This came up in a thread on AFR 205 cam recomendations.
does excess lift hurt velocity in a small port head like AFR's??
Any thoughts?
And at what level would "excess lift" be??
Over .550, .560, .570 etc.....>.600 ???

 

This is a very good question. I think excess lift could hurt velocity, especially if the peak flow lift number of the head is exceeded greatly by the cam lift. The only way to measure would be to take velocity measurements at all lift figures. Remember, going past the peak flow lift number slightly with actual cam lift can be very beneficial because the peak cam lift is only seen a very small amount of time (i.e. the peak lift number is only obtained once while all lift numbers lower then peak are obtained twice, once on the opening and once on the closing of the valve).

 

Bump, would like to hear others thoughts as well. Considering the same heads and .580 .580 lift

 

just like duration peak lift doesnt tell you much of anything.

you can have a head with good low lift flow, or one that needs the valve off the seat to move air, ones going to like a milder ramp and be able to control the valve for a fe hundred rpm more, and the others gonan need an aggressive lobe to get it up quick.

Too much lift id think would only be a problem if the port stalls out and goes turbulent, also - how long does a cma spend at peak lift? Not long, and peak airflow demand doesnt happen at peak lift by any means

 

Valve lift don"t really affect velocity of air flow in the port that comes from the cross-section area of the port.On the valve lift to peak flow lift you can go about .050 lift over the peak flow as was mentioned in the above thread you only reach peak lift once(say your heads flow peaks at 550 and your max valve lift is 550 then, you only hit max flow once per intake cycle).If you go .050(or .600) over then you hit peak flow 2 times per intake cycle once on the way open and once on the way closed,so it is beneficial to go over peak lift with your cam.There are reasons why a heads flow will decline as valve lift reaches a certain point,for one the air will flow around the head of the valve and seat differently after it gets off the seat a certain amount and second the head of the valve can be shrouded by the chamber or the cylinder itself.On the valve lift flow decline, what happens is the air is flowing a certain direction say mostly toward the center of the chamber or toward the exhaust valve,but as the valve lifts more the flow my tend to go in another direction, then as the valve closes the flow wants to go back toward the center of the chamber as before.This change in direction is what causes the decline in flow as valve get too high off the seat.I"m sure I will get some responses to this but its the best I knew how to explain.

 

Provided the intake port doesn't go massively turbulent, then no, going past peak flow lift on the cam will not hurt. It will actually help cylinder filling. A head can have great flow numbers, and can have lots of velocity when measured with a velocity probe on a flowbench at max valve lift, but that is no guarantee of having good velocity on the engine. If the cam is too small, the intake valve will never be open long enough or high enough to allow enough air to flow through the port to ever reach "critical" velocity on the engine. You must always remember that a port doesn't flow anything unless the valve is open. Heads and cams must match. A larger cross section intake port requires more valve opening, or "area", to keep the air speed up.

Peak piston speed is where the intake port sees the greatest signal. That's around 80 degrees ATDC. You're not at full lift there, but almost. That's why improving high lift intake flow is almost always beneficial to power. Highest valve lift occurs during the time of highest draw on the port. But, it's not enough to fill the cylinder completely. That's why the mid lift flow numbers are so important. They supplement the high lift numbers. What you need to be looking at is total cfm available to the cylinder at every crank angle. There are two factors that determine it, flow and valve lift. What you're looking for is a high average flow. To get this, you want a head that has great mid lift ( valve job angles and combustion chamber shaping greatly affect this) and high lift numbers, along with a cam that has fast ramps and high lift, within the limits of valve train stability.

Let me give an example. The old NHRA Super Stockers that ran 350 Chevy engines had unported heads that peaked out at .450" lift. The cams they ran had over .750" lift. Why did they do that? They ran faster, but why did they run faster? Duration at peak port flow was much longer, and filled the cylinder much better with the higher lift cam. Most people I talk to think that the valve lift should match the lift where the port reached peak flow. That leaves some power on the table in my opinion. It's ok to go with higher lift. All you're doing is increasing the duration of time the port spends at its max flow rate. That helps cylinder filling.

A small port head is more tolerant of a small cam. High intake port velocity accelerates a car very well. In my experience, conservatively sized heads that flow well, meaning having a port that is "just" large enough in cross section to flow it's numbers, run the best at the track.

Comp Cams has a new family of lobes for the LS1 that have a lot of lift and area. They are called their LSK series. I don't have the list in front of me but the 224 @ .050" lobe had about .635" lift with a 1.7 rocker.

 

Great post Greg!

 

Owesome Post, keep the good info flowing....

 

New Comp Cams Xtreme Energy "LSK" for LS1 lobes

Lobe#, Dur. @ .006", .050", .200", & Lift w/1.7 rocker
2124 265 215 142 .629"
2125 269 219 145 .632"
2126 273 223 149 .636"
2127 277 227 153 .639"
2128 281 231 156 .643"
2129 285 235 160 .646"
2130 289 239 164 .649"
2131 293 243 168 .653"
2132 297 247 171 .656"
2133 301 251 175 .660"
2134 305 255 179 .663"
2135 309 259 183 .663"
2136 313 263 186 .663"

 

Great first post Greg. Welcome to the site. THanks for posting those Comp cams lobe numbers. Those look very aggressive, I assume they are still hydraulic?

For comparison - 224 cams:

Comp XE -
277 @ .006
137 @ .200

XE High lift -
273 @ .006
143 @ .200

Comp XER -
273 @ .006
145 @ .200

new Comp LSK 223
273 @ .006
149 @ .200

Thats a big jump in .200 lift duration. Idle of the 223 LSK should be similar to an XER 224 but have potiential to make more overall power. Interesting....

 

Yeah, my tech guy there faxed this over the other day. Hydraulic rollers for the LS1.

 

Greg, you have a new private message.

 

Are those single patterns?

 

Those are just the lobes. You can mix and match them for a custom cam. I'm sure Comp Cams, and a few tuners also, will come out with some standardized "shelf" versions soon.

 

I hope they have some springs to go with that 663 lift on a hydralic lifter,I mean a spring that is matched to each lobe.Thats some big lobes,there is a NEED!!

 

theres a better way to use that lobe then try and find a crazy spring

 

I don"t have a clue about what you mean,please explain.

 

a lower rocker ratio, make it a bit easier in terms of ramprate, and make them 590-625 lift

they look lke the exact timng of the gnarly SBC lobes Comp has, but more lift

 

Where did you find information on this family of cams. I did not see anything on their website.

 

Well there is a rocker made by Crane and sold by VHP that is a 1.6 throughout the lifts untill peak lift where is settles at 1.7.
They develloped it in conjunction with their low durations, tight lsa cams to help with idle characteristics.