A long-duration performance cam opens the intake sooner and closes it later. As an example, it might open the intake at 40 BTDC and close at 85 degrees (40 + 84 + 180 = 305 degrees). Note that this longer-duration cam closes the intake valve a whopping 35 de-grees later than the mild cam. This later-closing intake effectively moves the peak torque to a higher engine speed to make more horsepower. This later-closing intake also demands a higher compression ratio to make acceptable power below the peak horsepower point because the later-closing intake will bleed off cylinder pressure. The reason for the longer-duration and later-closing intake is to give the engine more time at higher engine speeds to fill the cylinder.
Crower's plan involved radically increasing the compression while closing the intake valve much later. The later-closing intake valve point bleeds off cylinder pressure at lower engine speeds so the capture ratio of air and fuel in the cylinder is lower. But because we're squeezing what remains much harder (with the higher static compression ratio), Crower's idea was that the high compression would improve fuel mileage. Crower actually designed a kit for the small-block Chevy that utilized this technique. It did work to some extent but didn't really deliver on the improved fuel mileage idea as much as planned, and the concept never caught on.
Now let's jump ahead to today when the L99 engine used in the '10 Camaro uses variable valve timing (VVT) to either retard or advance the camshaft as much as 52 degrees to enhance performance and mileage. In an ideal world, you would want to separately control the intake lobes from the exhaust lobes, which is what the '08 to '09 Viper V-10 does with a cam-within-a-cam design that allows the engineers to change the lobe-separation angle. This allows the computer to then vary the amount of overlap. The advantage is that widening the lobe-separation angle at idle and low speeds will improve fuel mileage and then narrowing the angle at higher engine speeds will improve power.
The aftermarket has always been more concerned about performance, but it appears that tuning these engines beyond the stock settings might enhance power and fuel mileage by altering the conservative OE parameters. For example, you could increase an engine's static compression ratio and then tune with overlap and intake valve closing to see if there are advantages to fuel mileage. It appears that EFILive addresses the VVT engines, but the tuning is, as you might guess, complicated. But hey, if it were easy, everybody would be doing it, right?
HP Tuners; Buffalo Grove, IL; hptuners.com
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Car Craft Summer Nats!
Summer is almost here. Join us for the CCSN at the Minnesota State Fairgrounds July 16-18, 2010, and have the time of your life!
A Serpentine Answer
Mickey Cummins, via CarCraft.com: I've been reading Car Craft since the '70s and got really excited about the budget serpentine beltdrive article (Nov. '09), as I would like to do this to the 350 in my '86 Silverado. However, the story brought up a few questions. You stated the beltdrive system for this story was from the late '80s/early '90s engines. Will this bolt right up to the '70s block I am using? Will the brackets bolt up to my stock heads? What heads were on the 383 you used in the story? What year was the block? I haven't had a need to learn anything about late-model blocks until this time, and I am woefully ignorant about them. I am guessing the late '80s to early '90s engines share the same block as mine? I thought the tip about changing fittings on the PS pump was trick. Can or does something similar need to be done to use the late-model A/C compressor?