Thread: Cam Question

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Posted by jimmyjeep:
"I understand the closing point of the intake valve is the most imortant, correct? "
Yep, in order of importance they are: intake closing, then intake opening, then exhaust closing, then exhaust opening.

Here's some very good info from the mind of Dema Elgin of Elgin Cams:

Engine Cycles and Cam Timing

Once you have the particular performance characteristics of the heads carefully mapped, use that information to select the one cam that makes the engine make power like it wants to. Compare your test results against what the engine experiences while it goes through all six cycles.

Engine cycles are determined by the direction the piston is traveling and the timing of the openings and closings of the valves — collectively termed valve-timing events. Timing these events becomes complicated because a lot of compromise must be made in order to balance out all of engine operating cycles. The May 2004 article outlined the six engine cycles of a four-stroke engine. This time we’ll walk through each cycle in turn, now from the viewpoint of determining how changing every valve-timing event affects other cycles, and how balance builds power.

· The Power Cycle: TDC to Exhaust Valve Opening

By the time the crankshaft reaches 90° ATDC, cylinder pressure has dropped greatly and most of the power that can be recovered from it already has been. So opening the exhaust valve well before BDC loses less power from the power cycle than it later gains across the following cycles. The lower the rod ratio, the faster cylinder pressure drops.

· The Blowdown Cycle: Exhaust Valve Opening to BDC

The blowdown cycle relieves excess (but unrecoverable) cylinder pressure and begins clearing exhaust gases off the energy of their own pressure. Otherwise the piston would have to push all the exhaust gases out of the cylinder on the next up-stroke, lowering horsepower from a pumping loss.

The timing for Exhaust Opening is the least important of the four valve events. It can be anywhere between 50° and 90° BBDC, so its timing is easily adjusted to match the performance characteristics of that engine.

With higher compression ratios the burn rate is faster, so the exhaust valve can be opened earlier, which aids in the cylinder blow-down. With lower compression ratio (static 8:1 or lower) you want to delay the exhaust opening as late as possible in order to utilize the last usable bit of pressure that is on top of the piston. But that hurts the top end horsepower, because the blow-down period is no longer as effective.

· The Exhaust Cycle: BDC to Intake Valve Opening

The piston reaches maximum velocity at about the same number of degrees BTDC as it did ATDC on the way down, or a degree or so sooner with offset wrist pins. The exhaust valve must be open sufficiently by this time so that spent gases in a hurry meet little resistance against being pushed out.

How far the valve must be open is known from flow-bench data. The proper cam meets that need from a combination of timing, total lift, and its rate of lift (its “velocity”).

· The Scavenge Cycle: Intake Valve Opening to Exhaust Valve Closed

The scavenge cycle occurs during the overlap period, when intake and exhaust valves are both open at the same time. The intake valve is just opening. The exhaust is closing but not yet seated. Overlap is what the cam and valves are doing, dictated by the combination of total cam duration and the locations of lobe centers. Scavenging is what the engine is doing with that.

A good number of engine processes (and a few unsolved mysteries) are going on now simultaneously. The most important are (1) scavenging the last of the exhaust gases as much as possible from the clearance volume, where the piston cannot reach to push them out, and (2) initiating intake flow into the cylinder without wasting very much of it out the open exhaust valve.

Overlap duration increases as total duration increases, and it also increases as the lobe center decreases. Increasing the time for Overlap makes more time for scavenging at high rpms. Residual exhaust gases kill power twice over: they displace their volume in incoming charge, and later during combustion they absorb heat that should have gone into making power. At 5000 rpm an engine with a high-performance cam carrying 55 degrees of overlap must complete the entire scavenge cycle in less than two thousandths of a second.

In standard engines, valves are open together for only 15-30 degrees of overlap. In a race engine operating between 5000 and 7000 rpm, the overlap period is more like 60-100 degrees. The penalty for so much overlap in a street engine is very poor running at lower rpms, when a lot of the intake charge has time to sidetrack directly out the open exhaust valve. Mileage goes South. Heads overheat from fuel burning in exhaust ports. The engine runs hot. The exhaust system gets fueled like a blowtorch. The tailpipe turns white. Catalytic converters fry. The buyer blames the cam grinder.

Timing Exhaust Closing must be balanced against flow through the intake port. If the intake port flows poorly from being too small (or too large) then later Exhaust Closing might help to initiate intake flow. I consider this only as a last resort for kick-starting a lazy intake port. It always carries some charge out the exhaust valve, wasting fuel and all that.

Make the overlap period as short as will complete the job of scavenging. Factor in the effects from the combustion chamber size and shape (including the shape of the piston top) and shrouding near valves. Balance power goals with other requirements for the intended usage, such as idle quality, low-speed throttle response, fuel economy, and smog test compliance.



· The Intake Cycle: Exhaust Valve Closed to Intake Valve Closed

I consider Intake Valve Opening the second most important valve timing event, because that does two important jobs. (1) It initiates the Scavenge Cycle and (2) it begins lifting the intake valve out of the way of the incoming charge. The air/fuel mixture began entering the cylinder during the Scavenge Cycle, builds to a maximum, tapers off, then packs in a final gulp.

The intake valve is in a race with that pressure differential at maximum piston velocity that drives intake flow. The valve always loses this race, because max draw happens between 70° to 80° ATDC, yet the intake valve does not open fully until it reaches centerline, down around 105° to 115° ATDC.

When you can’t win, do your best. Get the valve out of the way as far as possible by giving it a fast rate of lift, a “high velocity”. Much the same could be accomplished by more valve lift, but then the nose of the cam gets pointy and real stiff springs are needed for closing the valve – a combination not favorable to very long service life.

The Intake Closed point – when the valve seals on the seat – is the most important valve-timing event. This event governs both the engine’s rpm range and its effective compression ratio. Closing the intake valve later optimizes intake flow for high rpm and allows inertia to pack in its last gasp of air. The drawback to that is back-flow at low rpm. But closing the valve earlier shuts down rpm. Pick your operating range.

Jim, use the Crane Cams site to get close on the duration and closing point relationship to static compression ratio so you can begin to get an idea of how to choose a cam. Getting your head wrapped around these relationships will prepare you to use a dynamic compression ratio calculator and zero in on the correct cam. Start with the first cam on the list, click on it and note the recommended c.r. in the upper right hand corner of the card and how it relates to the intake closing point and so forth. At the bottom of each card, you'll see the centerlines of the intake and exhaust lobes. They are shown on the card as "max lift intake" and "max lift exhaust". Add these centerlines together and divide by 2 to find the lobe separation angle. Subtract intake lobe centerline from lobe separation angle to determine how many degrees the cam is ground in the advance position. We'll use the Gen I SBC charts to keep things simple:
http://www.cranecams.com/index.php?s...=262-400%20C.I.