Please help me understand this stall speed discussion

[nanbuicker]

I'm in contact with one of the respected site sponsors regarding a new 2004R and lock up converter for me. I have problems understanding his point and i wanted to ask here for explanation, because i don't think he has the time to chit chat this out via email, i am in Germany, thats why i don't call. I wouldn't understand on the phone anyways.

I totally trust this sponsor, but it seems like i just don't understand it.

Here's our shortened email traffic:

Me:

Could you make me an offer for this combo:
2004R /w LockUp
Lock Up Converter
Regarding stall speed see the cam specs below, I’d go for 3000 to 3200 I think.

Here are the specs of my car:
71 Chevelle Malibu, full interior/weight, 383, 10.2:1, all forged, AFR 195 heads, HR-296-2S-12 Roller cam, around 450hp
Cam specs: Crane Cams 109841 - Crane PowerMax Camshafts - Overview - SummitRacing.com

Him:

I think your selection is correct with the exception of the stall speed.I would suggest you use a 2800 stall speed

Me:

Why do you think it will need only 2800 stall? The cam specs say that rpm range starts at 3000. At the moment I have a 2400 converter and it’s choking the cam. You know I thought if it has lock up in fourth it wouldn’t matter if stall speed was a little higher. I will listen to your advice though! If you say 2800 is better, then that’s what I will order.

Him:

Oliver you dont want the engine rpm at cruise to be above the stall rpm or the engine will not make efficient use of the gasoline.

That's where i can't follow him anymore.

Here’s my understanding until now:
- on a non-lock up tranny like the TH350 I have now, cruise rpm should be above stall speed rpm, otherwise it would be slipping during cruise and creating lots of heat.
- on a lock up I thought it doesn’t matter, because the tranny will look up in 4th gear, so there is no slippage regardless of what cruise rpm is?

Could anybody explain, or maybe also give his recommendation for stall speed? Point is, cam rpm starts at 3k.

Thank you very much!
Olli

 

[turbo nasty]

This is gonna be long ...

2800rpm is pretty much it on a lock up convertor as fars as it retaining good slip % on the big end (top of second gear and on) once it gets out of the lower gears and the gear multiplication lowers then the conv slop is gonna shine on a loose (2800 or higher) Lock up stall. Any more than 2.8k and its gonna lay on the slop/slip on the other end and slow the performance down.

Going with a higher stall and retaining lock up and forcing the Convertor clutch closed (locked) at WOT Wide Open Throttle is a way around the slop. But...
This isnt recommended for trans and convertor longevity IMO. You will need a multidisk clutch convertor to handle the abuse as well as the parts (billet, hardned, etc).
The multi disks need a pump mod, tend to rattle at idle and will frequently stall the engine when the fluid is cold when first put into gear.

To further elaborate on the mechanics of why this happens..... is due to the construction of the convertors themselves that have Lockup feature.

Way the convertor works in a nutshell is it is a fluid coupling with two halves that are not mechanically connected (except when the coinvertor clutch in locked) that has fins/blades in both sides that "cut" the fluid.
The fin angle/size/number of fins has an affect on the cutting (Cutting=slip for stall speed).

So when you have a lockup convertor the area inside the convertor has to allow so much space for the lockup parts. That cuts back on room for more fins to keep the convertor effiecient. Couple that with a smaller than 12" convertor and it gets worse since there is even more space constraints.

So when you get over 2800 rpm lockup conv and you DONT force lockup at WOT then its gonna slip on the slop.

You are at the crossroads...
You can get a NON lock up and get it set up for a high stall and still be effiecient at WOT on the big end. Gas MPG will drop some and it will tach higher and put off more heat.

Being that you are overseas go non l/u and have a your combo work together better. Unless you have the money otherwise

I highly advise against forced Lockup and longevity they dont mix.

 

[nanbuicker]

Thank you very much for your long explanations, uh, Nasty.
That helps me understand a lot. Especially interesting for me was the explanation of how lock up is influencing the slippage. I don't want to go the WOT Lockup route, that's for sure.

Maybe for your further suggestions I should say that my Chevelle is a 100% fun car for the weekends, meets and half a dozen of quartermile races a year. I was hoping to get her into the 12s so i can take part in the Pro ET over here, not really sure yet if that's doable though. My engine ran 12.0 in a Nova but the Chevelle is so much heavier. What is important for me is that i have decent performance on the track and to be able to take 6-7h trips to the races and shows without worrying about converter slippage / heat and fuel economy (thats why i want OD!). And that's why i wanted to go for a lockup. I don't wan't a trailor queen, i want to drive her across europe and still have some fun on the track. Not expecting low ETs though.

At the moment i'm thinking i'll take the builders advice and go with the 2800 L/U and see how it works with the cam. If it doesn't work, shipping for a new cam is not as nearly as expensive as for a tranny! :wink: I was looking at the Voodoo roller cams anyways.

Keep them suggestions coming, i really appreciate them.
I need to be sure what i want before i order, because shipping overseas is extremely expensive and there's hardly a way back. So it needs to be well considered.

Thank you!

Olli

 

[turbo nasty]

How much does your car weigh?

And as far as changing the cam around to make the convertor l/u better thats the wrong way unless you know the car is over cammed. Which is the case alot with folks wanting a certain lumpy idle sound but the rest isnt there foir the cam to shine.

Shoot for total package get it right a be done.

IMO the combo is everything. If your heads flow a certian amount, the carb, headers, intake and CAM should be matched to that.

Why put a cam that doesnt need the big heads or vice versa.

Of course the convertor is based off of cam (engine VE), car weight, rear gear and tire size.

Thats looking at it for total package.

So the 6 plus hour drive to the track with non l/u and OD can happen just make sure you have a a aux trans cooler thats up to the job form the extra heat. Dont skimp on a cooler get a GOOD QUALITY one!

Without a doubt roller is the way to go...You can get the advantages of less parasitics loss due to friction and take advantage of awesome grinds that a roller entails.

Dont forget to add ZDDP if your running a flat tappet

 

[chris718]

Stall speed is a derivative of input turque.The torque converter is a fluid coupling so there is always a lower output speed than input speed when the converter is unlocked.Input is via the converter cover which is attached to the engines flexplate and output is from the turbine which is linked to the transmission input shaft.The highest percentage of fluid coupling is at or above the stall speed.In simple terms the cover drives the turbine at a constantly varying coupling percentage based on engine torque.A higher stall speed allows the engine to bypass the point in the rpm range where air ingestion and torque output is low and jump to a point where these are higher so the engine can make the car leave faster.Keeping the cuise rpm unlocked 200 to 500 rpms lower than the stall speed allows the engine to make efficient use of the gasoline it sips.

 

[chris718]

If you were strictly going racing with that cam a 3600 or better stall would yeild the best 60 footers and ets.Because it is a cruiser 2800 would be a great choice.In my opinion that cam is a little too large for that compression ratio and intended usage.A 280/288 hydraulic roller would probably be the best choice for all around performance,efficiency and economy with the rest of the items you already have.Because of the lockup you can run up to 3200 or so on the street with that as there would be zero slip at that point but gas mileage would suffer.I f you want to light the tires at every stop light then use a higher stall but from your intended usage I suggest you look into the 2800 rpm stall speed.

 

[nanbuicker]

How much does your car weigh?

Not sure, it's in full trim, i need to drive to a truck scale some day.

And as far as changing the cam around to make the convertor l/u better thats the wrong way unless you know the car is over cammed.

It is. It has a GM847ish cam, see specs in the link of the original post. I've been looking at the Lunati Voodoo 60122 for a while and you are absolutely correct, i should match the converter to the combo i'm planning on and i very likely will swap the cam later on.

If you want to light the tires at every stop light then use a higher stall but from your intended usage I suggest you look into the 2800 rpm stall speed.

Hm, lets put it that way, yes in fact i definitely want to be able to light the tires at evey stop light. :biggrin: Should be possible with 2800, no? Being able to burn rubber is mandatory with that kind of car in Germany. Seriously.

Thank you all for your help, i appreciate it!

Olli

 

[chris718]

The 2800 will get the job done correctly.