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 Post subject: What are cams?
PostPosted: Thu Dec 07, 2006 3:43 pm 
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hey guys, i dont have a clue as to what exactly the cams do. Is there a good read somewhere explainin them. Also how do I pick what size I want, I see a lot of performance cams with different numbers, and what happens to the rest of the engine with those corresponding numbers??



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 Post subject: Re: What are cams?
PostPosted: Thu Dec 07, 2006 6:11 pm 
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OK. Did a little google search, and from what I found out so far, cams open and close the valves. Is that it, or do they do something else?



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 Post subject: Re: What are cams?
PostPosted: Thu Dec 07, 2006 7:02 pm 
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Happy to help here-

The camshaft is what controls the valves. It spins half as fast as the crankshaft. It converts rotary motion (it spins around) to linear motion, which is where the lifters play in. Lifters ride on the cam lobes, or the 'bumps' on the cam. They move up and down, or in a linear motion. From the lifters come pushrods, which are long cylinders that sit atop the lifters and go up to the rocker arms. The rocker arm is a simple lever and fulcrum that sits between the pushrod and the valve stem, and is anchored in the middle. When the pushrod is pushed up from the lobe of the cam, the corresponding valve opens up. It works just like a teeter-totter basically.

The cam is driven via chain on most older v8's, and via belt or gear on many newer ones, although that is just a general statement. It is with the timing chain that the movement of the cam is timed to the movements of the crankshaft.

Picking a camshaft is a whole other ball of wax. There are many things to consider, such as what the engine will be used for, what setup it has now, and what the potential is for that setup. Pick too small of a cam, and it will be a dog, go to big, and it will also be a dog.

I believe Norm reccommended changing your rear end ratio as opposed to getting a new reman engine. If you are considering a cam change for a performance upgrade, I too think that a rear end swap would be more worth your buck b/c you will put the powerband more in the range where it can be felt by the ol ass dyno.

Hope this helps-

Andrew



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 Post subject: Re: What are cams?
PostPosted: Thu Dec 07, 2006 7:36 pm 
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Thx andrew. This question really has nothing to do with my setup right now, I just want to know what they do, which I do now, THX!! :)
So now I want to know how to pick one. What do those numbers mean like 204/214 or 220/231. whats tat mean?

And what would the difference be in the engine with the above examples?



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 Post subject: Re: What are cams?
PostPosted: Fri Dec 08, 2006 2:10 am 
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GraveReaper0 wrote:
........ So now I want to know how to pick one ........

Although it is probably less complicated than brain surgery, an engineering degree would get you started.

First, you need to be reminded that the cam is only part of the total package.

In order to choose a cam, you will need to know the bore, stroke, head/manifold flow and the planned static and dynamic compression ratios. Among the many other factors, the weight of the car, the trans, rear gears, tires, and how the car will be used.

Once these are known, you will know where to go next. IE: If the trans used will be automatic, a 350, 400, or Glide, each might require a different torque converter, gears, tires, and (you guessed it) cam timing.

Then you'll need to know how any small change in those specs will require changes in the cam profile.

Most of the people you will encounter on the street, or at shows, will use whatever someone else thought would work.

GraveReaper0 wrote:
........ like 204/214 or 220/231 ........

If those numbers are in degrees, they show the amount of time the valves are open. 204=intake, 214=exhaust. Since they are different values, it is called a "split duration" cam. Some think it is better than a single pattern cam, some do not.

If they are in inches, they show the maximum cam or valve lift.

I'm guessing, your examples are generic Olds V8 cams. If so, they show the timing (in degrees) at .050" valve lift.

204°/214°@.050" might be a good choice for a stock 350 because of its impressive idle. Likewise the 220°/231°@.050" for a stock 455.

Here is the cam card for my 437.

Image

As you've seen here, it works well in my present 437. It may, or may not, work as well with the new heads. Its 39° equivalent did not work as well in my 461.

GraveReaper0 wrote:
........ And what would the difference be in the engine with the above examples?

In theory, the 220°/231° is bigger, so it would make more power. In actual practice, it could make less than it did before someone forked with it.

It is far easier to ask someone else what he/she uses.

Norm



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 Post subject: Re: What are cams?
PostPosted: Fri Dec 08, 2006 2:38 am 
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Oh ok, so the bigger the number is, the more air/gas mix will flow into the piston?
And even the tires matter, how so?

And what's a static or dynamic compression ratio? I thought there was just compression and thats it.



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 Post subject: Re: What are cams?
PostPosted: Fri Dec 08, 2006 7:39 am 
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GraveReaper0 wrote:
........ the bigger the number is, the more air/gas mix will flow into the piston? ........

Into the cylinder, theoretically, yes. There are many factors that keep it from actually happening that way. First is, will that much mixture fit where you want it to go? Next is, can you get it past all the restrictions in its path?

GraveReaper0 wrote:
........ And even the tires matter, how so? ........

Tire diameter has a direct effect on gears and traction.

GraveReaper0 wrote:
........ what's a static or dynamic compression ratio? ........

Static CR, is the mechanical number you are familiar with. It can not be changed without altering the internals.

Dynamic CR, is the compression (or cylinder pressure) that the combustion chamber actually sees. It is determined by the valve timing events.

If you had 12:1 CR and the valves never closed, there would be no compression. Or you could also get an effective 12.5:1, or more, by opening and closing them at the right time.

Optimum would be someplace between the extremes.

GraveReaper0 wrote:
........ I thought there was just compression and thats it.

If it was that simple, we would all be driving 10 second limos that get 90 MPG, because every fourth site on the internet would be telling us how to do it.

Norm



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