i know it might be a stupid question.... but this is the only way ill learn... how do u know wat lobe seperation of cam to use... in the car??? my is a 95 lt1.... thanx

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Valvetrain geometry can get very complex. Short answer: depends on how much power you want, and where you want to make it.

 

It's a fairly complex topic. LSA per se is not what is really the important variable - it's overlap. Overlap (OL) is when the intake and exhaust valves are both open at the same time. Overlap is needed to make power - it harnesses the inertia of the exhaust gasses exiting the cylinder to pull the intake charge into the combustion chamber. This increases power in two ways. Primarily, it increases the mass of the intake charge, IOW increases volumetric efficiency (VE). It is one of the contributing factors to allowing VE over 100%, which is counterintuitive until you take exhaust and intake tuning into account. Overlap also provides a cleaner intake charge, less contaminated by exhaust gasses.

The relationship between OL and LSA is simple. For a given duration, tighter (lower) LSA = more overlap and vice versa. So, why not just tighten up the LSA, thereby increasing the OL, and make more hp? Well, as you can guess, you can have to much overlap. If the LSA is selected to give the amount of OL which is optimal for peak power, you will have a very "racy" cam. Poor idle, bucking and surging at low engine speeds, high emissions, poor mileage, etc. So, all street cams have wider LSA than the optimum for peak power. Too wide and the power falls off. The range for a streetable LT1 is going to be ~110-117. I was going to say 110-116, but factory stock cams go up 117 so I went for an odd number. The factory cams have very little OL, which hurts power but helps drivability, emissions, economy, etc.

A factory LT1 stock cam has a duration of 205/207 @ 0.050" with a 117LSA. This gives very little overlap. In fact, at 50 there NO overlap - calculated in the usual way, using the specs a 50, there is 28 degrees of "negative overlap". By comparison, the LT4 cam is 203/210 on a 115LSA, which gives 5 degrees more overlap (-23). A "big" street cam, the CC306, is 230/244 on a 112LSA. The OL at 50 is +12 degrees - 40 degrees more than the stock LT1 cam.

How much OL is correct? Most people figure OL on advertised duration but I like to think of it at 0.050". Look for 10-40 degrees of OL at 50 for street/strip SBC cams. The bigger the motor, the more OL it can tolerate and remain drivable. Figure OL by averaging the intake and exhaust duration. Then multiply the LSA by two and subtract this number from the averaged intake and exhaust duration.

Example using CC306 numbers: 230+244/2 = 236.
112*2 = 224
236-224 = 12 degrees of OL at 0.050" lift

By way of comparison, the cam I have in my BBC race car is 272-280 @ 50 with a 110LSA. So, the OL at 50 is 56 degrees. You could use it in a hot street/strip setup. This is largely possible due to the addtional 120ci of displacemnt and the high compression.

Most discussions of OL use advertised duration - so be sure what you are looking at or it will get very confusing. The same equation will calculate OL at any chosen duration.

Rich

 

That's good info Rkrause. Helps me out too.

 

BTW: besides usage and displacement, as alluded to above, OL is also a function of the size of the intake valve relative to displacment. David Vizard elucidates this very clearly in his technical writing. The formula he has derived recommends LSA narrower than many other people recommend, but it is a good starting place. For one thing, it takes no account whatsoever of drivability, which you need even in a race car, especially if it isn't towed to the starting line and back to the pits. But the more restricted the valve size relative to displacement, it is clear that the more OL is needed (and the narrower the LSA). That's why large displacement BBC need more OL than SBC's - even beyond what you would expect with just the larger displacement. Also, cam specs for 4V engines look "mild" compared to 2V OHV motors. The large valve area relative to displacement is the main determining factor.

Take a look at Comps catalog. The way it is organized and presented makes some of these relationships clear if you study the duration and LSA numbers along with the rev range, etc. Most of their street and street/strip grinds have ~2-3 degrees of LSA beyond what is needed for peak hp in order (I assume) to preserve drivability. When you look at their race grinds, they are closer to optimal, though there too they are a little on the generous side with LSA. Compare Comps grinds to Crower, who tend to use a little narrower LSA for comparable cams.

When I spec out a cam, I will only approach the optimal LSA when I know the user has access to competent tuning and that they understand that you can't get something for nothing. Comps approach, erring on the side of a little too wide, makes sense for most users. Of course, there are a myriad of other things to consider when figuring out what will work best for a given driver in a given situation.

Rich

 

Great read Rich! Although I have a very general understanding of this stuff
it's great to have it in print and in more detail just to refresh the brain matter.

With this discussion on this board we are all assuming roller cams and take
their advantages for granted as compared to the older non-roller days. In particular
I like to compare my 1968 RA IV cammed GTO to the 1997 LT1. It's amazing
that the two cars have similar rwhp, yet one has a 230/240 cam and the other
has a 210/224. I believe both are at 112 too. It's really amazing what can be
done with fuel injection, computers and roller cams compared the past.

 

thanks for clearing that up rich, its something i always wanted to know

although my cam is slightly out of your normal lsa, it still drives pretty good

 

Couldn't agree more with changes in cam tech. This is an extreme example of what a valve train change can do. It's my BBC race car. Originally with a small, off the shelf falt tappet cam and then tested with the solid roller I specced out.



Rich

 

thanx a lot man.... great info....


smiley