I was looking through the back of Comp's catalog tonight (a dangerous thing to do ) and got to thinking about what kind of lobe to go with for an endurance/street BBC. It won't be a high RPM engine at all. Seven grand is a good max, if that high. Short seat duration is important also to keep overlap down.

Anyway, back to the point. Should I be looking at lobes with relatively low lift or slower ramps to get the longest srping life possible?

 

A Stock Eliminator camshaft has low lift but a long duration. The valves are slammed open so fast from the steep ramps that they're always changing springs.

A roller camshaft has high lift but from the roller design, even a long duration cam can have a ramp that's easy on valve springs.

Large diameter springs can easily offset the effects of fast ramps and high lift.

7000 rpm is high for a "street" BBC.

 

If you were referring to how short the "ramps" are during the first .050 of lobe lift, that really doesn't determine how much total lift you get for a given duration.

You really can't have it both ways. If you choose a duration (total or .050) and a lift to achieve your power goals, then that determines how aggressive the flanks of the lobes need to be. This is often expressed as lift per degree of cam rotation or "Cam Velocity". It's not engine rpm dependant. Now add engine rpm and valvetrain mass to that equation and you arrive at spring stress which has a lot to do with determining valve spring life. There's really no free lunch.

Sure you can have long spring life, but knocking off a thousand rpm and .100 of lift from what probably produces the good power numbers will get you back to near stock performance.

One way to get the longest spring life possible for your combination, is to choose valvetrain parts which are very compatible, lightweight, but stiff, and use the minimum spring loads which will control the valvetrain throughout the rpm range. This isn't always easy to do, nor are the parts cheap. Again no free lunch.

Another thing that should help is to limit your max rpm to something like 6500 rather than 7000. Of course a lot of this depends on your power goals and the other parts in the engine; better flowing heads need less cam and less rpm to achieve a certain power level, so valvetrain can be stressed less, and vice versa.

My $.02

 

Spring technology has advanced a lot, and the "rules" are therefore changing. But as always, there is no such thing as a free lunch. So, as Jon said, pushing the limits is going to cost. The advancements in springs have been in both materials and design. The "beehive" spring is an old concept that is experiencing a resurgence. The beehive design reduces spring mass without really compromisng other aspects of spring performance. This allows more rpm with less open pressure, which in turn promotes longevity of other valvetrain parts and allows lighter parts to bve used. Spring materials have improved the ability of a spring to resist fatigue. But there will always be tradeoffs.

How long is long enough spring life for your application? One season is enough for the Camaro (a street/strip car). That means ~5,000 miles and 30-50 runs at the track. But on my daily driver I don't ever expect to replace the springs.

Anyway, my "rule of thumb" is that assuming you don't float the valves, a max lift in the 0.600" range will give very acceptable spring life if premium springs are used. The duration (an hence velocity) is not really relevant to spring life as far as I know. So, choose the duration you need but limit max lift to 0.600". Use premium springs, don't over-rev it and I think you will be fine.

Rich Krause

 

The key here is BBC little bit different requirements from a SBC for what you are looking to do

Now 7,000rpm and a street BBC is not chump change, and if you are thinking hyd you better take down your RPM range. That's what it sounds like since the only BBC specific lobes are the hyd roller ones.

Now the other problem is the short duration (seat or .050) and low overlap are not going to work on a up to 7,000rpm BBC.

I would say take the RPM down to the low 6,000rpm range max since you want this to be a engine that works as a "endurance/street" "Short seat duration" and low overlap engine. The BBC likes less overlap anyways since it has canted valves but that's not all part of this right now.

To extend your spring life, slow ramps and less lift will get you the longest spring life.

As everyone said the problem with most BBC stuff is the valvetrain weight/mass. A standard 3/8 stem BBC valve can have a 25-30% higher mass valve than a typical SBC, so with that you need more spring or less weight to make it work. Also you will notice that the BBC lobes are very tame, not much lift for the duration and the ramps are pretty tame also, both of those things are to help with the BBC valvetrain mass issue. BBCs love solid rollers! Throw the spring pressure at it and spin it up!

Bret

 

what about if I throw turbo charging into the mix?

Yes I was leaning toward solid roller. I don't want to have to worry about lashing the valves and changing springs all the time. Clearance under the hood may be at a premium.

 

I use a solid cam and only check valve lash once a year. If they require adjusting more than that, you have problems elsewhere.

 

I realize that I just don't want to wear out springs too often.