High speed stability issues

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66Valiant528

66Valiant528

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So my Valiant ran 131 mph and that was at 6800 through the stripe. I was limited by my 2 step. Tach was out at higher rpms. The car was squirrelly. It is a brick....with a Harwood scoop. I replaced my upper control arms to QA1 units for some stability, but grenaded my 8.75 on the next run. Ready to go back to the track but was wondering if anyone has any suggestions for high speed stability. Lower cover? Cowel style scoop? The nose is already dropped for the 3 inch pan to ground clearance. Skinny 26x4.5x15 front tires ,more pressure,, less pressure? At 35 psi Carb is up higher than the hood line. This is a stock bodied 66 Valiant. No spoilers. I'm thinking my scoop is lifting the front end but I'm not sure. Havent taken any videos of the car at the top end yet. I know this is a lot of dksjunct info but I was hoping someone with an early A body has dealt with these issues before. Any experience anybody?
 
7 inches tall...

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My opinion isnt worth much, but....
I would not think the aero scoop is much of a problem.
First thing i would look at is front end alignment, then a chin spoiler to keep as much air from going under the car as possible, and a deck spoiler for some rear downforce, rather than lift.
Maybe study land speed aero for stability tips.
131 is a bunch fast for an aero brick, without a tweak or two.
 
At 120+, even though it’s tall, I’d wonder if the scoop is even getting into the air considering the blunt front of the car is creating a big vacuum area at the front of the hood. That’s why a lot of vehicles have vents near the front half of the hood, to suck the air through the radiator and then out.
That’s why a Chevy cowl hood works so well, back by the windshield, what air isn’t going up over the car is getting pushed down to the base of the windshield.
I’m thinking 33IMP is in the right zone. Keep as much air out from under as you can.
 
I thank you for your input. The front end was given a drag alignment and it was there that I saw that the upper ball joints needed to be moved back. Hence the QA1 uppers. Re aligned and gained the 5 degrees advertised. Havent run it at speed yet as the rear end blew up. Super 60 now. Took a big break as I had to get the clutch,bell and seatbelts recertified over the winter. All back together now. Just trying to avoid the skating at the top end. After the stripe I drifted over the center line.....glad I was way infront of the other guy . Not good
 
It was bad. The jersey barriers were flying by and close . I do believe that the scoop is pling the hood up as I have developed a crack at the peak of my glass hood. I have picked up a steel factory hood. I think a cowel scoop makes sense. I asked Harwood about an open rear scoop. They told me I can drill holes in the back of the scoop to let air pass through. That's a cheaper option! Just not sure if it'll do anything. I may try to do some sort of pan underneath. It'll be tough to fabricate and mount. Hoe about lifting the rear of the hood or punching vents in the rear?
 
Not so much a pan, but an air dam.
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maybe a bit bigger, though that might be enough & at those speeds the force trying to rip it off go up exponentially
I am by no means an aero expert, but I did stay at a Holiday Inn once. Maybe some others will chime in.
By pushing air around the car, it also works the same way as what I was saying about up top, it creates a vacuum zone under the car, also sucking the air through the radiator & out of engine compartment, taking heat with it.
PS, someone else’s beautiful Dart-not mine
 
Five extra degrees of caster will help.
How big a rear tire? And how much tire pressure? Bias or radial? A soft bias slick can become plenty unstable on the big end.
Edit: our local carb guru/mopar lover has experimented with air stack-up relief holes. His nine second 470 street driven dart has a sixpack scoop, sealed to the carb. He says three 1" holes hole-sawed into the rear wall of the scoop made a substantial (?!?) difference in the metering requirements on his dominator.
(I think i might have just removed the carb pan and see what happened).
 
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RogerRamRod said:
Front of car is lifting, unloading suspension and altering toe?
Click to expand...
That's a strong possibility. I have a street car that sails along at 120 all day long. But I spent many hours on the alignment rack fixing the toe pattern, and what-not.

As the suspension cycles up and down, because the UCA and LCA are different lengths, the camber HAS to change. And when the camber changes, the toe also HAS to change. With unequal arms it cannot be otherwise.
Your job is to just minimize it and then set your ride height in the best possible place to stabilize it at speed.
Your Scrub radius is another thing that can be affecting your steering at speed.
And of course the back of your car can steer as well, if one side or the other is growing or shrinking in length. This leads to a change in Thrust-Angle, which has to be compensated for, by the steering wheel. The wheelbase between sides has to always remain fixed, else, by definition, the car is turning.
 
My carb is not sealed to the scoop. Dialed in on an engine dyno. I'm aware of the suspension travel issues. I haven't checked the strut rods for bind, but I'm "only" going straight. My car is aligned with the requisit 1 inch extra ride height. It's all stock except for the QA1 upper arms, and the drag race alignment. I really need to take a video of the car in a few places down the track to see what the front end is doing. Not sure if my car "skated" when I let off the throttle or coasted. I have the 11.75 drilled slotted Monoco rotors with the Volare large single piston calipers in the front and 4 piston 12 inch rear Wilwood set up. 2 brake masters, and a clutch... but I don't recall when the car got unstable. I gotta say it had to be when I let up off the throttle. Probably before I hit the brakes. But I do believe it was suspension related and aerodynamics. I think a 66 Valiant is not a vehicle that does 131 mph well. I need the car to be safe.
 
as far as the chassis is concerned, about the only thing that changed in the later As is that the track grew, and the wheelbase as well. I suppose the later Ks, if nothing else, look more robust, but in your case that does not apply.
 
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I just went through that, with a B body, the car hadn’t been on the road since 1994 and all the suspension was new, we put our extra engine from our Dart in it with a mild cam. The car had stability issues at the top end at 117, we put spc uppers and aligned it a couple times and tried it at the track. We finally settled on 5 degrees Caster 1/8 toe and 1/2 degree of camber and changed to a little stiffer front shocks and it didn’t hurt the hook up. We had Calvert 90/10 but they we very soft both ways, so I had a set of cheap competition engineering shocks and put those on, now its perfect, pretty scary before. Maybe time for double adjustables
 
Funny, nobody has asked what rear tire you run or what air pressure you run in said tires. Please elaborate.
 
I think you should see what the extra 5* of caster does before you get too involved.

And per post #13, what air pressure is in the rear tires? How wide are the rear wheels and tires? (Too narrow a rim for the tire?)
 
30 by 10 by 15 on 9.5 inch wide wheel Hoosier slicks ,inner tubes 12psi. I have only made 7 runs. Progressively faster. Started to get squirrelly at 124 when I took my foot out of the throttle. Double adjustable Viking shocks all around. Set at there starting recommendations. Havent run the car enough or hard enough to deal with those yet. Cal tracs with there split mono leafs.i am going to go progressively faster for safety sake. Maybe the extra caster will have solved the issue. Just apprehensive and hoping for some guidance. Any thoughts on adjustable strut rods? Worth the money?
 
Wait and make a pass .
You could replace a dozen more items for nothin, cuz you already cured it, - and you'd never know which item was the cure, and which was money wasted ! !
 
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Probably the best plan. Let's see if my QA1 upper arms and alignment did the trick. Rain rain and more rain here in Massachusetts. The remnants of Else....
 
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