Imo Apita

Another thread has a poll about a torque converter for a ZZ4.
That one got me thinking; can anybody explain how to pick or calculate what converter you need with a specified HP/TQ or rear end ratio?
Right now this seems to be complete witchcraft to me.
Why would I need to change the converter if i switch rear gear ratio's from 3.08 to 3.55?
I am building what will hopefully be a 400-450 HP/TQ 383 and plan on switching from my worn out TH350 to a 700R4.
Currently a 3.08 gear but planning on maybe 3.55 later.

MelWff

isnt that choice usually dicated by how radical a camshaft you have. You need a higher lockup as the cam becomes too much to sustain an idle in gear.

DRIVESHAFT

Thats what they want you to think.
The secret is that unless you are racing the car, there is no best converter. What makes the converter perfect is all based on owner opinion.
It does have to be loose enough to allow your engine to be into its efficient RPM range, while being tight enough to still be efficient around town.
For an average hot rodder, that means that you will most likely get something with a stall between 2000 & 3500 rpm.

The reason that you might use a different converter with the gear change is that the 3.08 gear will make the converter feel a bit looser than it will with the 3.55, simply because its harder to get the car moving. That will cause the converter to slip a bit more while you are starting off. The actual true stall speed wont change.

Imo Apita

But is this Speed Shop Hokey-Pokey Witchcraft or is there actually a formula in use?
I have trouble understanding the relationship of the cam and the converter.
I would guess that a 3000-7000 cam needs a converter above 3000.
Is that even correct? Does a converter need to be 1/2 max TQ rpm?
Or is that a tall tale also?

63mako

Good rule of thumb is the convertor should have a stall rating 200 RPM above the lower number of the operating range of the cam. In reality a 4.11 might work fine with a stock convertor with a fairly radical street cam where a 3.08 might want a 2500-2800 stall with the same cam. Your intended use will make a difference also. If you race a lot your convertor needs would be different than if you mostly cruise around town. It would also vary if you do a lot of highway cruising. Best bet is to call a couple companys that specialize in convertors and tell them your power, cam, gearing, trans and your intended use and get a few recommendations. What is perfect for one user will not be the best choice for another.

DRIVESHAFT

There is no formula. Its done by experience.
Your correct that you will need a converter that stalls in the cams RPM range. Of course since the cam manufacturer probably didnt grind the cam just for your engine, you cant assume that their published RPM range will be exactly accurate.
I cant see the 1/2 max torque rpm being very useful for picking a converter. If for instance your engine makes max torque at 5000 rpm, you have a pretty big cam, and a 2500 RPM converter will not be enough stall for good driveablity.
For comparison, my engine should make peak torque around 4500 RPM. When I called around talking to various companies about what they would recommend for me, the recommended converter was typically in the 3500-3800 rpm range.

Imo Apita

No disrespect but I find that hard to believe. They need to base that decision on something more then just a "feel".
"Just because" doesn't jive very well with me.
I wish we could have like a "how to pick your stall converter" sticky so this stuff can be put to rest once and for all.

So I read in another thread that you need a "special stronger" converter if you do a lot of burn-outs since the fins(?) get damaged. So you need a $500+ converter if you plan on doing donuts all day long iso $200+ converter.
I need to do three different things with my engine/converter:

• Daily driver-city driving
• High way long trip cruise at 75-80mph
• Go sideways when I stomp on it

When a cam company states 2500-2800 stall required, where does that number come from?
I (think I) understand why they recommend a particular compression ratio for a cam, but why a stall? Does it matter/change if it's a lockable converter as in a 700R4?

DRIVESHAFT

The deal is, street cars are always a compromise.
If you were racing they would have pretty specific methods of choosing the right converter.
On the street, that racing converter would probably build up more heat than you want to deal with, but the closer you get to it the better your car will perform.
For street use they will take some stall out and improve the low rpm efficiency. That will cost a bit of performance, but will make it more livable as a daily driver.


If you stick with the TH350, in the RPM range that you've been talking about you can get by with roughly a $300 converter and still have a good one.
I know that Coan has an 11" on sale for that price and PTC converters will build you one for about the same price.
If you switch to a 700 then your probably looking at $500+. If you want to be able to lock the clutch under full power acceleration, then plan on $700+

The cam company wants the converter to be loose enough to allow the engine to be in its efficient operating range so that the engine wont bog and stumble from being loaded at too low an RPM.
The only reason a lock up converter would alter your choice at all is that you could get away with a looser converter and still be able to lock it on the highway. Otherwise it doesnt change much.