Lockup converter or Overdrive trans/adapter

I have a '64 Wildcat running a TH400 behind a 425 Nailhead and the problem is this. Going down the highway at 60mph puts the engine at 24-2500rpm, right in that harmonic zone where I can find every loose bolt in the car. I am looking for ways (other than slowing down) to drop my highway rpm a few hundred to maybe 2000-2200. I haven't been able to find any info if I can get a lockup torque converter for a 400 trans and my other option appears to be swapping out the trans to something with an overdrive or installing an aftermarket overdrive unit which will also mean swapping out rearend gears to keep it off of idle at speed. All comments and suggestions are very welcome.

ps. this is a stock setup, no changes except for electronic ignition

 

Instead, are you interested in help while troubleshooting the source of the vibration and fixing that, or do you have your mind made up regarding the upgrade? If worn or out of balance parts are the culprit, it would still be best to take care of that problem anyway before it gets to the point that parts are breaking whether you do overdrive/lockup/etc.

Devon

 

It's not really a vibration problem. The engine runs smooth and the driveline has been straightened and balanced. It is just that rpm where everything sort of 'hums'. So dropping the highway rpm is the ideal way to go, I think.

 

And worn parts aren't the issue. The engine, trans, rearend has been rebuilt to factory spec. with less than 10,000 miles on all. The car has only been back on the road since 2008. And only on sunny Sundays. I was thinking of trying a fluid dampener, though. Had heard some amazing stories in regards to them.

 

I still think something else is up. That car should be smooth as glass at any speed. What was done with the driveline (driveshaft), if anything? Is it a simple single shaft with two u-joints, or is there a double cardan (constant velocity joint) in the driveline in one or more places? Is it a two piece driveline? What method was used to balance the wheels/tires?

Sorry I'm not more familiar with the '64 so thus all the questions.

Devon

 

Perhaps it is just an exhaust resonance at that rpm, maybe different mufflers would help. Do you have a switch pitch turbo 400?

 

Driveshaft is not the problem, the engine hits that harmonic sitting in the driveway in park/neutral and revving to 2500. No to the switch-pitch. I am looking to put headers on this winter and that may help. Still wondering if I could get a lockup torque for the 400 trans?

 

Change your axle ratio to 2.41:1, and rpm should be 2000 rpm.
That is what I use, switch pitch gives me back some performance.

Otherwise a gear vender, etc overdrive. I don't think there has
been any lockup TH400, though the 4 speed 4L80E later version
has lock up. Bruce Roe

 

I hate to keep bringing it up, but that's a sign of some kind of engine imbalance, maybe a flexplate that's lost its counterweight. Wouldn't hurt to check that too.

Devon

 

Devon, I am looking to go to a fluid balancer when I can find one for the 425 Nailhead (if you know where I can get one, let me know). The resonance just brought my attention to the rpm at highway speed and I would still like to drop it down to the 2000-2200rpm range. Any help there?

 

Ok, found the answer in the forums (just took a little while to get to them) and answered my own question. Still interested in a fluid balancer if anyone can help.

 

So what was the answer?? Don't just leave us hanging.

 

Well, no lockup clutch for my th400 so that leave either swapping out the trans to an overdrive and changing the rearend, installing an external overdrive and changing out the rearend or changing out the rearend. None of which are too appealing at the moment. It'll have to wait until after the holidays (2015?). For now I'll live with it, there are more important things to work on on the car. (eg I've been putting off doing the wiring for awhile)

 

Around here, if you drive 60 on the highway, you will get run over by the traffic behind you. Try speeding up to 70-75! Should get you above the "harmonic zone", and it's lots more fun. Simple fix is always the best!

 

:laugh: Unlucky for me, the sheriffs around here watch the roads pretty close and speed limit is 60 on the two-lane. Nearest four-lane is an hour away.

 

What you wrote stuck with me and I went out and checked the engine last night. I haven't checked the timing since I got it running right and, would you believe it, the balancer is about 12-14 degrees out. I haven't had a chance to check the flexplate yet and may not get to it for awhile but I will definitely look into that one as well.