Im getting a cam cut for my upcoming project. My theory is as long as the valve events are correct for what im doing, it shouldnt matter what the LSA is correct? I was told it will be cut on a 107 and rev to 7500. Will still be hydraulic roller as well.

Give us your thoughts on this. Turbo setup btw, and please give some hard evidence on why it shouldnt or should be cut on a wider or narrow lsa. Im thinking it doesnt matter what the lsa is as long as the proper valve events are there because it is by product of valve events.

 

Yes and no.

You are correct that the proper cam is a combination of valve events that best meet your needs. LSA is just the result of those chosen valve events. But since some applications (like turbos) prefer a certain range of valve events compared to a max-effort NA motor, they tend to congregate within certain LSA ranges. So if you look at it from a purely general standpoint, the chosen valve events will typically dictate cams within certain LSAs, so the argument can also be made that LSA does matter.

But if I had to choose, I'd say it doesn't matter. The motor will want a certain intake valve closing point, a certain amount of overlap, and a certain EVO.

 

From what I gather what you say is correct. i see alot of guys using the wider LSA's but im wondering how much is being left on the table with their setups.

I feel comfortable with the specs he's given me so far but we are waiting on the head work to be completed to finish spec'ing it out.

 

Some cam companies, and performance shops, only look at LSA at the end. They mainly focusing on the parameters given to them, ie. race car vs. street car, and stuff like max rpm.

Some turbo cams are 112 LSA, since the builder is looking for a strong curve and not as much concerned with making another 20-25 peak hp.

 

Thats pretty much what he was getting at with the questions he was asking me yesterday. I told him radical yet simple street car.

 

That is entirely the wrong way to look at it BTW.

Bret

 

Bret, please explain

 

I guess I read it wrong, that they picked the LSA if it was a street car or race car.

Either way there is something to be said for the centerlines and where they are placed reguardless of opening and closing events.

Bret

 

So who would be the end-all guru to have build a perfect cam for any particular application?

By that I mean understanding the theory behind selecting valve events for each application.

 

My point was that they are not looking at LSA when they first think about the cam specs... I know that they do that, since I heard that from Comp and Cam Motion... I guess Bret is smarter than them.

 

Well the end all guru is a woman, mother nature. You have to give the motor what it wants to get the most out of the motor.

It seems odd to me that a shelf cam isin't picked off of a LSA first since 90% of the time if you look thru a catalog a certain group of cams all has the same LSA and differing durations all in the same lobe family.

Bret

 

Good article:
http://www.compcams.com/Community/Ar...ID=-2026144213
Link courtesy of EDC

 

i agree.but in it,Vizard gives a rough formula for picking the right cam for your motor.i assume he's talking about carb motors.i wonder how this could be applied to our fuel injected engines?anybody have any thoughts?

 

Why would the fuel delivery method matter? Surely optimal AFR is the same no matter how its delivered?

Head and intake design however can be optimised for certain rpm ranges, so in theory your overlap and valve bias with respect to TDC should be designed to compliment that.

 

"overlap and valve bias with respect to TDC should be designed to compliment that."

What exactly does that mean? I really want someone to explain to me the valve bias with respect to TDC one of these days. I know WHAT it means but don't see how it changes things substancially.

Bret

 

Im a bit vague on the topic myself, based on the articles Ive read its about cylinder filling optimization, leveraging scavenging etc.

Here are a couple of tit bits I found.

http://www.4cycle.com/karting/html/cam_theory.html
"Interesting things happen during overlap. Since the intake valve is being opened while hot exhaust gases are rushing out of the exhaust track, an effect known as intake draw-though can occur. This effect can actually 'pull' the intake mixture into the cylinder. This effect actually increases with rpm. The faster velocity in the exhaust during high rpm will create a lower pressure in the exhaust track(trust my physics prof). If the intake is opened as the last of the exhaust gases are being pulled from the cylinder the intake charge is sucked into the chamber. This effect can be enhanced by opening the intake valve earlier in the cycle. This is great until the pressure in the exhaust gets lower and draw though actual begin to pull some of the intake mixture out of the exhaust pipe! This is an important consideration in a restricted motor."

And an obvious statement, but not much help.
http://members.tripod.com/otra_1/id12.html
"The science involved with overlap is quite complex. Pressures, runner lengths, temperature, and many other aspects influence how well the overlap effect works. "

Slightly more info here,
http://www.wighat.com/fcr3/camtruth.htm
"During overlap, there is charge exchange possible between the inlet and exhaust ports (by definition). If the intake and exhaust timing events have been optimized to produce maximum cylinder filling and pressure, that means that the TDC overlap activity has to be a balance of intake opening and exhaust outflow. To increase cylinder filling, the intake was opened earlier; only to the point at which the exhaust closing caused a degradation of this filling capability. For a given inlet and exhaust effectiveness, cylinder volume, and rpm, there is an optimum exhaust closing.

Consider the low-pressure region behind the exhaust outflow. For a low-pressure situation to exist, the exhaust closing has to be timed correctly to sufficiently rid the cylinder volume during the exhaust process. Any earlier and the inlet charge is compromised by exhaust pressure and spent gases still in the cylinder. Any later and the cylinder volume will be contaminated with residual exhaust gases coming back toward the cylinder. This is due in part to the exhaust system seeing atmoshperic pressure, as well as the increasing cylinder volume creating lower pressure situation (vacuum). However, at just the right instant, it is possible to help draw clean intake charge into the cylinder (this is referred to as scavenging). With the analogy of the inlet and exhaust ports being instantly opened or closed, per the discussion so far, it is possible to reach maximum cylinder filling and emptying for the specific situation."

And some nicer info from Comp,
http://www.compcams.com/information/tutorials/
"The amount of overlap, or the area between the intake opening and the exhaust closing, and where it occurs, is one of the most critical points in the engine cycle. If the intake valve opens too early, it will push the new charge into the intake manifold. If it occurs too late, it will lean out the cylinder and greatly hinder the performance of the engine. If the exhaust valve closes too early it will trap some of the spent gases in the combustion chamber, and if it closes too late it will over-scavenge the chamber; taking out too much of the charge, again creating an artificially lean condition. If the overlap phase occurs too early, it will create an overly rich condition in the exhaust port, severely hurting the gas mileage. So, as you can see, everything about overlap is critical to the performance of the engine."

Im guessing the major issue is going to be the enertial effect of the gasses as the piston isn't doing much around TDC. (Open to correction of course). More info would be nice. I guess its all about EVC and IVO.

 

Right.... it's always about making the cylinder pressure lower than the intake port pressure. But the biggie is to WHY would you place the overlap more before or after TDC. At least in terms of the available curtain area that the cylinder sees from both valves....

What you posted is the basic laws of overlap, I get those completely. Just not the idea of why you need to bias the overlap area before or after TDC. I have my way of doing it and how it's interconnected with the other valve events and the amount of overlap being used.

Bret

 

Yeah there are hints of info in there, but not enough for me to understand fully. I can see the basic valve events being the big point. Most charts show overlap being bias to the exhaust slightly. Perhaps the piston motion does play a part in deciding which way to bias?

 

Most charts show it that way because most cams have advance ground into them. Take that advance out and the cam will have the overlap biased towards the intake. Hell it might have it there anyways even with advance.

Piston motion in this area has very little to do with it is my guess..... Take for example a TRex cam on a 5.7L LS1. It's advanced some but it's overlap peak is right at or very close to TDC. (assuming +2° advance), still since the exhaust valve is opened longer than the intake the cam is biased towards the ATDC in terms of overlap. There is more overlap ATDC than BTDC. This means there is more piston motion on that side of TDC when the valves are open, but we are not talking about a lot. The cylinder displaces, actually closes up 5.5cubes of volume on the way up during overlap and 7.3cubes on the way down. So 12.5-17%, so not a lot when you look at the aggregate. Overlap is not about discplacement at all, it's about pressure just like every other cylce in the motor. The whole goal of overlap is to open the intake valve at the time when the exhaust port & cylinder are at their lowest pressure, and then close the exhaust valve before the exhaust ports pressure rises above that of the cylinder. That's how you get the best effective scavenging. Only problem is that these conditions change every RPM. So you have to make the best comprimise for your RPM range for best results.

Bret

 

I do not know Bret but I sure appreciate his real world knowledge as opposed to an engineers theories ( no knock on engineers ). Thanks.