Featured Hot Rods 400 turbo vs 350 turbo

I've never been more than a scrub nurse for a trans rebuild. There's a part in the clutch pack area that's really easy to put in the wrong way, as in backwards or upside down. It'll work, for a while even yet you don't know when or how much load is gonna make it roll the case. Here in Motown we had "Joel's on Joy" and he was the man on glides and 400s for us racers. He did my last one, I went quicker than ever before with ONLY his rebuild being the difference. When I asked him what he did? "I fixed it." Ok, worked for me. Some of the true experts here will know what parts I'm talking about.

 

I like the turbo 400 and especially the variable pitch turbo 400 that was available from 64 to 67 in Buicks Oldsmobiles and Cadillacs. I've got one with a V6 vp converter and high stall is 3200 and low stall is about 1800 RPM.

 

You should see what happens to one, that the A Hole decides to put a 500 shot NOS on a BBC Chevelle, that has never been taken down the dragstrip!
On his first pass, the engine blew up going through the traps and locked up.
Crashed the car, sheared the bellhousing, twisted the input shaft, intermediate shaft, output shaft, broke the tail housing, blew a hole in top of the trans case, twisted the driveshaft.
Engine had a Howitzer hole in the side of the block.
I still wonder how many times the trans spun around before the car came to a stop!
He tried to blame the whole mess on my transmission failing and locking up! That didn't work!
Nothing is indestructible when a moron is behind the wheel!

Bill

 

I've got a 400 in my Model A behind the nailhead it was married to at the GM factory. On light throttle upshifts, you can barely feel it shift. You do feel the stator shift from high stall to low stall and you have to let up on the throttle a little to maintain your speed (25 -30 mph around town). The THM 400 was probably the best built transmission to come out of the 60's.

 

When I built my off topic car with a 427 the transmission guy talked me into rebuilding a TH350 I had. He said less weight, and less energy to spin it. He added that he could build a TH350 to be as strong as I'd ever need with my 425hp 427 BBC. I had him build it, and it was in the car for 35 years when I finally sold it. Many passes down the 1/4 mile, plus tens of thousands of street miles too. I'm a believer that a good trans guy can make a TH350 very strong.

 

I think it's a max loss. Maybe loss isn't even the right term. It takes X amount of power to turn that trans at max effort. And really, if ya think about it, it's no power loss in 1st and 2nd. It's multiplied torque. At 2.48 1st gear what do we have at the drivesahft/pinion pushing that car? Not 500 lb/ft, more like 1500. Then multiplied as well by the convertor so it's more. Then the ring n pinion, so by the time the rubber hits the road it's about 4000+ lb/ft. At 1:1 high gear we have parasitic loads and the old figures were like 45-50. From flywheel to tire the overall parasitic losses are like 125-150, but again in high gear.

 

There are things you can do during rebuild to help lessen the loss.

 

I have a 475 in the Studebaker. I don't hear it......but that 3" Blower belt and 3" exhaust may have something to do with it.

 

I've never had any issues with the gear train in a TH400 behind a 600-700 hp blower motor. Clutches...yeah, you can burn them up without much effort.

But back to the original question, deciding which transmission to use depends on a lot of variables. The only situation to use a TH350 instead of a TH400 because of less drag in the transmission, would be when you need to go as fast as possible, and cost is not a concern. Such as stock class drag racing, where you are competing with other guys who spent $5k on their TH350. But then they'll probably be running a TH200 instead, as they have even less drag and weight than a TH350, and can be built pretty darn strong these days.

If you are just building a normal car to have fun with, and you can get a TH350 built for a reasonable price strong enough to hold up, then go for it. Otherwise, you can probably use a mostly stock TH400 and it will do the job.

 

The power loss is from friction and rotational inertia.
Thrust washers/bearings and unengaged clutch packs rotating with fluid film between them. Bigger washers and larger, more numerous clutches will increase the drag.
Rotational inertia is the big one for drag racing where rpm increases quickly. The mass of the internals are a flywheel. The heavier it is the more energy it stores. This has no effect at constant speed cruising, but is significant for acceleration and deceleration.

 

yup, the geartrain in a 400 is heavy, and you have to get it spinning very fast. But if you are not racing in a situation where you need to go as fast as possible, who cares?

 

Agree with Squirrel. Turbo 350s are for the most part, just fine in most stuff their are put in.

Sure, back in the day 350s gained a bad reputation as not being so good, even in the transmission repair industry,
they called them "slush boxes" etc.
What most fail to realize is, they were in just about a gazillion GM cars from 1969-1987.
1969-1979 were non lockup versions. 1980 and up had a lockup torque converter.
Operating principals between the 350 and 400 are the same.

The best versions of the 350 were between 1976-1979.
They had needle thrust bearings instead of bronze thrust washers between components, a "steady rest ring" on the stator support to prevent the direct drum from contacting the support in the ring bore etc., which all helped to reduce wear.

So yes, we rebuilt a whole lot of them.
Not because they were a weak design but because of the sheer number of them, that were out there.

Gear ratios between the 350 and 400 are pretty close, 350 is lighter, smaller and fit into the stuff we are building much easier.

If you are worried about .100s of a second in the quarter mile, then maybe a 400 is not the best choice, due to parasitic HP loss.

Most of the high perf weak link failures we have seen in 350s are the intermediate roller clutch failing.
There are some not so high dollar mods to address that with a hardened intermediate race.
Is it indestructible, no. Everything has its limits.

Today the biggest issue we have building them is finding a good case. High mileage 350s have an issue where they wear out the splines machined into the case for the center support. There are some aftermarket repairs to address that issue, some not so and some more expensive.
Its best to have a not worn out case, but as time goes by that is becoming harder and harder.

A good built turbo 350 is good for about 400lbft of torque without crazy expensive custom racing parts.
And that only comes into play if the car that it is in, can actually "hook" well at the starting line.
If it just spins and fries the tires, there is no hook.

Bill

 

This should explain it once and for all.

 

HaHa, beyond my pay grade for sure!

 

I absolutely Love that he does all this with such a strait face!

 

It either took several takes, or he's an actor who has no idea what any of the words mean

 

Yeah, only real engineers appreciate just what the amazing wonder material "Prefamulated amulite" can do under stress, and can visualize the sinusoidal motion of a properly designed Reciprocation Dingle-arm.

 

I have had T350 and T400 transmissions in my cars over the years and thought both were great to drive with. My cars were just tough street cars and I never thought the T400 made a car much slower. As has been said, if your looking for 10ths on the quarter though?

I do recall the T400 were physically bigger to play with

 

Parted out a crusty c20 camper special
Had a th350 in it
JS

 

i have had a junkyard t 350 in my off topic drag car for 20 years with just a couple re clutch and steel rebuild it has a tci manual valvebody and a 4000 rpm tci converter other than that no other than stock parts it came with it runs 10,80 at 124 with a 145 short time