I have been playing with the thought of changing my torque converter. My thinking is that I have a stock TC in a very light (approx 3000or so pound) truck. It’s a 40 Ford pickup. As I have mentioned in earlier posts here and on GJ, the truck lurches when launched and then feels flat until the engine starts to wind up. I’m watching the tach, it looks like the flash stall is about 12-1400 rpm. Footbrake stall is about 1800 rpm. I noticed something while trying to determine the flash stall... as the truck launches, the tach rises and then the rpm plateaus as the truck moves out. That’s what I call the flash stall. It’s king of a subjective observation. My other observation is that as the truck accelerates and shifts, the rpm after the shift drops to the same speed as my observed flash stall as long as I don’t change the accelerator position from the initial distance I depressed it at launch. Is the rpm that it drops to after the shift a valid way to determine flash stall? Sent from my iPhone using The H.A.M.B. mobile app
What's the rear end ratio and rear tire height? If you've got something like a 3.08 or 3.23 rear end gear, although a higher stall will help some, it will still be boggy. Everything has to be working together, engine combination, stall, trans type, rear gear, etc. If there is one thing to change, that will make the most noticeable difference, it is rear end gear. It's why WW II military surplus 4 cylinder Jeeps can climb what they do. Course you can also forget about gas mileage. I am Butch/56sedandelivery.
Sounds like you 've diagnosed it pretty well. The numbers sound about right. As for your other observation, I'd say no, not a reliable test. Too many variables.
Thanks , I forgot to mention that the rear axle is 3.45:1 Sent from my iPhone using The H.A.M.B. mobile app
The cam, gearing, stall speed, vehicle weight, etc all need to work together to make it fast and fun to drive. If you want suggestions about what stall converter to get, or whether there are other things you could change as well to make it all work better, we'd probably need a lot more info.. Also, converter and cam companies both are pretty good at figuring out how to set up the rest of a car to work with their product, or how to select the correct part for how the rest of the car is set up already.
Beware! There is a downside to using a "loose" or high stall torque converter in a regularly street-driven hot rod. You will lose compression braking at around-town speeds and make the car miserable (IMHO) to drive on the street. It won't slow down when you lift your foot off the throttle, it will "coast". This gives the car a kind of "spooky" feel and you'll be using your brakes a lot more than before. Also, the car will have much more "throttle lag" at around-town speeds which also gives it a spooky feel. It does not smoothly accelerate with minor throttle input at lower speeds but instead waits for the torque converter to "spool up" before the car begins to move faster. Then it's usually too fast and you back off the throttle and go through the process all over again. In traffic this is majorly annoying. So, in my experience it's better to stay with a stock OEM torque converter unless you plan on racing your hot rod a lot.
^^^ What he said.... I'll mildly disagree and say it won't be 'miserable' but you'll definitely notice that your low-speed throttle response will be 'mushy' in around town driving. It will also put more heat into the trans fluid, so make sure your trans cooler is up to the task. If you have a lumpy cam with a high idle speed a high-stall convertor can help with 'creep' and get you into the cam's power curve faster, but is otherwise a PITA on the street.
Thanks gentlemen, 40 Ford PU around 3000 lbs 4.3 Chevy V-6 that is bone stock ‘89 model year 700r4 with locking converter, external trans cooler. 3.45 Ford rear gears. 26.5 diameter rear rubber I understand what is meant by mushy. But right now I have changed the accelerator pedal ratio so that it takes a lot more travel of the pedal to get the truck to move... the thing was really that squirrelly. At least now someone beside me can drive it. If my observation is correct, the truck is hooking up at about 11-1400 hundred with the stock converter (1800 or so stall) a converter with a slightly higher stall speed should allow the truck to work at a more normal rpm from a stop Sent from my iPhone using The H.A.M.B. mobile app
That 700r4 has a fairly low first gear so it should launch ok.Try a shift kit,and see if you can tailor the trans to shift when you want.If you stay in each gear longer it will have more low end torque.You still have OD to cruise in.
Maybe I’m not describing this correctly. The transmission currently has a torque converter with the factory (1800 or so RPM) stall speed. What I’m observing is that the flash stall speed (the speed where the engine revs to before the truck starts to launch) is between 1100 and 1400 RPM. It’s very difficult to get the truck to launch smoothly because there’s basically no space between when you start to step on the pedal and the truck lurches forward.It’s very difficult to feather the pedal to pull away from a stop gently. I’m hoping that a converter with a higher stall speed (which in actuality will be diminished due to the light vehicle weight) will allow the engine to wind up a little more before the truck starts moving out. So this doesn’t have anything to do with shift points and it’s currently the exact opposite of mushy. A previous poster used the term “tight “ that’s a very good description Sent from my iPhone using The H.A.M.B. mobile app
I don't think that is your problem,what carb is on it and do you have the correct ratio adapter on it ?
I bought a '89 S10 new with a 4.3 auto. It shifted hard from a start from the factory. Almost jump out from under you at a stop light. Just me but I loved it. Little truck would haul ass.
The 3.54 rear gears are a big part of your problem. That's pretty low for an automatic-equipped vehicle on the street, particularly a light weight one. A swap to something around 3 flat or maybe a 3.25 would help a bunch.
A 700r in that truck should move it rather well with that low a first gear... As said above sounds like ratio throttle to carb is off...
I have been giving that some thought as well. I know the original truck that this eng/ trans came from was something like 3,23 gears Sent from my iPhone using The H.A.M.B. mobile app
The stock '89 4.3 makes 235 lb-ft at 2400rpm per what I found online. If you raise your stall speed too much, it will flash too high and close to the peak torque and it won't drive the way you want it too. You should call whichever company you plan to buy a converter from and go over your setup, what it is doing, what you want it to do and trust their advisement on the stall for your truck. If you buy a converter rated at , say 2000 rpm, from a catalog it will be wrong, because that rating almost certainly going to be based on or assumed to be used behind a 350 in a vehicle of a given weight. You don't have a 350 and your truck is probably lighter than the assumed given weight used for rating a converter. You might find that a 3000 or 3500 rated convertor stalls at 2000 behind a less powerful and lighter truck such as yours. But you will be guessing. Or trial and erroring. Call the convertor maker and save yourself time and a headache and probably money by getting the right convertor the first time.
With an OD you don't want too high rear axle ratio (e.g. 2.75:1, 3.00:1 etc) as it will not be too user friendly at low speeds unless you pull it back to D. When cruising at 60mph the engine needs to be in the 1.8Krpm to 2.1Krpm range. I have a 3.54:1 rear axle with 235/75/15 tyres, 700R and stock convertor behind a similar 5lt SBC. The combo is adequate but no world beater. One thing to make sure off, ensure that your TV cable is adjusted correctly, that could be the source of your problem? OEM ratio needs to be replicated for it to work as designed or your transmission could be quickly toasted. 700R ratios are 3.06, 1.62, 1.00 (D) and 0.70 (OD). I've a friend with a pretty angry 383SBC with T350 with 3Krpm stall in his 34 PU, great combo and works well with engine etc however around town and in stop-start traffic it's too much convertor. As stated above, foot off the loud button and it coasts, you need to rev it before it gets moving. When he eventually stops driving it one weekend we'll swap out the convertor for a lower stall, this will make the car way more user friendly and drivable.
Do a search for "torque converter k factor". Converter stall behaviour varies with the square root of the torque it sees at that moment.
Thank you Ned, I'm going to check that out as well . And, I'm certainly going to work with the converter manufacturer.
I would look into your gas pedal ratio, there's no way a 4.3 with 3.50 gears is jumping out from under ya, or a 3,000 stall converter is to much for the street. I swear some of you guys have never driven anything more than grandmas stocker...lol A 4500 converter is loose for the street, I spent a year with one of those, but a 3,000 is not. You need to think about what you are trying to cover up with a higher stall and get that to calm down, not band aid over it. With your overdrive I wouldn't pull gear out of it or you'll be to low of an rpm at cruise speeds.
1st point ..A bone stock 89 S10 would be TBI, so no need to recommend carb /linkage changes. This could be a problem , in itself depending how it's controlled. I might get this thread tossed by bringing this up. 2nd, If it's the converter that belongs with the specific engine /trans, then yes it should flash stall to about 1800. With the 3.06 low and stock cam , it should burn rubber pretty good when matted. If it doesn't, I think we're looking at a tune up problem here, rather than a converter problem. Yes , you could get a few hundred more stall out of that size converter core, without making a heater out of it. As long as you retain the lock up, you should be okay. You don't need any more than that with a bone stock motor.
Hi, As mentioned before, I have changed the gas pedal ratio but, I should note that the entire linkage including the pedal all the way down to the transmission is GM factory stock, yanked out of the donor truck myself. It's got nearly 20K on this setup and I know the TV cable is set correctly. So, when using everything stock and the only difference is a couple of ticks on the axle ratio and probably close to a thousand pounds less vehicle weight, I have started leaning toward the stock converter hooking me up too soon because of the light vehicle weight. I'm going to call a converter vendor tomorrow and see what they say. The converter install was a cake walk when the engine was on a hook but once the whose thing is in the vehicle, I only want to play with it once.
Two quick ones: Have you actually weighed the 40? A loaded short bed /cab weighs about 3300#..Usually a 3.42 gear. What did you do for the fuel pump? Have you monitored the pressure, while diagnosing this issue?
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