How do you get an eng to live with 12.1cr and pump gas, i would like to run this much and drive the car every day. What are some things that i should look for or try and do...

i no the cam is a big part in this, but how dose the cam help?
I think you want SC @ 9.0 if i am right here.

What are some other things i can do??

Deck hight? putting the pistion .9 in the hole, and a .30 heads gasket
with a flat top piston/dish???

thx

 

Because "when" that intake valve closes determines the amount of intake charge trapped (= cylinder pressure) at low rpm.

Whatever you do, you want to stick the quench height at .035-.050. Leave that alone. So figure your static compression based on these build parameters, then choose a camshaft that will produce the dynamic cr you need.

-Mindgame

 

I second what MG says about the quench.

"Wanting" 12:1 isn't very logical. The CR you should "want" is the one that will work with the cam you selected. If for some reason you can't change the CR (IOW, you are just replacing the cam and the motor is already built) than you had better pick a cam that will work with that CR. If you don't, you will either lose power or get detonation.

Rich

 

Um ok. I was thinging of putting the quench height @ .40 anyway..So dose it mater if put the piston in the hole or zero drck the block as long as i have the 35-50 quench height.

i have not built the eng.. looking into I don't under stand picking the cam for the CR i want.

Why is 12.1 not a good choise...

Is there a formula that i can use to play with numbers

 

Which brings up a good question for me...I plan on running the 11.9:1 compression JE pistons, the block will be at zero deck height and the Fel-1074 gasket that I plan on running has a compressed thickness of .039"...will this be enough quench height in order to spin the motor up to or slightly past 7000 RPMS??? (AFR 210 LT4 heads, JE Flat Tops, Custom matched camshaft)

 

Schurter, please correct your decimals. ".40" is about 3/8 of an inch or .375. If you mean .040 then make sure you get that "zero" in there. Otherwise we're constantly wondering about your units of measurement.

Like I said in another thread. You are going to have a real tough time getting the quench in ideal range if the piston is much further than .010 below deck. At .010 and with a .039 gasket, you get .049 quench. That's as much as I'd care to go.... actually I'd prefer a tad tighter depending on the rpm.

Al the stuff is going to fall in place statically. Not much you can do about it other than choosing a piston with some dish. What I'm saying is this... the combustion chamber, quench, cylinder volume... they aint changing. So you can lower the static cr with the piston volume and that's really about it unless you can find someone who won't wreck a combustion chamber (not recommended).

So, let's say your static just happens to come out to 12.0:1 with a flat-top piston.... you will need a cam with an intake valve opening event that produces a dynamic compression ratio of 9.0:1 or lower (8.5-9.0).

If you're getting lost on the terminology, I suggest a search in advanced tech for "dynamic compression ratio". You'll aslo find calculators in those threads.

Dustin,
Your quench height is just fine.

-Mindgame

 

12:1 is not a bad choice, but whether or not it's the RIGHT choice will depend on the cam chosen. I suggest you do a search on "dynamic comnpression ratio" to get a handle on this. As far as the geometry of getting the quench right it doesn't matter much if there is a zero deck or not. But most off the shelf pistons are designed for a zero deck, so unless you want a custom piston, that's probably where you will end up.

Rich