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grumpyvette
04-11-02, 06:16 AM
Duration vs overlap ?
ok what your trying to do is keep the DCR as high as the fuel octane will allow and the duration and lift matching the rpm level you want to be most effective in, now lift per deg. of rotation is limited by the max feed ramp angle that the type of lifter your useing will allow without haveing the edge of the lifter contact the surface of the cam lobe as the cam rotates under the lifter and that is limited by the dia. of the cam lobes base circle to lift ratio and the dia. of the lifter or in the case of roller lifters the dia. of the roller wheel. thats why the ford guys have a slight advantage in that stock chevy lifters are 0.842" and fords are 0.874" this allows the ford cam lobes to be slightly more aggressive and thats also why those mushroom base lifters are used in racing engines in classes not allowing roller lifters. now the main idea is to get the valve open as fast as possiable and closed as fast as possiable within the limits the valve train can support in the rpm range you want max power in , since power is basically the engines torque x rpm and the rate at which it can be produced, and torque is basically cylinder pressure times piston surface area times leverage( max crank stroke) times the number of power strokes per minute your looking for max torque at the max rpm your engine can effectively flow air into the cylinders, so your looking for a ballance between the shortest possiable duration to maximize cylinder pressure that keeps the valves open long enough at that rpm level time wise for max air flow to fill those cylinders, ballanced against the least overlap that will provide efficient exhaust scavageing at that rpm level.now alot depends on the cylinder volume versus port flow numbers but in general (for racing)were talking about a rpm range in or near 6000-7500rpm and a intake duration of 245-255 for a 396-454 chevy with a 106-110 LSA and a .600-.700 lift for street use a rpm range of 1000-3000rpm drops you back to about 220-230 duration 108-112 LSA and .450-.550 lift but as I said before port flow and compression have a big effect on your choice here.ports that dont flow well may require a tighter LSA and longer durations in the cam you pick to allow greater time in which the cylinders can fill at those rpms and of course the 454 and larger engines haveing greater cylinder volume to fill tend to favor those longer durations and tighter lobe seperation angles more than the 396. also read these threads.



http://www.chevytalk.org/forums/Forum64/HTML/008208.html

http://www.chevytalk.org/forums/Forum64/HTML/008029.html


these are the valve timeing overlap ranges that are most likely to work correctly
trucks/good mileage towing 10-35 degs overlap
daily driven low rpm performance 30-55degs overlap
hot street performance 50-75 degs overlap
oval track racing 70-95degs overlap
dragster/comp eliminator engines 90-115 degs overlap

but all engines will need the correct matching dcr for those overlap figures to correctly scavage the cylinders in the rpm ranges that apply to each engines use range.

example
http://dab7.cranecams.com/SpecCard/DisplayCatalogCard.asp?PN=114681&B1=Display+Card

here is a hot street cam that works great in many CARBERATED 383 camaros with at least 10.5 static cpr with 3.5-4.1 gears
now the timeing is intake opens 29.0 btdc, closes 71.0 abdc exhaust opens 77.0 bbdc, closes 31.0 atdc so if we add the 29 to the 31 we get the overlap duration of 60 degs of which makes this cam fall in the center of HOT STREET