I need a little help here. I have chosen a cam for my TA and I figured the overlap on this specific one to be -7*. I understand that the overlap is when both valves are open but what does this mean to me. What does a negative overlap accomplish? What about a positive one? How will a cam that has 10* act compaired to one that has 50*?

Basically in all my searching all I have been able to come up with is that with a negative overlap you'll have a more emissions friendly cam. What a coworker and I have speculated is that the overlap of my cam will be "torquier" than if I went with something more agressive.

I don't know if I am just looking in the wrong places or what but I can't find any real answers one way or the other. BTW I'm sorry if this is in the wrong forum.

 

Overlap is usally figured on advertised duration. I assume that to arrive at a negative number, you were using duration at 0.050". This makes a certain amount of sense, but you wil have trouble interpreting the number resulting from calculating at 50 since that is not how most people define it.

Anyway, it's hard to talk about any cam spec in isolation. If you want to discuss the cam you choose, why not post the specs? In general, a SBC is going to want 40-80 degrees of overlap for street performance. Take the Comp 501 grind, a very mild performance cam. It has 42.5 degrees of overlap. The CC 306, a big HR cam for an LT1 has 74.5. They have the same 112 degree LSA, the overlap comes from more duration. That points to why you can't talk about overlap in isolation - it usually comes from changes in duration. These cams will have quite different valve events and ultimately the timing of the valve events is what determines how the cam will behave in a given motor.

Rich

 

Just to elaborate a little on the previous post. Besides the valve events, the lobe profile is a major factor. The valve events do allow comparison of cams with respect to when the valve leaves and returns to the seat. The lift of two cams can also be directly compared. But what happens in between, the valve motion in between the seat and max lift, is much harder to compare. You can get a hint by comparing the advetised duration, duration at 50 and duration at 0.200". This will give you an idea of the "intensity" of the lobe - how quickly the valve is accelerated.

If you actually have the cams in front of you and are comparing two grinds you can see marked differences in the shape of the lobes. The current trend is for high valve acceleration (steep flanks on the lobes, very squared off over the nose), especially for the intake valve. Roller cams v. flat tappet are a little slower off the seat but can be ground to accelerate the valve much more rapidly up the flank and over the nose. This can produce astounding improvements in power. So much so that I wouldn't believe it if I hadn't seen it with my own eyes. I have seen a roller cam swapped in for a flat tappet where the seat timing was almost identical and the valve lift just ~0.050" greater on the roller produce a gain of more than 100hp. Don't scoff, I have seen it (a couple of times). Obviously, that's an extreme example. But gains of 30-50hp are common when the swap is between two roller cams that spec out close to the same but are optimized in terms of the lobes used and so on.

Of course, TINSTAAFL. The 100hp example required a new roller cam, a set of $500 lifters, $200 pushrods, $400 springs, $200 Ti retainers, and a $900 set of custom pistons to correct a piston to valve clearance problem. Add in a new timing set and the cost of the parts for the upgrade approaches $3,000. However, that's an extreme example, both in terms of the gains and the costs.

I have spent a lot of time over the last few years learning about cams and I have only scratched the surface. Most people should use an off the shelf cam or pay someone to spec out a custom (which often amounts to simply picking the appropriate off the shelf grind and is still well worth it). It pains me to read some of what is posted about cams and most of it should be ignored. Do not go into a restaurant and tell the chef what ingredients to use in his sauce either.

Rich

 

Damn, my posts are red meat if I do say so myself

Rich

 

OK, here's the specs. It's a small cam but I had a lot of "restrictons" placed on me when I went shopping for one.

gross lift: .557/.562

duration @.006 tappet lift: 265/271

valve timing @.050 Open Closed
INT 5- BTDC 37 ABDC
EXH 48 BBDC 10- ATDC

111.0 Intake centerline

Duration @ .050 212/218
lobe lift: .3280 .3310
LSA: 115

 

 

Well, you already have the cam. The overlap using advertised duration is on the skimpy side at 38 degrees. This should be a torquer, not a revver. I think it will have great "manners" and low emissions. If it pulls past 5,600 I would be surprised. On a tighter LSA this cam would work better. The overlap on a 112 LSA would be in the 44 degree range and would be 48 degrees on a 110. It would be much happier over 5,000rpm.

Rich

 

That's what I was after really. I was looking for a fun street cam that would still make a little power. The wife drives this car too so I wasn't wanting to slap a stall in to accomodate for a bigger cam quite yet. That and I can see a couple hard launches on the stock 10 bolt and I'd be looking for a replacement. But if I can go from my 13.0 et to a ~12.6 w/ some minor work done to stock heads I'd be happy. It doesn't have to be stupid fast the only drag racing around here is bracket unless you've got a load of money to dump.

I would have went with a tighter LSA but I don't really want to get much chop in the idle. I'd prefer a nice stockish idle. I should be able to hit my goal with this cam shouldn't I? It's not that much of a dog is it?

 

Is there a cam overlap "calculator" out there? I can draw it out on paper, but thats the only way I can "see" the overlap in my head. I was also searching a few sites, and they listed cams AND their overlap specs.

I want to pick a good cam for my boosted 327 with minimal overlap ( to help keep the boost in until its ready to come out after combustion ). ---

 

It is extremely easy to do in your head. (intake duration/2 + exhaust duration/2)-(LSA*2) = overlap

Example A: a big cam is 312/320 seat duration on a 110LSA
156+160 = 316
110*2 = 220
316 - 220 = 96 degrees of overlap

Another example B: you are looking at a small cam and want the overlap at 0.050", where it is 215/225 on a 114LSA
107.5+112.5 = 220
114*2 = 226
220-228 = -8
This cam has no overlap at 0.050" because the number is negative.

The cam in example A is 270/280 at 0.050", so if you want to figure OL at 50, it would be 55 degrees.

What helps more than anything in tryong to understand cam are the actual valve events. It's fairly easy to figure out, but there are a number of online calculators that make it easy. Overlap per se has some importance, but you can change the LSA and duration so that overlap remains the same but the valve events differ markedly and the cam will work very differently. The simiplest was to get this concept is to think about the effect of advancing or reatrding the cam. The OL stays exactly the same but the valve timing differs.

Rich

 

^^^^^^ Ive wanted to know that for soooo long and couldnt find it anywhere....You Da May'n RICK!

 

TY,TY!

Here's an example of a very large cam, a typical "mountain motor" cam.
322/360 292/320 118°
seat OL = 105°
0.050" OL = 40°

Cam A above is the cam in my BBC bracket motor, which is a mild cam in a 468. That cam has 96° of OL. The point is that no matter how big the motor, 100-110 degrees is the most practical OL in an OHV motor. Even a blower car needs some overlap to help push out the exhaust gasses and get a full, clean intake charge. I think some of the blower cams I "designed" early in my experimentation had too little OL. It doesn't take much though, with the intake under positive pressure at all times. Cars with big Roots blowers (blown Alky cars) have a fair amount of OL - they move so much air that it's worth losiing out the exhaust to cool the chamber and have a clean charge. Nitrous cars definitely DO need substantial OL as one of the main issues there is the exhaust cycle being enough.

Rich

 

"red meat" = "verbose" ?

 

And tyvm for your valuable contribution to the thread.

Rich

 

If it's -7 using .050", then it doesn't have much overlap at all and would be good to use for low rpms NA or used as a forced induction cam.

Mike