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duration and lift clarification on camshafts

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Can someone explain in simple terms the meaning and performance differences of lift and duration (deg)? For example: A Schrick 284deg cam has a 9.5mm lift while a Korman 300deg cam has a 9.4mm lift. In addition, has ne1 used Korman camshafts? The shrick cams are just expensive. Thank you very much.

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The 284 # is the duration or the amount of time the valve is open. 9.5mm is how far it opens the valve at zero valve lash. So the Korman 300 cam is a more radical cam even though it has less valve lift because the valve is open longer it lets in more fuel to burn.

If you install a cam it must be degreed in so it is correct time with the crankshaft and you must check piston to valve clearence. The Korman 300 and the Shrick 284 are both good cams but the 284 is not much over the stock cam for a fun street car I would go no less 292.

John

http://www.iskycams.com/degreeing.php

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The other big 'Huh?' is that different

cam manufacturers measure duration in different ways.

Schrick measures at (according to CD He Sold It) 1mm, whereas

domestic manufacturers and regrinders measure at .050".

So a '300' from Schrick might be a '240' from Isky.

It has a lot to do with how the valve is accelerated open and decelerated on

closing.

But it makes apple pie to apple strudel compariasons pretty hard to do...

t

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ctrp_0701_04_z+engine_camshaft+lobe_diagram.jpg

Duration

Duration is the amount of time, measured in degrees of crankshaft rotation, that the valve-either intake or exhaust-is open. Most camshaft manufacturers list both an advertised duration and duration at 0.050 inch. We'll discuss this in more detail later.

As engine rpm increases, the engine eventually reaches a point at which it has trouble effectively filling the cylinders with the air/fuel charge in the short amount of time the intake valve is open. The same thing holds true with the spent exhaust gases. The simple answer here is to increase the amount of time the valve is open, which is referred to as increasing its duration. For example, to maximize flow during the exhaust stroke, many extreme performance cam designs begin opening the exhaust valve near the midpoint of the power stroke. This may seem harmful to power production, but the idea is to have the exhaust valve fully open when the exhaust stroke begins. During the power stroke, the burning fuel has used about 80 percent of its available force on the piston by the time the crank has turned 90 degrees. The bottom half of the power stroke actually provides very little in terms of engine power, and it can be better used to help exhaust the combustion chamber so that there is more efficient cylinder filling on the intake stroke.

Lift Versus Duration Versus Acceleration

Here's a statement that you already know: The valve is most efficient at allowing air (either intake or exhaust) to flow past it when it is fully open. Not to insult your intelligence, but we needed to get that out of the way. What that statement tells us is that in terms of achieving maximum engine performance, the amount of time the camshaft is either raising or lowering the valve is effectively wasted. In a perfect world, the valve would be completely seated to seal the chamber, then it would fully open instantly at the appropriate time to allow maximum flow.

To get as close to this as possible, maximum race cams use extreme lobe profiles that open and close the valve ridiculously quickly. This requires stronger valvesprings and lightweight valvetrain components to maintain valve control, and engine builders and cam designers alike are still researching ways to open the valves even faster.

A more aggressive cam with high lift velocities allows you to shorten the duration in certain situations, which can help power. "Aggressive ramps allow the valve to reach maximum velocity sooner, allowing more area for a given duration," says Godbold. "Engines with significant airflow or compression restrictions [often seen in Street Stock classes or other classes with small carburetors] seem to love aggressive profiles. This is likely due to the increased signal to get more of the charge through the restriction. The decreased seat timing also results in earlier intake closing and more cylinder pressure."

This diagram shows many of the critical areas on the cam lobe as well as the relationship between the intake and exhaust lobes. Courtesy of Comp Cams

Overlap

Overlap is the point in crank rotation when both the intake and exhaust valves are open simultaneously. This happens at the end of the exhaust stroke when the exhaust valve is closing and the intake is opening. During the period of overlap, the intake and exhaust ports can communicate with each other. Ideally, you want the scavenge effect from the exhaust port to pull the air/fuel mixture from the intake port into the combustion chamber to achieve more efficient cylinder filling. A poorly designed cam and port combination, however, can cause reversion, where exhaust gases push their way past the intake valve and into the intake tract.

Several factors influence how much overlap is ideal for your engine. Small combustion chambers typically require minimal overlap, as do engines designed to maximize low-rpm torque. Most current stock car racing engines depend on high rpm to take advantage of better gear ratios, so more overlap is normally helpful. When the revolutions per minute increase, the intake valve is open for a shorter period of time. The same amount of air and fuel must be pulled into the combustion chamber in less time, and the engine can use all the help it can get to fill the chamber. Increasing the overlap can help here.

Long rod/stroke packages, which are becoming increasingly popular in circle track racing, also have an effect here just as with the lobe separation. Because the piston dwells near TDC longer, it makes the combustion chamber appear smaller to the incoming air/fuel charge. Because of this, less overlap is needed to properly fill the chamber. Along with reduced vacuum and potential reversion problems, running too much overlap in your race engine sends unburned fuel out of the exhaust pipes, reducing fuel efficiency. For most short track racers, this isn't a problem. But if you run into a fuel-mileage situation to cut out pit stops, it can be helpful.

p><p>Duration is the period of time, mea

What is the difference in Advertised Duration and Duration at .050" Lifter Rise (Tappet Lift)?

In order for duration to have any merit as a measurement for comparing camshaft size, the method for determining the duration must be the same. There are two key components for measuring duration-- the degrees of crankshaft rotation and at what point of lifter rise the measurements were taken. Advertised durations are not taken at any consistent point of lifter rise, so these numbers can vary greatly. For this reason, advertised duration figures are not good for comparing cams. Duration values expressed at .050" lifter rise state the exact point the measurement was taken. These are the only duration figures that are consistent and can accurately be used to compare camshafts.

How does Valve Lift affect the operation of an engine?

Lift is the distance the valve actually travels. It is created by the cam lobe lift, which is then increased by the rocker arm ratio. The amount of lift you have and the speed at which the valve moves is a key factor in determining the torque the engine will produce.

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CD says it the best!

Bigger the duration, the bigger the overlap and the lumpier the idle due to turbulence in the intake.

John

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Schrick measures at (according to CD He Sold It) 1mm, t

That is not close to being correct. BMW & Schrick both use the same convention for duration, and while it is quite tricky to repeat accurate results with their method, the correct information on where to measure is revealed in the Owner's Manual that came in the BMW's glovebox, as well as the Service Manual.

When you measure a 292 degree Schrick @ 1 mm its duration is only 241 degrees. When you measure it correctly it will show 292

BMW and Schrick measure 4 cyl cams from ZERO with a special lash setting that is different than the normal cold lash clearances. The cam checking valve lash setting they both use is 0.28 mm between the cam base circle and rocker pad. BMW also shows a different (shorter) duration that can be found with 0.5 mm instead of the 0.28 setting. Six cyl cams use a different setting.

When measuring duration off zero lift, you need very sensitive tools and a lot of patience in achieving the special checking clearances spot-on, as a very small difference there will make a huge difference in duration as the "tails" at open & close are so shallow on the graph. I use a long travel indicator with resolution of 0.0001" (not that unusual) with a certified accuracy of 0.0001" (not that common!)

post-687-13667592073962_thumb.jpg

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Guest Anonymous

Schrick measures at (according to CD He Sold It) 1mm, t

That is not close to being correct. BMW & Schrick both use the same convention for duration, and while it is quite tricky to repeat accurate results with their method, the correct information on where to measure is revealed in the Owner's Manual that came in the BMW's glovebox, as well as the Service Manual.

When you measure a 292 degree Schrick @ 1 mm its duration is only 241 degrees. When you measure it correctly it will show 292

BMW and Schrick measure 4 cyl cams from ZERO with a special lash setting that is different than the normal cold lash clearances. The cam checking valve lash setting they both use is 0.28 mm between the cam base circle and rocker pad. BMW also shows a different (shorter) duration that can be found with 0.5 mm instead of the 0.28 setting. Six cyl cams use a different setting.

When measuring duration off zero lift, you need very sensitive tools and a lot of patience in achieving the special checking clearances spot-on, as a very small difference there will make a huge difference in duration as the "tails" at open & close are so shallow on the graph. I use a long travel indicator with resolution of 0.0001" (not that unusual) with a certified accuracy of 0.0001" (not that common!)

This is correct....anyone who has tried to replicate the factory specified duration of a BMW or Schrick cam knows hows frustrating this can be until you realize how they are measured. Much different then how domestic cams are recorded and measured.

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