Questions on camshaft LSA and overlap

[dhickey]

Hello all,

This is my first post. I have been reading for a while and have found a ton of useful information on this site. I have also been searching other places and reading as much as I can on timing events. I have only been able to find generic recommendations for LSA and overlap selection.

For overlap there are usually generic ranges based on idle quality and intended use. I have to believe there are better ways to get optimal overlap for a given combination. Anyone know of methods that take other variables into acount? I was thinking intake port flow at the low lift values that fall into the overlap triangle would be important. What about displacement or at least stroke?

LSA recommendations seem to be equally as vague. I have found charts that provide a recommended LSA based on displacement per inch of valve diameter. Wouldn't calculations base on curtain area of an open valve or some average intake port flow numbers get a little closer to an optimal configuration?

I am willing to purchase a books or even a piece of software if I could be sure that it provided me with more info than the generic stuff you get from Popular Hot Rodding, Hot Rod, Car Craft and the like. I do have dynoSim. Basically all I can do is look to see what configurations provide the best power, but can't rely on that information due to the limited options for intake and exhaust modeling. On another board it was suggested that I purchase Camshaft Reference Handbook by Don Hubbard but no one could verify that it provided what I was looking for.

If I can come up with a good spec for LSA and overlap duration is a simple calculation. ICP is easy enough to play by advancing or retarding the cam.

Thanks in advance for any assistance.

Dan

 

[Lou Scannon]

I doubt you'll find hard and fast rules for valve timing unless you boil it down to one particular engine configuration and application. What are you trying to accomplish?

 

[EngJW]

If you have not seen it already, find the book on cams and valves by David Vizard. It is widely available- Classic Motorbooks, etc.

I used to think only in terms of opening and closing points until reading this book. It has the best explanations of lsa, duration, advance and retard that I have ever seen, and now I prefer to use these terms. You can look at two very different timing diagrams, but the cams may be exactly the same except one is advanced or retarded a few degrees. There is a lot of application information too, like how much duration and lsa for street, racing, turbocharging, etc.

 

[dhickey]

Thanks for the tip on the vizard book. I have some of his other stuff and it's been pretty good.

Just trying to educate myself and evaluate options for my next project. It will be a small block chevy with a couple of small twists. I don't think an off the shelf grind is going to be optimal for the combo I am considering. I will be focusing on maximum power below 6500rpm while still remaining streetable and minimizing intake reversion. My intake selection may end up being restrictive compared to the rest of the system so I really want to maximize the intake events. It's going to be EFI controlled, so I'm not too concerned with getting it to idle well and be responsive.

 

[jbthiel]

I think that you don't find much detailed info on LSA because the average "hot" street engine is not super sensitive to it. I have seen a couple of tests in Hot Rod, etc. that showed some minor improvement when tweaking. The the overall affect was go 2-4deg one way from optimum (and I can't remember which way off of the top of my head) and you will only loose a little, go to far the other way and you loose a lot. So must people stay on the conservative side.

I do agree that I don't think there is enough attention given to overlap. Most people look at duration numbers, but this doesn't tell the whole story. The overlap triangle is influenced by the ramp rates (roller vs solid vs hyd), rocker ratio, lash, etc. And the effective overlap is also affected by low-mid valve flows, intake tuning, exhaust tuning, etc. It seams like the guys who finish well in the "engine masters" series have good low-mid lift flow numbers.

 

[dhickey]

I am a little disappointed in most of the magazine articles I read and the off-the-shelf grinds of the big cam manufactures. Both seem to be very conservative when it comes to LSA. I think they assume (rightfully so) that most people are going to over cam and they spread or recommend spreading the LSA to compensate. What % of off-the-shelf street grinds have greater then 110deg LSA?

Most magazine articles are testing 114, 112, 110 or maybe the occasional 108deg LSA cam. Most of them are with cams from big companies that have less than optimal ramp rates and conservative designs. Seems like every article uses Crane or Comp Cams hydraulic. I wonder if varying LSA on an Isky, Voodoo, or old Ultradyne cam would have greater effect. What about the solid lifter cams? I would also like to see some testing with tighter LSAs. Who knows if a lot of engines would respond well to 106deg LSA if overlap is kept in check? They never test it.

I would also like to see tests that didn't keep duration constant while changing the LSA. All of the articles I see first select duration and then vary LSA/overlap. It seems that may be a bit backwards. How about selecting more than one cam for each LSA. On any given engine I think you would need less duration as you tightened the LSA unless it was undercammed to begin with. At least advance or retard it a few degrees to get some indication if it is now overcammed.

I think to really do this right you're talking about a really long or multi-part magazine article. Probably better suited as a chapter in a book about timing events.

Anyway, I am sure guys are conducting this kind of research. I would sure be nice if someone published it for the rest of us.

 

[Lou Scannon]

Research costs money. Generally speaking, the people that get to see the results are the ones that paid for it.

 

[oab]

The Lobe Separation Angle, is the end result, not the starting point.
You choses the duration and lift. to suit the rev range that the engine has to operate at. You set the inlet lobe center to correspond to the maximum air speed rpms. Set the exhaust lobe center to suit the exhaust needs, to reduce pumping losses, scavenging.
What ever the LSA number is, it is just a number. If there are overlap problems, you have probably used the wrong settings.

Anyway thats the way I see it.
Harvey.

 

[boatmotorgod]

I like to use the dyno-simulation program from Performance Trends.It has inputs for the exhaust system, header primary length and diameter intake length and diameter.you can either choose pre-entered cylinder head data,or use flow bench data to enter data about your own heads. Runing conditions like temp octane ect. Enter your engine combination, then choose the operating range you are interested in using, and have the computer optimise the cam specs to achieve max avg tq or hp, or max peak tq or hp. You can choose to optimize intake or exhaust LC's cam advance, or lift and duration, or all at once. That can take 3 hours on my computer. It spits out Mach #'s, BSFC, Volumetric Efficiency and all kinds of good stuff. Then you can have a custom cam ground. It has always been on the money for me. oh and it gives you information on the effects of overlap on your current combination. Check it out.

http://www.performancetrends.com