I have been driving my 40 Ford pickup for about 5 seasons now. I guess we're never really "done" when you build a hot rod. Right now I'm trying to sort out a problem I'm having with the way the truck launces from a stop. In a nutshell: The truck would always lurch forward when accelerated. Part of this was the fact that I was using rear leaf springs that were longer from the axle forward than from the axle to the shackles and the springs would torque up when loaded and the truck would then lurch. I added a set of traction bars (not slappers) to control axle wind up and that helped a lot. However, the truck was still difficult to launch from a stop. It took very gentle use of the fast pedal to get it off the line smoothly. So, I incrementally changed the pedal ratio to require more pedal travel for a given amount of throttle opening. While this has pretty much solved the problem, it still bugs me as to why this had to be done. The pedal, cable and throttle linkage all came from the same donor as did the engine. A couple of years ago, I installed a tachometer. A couple of things I noticed by reading the tach were: The engine needs to idle at 1000 RPM in park to yield 600 RPM in drive and The engine never gets much past 1000 RPM before the truck launches. I have other OT vehicles with automatic transmissions and tachometers and all of them seem to initially climb to about 2000-2400 RPM as they start to move out. Another thing is that although the truck would always launch with a lurch, from that point the truck would seem rather slow on the uptake and my best acceleration would happen in 2nd gear as the engine would get into it's optimum torque range (235 ft lbs at 2400 RPM) Now I'm wondering that even though the transmission was a rebuilt unit(supposedly a rebuilt converter as well) Could it have been assembled with the incorrect converter? Could I have a severe mismatch of torque converter stall speeds that I can't get the acceleration to behave normally? This is an 89 700R4 transmission. I'm seriously thinking of buying a converter in the 2400 RPM stall speed range and trying it out, thinking that letting the engine get up to a speed where it's making some torque would help. Your thoughts please.
1, Tell us more about your engine combination and rear end ratio. 2, Try this. Put it in gear after it's warmed up. Stand on the brakes, and see how high it goes. You won't hurt anything. Might have trouble holding it back ,with the low first gear. Lowest stall 700 converter is about 1375 rated, I think , without looking it up. Let's go from there.
Thank you. I will try that. The engine/tranny/rear is a 4.3 Chevy V6, TH 700R4 and 3.45:1 Ford rear end. Thanks also for your info on the stall speed. It may explain what I'm seeing.
I have the exact same drive train in my daily driver. 3.42 rear. I'd have to say mine probably goes to 1800-2000. They might have given you a V8 converter by mistake. Could be the problem.
Wow, This is the most reassuring information I have gotten about this since I started investigating the torque converter as a possible contributor to the condition.
I've got a S 10, all stock besides gutted cat, Flowmaster, 3" tail pipe. It goes to 18-1900 on the brakes.
I just came in from the garage. I had the truck warming up...1300 was as high as I could get it before it would start to move. That can't be right
Mine started to move at about 1700. I cheated a bit and put it in second. Electronic 700's (whatever they're called) are actually in second gear in that position. It went a bit higher but that's all she wrote. Of course, I've got power disc brakes. Yours all stock? Computer and all? What year?
It's a 40 Ford pickup on an 86 Ranger chassis with a 4.3 and a 700r4. The drivetrain is out of an 89 Astro. While I consider it a hot rod (street rod) I don't mention it too much here because the TBI is taboo. The engine and transmission are all stock computer and all. Power brakes, disks in front. In my book, a hot rod is built at home with your own two hands from stuff you scrounged and made to work, so it fits my definition just fine. It's not an electronic transmission although it does have a locking converter. The 1300 RPM stall is certainly not right. Now, since I have been farting with the thing for 5 years, I doubt the guy who built the transmission wants to see me. And the guy who fixed the overdrive for me didn't seem to think the lurching was that big of a deal. I really don't want to lay out a couple hundred bucks for a converter but I might have to. I can imagine it really waking up if I can get the engine to wind up a little and make some reasonable torque when I take off. It might also solve the 400 RPM difference between idle in park and drive. I never recall having to idle anything so high to get a normal idle in gear.
Okay, mine's a Vortec, later model. They make more power than the TBI 4.3 They make a better manifold and bigger TB (305 or maybe 454) Might need a looser converter, or maybe just hop up..More torque = more stall
How much does you truck weigh, and how does that compare to the car the tq was designed for? If the TQ is designed for a larger heavier car, then the stall speed will be too low for your lighter 40 pickup, hence the condition you describe of the low stall speed and the slow acceleration. A higher stall speed will help with that. But I don't think that is the cause of the lurch you have on initial take off.
I do need to get the truck weighed but I'm guessing about 3000 lbs. The vehicle that the engine and transmission came from weighed 4300 lbs. Does anyone think that a looser converter will allow me to drop the idle speed in park from 1000? I'm seeing a 400 rpm drop when I go into drive (600 RPM in drive) but the truck doesn't creep on a level surface with my foot off the brake.
Get a convertor from an S-10 4x4 with a 4.3 and 700 . It will have a higher stall and a rebuilt should be cheaper than a performance convertor. I use B&M convertors in my AOD swaps and they work great. About 22-2300 stall and trans temp issues.
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