Figuring Cam Overlap
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From: Frisco TX (Dallas Area)
Figuring Cam Overlap I hope this an appropriate topic for the advanced guys. I'm busy as a bee trying to learn about cams for FI, particularly turbo apps, but there seems to be very little math posted for figuring the actual overlap number. I know overlap depends on lobe profile to a degree (low-lift characteristics), but almost everything is spec'd at .050.
So the specific question. What is the general math to figure overlap from a cam spec'd at .050 given the LSA and advance/retard installed position? I've heard some calcs, but they seem suspect. Who's got the straight poop?
A side question would be to calculate the overlap using the cam card with the valve events specified.
Thanks, guys and gals...
So the specific question. What is the general math to figure overlap from a cam spec'd at .050 given the LSA and advance/retard installed position? I've heard some calcs, but they seem suspect. Who's got the straight poop?
A side question would be to calculate the overlap using the cam card with the valve events specified.
Thanks, guys and gals...
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There is no fixed relationship between 0.050" lift timing points and the several low lifts points that approximate "advertised" numbers. It is specific to each cam or family of profiles. There are however reasonable approximations available, e.g. this link from elsewhere on this forum: http://users3.ev1.net/~black_ops/ca...0calculator.xls , which will also calculate the actual events at various lift points.
FI, especially turbo, has very specialized requirements, particularly when less than perfectly engineered. Turbo exhaust backpressure can be double the boost pressure, requiring negative overlap to prevent massive reversion. There are a couple of excellent books on turbocharging, one by Hugh MacInnes and one by Corky Bell (search Amazon .com for stuff by these authors)
FI, especially turbo, has very specialized requirements, particularly when less than perfectly engineered. Turbo exhaust backpressure can be double the boost pressure, requiring negative overlap to prevent massive reversion. There are a couple of excellent books on turbocharging, one by Hugh MacInnes and one by Corky Bell (search Amazon .com for stuff by these authors)
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Thanks, MadBill, but the link doesn't seem to work.
The calc I've seen (that is suspect) is a sum of the .050 duration/4 - LSA. That doesn't always seem to work though. I've taken in the info on the backpressure created by a turbo and I think I understand the theory pretty well. IOW, if the intake pressure ratio is 1.8 (about 12 psi) and the restriction of the turbo backpressure is 2, then reversion is worse the more overlap you have because the pressure of the exhaust can overcome the boost pressure. The books are something I have checked out, but they all seem to base everything on the duration at .005" lift and the event timing at that lift. Hmmm...
The calc I've seen (that is suspect) is a sum of the .050 duration/4 - LSA. That doesn't always seem to work though. I've taken in the info on the backpressure created by a turbo and I think I understand the theory pretty well. IOW, if the intake pressure ratio is 1.8 (about 12 psi) and the restriction of the turbo backpressure is 2, then reversion is worse the more overlap you have because the pressure of the exhaust can overcome the boost pressure. The books are something I have checked out, but they all seem to base everything on the duration at .005" lift and the event timing at that lift. Hmmm...
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From: Frisco TX (Dallas Area)
Originally Posted by Beast96Z
I've always used : duration + duration /4 - LSA *2.
(intake duration + exhaust duration)/2 - (2 x LSA)
I'm gonna bet that works out to the same thing as you just said. Thanks for the info guys!
The point of all this is that I have a "good" cam already installed in my built stroker for my street machine. The engine will be N/A for a while (2-4 months), but will see a turbo at 8-12 psi after break-in. I was trying to determine if the cam spec'd for me was decent for both or if I needed to budget for a turbo cam. The cam is 224/228 - .581/.586 @ 114 +0. It appears that it will do the turbo job pretty well since the overlap calc is -2°, although it's probably not optimum for either. Again, thanks for the help.
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Will try again with the link.(BTW, it's a thread under Advanced Performance Tech) As soon as I post it, I'll test it and try to fix it if it still doesn't work. (EDIT: OK, it worked twice for me, perhaps it will for you too...) Maybe it's just me, but I have a hell of a time finding a thread again after I leave it for any reason...)
https://ls1tech.com/forums/showthrea...light=formulas
https://ls1tech.com/forums/showthrea...light=formulas
