C4 Tranny Stall RPM

[BPerry]

So one thing I have noticed on my cream puff is the amount of brake force I have to use while stationary, wondering what the normal stall rpm on a stock c4 would be. If there is such a thing....

Particulars - 73', stock C4, D20, twin sticks, 351W "mild build, from what I know", 4 wheel drums "I know, I know", 4.11's, Sniper EFI

Current idle RPM 850-900

Thanks in advance!

Bp

 

[DirtDonk]

There could be contributing factors with the brakes of course, but your idle is far too high for a C4 transmission.
Well, maybe not really “far“ too high, but high enough to make a difference.
Get it down to around 600-700 in neutral or 550 in drive and you’ll be a lot better off I think.

Unless it won’t idle down that low?

 

[DirtDonk]

And by any chance did anyone add a shift kit to the automatic?

 

[BPerry]

And by any chance did anyone add a shift kit to the automatic?

I was going to lower the idle rpm, easy enough with the sniper set up. But I wanted to pose the question first.

I have no idea on the shift kit, tranny shifts great. No slips or hang ups, kick down works as it should.

 

[Yeller]

That’s enough too high to be hard to hold than necessary, it’s also creating a lot of heat sitting still in gear. At 550 in gear you won’t even notice it.

 

[BPerry]

That’s enough too high to be hard to hold than necessary, it’s also creating a lot of heat sitting still in gear. At 550 in gear you won’t even notice it.

Thanks fellas, currently on a 18 mile ice cream run... lol

Will lower my rpms when I get back to the house!

Bp

 

[badandy73]

Don't mean to thread hijack here, but curious as to the OP's original question of what the stock C4 stall speed is. Running one behind a carb'd BP 306 and they recommend a 1800-2200 rpm convertor for it. Guessing the stock one is somewhere down in the 1500 range, am I leaving anything on the table performance wise not going to a higher stall speed for a mostly street driven rig?

 

[BPerry]

I rolled my idle RPM back to 730 solely through the Sniper, which placed the in gear around 680. Much better!

I may try and lower a bit more, but need to play with the carb a bit.

Thanks again!!

Bp

 

[Broncobowsher]

Don't mean to thread hijack here, but curious as to the OP's original question of what the stock C4 stall speed is. Running mine behind a carb'd BP 306 and they recommend a 1800-2200 rpm convertor for it. Guessing the stock one is somewhere down in the 1500 range, am I leaving anything on the table performance wise not going to a higher stall speed for a mostly street driven rig?

It is all a compromise. The lower stall is likely hurting off the line acceleration (where the torque convertor is multiplying torque). But get a little speed/RPM and the tighter convertor is likely passing more power through more efficiently. Higher stall will let the engine rev to a higher RPM (typically where aftermarket performance parts make there power at) so it goes straight into power off the line and doesn't have to built RPM before getting into the power. But they slip more, are less efficient, generate a lot more heat at lower speeds. I go with the lowest stall that still performs good for a driver. Now if you are going racing, that is where you start looking at the higher end of the stall range.

 

[DirtDonk]

And though I don’t understand all the subtle dynamics of a torque converter, doesn’t any given torque converter have a higher stall speed based on the amount of power/torque that is put into it?
In other words one torque converter with for example a stall speed of 1500 RPM with 150 hp and 200 pound feet of torque in front of it, might raise to 1800 to 2000, or even more, when you put 300 hp and 300 pound feet of torque to it.
Is that how they work, or am I oversimplifying it?

 

[Broncobowsher]

And though I don’t understand all the subtle dynamics of a torque converter, doesn’t any given torque converter have a higher stall speed based on the amount of power/torque that is put into it?
In other words one torque converter with for example a stall speed of 1500 RPM with 150 hp and 200 pound feet of torque in front of it, might raise to 1800 to 2000, or even more, when you put 300 hp and 300 pound feet of torque to it.
Is that how they work, or am I oversimplifying it?

You are pretty much correct. Getting into the true engineering of it, K-factor not stall. But the service manuals were written to do field checks and that is done with stall speed.

The diameter plays into the impeller tip velocity, and the surface area available on the turbine for the fluid to impart the energy. The shape of the impeller is a further tuning aid. As the fluid comes off the turbine it goes to the stator, which can sometimes be trick and a twin stator can make for more multiplication, but as speeds increase one of the stators bypasses and goes to a single stator before that one also bypasses and it goes to a simple fluid coupling. GM had a super trick variable (OK, only 2-position) stator that gave 2 different stall speeds. There was a hgh stall for performance and a low stall for drivability. Some of the early performance convertors were little more than a stock convertor that had extra clearance between the impeller and the turbine, this made the low speed energy transfer worse. The degraded low speed performance allowed an engine with a bigger cam to not be dragged down as much at idle.

I remember being in discussions between an OEM engine supplier and an OEM transmission supplier. The engine supplier wanted a higher K-factor convertor while the transmission supplier wanted a lower K-factor.

 

[BPerry]

You are pretty much correct. Getting into the true engineering of it, K-factor not stall. But the service manuals were written to do field checks and that is done with stall speed.

The diameter plays into the impeller tip velocity, and the surface area available on the turbine for the fluid to impart the energy. The shape of the impeller is a further tuning aid. As the fluid comes off the turbine it goes to the stator, which can sometimes be trick and a twin stator can make for more multiplication, but as speeds increase one of the stators bypasses and goes to a single stator before that one also bypasses and it goes to a simple fluid coupling. GM had a super trick variable (OK, only 2-position) stator that gave 2 different stall speeds. There was a hgh stall for performance and a low stall for drivability. Some of the early performance convertors were little more than a stock convertor that had extra clearance between the impeller and the turbine, this made the low speed energy transfer worse. The degraded low speed performance allowed an engine with a bigger cam to not be dragged down as much at idle.

I remember being in discussions between an OEM engine supplier and an OEM transmission supplier. The engine supplier wanted a higher K-factor convertor while the transmission supplier wanted a lower K-factor.

bet ya'll can tell what that means... lol
I'm just gonna buy a bicycle, I can change a chain and tire!!

 

[DirtDonk]

I rolled my idle RPM back to 730 solely through the Sniper, which placed the in gear around 680. Much better!

Glad it's better. No reason I can think of though not to lower it even more as long as the engine likes it. Go with what works for your engine, but it's not unusual to be at the lower 550 range with a factory-ish cam.
A stock cam and carb setup will let you idle down to 350rpm without stalling. Done it many times, not always on purpose. But it handled it just fine and came right back to normal when I wanted it.
It might not like it all that much that low, but it'll stay running.

Personally unless your system just doesn't support it, I'd try to get it to the 550-ish range and see how you like that. At least then you'll know.

Paul

 

[Viperwolf1]

Don't mean to thread hijack here, but curious as to the OP's original question of what the stock C4 stall speed is. Running mine behind a carb'd BP 306 and they recommend a 1800-2200 rpm convertor for it. Guessing the stock one is somewhere down in the 1500 range, am I leaving anything on the table performance wise not going to a higher stall speed for a mostly street driven rig?

1750-1950 for the original Bronco converter.

 

[badandy73]

Thanks Viperwolf... have heard you're the transmission guru around these parts, and the table above doesn't disappoint! Looks like I'm right at the lower end of the correct range for my 306, good to know!

 

[blubuckaroo]

And though I don’t understand all the subtle dynamics of a torque converter, doesn’t any given torque converter have a higher stall speed based on the amount of power/torque that is put into it?
In other words one torque converter with for example a stall speed of 1500 RPM with 150 hp and 200 pound feet of torque in front of it, might raise to 1800 to 2000, or even more, when you put 300 hp and 300 pound feet of torque to it.
Is that how they work, or am I oversimplifying it?

Yes.
It's all where that torque band is. My '77 302 had its maximum torque at 1800 RPM. That max torque RPM changes dramatically with different cam, carb, and heads.
Also, getting an engine with a performance cam and heads to idle at 600 RPM can be difficult.