car porpoising in high gear.......

Finally got a chance to run the car hard.........3000 lb. '65 Dart. AlterKtion front end with QA 1 single adjustables , Cal Tracs in back with their 9 way adjustables.
I got it into third and it was still pulling hard but the car started to porpoise and it was getting more pronounced as it continued to accelerate.
It was pulling pretty good.
I lifted at about 3/4 track and rolled through at a 12.24 @ 110 , with a very soft launch.
I have the rear shocks at 6 and the fronts are at 4.
I really should scale the car and figure out the bias / balance , but have not done this yet.
I suspect that the condition is originating in the rear of the car , but would appreciate any ideas as far as what to look for or where to begin making changes to try and correct this.
What are the common causes of this ?
Thanks in advance for any help.

What are the rear tires (bias or radial) and how much pressure are you running?

Double adjustable shocks are in your future.
I had the same problem, you need to tighten compression up.

Mickey Pro ET radials.......I had 26 lbs in there , thinking since I wasn't going for a 60 ' time , it would do.
I discussed double adjustables with the wife on the way home...........warm her up to the idea that I might need to drop another chunk on the car. LOL

I would video the car to see which end is acting up before spending any money.

You'd be surprised just what a couple of clicks tighter on the rear shocks will do. Had a friend with a Nova at the track just testing. Just took one click out of his rear shocks and his car when from being fine to looking like Flipper bouncing down the track put the 1 click bad in and no more porpoise.

Tighten the rear extension

Next up is a go pro and some under car video , but I will definitely try tightening up the shocks first.
I'm not afraid to spend money on the car , but I'd much rather not if I don't have to.
I need a proper test day now.
Here's what it looked like on play day.
More than one hit would have been great , but I needed that one under my belt just to get the ball rolling and put my biggest concerns to rest.

If all else fails it could be the front and rear spring rates are too close together. The spring rate numbers won't match but the force to move either end might be too close which can cause porpoising. A stiffer or softer spring on either end would be the easiest way to test for this.

You might give Calvert a call.

More time than not its the spring rates

How smooth was the surface? You might take it to an actual track and see how it does before you jump on changing too much. Although I do think double adjustable shocks are a good investment no matter what.

The surface was smooth as silk........asphalt runway , but looked recent.
It was a mile long , so I didn't even have to try the brakes , but it was smoother than some drag strips I have been on.
I am going to wait on calling Calvert..........plan A is to tighten up the rear shocks and give it a hit.
A friend who has had years of Cal Trac experience in both A & E bodies is going out with me next time , but I really appreciate the suggestions.
I figure there is more experience here on the board than in customer support , and if there is a consensus among posters , then I trust that information to a very high degree.
Spring rates will be the next deal if I can't sort it out with a couple clicks.
I like the idea of having a little more stored energy with a lighter spring in the front , and I don't know how I would go about changing the spring rate in the rear with the split mono leaf in there.
If I am still having problems , well...........our season is winding down up here.
But I will be ready for the opening bell next spring.

Have you check to see if your topping( or bottoming out) the shocks on both ends at max suspension travel?
I had one incident of porpoising on my old pump gas Duster, it started doing it on the starting line and got worst as it went down the track, I finally had to lift to get it to stop
Your car has plenty of front to rear rake so the faster it goes the more the wind should push down on the car, maybe not
A close up vid from the front would help also

You can try to stiffen the shocks all the way in the rear,then you can determine who's shock to get!! I bought vikings all the way around.

Your results may vary...............

Adjustable Drag shocks are for launch control not ride control...fix the problem don't mask it at the expense of other functions

I have seen video of this happening with cars that reach the end of the travel with spring sliders.........I am not in that position I don't think.
I am also fairly certain that I am not bottoming the shocks on either end of the travel.
But I will get a Go Pro under there to be sure just what is happening.
I have looked the video over and it seems to be originating in the back of the car.
And there isn't any real sign of it till 1/2 track or so.
The wheelie bars are 3 " off the deck static , and the Cal Tracs are set to just touching , so I don't think there could be enough axle wind up to put them in contact with the ground.
But someone suggested that this might be a cause.
Video will give me a lot more information than seat of the pants , but I have never felt that before and the way it seemed to be escalating was not very comforting.

Three inches is very little room for wheelie bars. I would start at about six to eight depending on bar length and how much the pinion angle will change on the hit.