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Maximum Compression Ratio On Olds 455 - What's Your Problem?

Lobe-Separation Angle
Ron Cotter; via First and foremost, I love the magazine. My dad and I have been reading it as long as I can remember. I have a question regarding lobe separation. I was recently talking to a respected engine builder about an engine a friend of mine built that wasn't pulling enough vacuum. The gentleman suggested that my friend find a cam ground with a wider lobe separation to help with the vacuum. I understand the valves wouldn't overlap as much with the wider separation creating more vacuum, but I'm wondering what you sacrifice in the long run. Thanks, and keep up the great work with the magazine.

Jeff Smith: Your timing this month was impeccable, Ron, since this column earlier touched on lobe-separation angle (LSA). You are correct that a smaller (tighter) angle between the intake and exhaust lobes increases the overlap between when the exhaust lobe closes and the intake lobe opens. This creates an overlap when both valves are open in the combustion chamber. The triangular shape of overlap in cam degrees can be easily seen in the accompanying Comp Cams illustration. It allows a crosstalk between the exhaust and induction systems that tends to reduce intake manifold vacuum at idle and low engine speeds.

Overlap is generally something the new-car manufacturers tend to avoid because it drastically lowers idle quality. But the advantage to overlap is it allows the engine to breathe more efficiently at higher engine speeds, which dramatically improves power. This doesn't happen just at peak horsepower. Dyno testing has shown that just tightening the lobe-separation angle from 114 to 110 degrees with a mild street engine can be worth as much as 10 to 20 lb-ft in the midrange of around 4,000 rpm, and the engine will even see a torque increase as far down as 3,000 rpm under wide-open throttle. Pulling the lobe even tighter to around a 108-degree lobe-separation angle will improve the power even more, especially in the midrange.

But before we set this concept in stone, we have to also look at the actual number of degrees of duration and overlap. If you look at longer-duration camshafts, you may notice that these cams often use a wider lobe-separation angle of 112 to perhaps 114 degrees. This is because a longer-duration camshaft (280 degrees at 0.050 for example) with a tight lobe-separation angle will physically have far more actual degrees of overlap than a smaller camshaft of say 230 degrees at 0.050. So you have to be careful when referring to LSA angles because the physical amount of actual overlap will change with the size of the camshaft. Based on all this information, you can see why many camshaft companies prefer to use a 110-degree lobe-separation angle because it's a great compromise between idle quality and increased power. It is also why dedicated, normally aspirated, drag race engines tend to idle with a radical sound-they've tuned the LSA for best overall power. The problem with a tight LSA on a street engine is that it tends to be lazy and requires a rich air/fuel mixture to idle properly with a carburetor and an automatic transmission if the converter is a little on the tight side. This is what we've run into on our Lester Scruggs 404ci LS engine with the big camshaft with lots of overlap at idle. To make the engine run in gear, we have to tune the carb to a rich 12:1 air/fuel ratio in Neutral or Park so it will idle in gear without dying. That's part of the sacrifice of a camshaft with lots of overlap.

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Comp Cams
Memphis, TN

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