Coilover spring rate

[AccurateAuto]

Hi Everyone,
First time posting. Have a 79 Grand Am which is now stock, 301ci. Modifying for mostly drag racing not much street. Looking to do coil overs in front and rear. Double adjustable. In front will be using 70 pont. 400 block w/ alum heads and intake. Alum radiator, remove heat and a/c. battery goes to trunk. Going with 12 bolt or ford 9 " with upgraded rear arms and extra supports. What spring rates in front and rear should work well? Viking or QA1? I am thinking 400# in front and 150# in rear. ( i think pontiac block weight is some where in between a chevy small and big block). Any opinions?Thanks

 

[Mikes455Wagon]

I can't tell you for sure in the front. I am currently running an iron headed Pontiac with stock v6 springs in the front. On the rear, I have 150 lbs coil overs on afco coil over shocks mounted in the stock shock location. Keep in mind, I do have a wagon which may be a little heavier on the rear even though it is pretty much gutted.

 

[AllMotorMalibu]

A 150 spring is a little on the heavy side for the back. I typically use a 125 spring on cars say 3200 lbs and under. For comparison a stock 78-81 Malibu rear coil spring is 118 lb/in rate.

I've used as light as a 110 spring in my car at 3025 lbs, couldnt really see a noticable difference from the 125 spring.

I've never used the tapered coils up front like QA1 uses. I've ran true coil overs up front, Afco and Santhuff. With the Afco's I used either a 325 or 375 x 10" spring with good success, with the Santhuff a 250 x 12" spring. Granted an aluminum headed BBC, so nose weight on mine is slightly higher.

Be warned that some coil over setups in the front can lose travel compared to a conventional coil spring deal. Depending on how much power the car has a loss of travel could cause traction issues.

 

[AccurateAuto]

Thanks for the input on spring rate. I am using stock control arms up front. I have been told to use between 400 and 450lb springs up front even with the modifications I mentioned. Does afco make a direct bolt in front coil over?

 

[AllMotorMalibu]

Thanks for the input on spring rate. I am using stock control arms up front. I have been told to use between 400 and 450lb springs up front even with the modifications I mentioned. Does afco make a direct bolt in front coil over?

The stock control arms create packaging limitations in terms of shock length and stroke which affects travel. The Qa1 coil over uses a pretty short coil spring which forces you to use a higher rate spring to support the cars weight. When you do this you lose stored energy in the spring. I'm just not at all fond of that setup.

What makes you want to go to a coil over instead of keeping a conventional coil spring along with a better shock?

No, Afco does not make a coil over to fit in the stock control arms. A system can be converted but it's a fair amount of work to do so.

 

[AccurateAuto]

The car has around 40k original miles and sits well with stock coil in front but once I lighten it I think it will sit too high. I understand what you are explaining. Should I try stock spring, cut 1/2 lower coil off? Would QA1 coil overs work well in rear? I am using umi boxed lowers, adj uppers with roto joints. Thanks again

 

[AllMotorMalibu]

The car has around 40k original miles and sits well with stock coil in front but once I lighten it I think it will sit too high. I understand what you are explaining. Should I try stock spring, cut 1/2 lower coil off? Would QA1 coil overs work well in rear? I am using umi boxed lowers, adj uppers with roto joints. Thanks again

In the past I had good luck with the Moroso trick springs. Some like them, some don't. Santhuff has a line of stock replacement coil springs for the front that would be a real good option to look into.

Out back, given the parts you listed I would likely just put a good shock on there with a stock coil spring. A coil over isn't of much benefit unless you install them with a upper crossmember and adjustable lower mounts. That would give some ride height adjustability. My car ran 8.50's with shocks and springs in the stock location out back.

I ran QA1 double adjustables on all four corners for a while but later dumped them due to the valving not being all that great. For similar money look into the Strange shocks. Afco would be the next step up from Strange.

 

[AccurateAuto]

Thanks for the input. I thought using qa1 coilovers in front would give me some ride height adjustability and ease of shock adjustment. But not a fan of having all that pressure on lower shock mounting of original control arms. I will look into stock replacement. In rear will coil overs eliminate need for air bag on right side or because of the angle they aren't really adjustable? This will be my first car, about 700hp, th400 w/transbrake, 3.73 rear, 28 x10.5. Maybe in 10's eventually. Also whats your opinion: 12 bolt eaton posi or ford 9 w/ Detroit locker? 8 or 10pt cage?

 

[AllMotorMalibu]

Thanks for the input. I thought using qa1 coilovers in front would give me some ride height adjustability and ease of shock adjustment. But not a fan of having all that pressure on lower shock mounting of original control arms. I will look into stock replacement. In rear will coil overs eliminate need for air bag on right side or because of the angle they aren't really adjustable? This will be my first car, about 700hp, th400 w/transbrake, 3.73 rear, 28 x10.5. Maybe in 10's eventually. Also whats your opinion: 12 bolt eaton posi or ford 9 w/ Detroit locker? 8 or 10pt cage?

I never had much luck with air bags. Till you got enough air in them to make a difference, the car sat all kinds of crooked. A true anti-roll bar is the real answer to eliminate body roll. For a bolt-on deal the HRP Parts bar is a good choice. For a more race oriented piece the bar from TRZ Motorsports is about as good as it gets in kit form.

Either rear setup should work. The posi in the 12 bolt will be the weak link in the rear compared to a spool. Not only in terms of small parts failure inside the posi itself, but also the carrier is more prone to flex than a spool and can cause ring & pinion failure. Granted the cars I know running the posi still get pretty good life out of them, it's just the spool is a more long term bullet proof choice. 9" Ford is always a good choice as well, just avoid stock Ford center sections & pinion supports. They have a tendancy to break. There are many aftermarket center sections & pinion supports out there that are much stronger for not a whole lot more money.

Cage points doesn't really matter all that much as long as it's done to be NHRA legal and will certify. I do like to add the engine compartment bars as the front frame section tends to flex a fair amount without those bars. You can see this if you put the car up on jack stands with the stands under the body bushing area at the firewall behind the front wheels. Then jack up the front crossmember and watch how far the front bumper goes up before the frame comes off the jack stands. Again, these bars aren't required, but are a good idea to consider.

 

[AccurateAuto]

Thanks, lots to think about