BBC valve spring help, please?
 

Hello all,
I'm trying to finish my BBC swap on my '68 GMC. It's got a '69 427 of of a '69 Camaro.* It's got a new forged 3.76" stroke crank and has been bored to 4.280; +0.030".* It has forged rods and pistons, 10:1 compression with 291 heads, 2.19"/1.88" valves, 1.75" primary Heddman headers, Edelbrock RPM Air-Gap intake, Quick Fuel Q-Series 850cfm carb, MSD Pro-Billet distributor & MSD Digital-6 Plus ignition box.* I'm running a Comp Cams XE-274 hydraulic flat tappet cam.

Comp reccomends 624 or 630 valve springs, though I've heard of lots of issues @ or above 5000 RPM with the 624 springs.* I've also read about guys using Isky 8005-A valve springs and being very happy with their performance.*

I've got a set of Edelbrock #5745 valve springs that came in a set of Victor Jr. 24° heads.* I'm wondering if they'll be a good fit for my application.* If they wont, I'd probably lean towards running the Isky springs.

Here's what I've got on ea h set of springs:

Comp 924:
seat pressure- 112 lbs. @ 1.88"
Open pressure - 355 lbs. @ 1.200"
Installed height - 1.88"
OD- 1.509"
Coil bind - 1.175"
Open height - 1.28"
Spring rate - 347#

Comp 930:
seat pressure- 153 lbs. @ 1.900"
Open pressure - 383 lbs. @ 1.250"
Installed height - 1.9"
OD- 1.550"
Coil bind - 1.160"
Open height - 1.25"
Spring rate - about 354#

Isky 8005:
seat pressure - 135 @ 1.875"
Open pressure - 395 @ 1.225"
installed height - 1.875"
OD - 1.530"
Coil bind - 1.160"
Open height - 1.225"
Spring rate - 400 lbs./in.

Edelbrock #5745:
Seat pressure: 125 @ 1.975"
Open pressure: 330 lbs @ 1.425”
Installed height: 1.975"
OD - 1.550"
Coil bind - 1.150"
Open height -
Spring rate - 372lbs./in (1/0.550"=1.8181 x 205 = 372))
*
The springs look fairly comparable to me. The Edelbrock springs have a taller installed height, by about 1/10th of an inch, but lower seat pressure. So, by my math, these springs installed at 1.875" would be approx 125 lbs + 10%, or 37.2 lbs, which would equal 162lbs. But, I read a spec somewhere about these springs having a seat pressure of 135 lbs, but didn't specify the in taller height.

Any help would be greatly appreciated.

Nik

Re: BBC valve spring help, please?
 

FWIW, I'd run what Comp recommends. The 5745 springs have a much lighter opening pressure than either of the Comp springs. (the 5745 is measuring open pressure @ 1.425 vs 1.200, back the 5745 down to 1.200 and t has less opening pressure) This matters because you don't need the springs to open the valves, you need the close the valves (I know, obvious). You need the valve to close as fast as possible to prevent the piston from striking the valve as it's closing.

Re: BBC valve spring help, please?
 

Well, but the numbers between brands are kinda comparing apples to oranges, if I'm not mistaken. The springs have comparable (actually higher) rates. The Edelbrock springs are figuring open pressures at 0.550" lift. The Comp springs are figuring on 0.680" & 0.650" valve lift, respectively. The Edelbrock springs figures are more realistic. Plus, the Comp springs would be 302.85 lbs & 347.7, respectively, @ 0.550" lift, if their listed rates areaccurate.

Plus, the Edelbrock spring would be installed on my heads at 0.10" lower than figured, if I used my retainers.
So (again, if accurate), if installed at 1.875", at 0.550" lift, their open height would be 1.325". So, at their spring rate (372lbs/inch), the difference in open height (0.10") would add 37.2lbs. to their open height, making their open pressure 367.2 lbs. @ 1.325"

So, the Comp 924 springs would have 303 lbs. Open pressure @ 0.550" lift. The Comp 930 springs would have 348 lbs. @ 0.550" lift. The Edelbrock 5745 springs would have 367 lbs @ 0.550".

So, if anything, I would worry about the Edelbrock springs over springing, not being too light! Now, I've figured all of this up quickly while working, so I will have to crunch numbers again, but think that these are pretty accurate.

Re: BBC valve spring help, please?
 

Also, when you say back the 5745 down to 1.2, I'm confused. Sealed height is 1.875", so going all the way to 1.2" would be compressing the springs more, which would require a cam & lifter combo that would move the valve 0.675", or 0.775" if installed at 1.975".

My cam lobes lift 0.325" & 0.327". With 1.7:1 rocker arms, this would net 0.552" & 0.555" of valve lift. Again, I'm new to all of this, but I believe that I understand the math behind it. I just want to figure out if the Edelbrock springs will work as well as the 930 or Isky 8005 springs. I'll figure up the open pressure of the Isky springs @ 0.550" of lift at the installed height, for comparison. These are the springs that I've heard of guys running with much more success. I've read about this cam not revving above 5000 RPM, as it starts to float the valves due to light spring pressures.

Re: BBC valve spring help, please?
 

Quickly figuring the numbers for the Isky 8005 springs, I believe that they would be right at about 355 lbs. Open pressure @ 0.550" lift from installed height (1.875-0.55=1.325"). So, 355 lbs. @ 1.325", if I am figuring all of this correctly.

Re: BBC valve spring help, please?
 

So, I was figuring on buying some lighter tool steel or titanium valve retainers. Perhaps I should look for some retainers that would allow the Edelbrock springs be installed a little closer to their 1.975" installed height, which would bring their open pressures to exactly what works well with that Comp Cam, like the 8005 Isky springs. For starters, I need to pull some springs and measure exactly where my installed height is now with my current retainers. I've yet to measure that yet! I'm havin a valve job done and just dropped the heads off at the machine shop yesterday. I'll swing by there and see if the machinist would mind me borrowing my head to measure.

Re: BBC valve spring help, please?
 

I understand - you are correct, I am backwards on my math! I read the info too quickly without enough coffee. ( or I'm just stupid!)

Re: BBC valve spring help, please?
 

It's all good, I had to double check my thinking to make sure that it wasn't me that was wrong!

Re: BBC valve spring help, please?
 

I think you may be over thinking what your reading? I have a set of 291 heads on a 69 396 with a Comp 268H cam. That cam was installed with the springs, retainers and locks, roller rockers, and push rods Competition Cams recommended for those heads and cam combination. I dont have my spec sheets in front of me but I can easily and often do spin that motor up to 6000 rpm with no issues. I've had it up to 6500 a few times with still no issues but keep my rev limiter set at 6000. The way I have it figured with my 268H cam is, if I cant get it done by the time she spins up to 6000 rpm then theres no need to push her any higher. Those heads hit a flow wall somewhere after 6K and the springs will start floating shortly there after..."just sayin"

Re: BBC valve spring help, please?
 

Sounds like your combo works well. I'm not over thinking it, ive read many, many threads about guys having the XE274 cam fall on its face at 5,500 RPM with the Comp 924 springs that they reccomend. Many of these guys switched to the Isky 8005 springs and rev past 6,500 RPM. My cousin bought the Edelbrock Victor Jr. 24° heads and needed different springs for his solid roller camshaft. He gave me these springs and I'm just trying to see if they will work? I'm not trying to be too cheap. The price of any of these springs is a relative drop in the bucket of what I've got into this engine build. I just don't want to buy springs that are nearly identical to these for bo reason, if I can run these with success. I've got my heads at the machine swap having some work done on them. I've left the Edelbrock spring with my machinist and he's gonna measure and check the springs to see if they're gonna work for my application. Thanks for your guys' input!

Nik