Hello all, I am researching tranny stuff for my 1980 cutlass supreme with 260 cid, TH-350, 2.29 rear end gearing. did the th350 in 80' have a locking torque converter, (ya know, for better MPG)? I will have to check pan again the next time I am under her, but it is too dang hot here today.

I believe this set up on mine was for economy, small v8, low gear, 2 bbl carb. I do remember the last who rebuilt it said it was a th350, and I do remember checking the pan shape, but wouldn't it say on the OEM window sticker? The trans did run well for a year or so after guy rebuilt it, but fluid burned up fast... and I did not run it hard at all, but it did have a 350 olds (rebuilt to OEM specs) that is gone now though.

I checked window label and it just says auto trans "No Charge" and 260 was the option installed, (along with many others). below is a link of used trans options for my year and they all say "Transmission A.T., TH350, w/lock-up converter, 8-350 (5.7L), w/o multifit case; 1 wire lock-up plug harness
and they list those with all motor options... even the v6, but my 260 is not there. I dont plan on getting used, I just want to see if I can make this trans make my 403 get decent MPG and explore options when it is time to replace the wussy 260.

I want it to run low RPMs at 65 MPH, like 1,100 or so. does anypne have a calc for this?


http://www.woodfins.com/Parts/1980-O...nsmission.html

any ideas friends?

 

Here is your calculator.

http://www.csgnetwork.com/gearratcalc.html

As far as a TH-350, no they did not have an overdrive. 1100 RPM at 65 is really low. The a/c will not work well at that RPM. IMO you will be lugging that engine. I personally would look at a target RPM of 15-1600 RPM minimum. That way the motor can get you up a hill if need be without downshifting into second. Or you could do a 200-r4 swap.

 

 

Hey Clint, the calc is awesome. I think you are right about the RPM's being to low. but that would be on flat coasting conditions, since it is a non-locking trans, the rpms would go up to about the 1500-1800 mark wouldnt it? plus the 403 would have super torqe over the 260 I have now

I have been thinking about the 200-r4 swap, but my trans is paid for, other than rebuilding it, would the 200r4 cost about the same as rebuilding mine and would it bolt up, with no modding?

 

I am ok with keeping the 2.29 read end and non locking converter, and the specs on my 260 are 105hp xxxfp @ XXXXrpm. and the specs i have seen for the 403 are 185hp @ 3600 and 320fp @ 2200rpm so I dont plan on it getting over 4500 rpm.

my tires will be 215/65/14 @ 25.5 inch diam. 2.29 rear end. if that sounds decent. I think that is what I have on it now (non factory rims) I love the look/stance from looking at the rear of the car. plus, I dont want to race her, just want her to have the pep she deserves ya know.

 

If you let the converter do most the work climbing slopes you won't be getting best fuel economy, and the transmission will get hot, I recommend more like 1500/1600 rpm at 65mph with the converter lockup engaged.
Low rpm doesn't necesseraly equate to good fuel economy, if the engine is chugging along with the throttle held open gas mileage will suck.
Ideally the engine should be purring along with your foot barely touching the gas pedal.
Roger.