Ok Im new here and I'm not sure how this works but here it goes...
I bought a 81' Elcamino that needed lots of TLC but...I was assured it was mechanically sound...and very dependable.. I was told it was a 305cid/th200 and never doubted the guy...well I have drove the car back and forth to work for about 2 weeks now and noticed that the shift between 1st gear and 2nd gear was kinda long...and from 2nd to 3rd was fine...Well I figured possibly the modulator must be going bad so I climbed under to make sure I had no leaks in the vacuum line. After climbing under the car I noticed that the pan on this what I was told was a TH200 looked an awfull lot like a TH350 but not ever seeing or being around a TH200 I did some research on the transmissions.. Well I have a 13 bolt pan TH350 which made me fill better...but...Ok the next day on the way to work my Elky started running hot after about 10 mins of drive time. Not thinking about the trans because I do not have loud pipes or rpm gage...I checked the clutch fan and thermostat...
That night on the way home from work I started noticing the higher rpm's so I stopped the car pulled manually into 1st gear and started off and shifted into 2nd with no gear change...and then into Drive and it sounded then to shift into 2nd, then shifted back down into 2nd and trans went into neutral untill low enough speed to hit 1st gear automaticlly....So I change the Modulator valve, clean the governor and check my passing gear cable just to be on the safe side and still did not change the tranny problem...

I was hoping to drive her back and forth to work untill warm enough weather to ride my bike back and forth...

Am I missing anything or is it time to buy a tranny...
Oh and what are factory gear ratio's in these years rear ends?

Sorry for being so long winded but this is sort of stressful...and I want you to know every detail...lol Thanks alot

I'd say you are looking at needing it rebuilt. They're only about 350 to around 500 depending on where you go to replace that. Most transmission houses have them sitting around.

Generally the gears in the 5th generation El Camino were ether a 2.73:1 or a 3.08:1 but you could order other gears. There may have been some variation here and there. The 2.73:1 was for economy and the 3.08 was generally a performance rear end.

Actually the V8 1980/81 El Camino actually had a THM 350c transmission not just a 350. It was a lockup converter for economy. More like a "Stall Converter" than a lockup.

Ok thanks alot for the quick and informative answer..
This THM350c your talking about...is it better for street and strip because I have heard some some bad things about this lockup type tranny...
Do I need to get the lockup style or the non-locker style for better performance and tougher durablity under quick starts..
I was planning on building alot stouter motor and rear-end to have a line jumper or possibly 1/4 mile car on its days off...just for play...lol

Thanks again for your response...

The THM 350 what ever, meaning any version, is inherently weak in it's design. That is why GM has a THM 400 which is much stronger. Now the 350 can be made stronger, most transmissions can be made stronger then they were originally.

The THM 700R4 has a better 1st gear than the THM 350 and the THM 200R4 has a better 1/2 gear than the THM 700R4. So, if your asking for opinions, mine says the THM 200R4 is the best to build up. It's also a very weak transmission from it's design but it can be built to deal with a good 400 to 500 Lb/ft. of torque possibly.

I say that because the THM 200R4 one has a better 1 and 2nd gear ratio then the 350 or the 400 or the 700R4 and in highway cruising it's got a .68 final drive ratio whereas the the 700R4 final drive ratio is .72. Also the 200R4 can have the final drive ratio reduced even greater to say a .6 or even a .55. The is a company in NY that makes the gears to do it, you can't do that with the 700R4.

The final drive gear ratio is what is going to lower your RPM on the highway so you don't suck gas and it sounds like your engine is about to blow from the high RPM. With a 3.73 a .72 final drive ratio you'll look like (in RPM) like you have a 2.68/2.69 rear ratio with the .68 you'll look like you've got a 2.5:1 gear ratio. That is streetability!

Now that is just my idea of a good thing. I run a THM 700R4 in my 76 and it's a nice piece of work. Those are Lockup converters, the 700R4 and 200R4. They use an electronic switch to engage the lockup converter and that is good. That is what will allow you to achieve the lower RPM. You can run it without locking it up but you won't get a consistent RPM and it generates more heat in the transmission.

Ok Thank you once again you have been very informative...
I know the 700R4 is a nice chunk of money...so I may have to go with the 400 or the 350 for the time being untill the bank roll gets fatter..lol

Thanks alot for your assistance...