With out a doubt, do the gears. I ran a 72 with smallish cam, 700 Holley, single plane, stock c4/verter, and 4.11 gears. Everyone swore the whole combo would be a torqueless dog. Nope. Traction was horrible at any speed and torque was amazing. No stall issues. When it did hook, it was quick... I had another car with a stall and 3.00 gears, it was a dog if it hooked. I had to break traction to get to the rpms up enough to move it fast.
There is so much bad information (let's call it MISinformation) out there concerning torque convterters that it can get really confusing really fast. Basically you have to look at a dyno graph. Lets say your engine makes peak torque at 5500 RPM. Today's converter companies are setting up the converter to stall at 5500, or for a street/strip car, a few hundred RPM below that. Look at this dyno graph: You'll see that at 2200 RPM, there is around 100 lb-ft of torque. That means that with a stock converter, it's going to be a pig. Now, put a 3000-3500 converter in there, and it will be a LOT happier. Faster, more responsive, etc. What does stall speed have to do with camshafts? Well, most of the time we put in a bigger cam to get more HP. Well there is a trade off...more HP usually means you'll be spinning the motor faster. So if your engine makes 100 lb-ft of torque at 2000 RPM and 500 lb-ft at 4500 and you have a stock converter, your not taking full advantage of the combination. It'll be fun to drive around, yes, but not nearly as fun (or fast) as if the converter was, say, 3800 stall speed. That'll let the RPM come up, which means you'll be able to really feel the torque/HP. Make any sense? That said, there is no such thing as a "generic" or "universal" converter. What works for one guy may not work at all for another guy...or conversly what may not work for a buddy might work PERFECT for yours. You never know..it's a gamble! That's why custom-built converters are really the way to go. They're set up for YOUR combination, not joe blow's combo which might be 110% ass backwards of what yours is! Then we get into the 9" vs. 8" vs. 10" diameters. Yes, reducing diameter will increase stall speed because the fluid doesn't travel outward as far, which reduces it's speed and force. You'll find that most 10" converters are 2500-3500 stall speeds. That is where most of them work best. 9 and 9 ½ converters work well from 3500 to about 5000 give or take. 8" converters work best from 5000 and up. Don't let that fool you though, there are companies who can build a 8" converter to stall 3500 RPM. It just might not be nearly as efficient as, say, a 9" 3500. Different companies do different things. How do they achieve a set stall speed? Well, diameter is only part of the equasion. There's vane angle, stator design, pump-to-turbine distance, fluid type/viscosity and I'm sure a few other things that I'm not thinking of right now. That is yet another reason that a generic 3000 stall Dacco converter may not be ideal for you. They put the same stator in all of them, same fin angles, same end play, etc. 3000 stall speed is used "loosely" (pun intended). Put that converter behind a high-winding 289 and you'll be lucky to get 2200 out of it. Put it behind a healthy 514" BBF and you'll be rewarded with 4000RPM stall. I've seen it a hundred times. That's why I stress calling a manufacturer and discussing it with a technician that lives, breathes, and talks converters. It's really a science...not a guessing game (well...to an extent). Good example: My converter failed last season. I was in the 2nd round, car was working great previously and so was I. Had .010 lights all day. Did the burnout and really shook the tires hard coming out of the water. Tree came down, I whacked the pedal and noticed that RPM was WAY up, compared to previous runs. Usually I'm leaving at about 5000 ± a few, this run, I was up to about 6000. ET was off slightly and MPH was down about 3. RPM didn't drop maybe 100 ± RPM going from low to high gear. Won the round. Back to pit area, noticed that trans was hotter than normal. Infared temp gun on the pan showed 180 degrees...should have been around 140-150. I knew the trans or converter was shot...but when you have lady luck on your side, you keep going! So I did...won the even that day, but in the last round I noticed that back in the pit area the trans was extremely hot. Temp gun showed 250 and change on the pan. Paint was burnt off of the converter, literally. IT was smoking like new headers. I could smell sulphur...which was BURNT trans fluid (I mean BURNT!!). Got car home pulled converter and Trans out (easy on my car) and when I got the converter off of the trans I immediately noticed filings in the fluid as well as the tell-tale of a thrust bearing rattling around inside. Anyway...moral to the story, the converter was stalling higher, yes. Did it do anything for performance? NO. WHy? Because of the excessive clearance between the pump & turbine, failing sprag. The converter was no longer efficient. Had it rebuilt and picked up a tenth and a couple MPH. Stall speed didnt' change...actually, it decreased a little but due to the more aggressive stator (which helps multiply torque) I was getting off the line better and "locking up" on the top end, which helped MPH quite a bit. RPM was down to 6800 at the traps from 7300 but MPH was up. Go figure. Just goes to show that unless everything is matched to what you have, just bolting in a converter sometimes won't help a thing...except separate you from a couple hundred bucks. BTW if you end up goign with a higher stall speed converter, get yourself a GOOD (read: biggest one you can fit on the car) trans cooler and trans temp gauge.
Hey Jon, You raised a good question, one in which I've tried solving myself. Because like what Mavman is saying every situation is different. Mavman has excellent points and I would suggest you get as much info. as possible. Let me give you a real world situation. My daily driver is a '78 F-150 that I like to bracket race on the weekends while my '70 Mav is being restored. Bone stock with 2.75 gears, 460, 4 barrel and 29.5" rear tires my truck would run 15.70's@ 87 mph in 1/4 crossing the finish line in second gear at 4500rpm. I changed to a Weiand Stealth intake, Edelbrock 750cfm, & a Lunati Voodoo cam(256 duration adv. 503 int/525 exh, smallest one). With these changes only and the gear still the same at 2.75, I could only run 15.20's@ 91 mph in 1/4. I was puzzled and thought the combination had more in it. My C-6 was shot and I had it overhauled with a TCI transkit, billet servo, a very firm shift valvebody, and a PTC 2000 stall speed converter. I notice a big difference this time with the same set-up as before and still crossing the 1/4 mile finish line at 4500 rpm in second gear, my truck ran a 14.40 @ 93 mph. I was happy and ready to try more gear ratio in it. With my truck making more torgue than my 351W in my '70 Mav, I knew it wasn't going to take 3.73's to get it going. I tried a set of 3.25 and bingo, my truck is running 13.70's @100 mph crossing the finish line in third gear now at 5000 rpm. My gas mileage suffered a little from 16 to 14 mpg. I just wanted to share this real world situation with you and take notice that a torque converter & gears are torque mulipliers and if the combination is not right it can hurt performance. Take it one step at a time and remember it all has to work together.
I'd like to know if anything I posted was misinformation. This is my understanding so far, I'd love to have anything corrected. I understand engines quite well, but when it comes to trannies and converters, I feel I'm better than average, but I don't fully understand all of the ins and outs of converters.
What does "flash" mean...what does it look like on the tach? Or will I even be able to detect it with the stock converter?
as an attempt to give a simple example: assume you have a transbrake car: 1. stage, put on the brake & floor it. rpm goes to 5300. thats the stall rpm. 2. stage, but the car has a 2 step chipped at 4000 rpm. put on the brake & floor it. rpm goes to 4000. let go the button to launch. rpm goes to 5500 because the engine gets a running start before it hits the converter. assume a footbrake car: 1. stage, load car way up against the brakes, maybe 3500 rpm. you leave & converter flashes up to 4500 rpm. 2. stage, just bring the idle up to maybe 1800 rpm. you leave & the converter flashes up to maybe 4800 rpm. (first really noticed this when i helped a friend switch to a quality 8" ati converter & glide in his mav years ago. he pulled it out in the street & nailed it off idle just to try it. he got out & said " holy s**t, it never did that before." we went back & looked at the tire marks. about 6" of somewhat black, then it suddenly got solid black when it hit the flash stall.) hope that explains it better.
I hate to drag this on and sound dumb, but with my stock setup, what is the flash? When I get to the line, I stomp the brake, and gas it until the engine stops climbing, usually around 1800rpm. If I gas it more, the rear tires will try to turn. Usually, I just floor it and let off the brake at the 2nd to the last yellow light. The rpms will just "slowly" rise as the car takes off (unless the tires break loose, then there is a jump in rpms, but not what I am looking for). Where is my flash, or is it not even something that happens in my mostly-stock setup? I think what you are describing is that if I had a 3500 stall converter, I would not run it up to 3500, but up to 3000 at the light, then floor it and let off the brake and it would rev up to maybe 4000 as it takes off, with 3500 being the "stall" and 4000 being the "flash" (in this example, numbers may not be real-life, just something I threw out for example).
OK, makes sense now. So my final example was sorta on the right idea...sorry I sounded so dense, but I have heard the terms and never really knew what they meant.
scooper - if you can floor it against the brakes without the car moving or the tires turning, then what it stops at is the stall (1800?). if you left it at idle & then floored it, it might go to 2000 on the flash. or only 1900 maybe. there is almost always a flash factor of some degree. your entire setup wil determine it. the point at which you push thru the brakes is not the stall of the converter. it is the point at which your brakes became inefficient. change the pedal arm ratio, brace the backing plates, etc., and it would change. on a fast ss car, the brakes may push thru at 3500 rpm, but the converter might stall at 6000 rpm.
try flashing the convertor not brake stalling it might hook better (see my magnum opus on adjustable rear leaf springs for more of a idea of driving styles)
let me try and explain a complicated subject..(maybe muddy the mississippi ehhh) brake stall is not the true stall ( you can use second gear and see how high it will go but don't do it more then a couple of seconds the heat it's making would fry eggs) If you took say a rated 2500 stall convertor and stuck it behind say a 5L HO motor (225hp 300ftp) and brake stalled it you might get 2200 rpm out of it or left at a dead idle and flat stomped the gas and watched how high it went (say 2600rpm) you would have 2 different numbers and still be wrong (altho closer with the last type of driving style) now take that same stall and stick it behind a 6/71 jimmy huffed 408w (600hp 750ftp) it might brake stall to 3500 rpm and flash to 4500(not likely your tires would disentigrate) same convertor different results. It's as much a art as a science And if you ever wanted to know the true stall of a convertor with the car it's in you need a transbrake This is why racing convertors can cost a small fortune they have to be custom speced to your combo.. Generic convertors are like cutting onions with a chisel it gets the job done but the proper tool would make a easier job of it whats the old quote speed costs money how fast can you afford to go?
One thing to remember, when you stall your convertor, do not just let it idle back! You load up your convertor, your building heat and lots of pressure. Let your car "unload" your convertor by releasing your brakes or trans-brake. You can bust fins off or will bend them throwing your pitch off and changing your stall. That's if your lucky, if your having one of them days,,, well your convertor wil be ventilated! Ah, Thats not a good thing! Always a good idea to replace seal when r&r convertor also.
I was afraid of that. Your in your car an you have it all warmed up. Okay, lets see what this stalls at. You put car in gear an hold brakes, hit the tranny brake, and smash your gas to see how far your tach comes up. You dont want to sit there very long as your temp will climb fast. Now you see your tach wont go any higher then 3,200 rpm. Dont just let off the gas pedal, let go of brakes or tranny brake, let your car "launch" start moving, then let off gas. This lets the convertor unload. The inside fins do not spin at the same rpms as outside shell. You build up rpms to 3,200 then snap back to idle, inside fins are still spinning with fluid and pressure of the higher rpm's, forcing your fins to slow real quick in the high pressure fluid. That's when you may bend or bust fins off. Seen several convertors with dimpled case's and builder explained reason. Ah, better then I can explain it. Most street convertors will be ok, it's just not worth the chance, fin bending is gradual and will have you scratching your head on why stall speed is changing. Hope this explains better, I'll see if I can find a tech link on this or if anyone can rant better on this, feel free.
