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JATCO autobox and engine build


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Engine build recommendation for street build with the good ole JATCO autobox.
This is not a discussion about whether a manual box would be better for performance , rather what build would be better for the limitations of the autobox .
Customer wants to “go fast” , but his wife wants to enjoy it too. She likes fast cars also . 
Nice original matching numbers 2/72 with 64k miles .
Sometimes I’m inclined to say pull the motor and put it in a stand and build a L28 . 
It might be a matter of correct cam and to put whatever cam in there that plays nice with the limitations of the JATCO . I’m not positive of what those limitations are . I’m inclined to believe the motor needs to make all its power down low and not need to rev out for its potential power and that would be easier on the trans . I know those trans like to shift early unless you manually shift , but not sure how well that plays out . If I remember, there is a lot of slop in the shifter and manually shifting might lead to over revving because of missed shift points .
I know there are valve upgrading kits for the JATCO , but not sure what improvement that will give . 
What little I’ve experienced the L24 with the auto wasn’t terrible . 
Other points is that he has decided Z Therapy SU’s . Ignition has yet to be decided , but the stock points will be going . Header and full exhaust . A/C is very high on his list and this car came with the dealer A/C which will burden whatever motor .

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If running any serious power through it I would keep eyes open for an '81-'83 280zx turbo automatic. It has extra clutches and plates, a modified valve body, and modulator valve and a higher stall speed torque converter. This allows the engine to rev quicker off the start and the clutches will handle a turbo output. Any 'kits' just move the shift points and pressures and don't help the stock parts any.


A good 'bang for the buck' modification is a change of the rear differential gears from your 3.545 to a 3.70 or 3.889. This will rev the engine higher at the same speeds as your stock gears. Power through a shorter distance equals increased acceleration at the expense of top speed (but still well above any posted limit) and gas millage. (though around town in drive the automatic will always up shift into the best higher gear) This is way cheaper than modifying the engine and reversible in an afternoon.


You'll never over rev an L24. It has the same stroke as the L16 in the 510. Red line is 8,200 and no stock L24 is going to be making any power past 5-6k without major modifications. Automatics don't have 'slop' but there is a slight slippage in the fluid coupling of the torque converter. At low speeds this is an advantage when the engine is placed under load.... it revs a few hundred RPMs higher than a standard would. The automatic has a governor that selects the maximum shift point speeds to get the most out of the engine's torque curve. 


A/C is a slight load when on, like an alternator. When off it isn't detectable. EI ignition from a '79-'80 L20B engine is a good idea but get the matching coil for the higher output.


Something I put on all my L series engines is a KA high volume oil pump from the D21 Hardbody engines. The internal rotors are 13% longer and although the output pressure is the same when revved up, the low pressure as when idling is much higher. My old L20B hot idled at 17 PSI. Putting a used KA oil pump raised it to 29PSI and it reached maximum pressure sooner.

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On 12/9/2020 at 4:19 AM, Madkaw said:

Nice original matching numbers 2/72 with 64k miles .


Yup, 3.545 on the 240/260 automatic. From Dec. '74 on build dates with the new L28 engine automatic and standard transmission cars were all 3.454. Probably because the Z car was putting on weight. By the time the ZX came out it was seriously in need of a diet. This is the trouble with car makers.... they make something fun and sporty.... and then load it down with heavy options. 

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