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Join Date: Jun 2018
Location: Central CA coast
454. Is 3,000 RPM cruise so bad?
Longish intro, but the question is at the end:
'68 El Camino SS. When I bought it about a year ago it had a very weak and worn 396, a rebuilt, but of unknown quality TH350, and a nicely built 12-bolt with 3.73 gears and an Eaton posi. My "plan" was to replace the engine with something more stout, then replace the transmission with something with an overdrive. I figured I'd make any decision regarding rear gears once I got familiar with the engine and transmission.
Fast forward to now: I have a mild Vortecpro 467 peaking at 581 Lbs.Ft. of torque at 4,200 RPM and 553 HP at 5,400 RPM. The TH350 seems to have been built well enough to live behind this engine, although my mental jury is still deliberating its future. But something needs to happen, as the extremely tight torque converter is really holding back the engine. If I set my idle at 1,200 RPM just dropping the car into gear knocks it down to 800 RPM. The braking stall is somewhere in the 1,500 to 1,800 RPM range.
With my 28.5" tall rear tires my car cruises at 2,500 RPM at 57 MPH, 3,000 RPM at 68 MPH, and 3,300 RPM at 75 MPH.. This car is an around-town toy and the engine runs so smoothly that I'm rethinking the need for an overdrive or a re-gear. Even if I do make a road trip, I'm suddenly thinking that 3K RPM may not be such a big deal.
So now though, if I keep the 3-speed, I definitely need to install a better torque converter. I think the engine would perform best with something in the 3,500 RPM stall range, but with the 3.73 gears, I think I'd burn up any converter with a stall over 2,500 RPM with normal highway cruising.
Certainly if I keep the 3-speed I'll be on the phone with a few converter manufacturers before I make any decision.
So after that long story, what are the thoughts here regarding just keeping the 3-speed transmission and the 3.73 gears and letting the car highway-cruise at the 2,500 to 3,300 RPM range? Swapping in a 200-4R would drop my highway cruising range down to the 1,700 to 2,200 RPM range. I'm now wondering if that would end up being too low for this engine. I'm not concerned with fuel mileage as much as I am with keeping this car fun, but reliable.