Those that followed the Holley carb thread know I am fighting a Chevy 350 swap full of headaches. A 1978 Malibu with a Targetmaster 350/ T-400 combination that ran for 10 years, then sat for 20 years. The new Holley 600cfm carb will idle like a champ, 20" vacuum, and wants to melt the tires at WOT. But if you set the idle at 650, which it likes, it runs smooth, but then once you put the trans in D- the engine idle drops 500rpm. Then the engine stalls. We found the sweet spot at 950 rpm idle. It runs 'OK', 18" vacuum, a bit rough, no timing coming in yet. At that idle speed, when you put it in D- it drops 500rpm, down to 450, but stays running, but rough. Driving around, it runs strong, and does not stall at stop signs. But that 950rpm idle is not right, and something is wrong.
I read that a short stroke engine like a Chevy 350 does not have the low end torque to handle a T-400. A big, long stroke engine would just shrug it off. It seems the low torque engine causes the torque converter stall speed to occur at a much lower rpm. I also wondered if the T-400 was out of a huge land yacht the would have had a 455 or so in front of it. But on RockAuto I researched 1970 Chevy converters and found that the difference in stall speed was only 1200/1400 for a 454 vs 1400/1600 for a 350. A peek under the car revealed a short tail housing, so not from a land yacht. The transmission oil is old, but has low miles. I suggested changing it just because.
The Holley site has a cfm application chart. It is the maximum cam rpm, times the cubic inches, divided by a number. A 5000rpm cam 350 engine would want a 540cfm carb. A 6000rpm 350 would want a 600cfm carb. So this carb would seem to be perfect, although it had a 750 on it originally.
We have searched for vacuum leaks, found none, and everything else checks out. No overheating, no backfiring, timing is good, and once rolling, the engine wants to roar. Barring engine problems, the only cure I can think of is a torque converter change to a higher stall speed, or a swap to a manual trans [my favorite].
My question is what in the world would cause that 500 rpm drop?
I read that a short stroke engine like a Chevy 350 does not have the low end torque to handle a T-400. A big, long stroke engine would just shrug it off. It seems the low torque engine causes the torque converter stall speed to occur at a much lower rpm. I also wondered if the T-400 was out of a huge land yacht the would have had a 455 or so in front of it. But on RockAuto I researched 1970 Chevy converters and found that the difference in stall speed was only 1200/1400 for a 454 vs 1400/1600 for a 350. A peek under the car revealed a short tail housing, so not from a land yacht. The transmission oil is old, but has low miles. I suggested changing it just because.
The Holley site has a cfm application chart. It is the maximum cam rpm, times the cubic inches, divided by a number. A 5000rpm cam 350 engine would want a 540cfm carb. A 6000rpm 350 would want a 600cfm carb. So this carb would seem to be perfect, although it had a 750 on it originally.
We have searched for vacuum leaks, found none, and everything else checks out. No overheating, no backfiring, timing is good, and once rolling, the engine wants to roar. Barring engine problems, the only cure I can think of is a torque converter change to a higher stall speed, or a swap to a manual trans [my favorite].
My question is what in the world would cause that 500 rpm drop?
