G'day All, I am new to the scene and am undertaking my first SBC build. I have put together what I hope will become a solid engine for my boat over here in Australia. We started with an early 80's 4 bolt 290hp 350, which has been stripped, bored and machined 30 over for a 10.5:1 comp, which is being built now with the existing GM crank, scat I beam rods, speed pro hyper flat top pistons, vortec heads, roller rockers, an edelbrock air gap, holley 600 and a lunati voodoo 262 cam. Aim is to run it at 5500-5800 for 30 mins at a time under load. Its the cam that is my problem, and I'd really like some help to understand some of the specs associated with it. Issue I have is that Lunati recommend a strangely stiff spring (73943) for the voodoo 262 with the following specs: Seat 98lb @ 1.75" Open 330lb Spring Rate 462lb/in The springs I wanted to run are Isky (165-a) which have the following specs: Seat 130lb @ 1.8" Open 320lb Spring Rate 310lb/in. Why on earth is the lunati recommended spring so gentle on the seat, but so darn stiff everywhere else? I am worried about wiping the cam with such a high spring rate, and want to run the isky spring, am I wrong? As always, any advice/thoughts are greatly appreciated! Ash.
Couple thoughts. Use light springs for the break in then swap to what you want to run.Speedway motors sells 1.3 ratio rockers to break in Chevy cams.After break in switch to 1.5,or 1.6. The 1.3 lowers spring pressure on the cam. Its all about giving the cam a fighting chance. Run oil with Zinc.Cross fingers......
Do yourself a favor , ditch the FT cam , put a roller in it , stop waiting for it to self destruct ...... Yeah ,its gotten that bad .. 100-300 is about average for a FT cam
Definitely take some steps to use weaker valve springs for break in. Yes its more trouble, but not nearly as much as wiping a cam. I'd look for some used stock springs and stock rocker arms if they have enough travel not to coil bind. That way you keep the pressure on the lobe down some. Be sure you check valve to piston clearance before final assembly. Need to check it 10 degrees before TDC and 10 degrees after TDC.
Cheers all. Catch here is that I don't want to run a roller cam in this engine, I already had the cam sitting on the shelf, so would like to run it, especially given the reviews on the voodoo stuff I have read being pretty darn good. Its the spring rate that has caught me out. 462lb/in is a roller spring rate (in my understanding), and I am just not feeling entirely comfortable running such a stiff spring, which is why I thought of the Isky 165a or even a Comp 26981 spring which have slightly greater seat pressures but are a far more forgiving spring rate, 310lb/in for the Isky and 347lb/in for the Comp spring.
No one can tell you for sure that they will be better or not. I don't understand the logic that you really like the cam but don't trust the manufacturers recommendation for what springs to use with it. Seat pressure isn't what wipes a cam out and they are both close enough on open pressure that it shouldn't matter. You have to be sure about length and the ability not to coil bind as well as the pressure.
Exactly right. It isn’t the seat pressure that wipes a cam. Its the spring rate in this case. Has anyone run this lunati 73943 spring?
Just a thought: maybe Lunati is trying to prevent spring harmonics with the spring rate they recommend. That higher spring rate spring would also have a higher natural frequency, to help prevent uncontrolled spring surge, which is what destroys springs. You are intending to run that engine pretty hard after all turning about 5500 rpm for extended periods of time. The only type of oval track racing that turns essentially constant rpm, is superspeedway racing like at Daytona or Talladega in NASCAR, or Indy for champcars. You also will need to pay attention to oiling, as you might find that unless you have a dry sump system, all the oil ends up in the rocker boxes, causing loss of oil pressure. The flip side is that at your operating conditions, lots of oil upstairs will help cool the valvesprings, which is another problem with spring life.
When we read about "valve float" we think of a valve lofting over the high side of the lobe. What happens though too, what also robs power, is valve bounce, it is very important for the valve spring to minimize this too. A valve closing might bounce some but only once or twice, instead of 5 or 6 times. I bet in part that's why they specify such a heavy duty spring, because of the combination of continuous load & fairly high RPM.
Rate should control the slope of the cam ramp, total pressure on top of the lobe would be what I would be concerned of. I’ve seen several flat cams, but none that just ate out the side of the lobe(s). But it’s your engine, seems you have your mind made up that what the manufacturer says, has to be incorrect. Also seems you are beleaguering your point and not listening to a differing opinion. If I told you yes, run the other springs, would you then be satisfied?
Richard holdener has YouTube videos that says most of the springs that are supplied on a cams are too soft and they have valve float on the dyno Sent from my SM-J737T using The H.A.M.B. mobile app
Dave g and Truck64, Thank you for those insights. I hadn't considered harmonics, frequency, surge or the effect on valve control and spring longevity. You mention oiling too, which is something I have covered as best I can with the design of my oiling system that must stay as a wet sump to comply with my class rules (which prohibit dry sumps). Enhanced oil drains from the heads to the block, trap doored sump and windage tray is the best I can do and still say legal. I will speak with my machinist about the springs and go from there. I definitely have plenty to think on this afternoon. Again, thank you.
G'Day Budget36, I hadn't made my mind up either way. If I had, I wouldn't have asked the question. I am most definitely listening to differing opinions and experience that a few people have since posted. I am also most grateful for what everyone is sharing so far.
What are you doing with an engine that you need to run a constant 5500 for 30 minutes? Just wondering.
Specs from Lunati's page; (Int/Exh): 262/268; Duration @ .050 (Int/Exh): 219/227; Gross Valve Lift (Int/Exh): .468/.489; LSA/ICL: 112/108; Looks to be an aggressive lift rate for a flat tappet.
A few people have asked, and the answer is point to point powerboat racing. You get up on the plane and stay there until the race is over. One in particular here in Australia is 112 kilometres (70 miles) but I am only contesting less than half that distance at a time. The classes I am eligible for are capped at 60 and 70mph respectively, which on water, isn't exactly hanging around but can still be done in a quick social ski boat. That said though, the fastest boats have no issue whatsoever running the 70 mile course at more than 130mph (hydroplanes mostly at that speed)
Those spring rates were as shown on the respective company websites. I hadn't calculated them myself.
G'day All, I have decided to run the springs as recommended by Lunati. I could find no compelling reason other than my own concern about the spring rate. The way I see it is like this, if Harold Brookshire and the team at Lunati went to all the trouble of designing the cam the way they did based on their qualification and experience, and then specifying matched components to suit, I am in no position to argue against that expertise. If it fails, well.... that will be an altogether different situation next time around. The engine will be on the superflow dyno for break in and tuning and I will report back with the results.
Update. When push came to shove, the advice that I got from pretty much everywhere was cut away the FT cam and lifters, eliminate the risk and run a roller cam. So I did. I ended up with a Lunati Voodoo 20120711 270/276adv 219/227@50 .515/530 (which will end up as .549/.565 with 1.6rr's). Very similar specs to the Voodoo FT I have, just with a heap more area under the curve which should produce more power all around, but with a similar peak number at a very similar RPM. Went with Morel 5372 hyd roller lifters to match. Will still be a little while until I get to light the fire, but I appreciate the advice so far.
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