Its been a little while.......
So with low compression the lower duration increases static compression,correct? I'm assuming you want the most lift you can get while staying within your advised duration
Read Wyrmriders response carefully; it is correct.
Here; I'll expound;
So with low
a static compression
ratio, the
earlier closing intake valve of a lower duration
cam increases
dynamic compression
pressure,correct? Bold text added.
Simple answer yes.
But you don't ALWAYS have to sacrifice .050 duration to get that higher dynamic compression ratio. It really depends on the length of the ramps. As a fer instance; say you like a 223@.050 intake duration camshaft. The opening and closing ramps may jump this up by say; 38* to 58*, at advertised of from .008 to .006
tappet rise. This makes advertiseds of 281 down to 261. That is to say; all 223s, in a given engine, will make about the same absolute power at some higher rpm where they make peak power.. But obviously the 281 will idle like a race-car engine, while the 261 will be pretty smooth.
This also translates into how they will perform at low rpm, say below 3000 rpm; with the 261 making some 20psi more cylinder pressure, it will run circles around the 281 at these lesser rpms. But the 281 will not make any more power than the 261 cuz it still just a 223@.050. So getting greedy for intake duration, depending on the .050 and the combo, can make or break your smile-factor after installation.
But hold on we're not done yet. The engine cannot begin to build cylinder pressure until the intake valve is on the seat and not leaking. And at the advertised specs of .008 to .006 TAPPET LIFT, the intake valve, being on the other end of the rocker arm, is still OFF the seat from .009 to .012 !
And there is no way to know how much more duration it will require before it finally closes. The assumption is that the faster 261 cam is gonna close first....... but there is ZERO guarantee of that. That 223@.050 cam, therefore, may not close the valves until very very late in the cycle. This does not matter much to a race engine that might operate between 4000 and 6000. But at stall-rpm to 3000 it makes a really really big deal; and the lower the stall, the worse it gets.
This is why shopping for a cam is no small matter when the STATIC compression ratio is in the basement.
Hope that makes it clearer.......lol.
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Just for something to think about the fun place to start at is 130/135VP.
It doesn't much matter the engine size or the cam size or the pressure, none of that matters; all engines with similar VPs will run at about the same performance level from stall to 3000,in a given combo, depending mostly on the TC stall.
By 140/145 you are having fun, and the bigger the rear gear, the more fun you will be having. With iron heads it will take more cubic inches to get much more than 145VP (read about VP by clicking on the blue link at the bottom of this page.) But 145VP is easy with a zero-deck(or close to it) 360 and closed-chamber aluminum heads.
The smaller the engine, the harder it is to make bigger VP numbers. So the work-around is to just get a hi-stall TC and not operate in that sub 3000 zone. That means you can still have fun with a smaller engine; you are just gonna need more stall and more TM. I'm just a fan of more cylinder pressure for the way it wakes up the engine at lower rpms.Also, to my way of thinking, if you have to buy a TC and gears, sometimes it is just cheaper to buy pistons and fix the low Static compression ratio, in the first place; it has a bigger pay-off.
But some of this philosophy does not work on a manual trans car with streetable gears, which operates in that low-rpm zone almost full-time. For example 2800rpm with 3.23s is about 23.5 mph in first gear. So that combo is stuck with the engine married to the rear tires every single time it is accelerating from zero mph and it ain't getting any better until after 30 mph. Then, when the operator shifts into second at say 2800, like I do,the Rs fall back to 2000, and there I am stuck in la-la land again. And it happens on every shift. So I said to heck with that, and went straight to Aluminum heads which allow up to(and beyond) 185 psi, which jumps the VP up to 160 or more, depending on the cam's Ica. Now when the rpm falls to 2000, there is plenty of VP to accelerate with.... normally, lol.
Ohchit, I bet I muddied the waters right there, lol.
Here is a calculator for you to play with. Enter your engine specs, then play with the Static compression ratio, and then with the Ica, and see how big you can make the VP to be. To make working it faster, click on "page back" in the upper left corner, and you won't have to keep re-entering all the engine data.Enter your operating elevation to be accurate; this also makes a really big deal.
For iron open-chamber heads, do not exceed 160psi, as for a newbe, that is pushing the limits for detonation resistance for best gas. With aluminum heads you can target 185psi, but it would be nearly impossible to achieve that in an actual build, with a 318, for a reasonable amount of money.
As to the Ica, stop at 68*. For a 318 streeter this is just asking for trouble. Likewise the stock 318LA cam has an Ica of about 50* so no point in going any smaller than that.
Just remember these things;
1) as for acceleration;low VP numbers can be circumvented with higher stall TCs. By 3000 stall, even 2800 with some combos, Big VP numbers are no longer important.
2) However, the usually associated higher cylinder pressures always produce a stronger midrange, and more power. And
3) cruising with low VP numbers definitely reduces your cruise fuel economy.
At 652cc the 318 is severely handicapped. By contrast the stock 360 is around 737cc. and a 408 is ~832. So what this means is the same total chamber volume on each of these, will quickly drive the STATIC compression ratio to the moon. The commonly available parts just happily work well on a 360+.020; like this ;
(744.6+78)/78=10.55Scr The same 78cc on a 318 looks like
(652+78)/78=9.36Scr. and on a 408 it is
(832+78)/78= 11.67Scr
10.55 on the 360 just happens to be about perfect for aluminum heads and a small street cam; read early closing intake, of about 58 to 62 degrees advertised. 9.36 is already limiting the 318 to iron heads and a small cam...... just like the 318 Magnum runs.
Strokers usually run dished pistons to reduce the pressure
TripleL,Happy HotRodding