allows the engine to turn more revolutions than the transmission input at low RPMs. It's stall speed is where the converter reaches the point where the engine and transmission input turn at a 1:1 ratio.

That said, I doubt your engine has been modified to the degree to warrant changing the stall speed. Fuel mileage improvements would be minimal and it could create issues when starting off going up inclines or accelerating from a stop with a large load. Under those conditions, if the torque converter is locked up the stress gets transferred to the clutch packs within the transmission and there is the potential for slipping and wear.

Your intuition is right here...stick with a stock torque converter.
 

·
gazing from the shadows
Joined
·
27,287 Posts
So, do any of you know why a lower stall speed would be beneficial?
Far from an expert, but I think it would be better to ask WHEN it would be beneficial. In a perfect world, the stall speed will closely match the peak torque of the engine. So if your engine modifications lowered the peak torque of your engine by 200 RPM, then it should be beneficial. You do know what those modifications did to the power curve, right? Theoretically, if not actually measured?

It's actually more complicated than that. Different levels of torque from different engines will alter the actual lock up for the same converter. Vehicle weight, tire size, gearing all change things a bit..... thus the black magic vibe you got. But boiled down, you should be able to go from changes from stock power to figure things out in terms of it being a good or bad idea.
 

·
Registered
Joined
·
8,224 Posts
Far from an expert, but I think it would be better to ask WHEN it would be beneficial. In a perfect world, the stall speed will closely match the peak torque of the engine. So if your engine modifications lowered the peak torque of your engine by 200 RPM, then it should be beneficial. You do know what those modifications did to the power curve, right? Theoretically, if not actually measured?

It's actually more complicated than that. Different levels of torque from different engines will alter the actual lock up for the same converter. Vehicle weight, tire size, gearing all change things a bit..... thus the black magic vibe you got. But boiled down, you should be able to go from changes from stock power to figure things out in terms of it being a good or bad idea.
Given the type of vehicle I doubt the engine has been modified to any significant degree. The OP speaks of it being "bored / stroked". I expect it has been bored to clean up worn cylinders at some point. That would only change the displacement by a small fraction (2% or so) and have little impact on the torque curve.

Changes in stroke and camshaft profile would impact the torque curve to a larger degree. I highly doubt the engine has been stroked. That is not a procedure done to repair an engine, it is done to modify it and can be pretty involved. I also doubt that the profile of the camshaft has been changed.

To answer the OP's question, a lower stall speed converter could be beneficial for fuel economy, but this would be marginal. The order of magnitude would be in the .1 mpg range. That's it for benefits.
 

·
Registered
Joined
·
3,557 Posts
Discussion Starter · #9 · (Edited)
Given the type of vehicle I doubt the engine has been modified to any significant degree. The OP speaks of it being "bored / stroked". I expect it has been bored to clean up worn cylinders at some point. That would only change the displacement by a small fraction (2% or so) and have little impact on the torque curve..
Not quite. It wasn't actually "bored" in the sense that they cleaned anything up, it uses brand new pistons/cylinders and a stroker crank. This took displacement from 2.1L to 2.3L. It has a mild performance cam, 4-angle valve grind, etc.

So, instead of the stock 90HP I'm now rocking a whopping 115HP, in a 2 ton vehicle. It's still slow, especially up hills, but it's noticeably quicker than a comparable stock van. I do agree that it's not anything as extreme to warrant crazy tranny mods but I want to future-proof to some degree.

Future plans are to swap the VW waterboxer for either a 2.5L Subaru or 1.8 turbo VW engine but I'm hoping to put that off for at least 5 more years.

These aren't broken down "hippie busses" anymore, they're cult/collector vehicles. Most nice ones start around $20k and they're appreciating (it's absurd and not common but some 4WD versions have sold for $100k+ Vehicle Sales History | GoWesty | Parts for VW Vanagon, Eurovan, and Bus). Lots of aftermarket support too.

This is the only picture I have here at the office.
 

·
gazing from the shadows
Joined
·
27,287 Posts
...mild performance cam...
Generally means power comes on at higher rpm, not lower.

I asked this question on my VW forum ...
Ask that forum what your new cam/displacement/etc will do to the power curve. They should be able to talk peak RPM for torque and horsepower from the mods pretty easily (if not, find a new forum :D ), and that should give you the info you need to make a call.
 

·
Registered
Joined
·
8,224 Posts
Generally means power comes on at higher rpm, not lower.



Ask that forum what your new cam/displacement/etc will do to the power curve. They should be able to talk peak RPM for torque and horsepower from the mods pretty easily (if not, find a new forum :D ), and that should give you the info you need to make a call.

^^^agreed.

Most performance applications call for a higher stall speed, not lower.

"Getting power to the ground faster" is a poor description.

You might call the shop that built your motor to see what they recommend. If you go elsewhere you will want to be armed with some very specific information:

1.) Vehicle weight
2.) Gear ratios
3.) Engine displacement (bore & stroke)
4.) Engine compression ratio
5.) Cam specs, including lift and duration (intake and exhaust) along with overlap.
6.) Intake & Carb/FI info
7.) Exhaust setup

In short, the whole thing works as a system and needs to be looked at as such.

Good luck, it looks pretty cool. Having followed may a Microbus over the Sierras, it's nice to see you have 3x the HP. :)
 

·
Registered
Joined
·
3,557 Posts
Discussion Starter · #12 ·
^You might call the shop that built your motor
That's what I ended up doing. Their take on it is that it *might* be an improvement and there isn't much risk to trying it.

Regardless, I'm just going to go with a stock TQ. I've been happy enough with it that way all this time...
 

·
Registered
Joined
·
5,374 Posts
My brother modified and raise the stall speed on his corvette, was that ever a mistake. It would rev to the stall speed and then hit like a dropped clutch. When it was running he drove it mostly on the street, and it was not nice.
I would keep it stock unless someone knows what they are doing.
 

·
Registered
Joined
·
3,394 Posts
My brother modified and raise the stall speed on his corvette, was that ever a mistake. It would rev to the stall speed and then hit like a dropped clutch. When it was running he drove it mostly on the street, and it was not nice.
I would keep it stock unless someone knows what they are doing.

It is basically great for drag strips and straight line pulls, otherwise not very useful.
 

·
Banned
Joined
·
19,737 Posts
I wouldn't waste the money.

I don't know if you know the basics of how a torque converter works, but it's basically analogous to two fans facing each other. When you turn one on, the moving air spins the other. Now imagine one fan attached to an engine crank, and one to a transmission, and instead of an air coupling, it's fluid. Stall speed refers to the speed at which one of two things happens - either the fan attached to the transmission starts to turn, or the engine RPM no longer increases.

The purpose of changing the stall speed of a torque converter is to more closely tailor it to the power curve of the engine, so that the converter is locking up at the RPM where the engine is producing usable power. This sort of modification is always necessary with high power high revving engines like those used in drag racing, so that the driver can rev the engine to an RPM where it produces usable power before launch. Engines like that are built to start producing power at higher RPM, so the converter will have a high stall speed. Such arrangements are absolutely miserable to drive on the street. Conversely, engines that produce power at lower RPM, such as diesel truck engines, will have a much lower stall speed converter in order to take advantage of the lower RPM power.

The one advantage to a lower stall speed is lower overall transmission temps, but this is really only a problem for the aforementioned high stall speed converters. Cars with a stock engine or cars with streetable modifications typically do not have a problem with excessive heat. Unless your engine modifications have significantly altered the power curve in terms of what RPM the power comes in, the money would be better spent on other modifications.
 
1 - 15 of 15 Posts
Top