73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Performance => Topic started by: JohnnyPopper on June 22, 2020, 04:57:22 PM
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So I'm just about to land the 383 in her special spot, just had a thought about the TC while it's still open.
Bone stock TH350, appeared to be in good shape when I bought her.
I don't know what the factory RPM stall is/was.
Should I consider digging into that factoid, or just take recommendation from the forum, among those who have forgotten more about automatic transmissions than I know?
I'll get the ratio of the rear end and add to the mix... ::)
Thanks!!!
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You did such a good job! Wish I could give you some good info. I put a HP built 350 in a car I owned and had to get my TH350 built with a shift kit. Hope someone can provide some good info for you.
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The selection of a TC hinges greatly on the lobe separation and profile of the camshaft.
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Thx bd, I'll get the specs and post them.
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Here's what came with the cam...
I have to admit, this part of mechanical science is above my pay grade, so I really appreciate any direction.
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I recommend a stock converter and a shift kit if it doesn't already have one.
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Agreed, that is a very mild cam. Keep it stock
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Thx guys!
She's going in!! 8)
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Spoke with a guy at ATI, he recommended their model 408320, Super Street Master.
Stall at 2800-3000 RPM.
Said it would be better than stock.
I have to admit, I am way out of my wheelhouse on this, anyway, I'm trying to understand how this works so I can explain it to my grandson.
I'll take a stab: Holding the break down followed by WOT, the TC will allow the engine to rev to 2800-3000 before engaging in a positive transfer of power- Am I getting this right?
If correct, the question that follows is, in everyday street driving, will I expect the truck to have to rev into that range before she moves?
Thanks for your patience... ::)
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I take it you don't trust our advice over a sales dweeb who's never wrenched a day in his life? He's trying to move product. Your cam is basically a bump over stock and will not benefit from the converter he recommended.
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JohnnyPopper - If in fact your transmission is original and stock, it is NOT a TH350 but rather the less common and arguably less desireable TH350C which has a lockup converter unique to that transmission. I'm also a 1980 owner, and made some poor decisions early on because I was unaware of the difference. I'm now running a TH400 and quite happy with that but my wallet is considerably lighter.
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Rather than questioning my trust and appreciation for the information I and others have gained by joining this site, can anyone offer a thumbnail sketch of Torque Converter theory and design to an admitted ignoramus?
Something like 'a stock TC stalls at X RPM'...'the definition of stall is'...'a lockup TC's purpose was'...'and for that reason it's less desirable'...
I don't mind constructive criticism, I'm still trying to learn.
Silverminer, thanks for the heads up, I will look closer at what is there.
Again, thanks in advance for you patience.
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The spec card you posted looks like it belongs to a Schneider Racing Cams 262-70H SBC Hydraulic Lifter Camshaft (http://schneidercams.com/262-70H_sbchydraulic.aspx) with a 112° LSA and 110° ICL for use in carbureted engines. According to the manufacturer, the power curve for that cam when used in a 350 CID engine is 1,600 - 5,000 RPM. Installed in a 383 stroker, however, the power curve will shift downward by ~200 RPM due to the increase in engine displacement and greater VE, lowering the power range to ~1,400 - 4,800 RPM.
A TC should “stall” ~500 RPM higher than the start of the camshaft’s effective power curve for optimum performance. That means the optimum TC stall for the Schneider 760-70H installed in a 383 engine amounts to ~1,900 RPM. Clearly, a 2,800 - 3,000 RPM stall is WAY too high! I think you would hate it within 100 yards.
The OEM converter installed behind a factory 350 engine stalls at ~1,300 RPM. Behind a 383 engine, due to its increased torque, an OEM TC stall might increase to ~1,400 RPM, producing crisp launches. Bear in mind that trucks are heavy and need high torque development to accelerate them. Lower RPM engagement is better so the engine has adequate working room before redlining.
Read through these:
How a Torque Converter Works (https://www.youtube.com/watch?v=bRcDvCj_JPs)
Understanding Torque Coverters (https://www.hotrod.com/articles/0808rc-torque-converters/)
Choosing the Right Torque Converter (https://www.rodauthority.com/tech-stories/drivetrain/torque-converter-basics-do-you-know-how-to-choose-correctly/)
A real-world example:
The "mild" roller camshaft in my 355 CI MPFI truck has the following specs:
- Duration: 202° Int / 212° Exh measured @ 0.050" lobe lift
- Lobe Separation Angle: 113°
- Intake Lobe Centerline: 109°
- Measured Intake and Exhaust Valve Lift: 0.590"
- Power Curve: 1,000 - 5,000 RPM
The engine has AFR CNC aluminum heads with 2.05" intake and 1.60" exhaust valves. Static compression ratio is 10.0:1 @ zero deck. The engine screams with invigorating torque and acceleration. Final drive is a 3.42:1 locker, twisting 31" tall tires. I run a stock TC. You couldn’t convince me to migrate to a higher stall converter!
Unfortunately, until you have the experience, you will continue to agonize and second guess peoples' recommendations.
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Thanks bd, you made my day
After 45 years of wrenching I am getting closer to understanding that which I had not been tasked to 'know'.
You Sir, are a gentleman and a scholar!
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Thinking about my first excursion with a Torque Converter...
By happenstance my first restoration was a 1967 Olds Toronado.
Back in the early 80's we were compelled to drive 55.
I was told it had a Variable Pitch Stator, (no clue) that was activated by a kick down switch linked to the carburetor.
Was told the purpose was, upon WOT, a 12v solenoid valve directed ATF to a piston in the TC, which would cause the Stator's fins to take a more neutral angle, instantly releasing the engine to rise by 400 RPM.
Started sniffing around, and did some testing.
Sure enough, my 3200 RPM at 55 dropped down to 2800.
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Bizarre how ATI would recommend that converter. You did give them the engine and cams specs? The cam is very tame for a 383. (nothing wrong with that as this is the beauty of cubes---you get good torque AND good top end without needing a wild cam.)
Let me pm you when i get a chance(i know a few guys there).
Also, as discussed above, let's make sure this is a "regular" TH-350 vs. a lockup TH-350C
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Can say Stewart, perhaps VZ nailed it, calling him a box mover.
Although, he did ask me for all the specs that are listed on the data sheet, sounded like he was using a calculator, and then made that recommendation.
I can usually sniff out a poser, but he acted like he knew his stuff.
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Take the advice on here, 1800-2200 stall max. I made the mistake years ago running a 3000 Hughes on a mild 350 w/ a Crane 272 cam similar to yours. Ran hot, inefficient and not very derivable. Wasn't even decent on the track.