The day is almost finally here, the Olds 455 and TH400 are going into the 79 Cutlass this weekend. Then the real work of figuring out accessories and how to put the radiator support and header panel, both of which I removed years ago, back together.

Anyway, I think I did most of my homework, got what I need. Reflexively, I got a performance torque converter, the lowest stall of which is 2200-2400 RPM - that I could find. I bought a Jegs unit, and it's been sitting, waiting to go in, for a couple of years. The engine builder recommended 1400-1800, saying I would have plenty of low-end torque. My friend, who is a physics guy/motorhead was concerned about slipping if this cruiser is driving for periods of time below stall speed, saying it could heat things up. I am running stock 14" tires, have a stock 7.5" rear with maybe 2.43 at best, and I'm wondering, before I bolt this all together, if I should get a stock stall converter?

I've picked a lot of brains here over the past couple of years, and I hope that someone here will weigh in on the issue.

Years ago, I put a 350horse Chevy 350 into a 82 Z28 with th350 and 3.42 rear. With the three speed trans, it revved like crazy on the highway, like 3K at 65MPH, and was so-so off the line, and I always wished I had put a hi-po torque converter in -- and maybe a 3.08!

This Cutlass is going to see some highway, but mostly for cruising, stoplight to stoplight stuff, never a daily, like that Camaro was years ago -- out of necessity.

Any advice, as always, is appreciated!

Tom

 

It really depends upon how the converter is built. If the converter is made correctly, it will not slip excessively unless there is torque applied, and during normal driving it will be very similar to a "stock" converter. I have a Hughes Performance 2500 RPM converter and at one time I had to put the 2.56 rearend back under the car. I did a lot of driving at speeds well below the stall RPM and I saw no difference compared to when I had the 3.23 rear. I even calculated the amount the converter slipped by comparing engine RPM above and below the converter stall RPM and it was very consistent at around 300 RPM. I have o idea if that converter you bought would operate similarly, though.

 

Stall speed is directly related to load. There is a ton of science that goes into converter design.

Let’s say you have a converter behind a 350 engine making 300 lbs/ft of torque in a car weighing 4000 pounds, with a 3.08 gear. The converter stalks to 2500 rpm.

Take out the mild 350 engine, swap in a stout 455 that makes 550 torque. That same converter now stalls to 3200. Take out the 3.08 gears, swap in 4:10, now your down to 2800 stall speed again. Put the entire drivetrain into a car that weighs 3000 pounds, now the stall speed might be 2500. Same converter, change the variables and the stall speed also changes.

Every aftermarket converter started out as a OEM converter. Most 10 inch converters sold today are based on FWD GM 4T60 converters. Years ago they were Powerglide converter cores. The same converter “family” could have the internal vanes welded at different angles, or the stators machines differently, all to tailor the converter to its intended application.

Obviously OEM converter cores were never designed to fit older transmissions, or handle to power. They need lots of modifications. The converters are cut apart, the vanes bent or the combinations changed, different stators or different machining, the vanes are welded or furnace brazed to increase strength, then the parts are modified to fit the desired application, carefully welded together on special machines, leak tested and balanced. All that labor is expensive!!

A good quality converter won’t slip or feel mushy cruising around. The Coan 10 inch converter in my car flashes to 4200, and almost feels stock. My wife drives the car all the time, the trans never gets hot, it’s not that big a deal.

I don’t know who builds converters for Summit or JEGS, I’m betting it’s Transmission Specialties out of Pennsylvania. Look the converter over, I’m sure there are numbers stamped into it. Call them, give them the numbers and as much vehicle information you can, they will be able to predict fairly closely what the converter will do in your application.

Im betting 2500 will work great in that car. A trans cooler isn’t a bad idea.

 

All of that. Good info.

 

What cam does the 455 have? Is it this one? Pretty sure they are made by B&M.
https://www.jegs.com/i/JEGS/555/60402/10002/-1
The stock 1600 stall is pretty awful and too low for anything but bone stock. Even the tight factory slip a couple of hundred rpm at minimum, I saw 220ish rpm, many of us see the 300 rpm number on aftermarket converters. If you are seeing much more than that, there is an issue. A trans cooler is a goid idea along with a big two core aluminum radiator gor that 455. The 5% slippage number in calculators is pure fantasy, especially with aftermarket converters. That 7.5" rear is really the weak link. Inspect the spider gears, I exploded a set. The used replacements were showing wear, they factory ones are soft. Yukon makes hardened spider gears.
https://www.summitracing.com/parts/yga-16048
A cheap and easy upgrade, the 455 will find the weak points. Good luck.

 

With torque monster engines like a 455 Olds, you can plan on the advertised stall speed speed being on the high end, and most likely a little beyond.

The Grand National came factory with a converter that would go to 2600, and peole didn’t complain. GM wouldn’t have used a converter design that would result in warranty or customer complaints.

 

Thank you for all of the information!

Now, a question on installation: I only got 2 clicks but spent several minutes spinning, rocking, and gently pushing on it. I believe it is seated all the way, but if it's NOT, would I be able to tell upon bolting to the block? Or would the air gap be really small or non-existent? Thanks again!

 

They can be tricky on getting the final click. Wiggle as you spin. Be careful, it will be very tight against the flex plate, if not seated. You must be careful not to damage the pump.p

 

When in doubt, slowly tighten the bellhousing bolts, stopping occasionally to make sure the converter still spins. The converter might hang up if the trans is at an angle, but once the bellhousing is tight against the back of the block it should spin.

The converter mounting tbs will most likely drag against the flexplate balance weights, but other than that you should have 1/8-3/16 inch of space between the mounting ears and the flexplate.

 

307/403, Matt
Thank you both. Obviously, I am concerned that it's correct. I bolted tthe engine and trans together as a unit, and installed. I haven't installed the cross member yet, or really anything, so if I made a mistake, I can still easily get the trans out and double-check. For now, I will see if the torque converter spins and if the proper air gap is there.

 

Hi Matt,
Thanks for replying on two of my ongoing posts! Very helpful.
So I crawled under the Cutlass today, and the torque converter does indeed spin freely, and it's slightly more than a quarter's thickness between the pads and the flex plate. A quarter is 1.75mm/0.068",and the minimum air gap you want is 0.060(??), so I'm in the range. I guess what I'm asking is could my air gap be OK, and the TC still not seated completely?
I'm wondering if I should pull the trans out and make sure the converter is all the way on, rather than get everything buttoned up and realize something ain't right.
I will try to spin and push it on further while still in the car, but I won't get much leverage -- worth a try, I guess. Better safe than sorry?

 

If the converter wasn’t seated correctly, the converter wouldn’t spin, or you would have far less than the gap your seeing. Relax, it’s good.

 

As said a gap is good and means it is seated. A quarters thickness or more is OK. I had one aftermarket converter that I could fit 4 flat washers in between the flex plate. I pulled it snug without washers and leaked it like a sieve. It later let go inside at the track. That amount is fine and will allow for converter expansion.

 

Thanks, all. Was losing sleep over it, just don't want to have to do anything twice!

The last time I did this was 20 years ago, installed a rebuilt Chevy 350 and th350 into an 82 Camaro. I did everything right that time -- unfortunately had to pull the engine a few months later due to a ring problem -- poor rebuild!