Have a 200r4 in a street rod with an Olds 350. Any particular 4th gear lock up kit you are partial to? Thanks.

 

I have used the one from Bow tie Overdrive. It uses a 4th gear pressure switch, and a vacuum switch to control lockup with engine load. Unfortunately, any lockup system that uses a vacuum switch will be very touchy as to when it locks/unlocks. I’m guessing a lightweight street rod will be even worse. Ideally, once 4th gear engages, and engine load is light enough to need lockup, a short delay is good to allow everything to “relax”. When you push the brake and release the brake (which should unlock and then lock the converter again) there should be a short delay before it locks.

I recall someone on ROP who designed a control box that used inputs from the brake lights, a vacuum switch, 4th gear switch input, and then a fancy algorithm to smooth out the converter lockup.


It’s hard to beat a simple toggle switch. Wire it with a 4th gear pressure switch and a brake switch to make it a little more idiot proof

 

The TCI works quite well for me, adjustable to different size cams.

 

Any chance you would have a schematic for that? Thanks.

 

Personally, I think a vacuum switch is overkill. If you really need to accelerate harder than your engine is capable of "grunting" through 4th gear locked-up, the trans will kick down into 3rd and unlock the TC. I use a brake-light triggered relay to interrupt the 12V going to the trans where a 4th gear switch is in series w/ the lockup solenoid. I just leave it in Drive and it takes care of itself. I've not yet encountered a situation where I wished that the TC would unlock in 4th gear. It might smooth the down-shift to 3rd, but that's about it.

 

x2

 

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The converter clutch isn’t designed to take a bunch of torque. Enough throttle to force a 3-4 downshift can slip the clutch.

 

I've not yet encountered this w/ my stock 350-2-bbl. But after it gets rebuilt / modified, it sounds like a vacuum switch should be considered (it'll be getting a new TC at that time anyway). Thanks.

 

The lock up clutch is maybe an inch wide, and just a little smaller than the converter diameter. Just not a lot of surface area for the clutch friction to grab. The converters that are designed to be locked under WOT are much bigger, and use multiple discs

You can see the converter clutch friction ring here.

 

Zoiks! I see now why a vacuum switch would be critical on a torquey engine. Good info, thanks.