This reduced overlap improves the idle quality, but also tends to hurt the top-end power slightly. Idle is set at 800 RPM. This is not a strict necessity; you could go with an old-school flat-tappet camshaft, but you are losing out on horsepower, torque and quite possibly fuel efficiency. ©2021 SAE International. It has a radical idle and low vacuum. jack vines But really, the biggest issue is the valve overlap between the intake/exhaust profiles, which can't be changed unless you change cams. Some engine specialists refer to it as that 106 sound—the NASCAR Cup and short track oval sound where lobe separation is set at 106 degrees. but lost 2 mph. The narrower separation makes the engine sound choppier. As the LSA number gets smaller (from 114 down to 106 degrees), more overlap is created where the exhaust valve is closing at the same time the intake valve is opening. Free play through hundreds of levels, or race against the clock in Time Trial mode. idle). A higher LSA reduces overlap, which improves idle quality and intake vacuum, however using a wider LSA to compensate for excessive duration can reduce power and torque outputs. Tighten (smaller LSA number) Max Lift or Nose; Flank; Opening … A tighter centerline starts torque curve sooner, and doesn't give a wide power band. Avoid any cam with more than 30-degrees of overlap. This is also why stock GM LS camshafts use an LSA between 116 and 122 degrees – to smooth the idle and help the O2 sensor. In reality, a cam with 220º at 0.050" and a lobe center angle of 112º will have the exact same mechanical overlap as a cam with 217º at 0.050" and a lobe center angle of 109º. As overlap increases, this tends to make the engine idle quality more erratic or lumpy, while improving mid-range and top-end power. As the LSA number gets smaller (from 114 down to 106 degrees), more overlap is created where the exhaust valve is closing at the same time the intake valve is opening. Camshaft Idle Quality:How to tell from Specs. The objective of the present work is to determine a simple methodology to calculate, during the pre-development phase, a geometrical parameter correlated to the combustion quality at idle. This is because it also opens the exhaust valve a little sooner; perhaps before the cylinder pressure has fully expended its energy pushing on the piston. Engine stability at idle is an important factor that influences the behaviour of an I.C., S.I. Camshaft Overlap Calculator: Use Advertised Duration Numbers and the Lobe Separation Angle. Lobe separation affects valve overlap, which affects the nature of the power curve, idle quality, idle vacuum, etc. With the HP carb you should have a good enough idle circuit to idle clean at the higher speed. simply put. A cam with timing of 25-65-65-25 has (25 + 25) = 50 degrees of overlap. 15 Comments. Comparing the idle vacuum between these engines is a direct reflection of the effect of overlap on idle quality. Can you determine overlap just by knowing a cams intake duration,exhaust duration,and lobe separation ? For a longer cam, the only way to restore idle quality is to spread the LCA, which reduces the overlap. “It affects the amount of valve overlap; that is the brief period of time when both the intake and exhaust valves are open.” For example, an oval-track small-block that needs to motivate the car off the turns quickly might have a narrow LSA of around 106 degrees, resulting in more overlap. Cam Lobe Separation Lobe separation can be measured using a dial indicator and a degree wheel, but is usually calculated by dividing the sum of the intake centerline and the exhaust centerline by two. Any camshaft with a significant amount of overlap is not recommended for street-driven cars. However, since tighter lobe separation increases overlap, idle quality will be degraded. But what about blown or supercharged engines? Control of valve overlap is also the key to good idle quality. What does it do? Engine stability at idle is an important factor that influences the behaviour of an I.C., S.I. Let me clarify my situation a little further. How to Choose Your Cam.. engine, in terms of fuel efficiency, exhaust gas emissions and customer comfort. The more overlap a cam has, the rougher the idle. Power brakes generally require between 14 and 17 inches of vaccum at idle to work properly. It causes a lot of intake reversion which causes the engine to basically choke on it's own exhaust during idle. Overlap causes misfires at idle and that makes the lope or choppy idle. better cylinder filling = more compression, bigger BOOM sound. Increased overlap equates to reduced idle quality, vacuum, and harsher running prior to coming up on the cam. Bahloooq on April 9, 2018 at 10:57 pm I have … View Full Version : Camshaft Idle Quality:How to tell from Specs. Watch out for overlapped branches, it will break if cross or overlap! For normally-aspirated engines, overlap generally improves engine performance by starting the intake cycle before the exhaust cycle has finished. Subscribers can view annotate, and download all of SAE's content. At high rpm, overlap takes advantage of the Scavenging Effect. A narrower LSA adopts more overlap and with it a lumpier idle and a narrower more specific power band. If you choose a cam with too much overlap, the idle and, hence, street driving manners suffer. That article is very informative ! In particular, the increasing daily use of vehicles in urban traffic bestows more and more importance on the engine idle quality. Low-speed throttle feel was good, and at the drag strip it ran the same E.T. The resulting internal EGR from high valve overlap tends to reduce engine stability at very light load (e.g. 2/20/2021 10:19:26 AM PONTIAC POWER RULES ! The closer the Lobe Seperation Angle the more the overlap, the more duration the more (typically) the overlap. Adding more overlap helps mid-range torque and top-end power, but it kills low-speed idle quality, which is why your engine only idles at 8-9 inches of manifold vacuum. However, at idle and low rpm, overlap will produce low Engine Vacuum and a rough idle. The lumpy-idling 101 LSA cam churned out an average of 484.0 hp at 6,100 rpm with 493.9 lb-ft of torque at 4,400 rpm. COMP Cams - The effect of changes In cam timing and lobe separation angle Select Store. The Ford ModMotor SOHC guys would be your closest cousins and best source of info on cam duration idle characteristics. ... which affects many performance factors particularly idle quality, peak torque that can be moved from a lower rev range to a higher range and power bands that can be narrowed or broadened. Yes, you can calculate the seat timing events using advertised duration, lobe separation, and the intake lobe centerline (most cams are ground 4 degrees advanced, so the centerline will be 4 degrees less than the lobe separation angle). A wider lobe doesn't start the torque curve sooner, but it continues to make torque longer and has a broader power band. Streetable cams usually range from very mild and close to stock idle … Choosing performance cams can often seem like a difficult task for many, considering the many variables that can apply to any one camshaft profile or application. cam controls the valve action and timing. A Calculation Methodology for Cam Overlap Optimization Towards Combustion Quality at Idle in IC SI Engines 2003-32-0040. engine, in terms of fuel efficiency, exhaust gas emissions and customer comfort. Keeping this in consideration, what gives a cam a lopey idle? All rights reserved. This cam idled at the identical rpm, with the vacuum about the same, but it was slightly rougher at idle. Purchase a cam designed for use with roller-lifters. A tighter lobe has more overlap. Overlap is the span (measured in crankshaft degrees) when the exhaust valve and intake valve are both open. In vehicles with nitrous or superchargers, a wider lobe separation angle is often recommended. Combustion characteristics play an essential role on the overall engine quality level at idle. Camshaft overlap is the angle in crankshaft degrees at which both the intake and exhaust valves are open at the same time. A given duration and overlap will behave completely differently in your application than it would in a larger displacement SBC or BBC with antique induction and ignition. As overlap increases, this tends to make the engine idle quality more erratic or lumpy, while improving mid-range and top-end power. The result is a choppy or lumpy idle.
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