I've known the basics principles of cam profiles for many years now, but with all the more aggressive ramps out now, I need a refresher. I've been thinking more into when the valve events should happen here lately, instead of just duration/lift/LSA/IC. I saw on here awhile back that about 43-45* ABDC is a good point for IVC, and it makes sense to me. And I think I could figure out decent EVC and IVO points to a custom grind, but what's a good EVO point? Basically, when have the expanding gases pushed on the piston long enough that any later EVO would cause them to push for TOO long?

I understand that a wilder engine will need different profiles, and I'm not planning to see anything faster than mid-11s(don't want to hack the car up to put in a cage), so let's look at this as a stock-cube NA engine that might see 6500-6800 rpm max. Thanks a ton for y'all's input.

 

Here is something to think about.

You want the valve to have its greatest point of valve lift at the greast point of acceleration of the piston. This will lead to the strongest vaccum signal to fill the cylinder. In most engines this occurs around 75 ATDC. But, you can play with one of those neat plots to see where this takes place, and shoot for that. It has to do with rod length, bore, etc...

Its just one of the things that folks ignore when they stick to the "Everything has to be 112 or 114 +4" argument

 

Right on J-Rod, with one correction, peak piston velocity is around 75 deg. before and after TDC, not acceleration. I have been telling people that for a while. The perfect cam would mimic the piston velocity. This is because the airflow follows piston velocity, plain and simple.

But still, the first thing when choosing a cam is the IVC. This determines how much A/F mixture you are going to be able to trap.

If anyone has any questions where there peak speeds of the engine are at, send me a message. I have put together an Excel spreadsheet that calculates all of this for me. Just need some basic info.

 

Good info so far... This is why I love LS1Tech and especially this forum. So much to read, so much to learn!!!

 

What about pressure differentials?

 

Yeah guys, thanks for the info. Patrick G's new cam profile got me thinking deeper into custom grinds, and I think something like that with a little less lift(still have the stupid-fast ramp, just hold the valves open instead of so high lift) might be to my liking. Less lift mainly because I'd like to be able to stick with single springs and be reliable. Still, I'd like it in the .570" area.

Am I in the ballpark of a good choice???

 

If piston velocity is zero at TDC and BDC, why do most engnes, especially high output ones have the intake valve opening before TDC and closing after BDC of the intake stroke? Inertia perhaps?

So if the air has inertia because it has mass, how do you get it to follow the piston velocity over a broad range of rpm, say 2000-6000? IOW, the slug (pun intended) of injested air isn't attached to the piston with a "cable" but rather a "rubber band". To my mind that makes the real world optimum valve lift profile less simple than you stated.

My $.02

 

Well the best way to get a custom ground cam is to know the flow numbers of the head. That way you can get the cam lift numbers matched up to the flow characteristics. If the heads flow the most between .550 and .600, or pretty much max out around .550 or a little higher, then the fast ramp lobes, and a lift number right around the maximum flow number on the heads will keep cam dwell at high lift for longer and maximize performance.

 

Wrong Fenster... make sure what you read and learn is from the right places. A good BS filter will teach you the most the fastest.

The cylinder heads are only ONE part of the situation on what the camshaft specs are based around. 100% of the time the more you open up the valve the more the heads flow, but if the flow doesn't increase the more you open the valve there still is good reason to open the valve even more.

The max valve lift of the cam is dependant on the valvesprings, but if you don't have limits in that situation then it's based on the valve size, engine RPM and displacement.

As for the pressure differentials that's the camp I'm in. There are the piston velocity determines the cylinder filling guys, but that doesn't explain all of what's going on in cylinder filling.

Ric,

If you are looking for increased valve spring life, this is the wrong way to go. What you describe here has lots of accelerations and jerks (jerk is v^3, so it's the rate change of accleration) and will cause hell with the valve control AND the movement of the coils on the valve springs. The idea is good, but mother nature doesn't like it.

Bret

 

Ok, I was a little open ended. What I mean is that if you were able to supply the piston with enough air to take up its change in displacement you actually wouldn't need the cam to open btdc. You would still want it to close abdc because of the inertia that the air carries. This is how you achieve over 100% VE.
Since everything takes time, you need to open the intake valve btdc to get the charge moving by the inertia that the exhaust has on the chamber, ie overlap.
Once everything is going, you want a cam that opens very aggressivley from 0 to 75 deg of piston motion, with peak lift occuring at 75 degrees. You also would want your peak lift to be beyond peak flow of the cylinder head because like I said before things take time, so you could hit the peak flow of your cylinder head twice instead of once.
Then, you have to think about how much ramming effect you are going to get from the inertia of the incoming charge. You want to close the valve soon enough to trap the best overall charge. This is obviously dependant on RPM because the inertia of the incoming charge rises with RPM. So, you would want to close the intake valve sooner down low and progressively retard it towards torque peak, and then, after torque peak you want to start closing it sooner again because VE is going down.
Oh man, I could go on for days about this stuff.

 

I'm no expert, but you're negating a lot of other events going on in the cycle.

 

Please don't.

I'm not convinced you understand everything you know about this. No offense intended.

 

What would you like me to elaborate on?

 

I'm with the strokers on this one, don't just say "well my head flowed too .600 so I need to stop my lift point there". I've been looking into this for some time myself. On a running motor flow numbers at 28" just don't count for all the small dynamic conditions which exist. Further, valve train deflection or geometry issues will not cause you to see the gross lift (lobe lift x rocker ratio) anyway. Like said previously, just about all the time you'll gain power when lifting the valve more unless you have some problems with your valve train when doing so or your heads ports to crazy crazy at high lifts. If you want single springs then look at comps xfi series of lobes, they are specifically designed to work with the 918 single springs comp offers and have lift over .600 and are nice and agressive for some good power.

 

MSU - you are looking only at the intake stroke. The intake and exhaust cycle work together to create over 100% VE. On a well tuned exhaust system, excellent scavenging will keep drawing air out of the cylinder as the intake valve is opening. Despite the fact that the piston is traveling upwards, against the flow of incoming air, you are getting more fresh air into the cylinder earlier, allowing the inertia to build more velocity. These are good reasons why narrow lobe separations and low lift flow are so much more important than people realize.

As the piston begins accelerating away from TDC, the exhaust valve continues to close, preventing too much of our fresh air and fuel from leaving the chamber. Meanwhile the piston begins accelerating our column of air into the cylinder, as the valve lifts farther off the seat and flow potential continues to rise.

Now lets say that your mild heads flow 290 cfm at .550 lift, but the port doesn't introduce undue turbulence or stack up air at higher lifts up to 600. If you can still flow 280 cfm into the port up to .600 lift, inertia is going to continue filling at a faster rate, even though the flow sheet says its past its max.

Others have already established where we want the valve to close, and I'm sure they could do a better job than I explaining why. But it seemed from the explanation you had given that you are leaving a lot on the table, before we got to the closing point.

There are a lot of people here who know far more than I do, so if I'm missing something here, please chime in.

 

http://www.speedtalk.com/forum/viewt...161&highlight=

The above thread may answer many questions for you guys. Like said previously I asked this quite a while ago regarding my combination. My ports don't pick up flow from .600 to .700 but my cam lift is .754, hmmmm, wonder why

 

Do did well in your answer except for one thing....

LOW LIFT FLOW....

The reason it's not as important is due to the pressures difference between the port and the cylinder when the valve is at these lifts. You can easily see a 200" of H2O depression difference or more at these lifts. A change in 10cfm @ .100" is pretty large in terms of percentage of how much the port flows at .100" but the motors demand on the port is more than taken care of at depressions that large even with the lower flow @ 28". Say if your port flows 70cfm @ 28" when the valve is at .100" lift. At the higher depression of 200" the port can easily flow 200cfm but the demand is not that high.

Bret

 

I am no Guru, but I will throw in a couple of points.

A perfect world would have a valve open instantly and close instantly at certain points. Since that would break the valvetrain at high RPM, the valves are opened gradually. This is called ramp rate. High ramp rate cams are noisier, break sooner, and can't be spun as quickly (assuming everything else is equal), but they do flow more. Since, you can't open your valve instantly, you can get more "area under the curve" on your flow by opening a little early. Doing this too soon will get blowback, but in small doses it works because: 1. the piston is barely moving, so there is little pressure (comparitively) and 2. the valve is barely open at this point. This way, when the piston starts sucking more air, the valve is open a little already.

I'm told that opening intake early when the exhaust is still open can help scavenging by letting clean air help push exhaust gases out and letting the scavenging effect fill your cylinder with clean air slightly. I've heard that you can shoot some unburnt fuel into your exhaust this way. Maybe there is a way around that?

That's about all I "know" about it. I won't be hurt if someone wants to disagree.

 

I am another who is not an expert but will throw in my $.02. I agree with you completely that overlap is an important consideration. One of the big benefits of 'camless' technology (ie solenoids to open and close valves) is the fact that you can have more overlap higher in the RPM range (without sacrificing idle) to help with scaveging and cylinder fill. Obviously you dont want so much overlap that lots of unburnt A/F mix passes through the combustion chaimber, but a little will help for the reasons you point out above.

The point of this thread is that there are hundereds of variables that you would have to take into consideration to design the 'perfect' camshaft. And that 'perfect' camshaft would only be perfect for setups identical to yours. This kind of research would take resources that most of your average joes (like me) dont have.

 

I'm another 'wanna be cam guru' that has some change to throw into the discussion.

That range of IVC would be specific to a certain engine combination and
would not necessarily be best for an ET of eleven seconds. SCR has a big
impact on IVC.

I believe the overlap considerations are just as important as IVC. Overlap
helps get the waste out, and the new charge into the cylinder.

According to Vizard, the exhaust gasses in the correct tuned RPM range can
begin filling the cylinder even before the piston creates a depression.

As the exhaust gasses leave, there is a low pressure area created which begins
to pull in the intake charge. The greater the pressure differential, coupled
with correct valve timing and pulse arrival at the respective ports will increase
VE substantially.

Reading further up, J-Rod touched on optimal ICL. Would ICL occur at max
piston velocity, or a little later? I would think the air is lagging the piston,
therefore ICL would happen a touch after max piston velocity?