gaining cam duration

[Barry Donovan]

I am deciphering something simple...but don't know how.

this is my cam. it is a simple cam, around .4 lift..very full fat profile.(injection)

spfi
10/54 49/15



a guessing game on where the duraton is coming from.. I simply added 2 degrees to everything
12/56 51/17




this is where I stopped. with a height of 1.56 inches, and a rocker ratio...just how long is two degrees, and why would lift start sooner and close later?

the lifters the cam came from were very very squishy, I mean absurd.. almost .25 inch squashy. the car was crazy with them. having solid lifters has made alot of noise when it has to..but am wondering how to decipher the gain in lifter, and duration together..just by being solid for the whole cam profile.

I learned generalizing..
beyond 260 gets rough. this would need more than 5 degrees gained in duration.
can lifters be that bad?

[Huskinhano]

I'm not quite sure of your question. But if you're trying to compare hydraulic to solid cam specs, from what I understand, you can't directly compare the two. If you do, the solid cam tends to act smaller. I recall seeing a formula to roughly compare. With the solid cam, you must have a "take up" for the tappet to come in contact with the lobe while the hydraulic doesn't need this feature. As far as the hydraulic tappet being able to compress, there's no oil in it. Since you can't compress a liquid, it basically goes away. They do leak oil though which does change things a bit.

It's been done running solid tappets on a hydraulic profile to try to make the cam seem bigger because the hydraulic's profile gets with the program sooner then a solid profile. The down side you have to run very, very little clearance to the point that expansion from heat in the components may not allow the valves to seat. it becomes a delicate dance, basically left to some one who is looking for something very specific and keeping a very close eye on everything.

[Barry Donovan]

yes that s where I am at.
hydraulic injection cam on mecahnical lift..
very robust round cam.. comparing to the pointy carb cam is something from american 50s to look at. could not wait to change it.
I forgot to mention this is sohc with a ratio rocker. the mechanical taps still get oil to help. very smart little setup.

I may be tricking myself... as overlap for injection likes a pulse back a little.
the carb has to rough idle to keep emulsifier happy. I don't recall ever having to be that precise. the part throttle adjust. seems a bigger cam than it is not. or maybe it is big.

starts up cold and drives. I'll go with that. gets better warm.

it would be interesting to profile it...a combo not built often.

[anotheridiot]

the more I read the less I learn. Overlap is the amount of time the exhaust and intake valves are both open, the exhaust valve is closing, the intake valve is opening to get the next cycle going. more overlap has the intake sucking harder since its getting some help from the leftover heat in the cylinder. the heat in the cylinder is the reason it gets better when its warm.

[Barry Donovan]

the more I read the less I learn. Overlap is the amount of time the exhaust and intake valves are both open, the exhaust valve is closing, the intake valve is opening to get the next cycle going. more overlap has the intake sucking harder since its getting some help from the leftover heat in the cylinder. the heat in the cylinder is the reason it gets better when its warm.

I blew the engine I was trying to figure out..about 6 hours ago.

back to hydraulic lifters for the next one..that is tomorrow.

there was too much of something.

the cams were just right, the casting for the block was an old error. almost like the forgotten 350s of the 70s...but worse.

duration needs tough stuff. regardless of static compression.