Damian

iTrader: (89)

Just like the topic says. I have shops telling me Duration is the culprit, and other shops telling me Duration doesn't mean **** and it's about how much lift the cam has. I have always thought that duration and ICL were the most importmant factors. Opinions?


josh

mastrdrver

You can have lots of lift and lots of duration as long as your LSA is wide enough. I actually think that ATDC point has the biggest impact on PTV clearence, which duration and ICL have the effect on that point.

LOnSLO

iTrader: (15)

J, the combination of high duration with a higher ICL will lessen the clearance. Whoever told you that lift is the only culprit needs to rethink it. A higher ICL on a given duration means that the valve will remain closer to peak lift while the piston approaches TDC.

Ron@Vengeance

iTrader: (51)

Just put the damn thing in and spin it over already!!!!!!!!!!!!!!

SStrokerAce

iTrader: (2)

I had to fix that. Basically a LOWER LSA or more advance will drop the ICL and combined with more duration that will give you P/V problems faster since the intake valve is bigger than the exhaust valve and it will move it closer to the piston top.

Bret

J-Rod

Some of this has been discussed before, but here is the gist of it.

You can have a cam with 1" lift as long as the timing and durration doesn't put the valve at max opening while the piston is at TDC... Last time I checked, in most cases the intake valve is just BEGINNING to open (nowhere near max lift) slightly before the piston reaches TDC and isn't completely open until the piston is near BDC. It's simple, MAX LIFT DOESN'T MEAN JACK SQUAT WHEN TALKING ABOUT PTV CLEARANCE!


If you don't believe me on the timing sequence, take a look at this webpage <http://auto.howstuffworks.com/engine.htm> and pay close attention to when the valve reaches it's max opening. Then you will see why PTV isn't necessarily determined by lift. I know it's a DOHC in the illustration but the concept is the same.

The LS1 motor is NO different than any other 4-stroke, internal combustion engine?? If PtV was an issue at max valve lift, all we would do is check for clearance at that point and call it good. This would lead to many engine failures as it would show you have a MILE of room. PtV becomes an issue during the overlap cycle, where the exhaust valve is closing and the intake valve is just opening. Your tighest clearance is going to be roughly 10* BTDC for the exhaust valve and 10* ATDC for the intake valve.

Duration, intake centerline, ramp rates, etc. play a MUCH more important role than max valve lift. It is ALL in the timing events.
If you have too much durration then your exhaust valves will be too far open for too long and the piston will tappy tap on them on their way up on the exhaust stroke. If there is too much advance ground into the cam and you have too high of a ramp rate then the intake valves will be too far open too soon and again mr. piston and mr. valve will kiss. These are just two examples of many that could cause problems, and they didn't even take into account many other factors. You only have to worry about PTV when the piston is nearing it's TDC, at TDC, and slightly after TDC, not when the valve is at full lift. And once again, "Duration, intake centerline, ramp rates, etc. play a MUCH more important role than max valve lift. It is ALL in the timing events." Picking a cam for a motor isn't as simple as putting a round peg in a round hole and when it fits, patting yourself on the back and calling it a job well done.


what he is saying is that you wont have a problem at max lift. piston to valve problems happens when the valves are opening and closing not at max lift!

LOnSLO

iTrader: (15)

Bret, I think we are both correct. I agree that a lower LSA will give you clearance problems. I also believe that an ICL that is closer to the LSA is the culprit.

MyLS1Hauls

iTrader: (2)

The smaller the ICL is, the closer the PTIV clearances will be with everything else the same. You advance a cam, the intake valve gets closer to the piston. This happens because the valve starts opening earlier than with a higher ICL.

Duration, ICL, ECL and ramp rates all play a role in PTV clearances.
Also, I've had some pretty respectable sponsors tell me its peak lift that determines PTV clearances and insist that it was so. I just said, "thank you" and hung up the phone

LOnSLO

iTrader: (15)

I agree. Smaller ICL and LSA will present clearance problems. What I am saying is this:
Let's say you have an LSA of 112, and you have an ICL of 108. If you move to a higher ICL of 112 then will you have problems?

Jason99T/A

iTrader: (21)

It all depends on what kind of clearance you had before you moved the cam. Retarding the cam opens the exhaust valve earlier and the intake valve later. This gives you a few thousandths of an inch more clearance on the intake side, and about the same amount less on the exhaust side. If your exhaust PtV clearance was tight before, you very well may run into issues.

Advancing the cam will bring the intake valve closer and the exhaust valve slightly further away from the piston during overlap period (which is where PtV comes into play, as the exhaust valve is closing and the intake valve is opening).

Brad pretty much summed it up though. Duration, ramp rates, ICL, valve size,etc. play WAY more important roles in determining PtV than max valve lift.

Jason

SStrokerAce

iTrader: (2)

Basically, the higher the lobe centerline is for either valve the farther away it is from the piston.

So a ICL of 110 vs 114 the 114 will have more room. The ECL of 110 vs 114 the 114 will have more room.

Advancing the cam will move the ICL down and the ECL up, so the intake valve will have a tighter P/V and the exhaust valve will gain clearance.

Retarding the cams is just the opposite.

Yeah, It's a little scary to hear some who should know better say the P/V depends upon the lift.

Bret

LOnSLO

iTrader: (15)

I need to be shot for thinking backwards. My thinking of ICL and clearance was 180 degrees in the wrong direction Bret, is there a way to estimate a maximum duration for an intake valve, given a lift and LSA?

SStrokerAce

iTrader: (2)

Yeah but envolves a dial indicator on the motor and a degree wheel.

On the other hand if you are given a certain combo that is running a camshaft with correct clearances then you can estimate from that where you can go. Basically it gives you a intake valve opening point that you can't go past, and a exh valve closing point that you can't make any later. From there you can play with the LSA, ICL, ECL and durations to get you where you need to be.

Bret

LOnSLO

iTrader: (15)

OK thanks. I'll give that a shot.