ddnspider

iTrader: (26)

I understand that you can only have so much lift for a degree of duration.But take for example the original LPE GT2-3 cam its around a 207/220 .575/.586 119.5.It idles like stock and makes pretty good power for being such a small cam.probably because it has decent lift.What would happen if someone made like a 215/215 .605/.605 cam?i would think it would/could make even more power and still sound really close to stock.discuss or explain.

Sammyboy

iTrader: (9)

The sound isn't a product of lift rather that of lobe separation. Changing lobe separation change power bands, cylinder compression, increase or decrease engine knock, change idle vaccum, open-valve overlap, and of course piston to valve clearance. But let us consider duration and its connection to lobe separation. As strange as this may sounds, more duration can be helpful in high RPM engines but not low RPM engines. The extra degrees of open valve time in high RPM engines gives the air flow a little more time to get into (or out of) the cylinder in spite of the pi ston's stroke. However, at lower RPMs, more duration can cause less power because the valves will be open at the wrong time in relation to the piston's stroke up or down in the cylinder. So what has to happen when you increase duration?

ddnspider

iTrader: (26)

i am aware of a wider LSA sounding more stock and how more duration can cause reversion at low RPM's,etc.I also realize that it would require a fairly sharp lobe.maybe i should have given more detail.....if you can have as much lift as you do on as little duration as you do for the GT2-3 cam....then why cant you go bigger on both lift and duration,ie increase by some proportionate amount.a 215ish duration is a decent amount of time to allow air in,especially if the valve is being opened .600 in lift or a bit higher.

jimmyblue

iTrader: (11)

Part of it is head and intake runner "stall", more lift
stops helping after a point and with stock 241 heads
that is somewhere in the .5xx lift range. The only
thing that helps beyond this point is more open-time.

Because of finite ramp-rates, a little more duration
gives more than proportionate improvement in flow;
the duration numbers are at barely-open and the
ramp degrees are subtracted "off the top" as far as
real airflow is concerned. That's where all the fancy
lobe profiles come in, how hard can you slap it up
and pull it down with the valvegear you can afford?

Because exhaust and intake stroke are adjacent,
duration and separation can't be "separated". Push
duration up and keep it in the place it needs to be,
then you chew into separation angle with negative
effects on low RPM behavior.

gametech

iTrader: (1)

Is this a joke thread, or did you just want to start a theoretical discussion? I only ask this because I have read some of your other threads, and it seems that you probably already know the answer to your question.

ddnspider

iTrader: (26)

lawl,caught me!J/K.the thought actually stemmed from a local debate about my old motor that had the GT2-3 cam thats now in an 1982 z28 with a TH400,lol.I was mulling it over in my head and realize there are limitations but if you look at the specs of that cam,theres more than enough room to play with the idea.hell going to a 114 LSA gives you 4.5* of angle to play in reference to duration and such.I was just bouncing the idea off to see what others thought.I think it can be done.

quikrnmost

The answer to the original question is because high lift, smaller duration is alot harder on the valvetrain. Personally I think the high lift cams are over-rated. A cam with the right valve timing is far more important than lift.