What Replacement Stall Convertor?
 

My 67 convertible has a great, nasty, lopey sound to it. TH400 auto,4v Rochester,3.23 posi, 225 / 70 R 14 tires. When starting, I have to nurse the throttle or stall out. Nurse the throttle for a minute or so and she keeps running. Then, when I go to back out, the car jerks into reverse. As I stop to turn the wheel and go forward I have to slip into neutral, rev a little and then shift into Drive. Underway, she runs like a champ. Come to a stoplight, I have to put her into neutral until the light change and then shift into Drive. Once again ,when underway she runs like a champ. Hit passing gear and she jumps, roars and flies like the barbarian horde as they begin sacking a defeated city.
I want to cure the jerking when going into reverse and drive and having to put her into neutral when stopped at lights.
Three of my car buddies keep telling me that the problem is the stall converter in the car. Big cam. The fellow who built the engine tells me "no, it's not the stall converter, check the vacuum actuator on the side of transmission. Could be that a vacuum hose has slipped off or has cracked".
So up on the rack. Checked the transmission vacuum actuator. No problems with leaks or hose connections. Checked twice by two different geaheads. Which leaves the stall converter. Waters I've never sailed before.
I've studied stall converter on line(educated but not) and know that info about what camshaft is in the engine is an important factor when choosing a stall converter. But I don't know the specs on the cam in the car. The best I can get from the builder is that " It's a custom grind I had made up between a Ram Air III and a Ram Air IV cam."
I contacted a very well respected and experienced engine and torque convertor builder who suggested that ,short of taking the engine out and confirming the cam specs, that he could build a 9.5"converter that would stall when torque is applied at 3000 rpm. He further remarked he would" have to degrade that positive feel slightly by opening up the end clearance to accommodate your big camshaft". I'm trying to figure out what to do. I will not be doing the labor on this. Ie: pay a mechanic. I want to "get it right" the first time. Assuming that this is the problem and the cure, what will be the change in the "feel/operation" of my car after a stall convertor is fitted.
Please pardon the length of this post ... I did not want to leave out anything pertinent . Thanks in advance for sharing your thoughts!

"I" would be verifying the carburetor, especially the choke before I blew hundreds of dollars plus labor on a new torque converter. The converter may not be helping...but I bet you've got choke problems that are the main cause of trouble. Once you're sure of the carb, look at the spark advance--centrifugal and vacuum.

AFTER the carb and spark advance are sorted-out, IF you need a converter, spend extra to get a quality higher-stall unit, not a repurposed stock Vega converter, or one screwed-up by mass-merchandizers. Good converters are not cheap. And, yes, you should verify cam specs, compression, gearing, etc. before ordering a proper, decent torque converter.

Once you get your carb and distributor in proper working order call Lane at PTC.
The man knows what he is doing. Expect to spent at least 700-800$ on a good converter.

The TH400 Shift-Kits or re-programming can cause a hard Reverse. There is a way to make Reverse soft-apply by adding a Restrictor in the Servo feed path.

The 400 engine might need the Dizzy intial ADV set to complement the cam. Sounds like the Total ADV is set right. Also Rotor-Post phasing may help that idle.

Converters all revert to Neutral at idle. Maybe lower the idle one the above TUNING is done.

Tuning is oft overlooked part of the hobby. Complete tuning anyhow.

I agree this sounds more like a tuning issue than the converter. As mentioned I would be checking the choke and idle mixtures on the carburetor. It also sounds like the engine may need some tuning on the timing curve. Depending on where the engine is idling and when the curve starts to come in, dropping down to idle may be altering the ignition lead enough to cause this behavior.

As far as the rough input into drive and reverse, did the TH400 build include the addition of extra friction material? This is common, but to do so typically coincides with removal of the wave plates in the clutch packs. Removing those will cause the transmission to jerk fairly heavily going into drive or reverse.

In regards to the torque converter, it sounds like you may be talking about a TSP converter. While I don't believe that is actually your issue, I had a TSP 9.5" spec'd for my combination in my TH400 and it was a great converter. Drove plenty positively under low load and flashed right to where I needed it under throttle. I opted to do a TSP in my 4l80e swap as well. I don't have any real drive time on that yet though.

I'm far from an expert on this kind of stuff but anytime I hear of a car jerking when dropped into gear, I think that the idle is set too high because of the carb needing more fuel in the idle circuit.

I am in the camp with the other comments. It has more tuning too be done. Probably start with a bit more initial advance and richen up the idle some. Look for small vacuum leaks.

I will say though, I would have a well built 9.5” stall converter 3000 plus converter in all my performance cars if I had a choice. But, those little 14” tires will be pretty pointless with much converter.

If you go forward with the stall swap, get a well built stall custom made stall for your set up. Some of the bigger diameter (11” or larger) 2000+ high stalls that are tight for the street driving can hit the engine a little harder than a stock stall at idle. You can actually make your idle problem worse. I don’t think a 9.5 would have that trouble though.

Ultimate Converter Concepts is a good source for custom torque converters. Lenny who started the company was good friends years ago with Kriss at Continental Converters.
Last I heard they still had some 10-inch cores.

https://www.ultimateconverter.com/



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...gokitty done left left the Thread huh

No Sir! I'm soaking in the info! I appreciate the advice.

GM put mild stall converters in their performance cars equipped with hot cams, Pontiac used what is most commonly referred to as the L88 converter in RAII & RAIV auto trans cars.

Stall was in the neighborhood of 2500 - 2800 if I’m not mistaken as compared to 1600-1800 for standard converters. The Hughes GM25 is pretty much patterned after the L88 converter and has a fairly good reputation.

From what you’ve described about your engine I’d want some kind of higher stall converter along with some better engine tuning. Link to GM25 converter below:

https://www.summitracing.com/parts/h...0aApCpEALw_wcB

I have a friend that has had similar idle issues with a Hughes GM25 (12”) as this OP for this thread. He thought it hit the engine to hard when he put it in gear, maybe even a little harder off idle than his stock converter. I helped him tune it, and thought the converter worked as it should. Converters vary some, if you are looking for a converter that doesn’t hit the engine as hard as stock, to help a stalling issue, the GM20 or GM25 Hughes probably aren’t the best choices. Hughes 10” would solve it for sure, but would be too loose on the street with your setup.

Thanks,y'all... I'm getting some great suggestions. What is the difference in the feel/seat of the pants of a stock engine/convertor and a hot cammed engine and a matched up replacement torque converter

A properly tuned performance engine with a RAIII type cam and good compression will idle at about 850-900. When put into gear, idle may drop to 750 as the engine loads up on a stock converter. It should still run, should not stall out. What you're describing is a tuning issue, or too much camshaft for the static compression ratio. Quadrajets usually need a LOT of help when used with a cam that has a rough idle. The idle circuits usually require a lot of re-calibration and bypass air to behave properly.

Having to put it in neutral at a light is a big clue. Typically that means the engine is loading up the intake plenum with fuel or the idle speed is too high or low due to improperly adjusted carb. Either way, it'll want to stall unless you allow the engine to idle high and unloaded in neutral.
None of the above is what requires a "high stall" torque converter. The purpose of a higher stall torque converter to to allow the engine to enter the beginning of the engine's powerband before it tightly couples the engine and transmission together. A stock motor/stock converter and performance engine/performance converter should both be perfectly streetable and have good manners. The performance engine should have just as good throttle response down low. The idle may be rougher with a big cam, but once you're off idle ~1200 RPM the response should be pretty crisp. This assumes a matched engine combo that's tuned properly.

1200 RPM and below is pretty much a wash between a stock and a good tight high stall converter. A cheap crap stall converter will allow the engine to revv above 2000 at light throttle. It'll unload the engine at a stoplight but you'll hate it. It feels like a CVT, the engine revvs up but the car doesn't respond immediately, like the engine and car are connected by a long rubberband. Throttle feel is garbage with a cheap converter, its like the transmission is slipping. A tight converter ($500 and up) is a revelation. It acts like a stock converter at light throttle, so the car responds when you tip into it from a stop.
Stomp on it and the engine RPM flashes to the flash stall (which should be matched to engine powerband) and then immediately couples tight, the car acts like its shot out of a cannon. That's a converter with a tight stall torque ratio, there's a tight relationship between stall speed and input torque, up to the RPM point where the converter "flashes" and couples up completely.

I used to date this country girl that had a 79 Z28 camaro with a 327 out of a truck. Pretty car, blue with the light blue accents. Engine had a set of low compression heads and some sort of hawg-azz cam. Double pumper, high rise intake, stock converter, TH350, 3.42 gears. It idled rough, stank like gas, the power brakes were marginal, it was a dog until 2600-3000 RPM. Poster child for what we're talking about. Even it would idle at a stoplight without dying. Around town was OK. If you stomped it, it would struggle to accelerate until the engine "came on the cam" and then it would run top-end pretty good.

I told her I'd help her with it, but she soon traded the car to some locals who let me drive it after they worked on it. They put a cheap, loose converter in it. RPMs had to be raised to about 1800 before it would even move. You could go down the road and blip the throttle and the tach would just jump but the car wouldnt respond. Felt like the transmission was slipping, but above 3500 the thing was scary fast. The whole combo was just too miss-matched. These same rocket surgeons took it to a local track and immediately overreved it and threw a rod. The girl was heartbroken because she wanted to buy the car back.

We do NOT select "loose" converters to correct tuning issues and a higher than "normal" RPM at idle speed.

Often the high idle speed problem is compounded by a stock or "tight" converter and a TH400 missing the wave plate in direct drum. The "cure" is to tune the engine so will idle in and out of gear at or close to factory idle speed. I would say that you still need a better converter with the "hefty" cam that's causing the issues in the first place, but do NOT expect it to solve all you issues all by itself. Matter of fact even smaller diameter converters can "hit" the trans pretty hard if the idle is higher than it should be.......FWIW......

Thank you,guys! There is more going on with Kitty (my car) than simply "throwing parts " (stall convertor,etc.) will cure. Gotta figure out a plan ( first this step , then second step, ....). And then finally a suitable stall convertor as Cliff recommends.

As far as the jerking into reverse it may have something to do with your idle RPM. If the idle speed is too high it will jerk and if you have slop in your rear end and/or bad u-joints. It is likely your carburetor is not getting enough air through the idle circuit and you're turning up the idle speed to the point that it is coming out of the idle circuit, this in conjunction with your ignition timing at idle is most likely the issue.

If you can maintain a reasonable curb idle speed (750 Rpm in park) and dropping the car in gear pulls your idle RPM down to the point of stalling, then yes, the convertor is too tight for the cam's duration and overlap.

As far as carb tuning being the issue, a quick check for idle air deficiency symptoms are dripping primary nozzles at idle, smoke on startup, a gassy smelling idle and idle mixture screws that have no effect on idle quality and vacuum.

What is your idle RPM in park and in drive? What is your vacuum under both conditions? The vacuum reading will tell you a lot about how much duration and overlap your cam has and where your convertor stall speed should be. I run a 234/242 @.050 hft (RAIV ish) cam and a Hughes GM25 convertor. My curb idle is around 850 rpm and 750 rpm in gear and can idle as low as 550 Rpm in gear without stalling.

Misfires like crazy at idle. Wasting fuel, destroying manifold vacuum and power.

Worse when the engine is cold/cool, but still a problem when warm. Runs good when warm and the throttle is opened. Thus likely to be choke-related, but with other issues as well.

The guy who built the engine suspects low vacuum at the trans modulator. He blames a disconnected hose, but in fact it's probably a matter of low engine vacuum at least at idle.

But we're assuming that it was installed correctly, isn't overly-retarded or overly-advanced. We're assuming the engine has suitable compression ratio for the "custom" cam. A cranking compression test might be helpful.

And we have no idea at all about the state of the tune--idle mixture screws, Choke and pulloff adjustment, ignition timing/timing advance, jetting, air-bleeds, plugs fouling at idle, etc.

IF the engine were properly tuned, this would be true. But we don't know about the engine tune. Screwed-up idle mixture needles could cause this problem and have nothing to do with the torque converter.

I'll make a guess that the engine is too lean, rather than too rich. So dripping nozzles, smoke are probably not applicable. It's certain to have a "gassy idle" since we know it's misfiring, which pushes the entire cylinder-load of hydrocarbon out the tailpipe. But if it's lean misfire, it'll smell rich but actually not have enough fuel.

Excellent questions, but how much they reveal about the cam and converter is debatable until we get the engine tune correct.