If I upgrade the motor in my '79 to something aroung 350 - 400 hp, should I upgrade the rear leaf spring also? According to the Duntov site, the stock spring rating is 260, is the leaf spring rating based on weight only? If upgrade is necessary, is 315 OK? I don't want the ride to be too stiff. Thanks.

Maj D

 

ttt

 

I'm now in the process of making that very decision. Ironic, huh? I have a 74 convertible and am in the process of an LS2/4L60e engine swap, and know this is an area that will need addressed. With HP in the 450 range, I presume there will be a few more mods to the 3rd member and driveshaft as well. I'm closing a deal on a complete Steeroids rack and pinion kit, which should clear up the inherent issues with bumpsteer that C3's are so noted for. I'm also leaning towards converting the monoleaf rear to adjustable rear coil overs. When I say adjustable, I mean adjustable. One can even adjust spring travel, both high and low if so desired. Of course, one should be able to...at what the kit costs anyway. Maybe I'll get lucky and find one "for the money", so to speak. I'm curious for the experts to chime in on the suggested spring rate for your application.

 

Patro, thanks for the response. It seems to me that any sufficient increase in hp would require an upgrade to the leaf spring for two reasons. First, a stiffer spring will force the tires to the ground and give more traction. Second, if the accelerator is mashed to the floor, I wouldn't want the trailing arms banging against the bumpers. Unfortunately, a stiffer spring usually requires adjustments to the ride height, which is not always trivial.

I'm not too concerned about weight since I won't be racing and just want to wake my 'vette up, so I'll probably stick with steel springs because of their cost effectiveness. 400 hp is a nice, streetable vehicle, but not any great shakes in the hp scheme of things so no high tech solutions are needed.

My understanding is that with over 400 hp, you could be looking at mods to the differential as well. What is the "bumpsteer" issue you're talking about?

Dan

 

The bumpsteer issue is more or less pretty common with the old C3's. You'll undoubtedly end up hearing both sides of this issue I can assure you, as some swear by their factory steering. I've done no more than drive mine to the end of the driveway and back, took one look at the steering (mine has power assist), and decided it needed something ...better. Plus the cost of replacing leaking pistons, hoses and a possible steering box overhaul netting me a fresh steering ssytem that could be improved upon...bigtime. The factory steering can tend to drift a tad and when you hit a dip it can kind of toss you to the left or toss you to the right. I can remember riding in more than one that displayed this characteristic. Mind you, a loose front end or a loose rear rear end, bushing wise can cause the same. So can the little bearing at the end of your steering column. They also can tend to want to drift off centerline, leaving you with a less than confident feeling. Keep in mind, the technology then was rather primitive by today's standards but it CAN be improved upon. With the Steeroids rack and pinion I'm using, and others like it, the steering is far more precise than factory ever was, whether it was manual or power assist. Oh, and you simply can't get 2.7 turns side to side with factory steering. Puts that sport car handling feel back into the equation or should I say where it belongs.

 

Bumpsteer: The car wheels hit a bump....then the wheels "decide to steer the car for you".....usually to the left or right, anywhere from a few inches to a few feet! On a manual-steering-box car, a steering damper (horizontal shock absorber) can reduce greatly the bumpsteer effect.

 

You can look up the stock spring rate and a whole lot more on:

gmheritagecenter.com

I have explained to folks many times that going from the stock 7 leaf steel gymkhana sport rear spring (or steel base spring) to a composite mono spring is not exactly apples to apples in terms of ride smoothness, simply since the fiberglass composite has MUCH better ride (and handling characteristics) than a steel spring, the materials are very different. Coupled with a top quality shock, the composite of MUCH higher spring rating will actually ride MUCH better than a much lower rated steel spring. Another words, a steel sport spring rated at 260 lbs cannot necessarily be compared to a 360lbs mono spring in terms of ride comfort.

My 78 L-82 4 speed came from the factory with the gymkhana suspension. After reading about the 81/82's with the rear composite back in those years, I decided to replace my 7 leaf steel spring with a VBP 360 mono spring in 1986 when they were first introduced to the aftermarket. I have replaced/upgraded ever single item in my front and rear suspension since then, a little over time, over all those years. Trying to give you the complete picture here:

Front:

Speedirect spreaders bar
Poly upper and lower control arm bushings
OEM stock 1 1/8 inch front sway bar-poly mounting and end link bushings
550 front coils, 1 inch lower than stock
Bilstein Heavy Duty shocks with poly mounting bushings
255/45/17 ZR ultra high performance tires

Rear:

360 mono spring with poly cushions
Competition adjustable struts rods with heim joints-no bushings
3/4 OEM type rear sway bar with poly mounting bushings
Bilstein Sports-30% stiffer than front HD's
255/45.17 ZR tires

Comparing the ride/handling and steering (I will get to this in a moment) to the way the car came from the factory is futile-it simple has a MUCH superior ride to the stock suspension-no contest. The ride is firm BUT not harsh or jarring. It is shocking-no pun intended.

As for the steering, I have a custom blueprinted/rebuilt OEM steering box (EDITED) , and it is superb-zero play/slop now in the box! The problem issue with the OEM boxes from the factory and later overtime is that recirculating ball steering is inherently sloppy UNLESS setup up correctly AND some of the design sloppiness is addressed in a rebuild. EDITED

Hope that helps!

 

I am in the the same process. Just posted another thread on this topic too. jb78L comments are great. I guess I am wondering how a stiffer rated spring can ride better that a lower rated progressive spring. Not doubting it, just seems strange. Either way nice to hear real experience on the issue going back quite a few years

 

If your not racing, a 350-400 Hp engine alone does not dictate that you need a stiffer spring. If you drag raced and experienced wheel hop, then you might think about it. If your spring is in good shape, I would first swap the engine and then see if your spring is still capable of handling it. I have a ~400Hp (NET) engine and run my adjustable fiberglass spring on almost the softest setting (can't remember exact spring rate).

As for the bump steer issue with the front. If your system is still in good working condition and you don't want to go the drastic route of ripping the entire system out for a rack and pinion, you could just get a "anti bump steer" kit that adjusts the location of the outer tie rod joints for a bit over $100. However, these kits usually require that the car be lowered an inch to get maximum benefit.



http://www.vbandp.com/auto-parts.htm...category_id=47

 

jb78L and Jason, thanks for all the info. I've heard great things about the mono springs and haven't ruled them out completely. It seems like more folks like them than don't...at least those that have changed them out anyway. It's encouraging that Jason runs a low setting on his spring, it seems I can keep the stock spring for now without any detriments.

As for bump steer...how common is it? I haven't heard of it before until posting this thread, and wasn't even aware there was a correction. Is it common, or something only folks with high performance engines experience? I've had my 79 for nearly six years and cannot recall a situation that corresponds. thanks for the info.

Dan

 

Bump steer is inherent to the design of the front suspension. Once you remove it, it's a noticeable change. However I drove my Vette for years before making the modification.

The below graph illustrates the change. Bump steer makes these older cars more "nervous" when you go over bumps. It's the change in direction of the front wheels as they travel up and down. In essence the car is making steering changes all by it self. Removing it just makes the car more stable. The original suspension would change wheel direction by over 0.400" for 3.5" of suspension travel. After the modification, this was reduced to 0.075" (much closer to modern cars). The VIP reference is an article written on C3 Vette suspensions that I was using to make sure my measurements were fairly accurate.

One thing I forgot to mention. Depending on the kit, you may have to drill out the tapered holes in the steering knuckles. Or for additional cost you can pick them up with the blocks already installed from Duntov Motors.

http://www.duntovmotors.com/vintage-...s-steering.php

 

I put the bumpsteer blocks on my car many years ago. (Purchased from Dick Guldstrand) My car is still at a close to stock height, and the driving feel improvement is, IMHO, absolutely worth it.
I never tested it like the previous poster did, but I also notice when I jack up the car, and the front wheels hang free, everything looks straighter now.

 

http://www.ecklerscorvette.com/corve...1963-1982.html


I 've got a car similar to yours. Put a hyperco 250# spring in this winter. Seems OK so far.

While I was under there I installed a heavy duty rear cover. Cheap insurance.
http://www.dragvette.com/products.htm

As far as bumpsteer - My steering was twitchy too at highway speeds.
I installed the "Borgeson Power Steering Pressure Reducing kit #899001
for about $20. Good bang for the buck and easy install.