New Tranny

[sg01fc]

I have an 80 with a 3 speed auto. The problem is that it's horrible for driving higher speeds, and I really think that it causes the engine to overheat with it's high revs. That's the only thing I can figure at least. Are there any good 5 or 6 speed manuals that you can put into it without too many mods?

 

[crause]

My 75 with 350ci auto trans., turns approx. 2500 rpm at 60 mph. I don't know how many rpms you are turning but unless the transmission is slipping only in 3rd. gear, you should be able to notice the slipping while accelerating from a stop. I think your running hot may have another cause. Mine was running just over 200 degrees until I put in a 160 thermostat which helped a lot. I also have a new radiator so you should make sure yours is in good shape. As far as switching over to a manual trans. I would think this to be a very involved job. I don't know your level of skill but there are a lot of modifications to make. Connecting the trans. to the engine is not to tough its the other mods. like installing the clutch linkage, shifter, and removing all the auto trans. wiring and associated plumbing. My suggestion would be to have the auto trans. rebuilt if that turns out to be your problem. It will be much cheeper and less stressful.
You said something about driving at higher speeds being horrible; The c3's are more fun from 0-60 but when I drive 60-70 I wouldn't call it horrible but it is not the same as the ride of the cars of today. Keep us posted on your progress.

Andy

 

[78SilvAnniv]

There are other automatic transmissions with overdrives that can be dropped right in. A manual tranny can be put in, but it is not a simple process.

FWIW...I have read numerous opinions that the TH350 trans is very dependable, reliable and strong...and I have to agree. My own is 25 years old with approx. 250,000 miles on it and still going strong!

I used to have a run-hot problem that lasted for 8 years. The run-hot problem occured every summer at any sustained speeds over 55mph. We replaced the clutch fan yearly, finally replaced the engine (GM crate@250hp) and cooling system and STILL had problems with it running hot in the summer.

When we moved to Georgia, I had finally had it with the run-hot problem and bought a flex fan at a speed shop when I refused to install yet another clutch fan.
I have had no problems with overheating at all since then. I can drive at the faster hwy speeds, use my AC on the hwy or in town (temp to 240* w/AC pre-flexfan) and have made several trips into the GA mountains and driven aggresively through the uphill twisties and the temp stays below 195, no matter how I drive the 78.

In previous threads regarding my use of the flex fan, it has been pointed out that the flex fan is not the cure-all for every engine/cooling application, but it worked for me.
Heidi

 

[sg01fc]

thanks andy and heidi. both good suggestions. the radiator is fairly new, and the thermostat is new, but a 190 degree one. i'll have to try the 160 and a flex fan and let ya'll know how it goes. thanks again.
andy

 

[MaineShark]

A 180-degree thermostat is safer for the engine. If you cooling system is actually good enough to get it that cold, that's all you really want. Cooling it the extra 20 degrees will gain you a few horsies, but significantly reduce the engine life.

But the thermostat won't have any effect on the operating temp of the engine, if the radiator, fan, etc. aren't in good enough shape to get it down to the thermostat temp.

Joe

 

[crause]

Joe, why would the lower temp. cause a shortened engine life? I know the newer engines are all computer controled and the temp has to be right for them to work right but the older engines don't have to deal with that. When oil gets to those 200+ temps the oil will actually start to boil/cook in the parts of the engine that don't have good oil flow. There are places in the heads and block where the oil pools and is subject to these high temps for periods long enough to "cook" the oil. I have always been told that the lower the temp. the better for the motor. I am not saying that this is gospel just what I understand.

Andy

 

[UKPaul]

Cool oil may not boil off condensation & combustion byproducts (depending on the type of use & ambient temps etc). I regularly see oil that is white with water condensation (not SBC's) because the engines are being run too cool. I've fitted a 180stat in my Vette which is the best compromise between engine longevity & power (cooler engine=cooler intake=higher density=more power). Even with a 180 stat I was regularly getting over 200 degs, sometimes over 220.
For the tranny question, I had a th350C with locking converter & it was great. Then the locking converter stopped locking & I was turning 3000rpms at 70mph, which isn't fun with most modern traffic cruising at between 80 & 90mph. Cruising at 85/90mph for about 50 miles caused the coolant temp to stay above 220 all the time. So I went to Bowtie Overdrives & got one of their Level 2 TH200-4r trannys. Fitting it was easy (if only it hadn't snowballed into cleaning/painting/refreshing the entire underside of the car!). I've now got an overdrive gear with locking converter (2200rpm stall) & I love it. It's brilliant, best mod I ever did to any car. Ever. The car is a joy to drive at 80/90mph, it's gone from being a 20 year old car to a modern feeling car capable of cruising at modern highway speeds in comfort. Brilliant! With 2.87 rear gears its doing 75mph at 2000rpms. The 1st & 2nd ratios are lower than the th350 which makes for more stop light fun I cannot rate fitting an OD tranny highly enough. It doesn't cost the earth either, it probably cost me about the same as a pair of decent heads (eg AFR). An added bonus is that fuel economy is now better than my daily driver (which is a 2800cc V6) & I'm regularly seeing economy figures in the high 20's when cruising. With fuel prices here being so high, this tranny will pay for itself at some point. For best effect I should be running 3.55 rear gears with the tranny as the OD gear with 2.87 rear is a bit tall for decent acceleration (I just knock it into 3 if I'm "in a hurry" ).
So, back to hot running. An auto tranny should have it's fluid at around 150deg in the pan. I fitted a tranny fluid temp guage with my 200-4r & it's surprising how high the temp gets at times. With a locked converter it's fine. Running around town in lower gears really heats it up. I then fitted a tranny cooler (a small B&M one) in the line coming out of the tranny & just before the rad. So tranny fluid goes through the cooler before going into the main rad, ensuring that when the engine is started from cold the tranny fluid isn't overcooled & actually gets warmed by the engine coolant. The big surprise for me was that the engine coolant temps plummeted after fitting the tranny cooler. Most of the time it sits bang on 180 & I only managed to get it to increase to 190 briefly by some "spirited" driving on a day of record breaking temperatures. I think what was happening was that very hot trans fluid was getting pumped into the main rad & was preventing the coolant from being cooled enough.
So, rather than spending lots of cash on new fans, aluminum rads, etc, etc, try fitting a tranny cooler for about $40. It sorted out mine OK

 

[MaineShark]

Crause: UKPaul answered it pretty well. 180 is the best compromise. Actually, 180 plenty to prevent excessive wear. Modern engines only run hotter than that to reduce emissions.

Of course, if your cooling system isn't capable of dropping the temp to that point, then the only thing that the thermostat will affect is warm-up time.

Joe

 

[JohnZ]

Joe, why would the lower temp. cause a shortened engine life? I have always been told that the lower the temp. the better for the motor. I am not saying that this is gospel just what I understand.

Andy

Our OEM testing has always shown that the rate of cylinder bore wear at 160 is DOUBLE the wear rate at 180, and running at 160 (assuming your radiator will cool it to that level, which is unlikely) doesn't let the oil in the pan get hot enough to boil off condensed moisture and combustion blow-by contaminants suspended in the oil. "Cooler is better" is an old wives' tale, not based on facts - just hearsay. Also, as Joe stated, thermostats have absolutely nothing to do with max operating temperature - they only control MINIMUM operating temperature.

 

[78SilvAnniv]

...the locking converter stopped locking & I was turning 3000rpms at 70mph, which isn't fun...

How do you know when the torque converter is not locking? I feel that my hwy speed rpms are about that high. I know my tranny is due for an overhaul, should I perform it ASAP?
Heidi

 

[UKPaul]

Heidi,
I think '81 was the 1st year that a locking converter was fitted (maybe the '80 Calif??). If you've had one fitted in yours then there should be a switch to cut the power to it when you brake (prevents stalling the engine if you lock the rear wheels when braking). The simple way to test if it's locking is to very slightly apply the brakes at anything over 50mph in "3". If the rpms jump up by about 300 then it's all working OK.
rgrds

 

[78SilvAnniv]

I don't think anything has been done to the original tranny. Thanks for answering my question, I did not know that some did, and some did not, come with the locking converters. I love learning new things!
Heidi

 

[Bob Chadwick]

Nut pointed this out to me on our last trip. That's why his later model car gets better gas mileage and slightly lower RPM's.