Suncoast Stall Question

bhysjulien · **Posted:** Mon Aug 04, 2008 12:00 am

If I remember correctly Suncoast sets the stall on the CRD converters at approximatley 1500 RPM. Has anyone tried having it set at 1200 RPM? I'm wondering if fuel mileage would increase more with the stall speed set at 1200.

gmctd · **Posted:** Mon Aug 04, 2008 1:12 am

Lowering stall any further results in wasted fuel at stop events, where engine has to work harder to overcome increased hydrualic coupling - best way to increase fuel mileage is to decrease shift times, increase shifting events to be in higher gear at lower rpm, and engage TCC at lower rpm, let trans gearing split the torque chores, rather than hydraulic slippage

Sir Sam · **Posted:** Mon Aug 04, 2008 1:58 am

gmctd wrote:

Lowering stall any further results in wasted fuel at stop events, where engine has to work harder to overcome increased hydrualic coupling - best way to increase fuel mileage is to decrease shift times, increase shifting events to be in higher gear at lower rpm, and engage TCC at lower rpm, let trans gearing split the torque chores, rather than hydraulic slippage

So for those of us that always shift to neutral during stops lowering the stall speed would be advantageous.

gmctd · **Posted:** Mon Aug 04, 2008 9:03 am

It may become a trend for eveyone to do just that, but for those that tend to forget, trans fluid temps could be 280-300deg, just from a simple stop sign in mild traffic - Dodge had that same problem with the new Cummins installation in '89 - the TC was identical to the GM D4 for the 454 and 6.5L, made by the same oem, but it slipped too much with the excessive low-end torque of the big 5.9LCTD, so DCJ spec'ed the internals for 11-1200 rpm stall, and soon the big torqueflyte transmissions were dropping like flies - it was a really big fiasco, as the big six likes to idle at 750rpm and it became a chore to keep the vehicle stopped - solution: huge separate self-contained cooler with coolant flow from waterpump output directly from radiator cool-side - helped, but wrecker-drivers could still go thru 727 rebuilds with the greatest of ease - best method is to modify transmission response to use the gears rather than the slush, and allow earlier lockup, say at 30mph, lighten up on shift duration while in lockup - be a lot easier than the mush we got now, but somebody needs to break the code

Joe Romas · **Posted:** Mon Aug 04, 2008 9:46 am

gmctd wrote:

best method is to modify transmission response to use the gears rather than the slush, and allow earlier lockup, say at 30mph, lighten up on shift duration while in lockup - be a lot easier than the mush we got now, but somebody needs to break the code

So if I understand what your saying F37 took care of the "lighten up" part right? Then about sharing the load with the transmission :idea:

Is the stock transmission durable enough to share the load, take on more of the work, maybe with extra cooling :?:

After my recent 1500 mile round trip vacation pulling my trailer through some mountains in Va and W Va it became very clear that it was with the "OD" button pushed it was 3rd with TC locked or without the button it was 3rd period. It seemed that 4th locked or unlocked and 5th unlocked were avoided and there were several instances where they would have been a better choice :cry:

Are they too wimpy for the load? My trailer was loaded to about 3500 pounds and has a small rounded frontal area :wink:

Joe

chrispitude · **Posted:** Mon Aug 04, 2008 10:29 am

gmctd, I realized just how "immature" the CRD's transmission programming was when I switched to towing our 27' travel trailer from the CRD to my 2005 Toyota Tundra V8. On the highway, the Tundra effortlessly slipped between the top three gears - always locked up - moving the RPMs from 1800rpm to 2000rpm to 2300rpm between those three gears. Shifting was seamless, and even off the highway you were almost always locked up in some gear or other unless you were at low speeds.

I now liken towing with the CRD to the real big gawky kid on the playground who's taller and stronger than everyone else, but doesn't have a lick of athletic skill. Sure, the CRD with the Inmotion tune out-torques the Tundra up the slight grades, staying locked up in 1800rpm. But as soon as it falls out of lockup, the RPMs scream and it flails about trying to figure out what to do.

The Tundra has a bit less torque but far better towing manners. The CRD could be a relative tow beast with a real tow/haul mode.

- Chris

gmctd · **Posted:** Mon Aug 04, 2008 11:29 am

The 545RFE is shared by the KJ CRD, the Grand Cherokee 4.7v8, and the Hemi vehicles, incl the Ram trucks, with no differences other than the excellent Hemi torque converter which Suncoast modified for us, and some programming changes due to the higher-revving V8's - the Hemi's are pushing way more power into a heavier vehicle, so Suncoast rebuilds the trans for racing with clutches designed with friction material that can transmit more torque - I don't see any problems with this trans except the programming, which was geared towards efficiency and fuel economy: a clutchpack with 65psi applied pressure will transmit less torque than the same pack with 120psi - but, the hydraulic pump supplying the pressure will take more power to make 120psi than 65psi - we got a two-phase hydraulic pump, which will supply low pressures at reduced power for slight loading and hi pressures at increased power for towing and such - this is determined by the amount of 'slip' between engine rpm and the 545 input speed sensor, indicating torque converter slip - further slip, determined between the input speed sensor and the output speed sensor, indicating clutchpack slip, calls for more pressure, or reduced power - F37 gave us reduced power in each instance, likely due to the plastic stator - the stator is effective only to ~20mph, depending on final drive ratio, then freewheels - torque-multiplication is required only from standing start, to get the load moving, and the greater torque of the CRD turboDiesel is a greater load on the stator than the other vehicles because the Diesel will be trying to get a towed load moving, which torque is required for much longer duration than, say, a Hemi-powered vehicle drag-racing, where 20mph comes and goes in a brief instant - we work the stator harder than they do, so we get F37 for reduced torque input - now we have the Suncoast TC, and no need of F37, and need somebody to make some further changes in pressure control and shift duration and shift event timing and TC lockup so we can take full advantage of the 5sp transmission and the new low-stall TC, which a Diesel engine can use to great advantage, where efficiency and economy are concerns

Suncoast Stall Question

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