73-454

I have a Comp Cam with a lift of .55 and a duration of 292, The 454 does not idle very well . The local speed shop guy said the lift is not my problem it is the duration of 292. Can some one explain and why would some one install such a cam. I bought the vette this way. I would not recommened this cam for street use on an Auto Vette, may it would work better on a 4-speed standard

Ganey

Lack of knowledge & experience. A shorter duration would have less overlap when both valves are open & more low end HP or TQ.

Duration is the length of time in degrees the valve is open & lift is the max. opening of the valve.

:cool:


[Modified by Ganey, 9:59 PM 1/15/2002]

ddecart

Ok. Let me try to explain this simply.

Lets look at a simple 4 stroke cycle cylinder.

Start with the cylinder at top dead center on the compression stroke so the fuel/air mixture is compressed and ready to be ignited. Spark, burn, and down goes the cylinder.

180 degrees of crank angle later, the cylinder is at the bottom, full of burnt gasses. Now we'll start to open the exhaust valve (we can't open it instantly, we need to ramp it up or we'll be whacking things around in the valvetrain too much).

Up goes the piston with the exhaust valve opening to it's maximum lift and then closing when we get to top dead center (gradually again...gotta watch the valve seats). So the exhaust valve has been open for a DURATION of 180 degrees, we've pushed out the exhaust gasses and we're ready to start the intake stroke.

Slowly again we'll open the intake valve as the piston moves down, drawing air in. 180 degrees of DURATION later the intake valve will have closed completely and gradually.

The piston moves up and compresses the fresh fuel/air mixture and we're set to start the cycle again.
*********

Now in reality things don't quite happen that way.

We actually start to open the exhaust valve BEFORE the piston is all the way down. This allows it to open gradually and maximizes the amount of time it spends at maximum or near max lift.

We don't close the exhaust valve before top center either. Gasses are still flowing out the exhaust and that flow actually helps draw some air into the cylinder (or get it oving that direction anyhow).

So you might have guessed that we open the intake valve BEFORE top center as well to do the same thing on the intake side. And since that piston is moving pretty quickly on the down stroke, we leave the intake valve open PAST the bottom of the stroke to draw as much in as possible.

The DURATION in crank degrees that the intake and exhaust valves are not closed is the duration. The 292 number is at zero lift. A standard number is to measure at 0.050" lift, which is more realistic for the valve being effectively open at all. Done this way that 292 is probably more like a 240 degree duration.

So this means that your intake valve is open for 240 degrees of crank rotation instead of that theoretical 180 degrees. Same for the exhaust.

So what does this mean to your idle? Well, look at that engine I first described and imagine it's going REAL slow. You don't want the valves to overlap in that case because things aren't moving fast enough to take advantage of the benefits of valve overlap and a long duration.

This is kind of what's happening in your engine. That long duration is designed for higher engine speeds to take full advantage of the scavenging effect of having valve overlap. Try to run that thing slowly and the engine doesn't really like it. In return you get a rough idle, low vacuum (since you don't have a full intake stroke to draw air in (some is moving back out on the little bit that the intake valve is open past bottom dead center).

So a long duration cam is better for higher speeds and a shorter duration is closer to that slow theoretical engine. Is that making sense?

73-454

Thank-you fore taking the time to explaine, I was worried that I was damaging the engine with such a rough idle. I also understand why my power brakes do not work very well.

MARKUS_P

dedecart, this was a very good explanation.
I really enjoyed reading it :yesnod:

ddecart

Thanks! :)

Alwyn678

Very well written......Explanation

evelkevin

hey '73, i would look at advancing the cam 4-6 degrees and see if that helps the situation...if that is a Comp Cams grind? it probably has a 110 degree centerline and that is what makes it idle so rough. :chevy


[Modified by evelkevin, 8:44 AM 1/17/2002]

clem zahrobsky

go to http://www.newcovenant.com/speedcrafter/ for all you would want to knnow about cams

Maurice

Great explanation!

74-Roadster

ddecart,

That was an excellent explanation and I to enjoyed my afternoon reading. :yesnod: Now, as in the real world, those who perform well are always tasked to do more. Can you please explain the comment about centerline and how or why the 4-6 degrees of rotation may help calm the rough idle? :confused:

Oh, and as in the real world, no extra pay, just more appreciation. :D :D

zwede

Advancing or retarding the cam is a crutch for when you realize you're running the wrong cam.

What advancing the cam does is to close the intake valve sooner. By advancing the cam you change the cams location relative to the crankshaft by use of a special cam sprocket with a built in offset.

Closing the intake valve earlier means that the piston will not be able to move as far up on the compression stroke as it would have with the cam in the normal position before the intake valve is closed. This in turn limits the amount of reversion (intake gasses pushed back into the intake manifold), and improves idle.

Unfortunately this makes the engine less efficient as now the exhaust timing is not optimal and it also leads to undisered effects as the intake now opens too soon on the intake stroke.

I'd also like to shortly discuss LSA, as many do not know what it is and what it does (even poeple that have built several engines). Many only look at duration and lift and ignore LSA, which is a mistake.

LSA stands for Lobe Separation Angle and is the angle measured in camshaft degrees that the intake and exhaust lobe are separated.

A small LSA increases overlap, with duration and lift staying the same. Overlap, as discussed above, is the time both the intake and exhaust valves are open at the same time.

Skipping some of the theory (which I can go into if anyone cares), here are the real world effects of LSA:

All this assumes that the duration and lift stays the same.

Wider LSA (112-116) will improve idle. Idle vacuum increases. Off-idle driveability improves. Torque curve flattens out. Low rpm torque increases. Emissions decrease.

Narrow LSA (106-110). Worse idle and idle vacuum. Less driveability (more bucking, surging). Torque curve develops a peak. Although average torque is the same, the torque peak will make the engine seem stronger.

Narrow LSA's are very popular among the aftermarket as it accomplishes what most look for in a performance cam: Lopey idle and a sudden 'kick-in-the-pants' feel in the 3000 rpm range.

Personally, I'm a big fan of wide LSAs, which means I have my cams custom ground. I like 114 LSA with its smooth idle, and it works much better with EFI.

ddecart

Naw. I don't think I'll answer. In fact, I'll just delegate this one to Markus :D

zwede

I thought you might appreciate some assistance. :)

Scary parts is that we just scratched the surface of camshaft theory. We haven't discussed lobe intensity, proper duration depending on exhaust vs intake head flow ratios or roller lifter cams. Just to name a few...

PRNDL

:confused: :confused: are we talking about sexual dysfunction??? :lol:


MJ

zwede

Hehe... fitting coming from MNJACK, or should I call you "MaNual jack"! :)

Ok, time to go to bed.

ddecart

:lol::lol::lol::lol:

Thanks for the help Markus. I haven't been on all day today.

I know I didn't hardly get into camshafts at all. Didn't want to confuse the poor guy :) There is SO much more as you said.

shoptek

ddecart and Zwede-----EXCELLENT explanation!!!!!:cheers:

73-454, I'm glad you brought up the subject. It is one of those "but afraid to ask" questions, and a lot of us were afraid to ask. :confused:


[Modified by shoptek, 11:42 PM 1/17/2002]