Q: I am choosing a torque converter for my 1970 Nova with a 430 horsepower 383 stroker and a 4L60 transmission. I see lockup and non-lockup options. My understanding is that a lockup converter is intended for fuel economy, so would a non-lockup be better for performance?
A: Lockup torque converters became popular in the 1980s to meet federal fuel economy standards, but they are also beneficial in performance applications. When a torque converter clutch is engaged, it creates a near direct-drive effect and slippage becomes virtually non-existent, regardless of rated stall speed.
A non-lockup converter can build excessive heat during normal cruising, especially in overdrive when engine rpm drops below converter stall speed. Installing a non-lockup converter in your 4L60 would also require a conversion valve to be installed into the front pump of the transmission.
|4L60 Trans. Ratios||Rear Gearing/RPM|
|28" Tires||@70 mph||3.08:1||3.27:1||3.42:1||3.55:1||3.73:1||4.10:1||4.56:1||4.88:1|
Above is a chart showing examples of a vehicle cruising at 70 miles-per-hour with a 28-inch tall tire, a 4L60 (700R4, 4L60E, and 4L65E are the same) transmission, and popular rear end ratios. Notice the rpm difference between drive and overdrive, even with lower (numerically greater) gear ratios. Use an online gearing calculator to find the numbers for your specific combination.
Choosing converter stall speed is largely based on camshaft operating range, and a lockup converter allows you to lean toward a higher stall speed for a better launch. Be cautious with this, though, relying on a lockup converter to compensate for a stall speed that’s way too loose will put an excessive load on the clutch during engagement, significantly shortening its life.
This is another in a series of weekly Q&A Mailbag sessions with Summit Racing‘s tech department, in which there are hundreds more. Click here to see them all.
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What is acceptably loose and way too loose for a lockup converter? I have a T bucket (2000lbs driving weight) with a 406 SBC making over 400 LB Ft of torque from 2000 to 4800 RPM with a Comp 12-466-8 roller cam.The tires are 27″ tall, a 700R4 and the rear is a 3.73.
where is RPM indicated on the “chart”?