so i know everyone says the compression ratio determines the octane ratio, which is true, but which compression ratio?

dynamic or static?

i understand that the static ratio is very important, however the distance the piston travels after the intake valve closes is the actual pressure building until it sparks and explodes.

i have a 300 with 390 pistons. number h395p std( no overbore)
that means 4.05 bore
3.98 stroke(effective stroke is 3.812)
rod length is still the same at 6.2097
crank throw is 1.99
head gasket is, if i remember correctly 4.128 x .039
pistons should be stock 390 pistons with a 8cc valve reliefe on each side, so essentially a 16cc
76cc combustion chamber( heads been milled .01 to true up so i figure 70cc)
intake closes at 28 degrees abdc
deck clearance is .0257 due to the 390 pistons
gives me a static of around 9, on every calculator.

so why do people insist this will be a high compression motor?
it has bigger valves, 1.75 rockers and will be getting head work.
not broken in yet.

im worried about this because it has a rv cam.
int/ext
204/214
.449/.473
opens: -4/41
closes: 28/-7
110
-11 overlap

the cam im looking at is a howards cam
int/ext
209/209
.448/.448
opens: .5/36.5
closes: 28.5/-7.5
108
-7 overlap



according to DD it will have the same numbers trq is 350 hp is 230 within one or two
the rv having like 2 more trq, and 2 less hp

im more worried about the cr and if the howards cam would bleed too much off, and would it lope? i can get a custom ground cam for 96$ to match the howards cam

i used this https://www.uempistons.com/index.php...315f663c4f1a52

and got the exact same 9.052 dynamic cr and 9.4 static
which is not really that high of a cr, i can run 89, and keep a few octane boosters for bad gas in a small town.

also a good cleaning of the head should give me a couple more ccs which will give me a lower cr, with a 76ccs CC it will bump it down to 8.67 and a 9.0.

if im wrong on ANY of this, please let me know.
what are your opinions of this?
who else has used 390 pistons?

 

That dynamic number is off because you're using valve timing events at .050" lift. Stock cam closes the intake valve fully @70° ABDC, Comp 268h @60°, that Howards @~56° I'd guess. With a Comp 268h cam the engine I'm assembling will end up 9.5:1 static and right at 8:1 dynamic. Dynamic CR above 8:1 usually means premium fuel depending on the vehicle.

Both of those cams have less duration and less overlap that the stock cam - i.e higher dynamic CR.

I doubt either has a lope, but my definition of lope starts with a lumpy idle at 800-900 rpm. I can make the stock cam idle like crap too if I back the screw and timing down enough.

Taking .010" off the head doesn't remove 6cc of volume from the CC, maybe 1 or 2. Maybe.

 

how would i find out what it actually closes at?
im going to get a thicker head gasket i know, hopefully not stacking one.

went to go back and mess with numbers and saw this
"Intake Closing Point (degrees) ABDC @ 0.050 lift plus 15 degrees"

should i add 15 to it?


using the old school method:
Uou take the advertised intake duration and divide by 2. 102
Then you add the LSA to this number. 212
Then you subtract out the ground in advance. 208
Then you subtract 180 28
We will assume the cam we are installing is installed straight up.
This gives you an actual intake valve closing point to use in the DCR calculator.

i still get 28

 

Assuming a 300 head milled .010" and 16cc valve relief on pistons .025" below deck you have nothing to worry about. Static CR will be 9:1 at best.

Lemme ask you this, since there is essentially no difference between the two cams when you run dyno sims why change it?

 

i was looking at the howards cam for my engine rebuild, then i bought this built motor.
and not being fully trusting of people, im going to have it torn apart to looked at so they can do a dyno break in and tune the carb and work on the head which already has bigger valves, and since its apart, i figured it would be easier to swap to a nicer cam, but then saw the numbers were the same.
Im still unsure of the exact cc in the piston, and very uncertain of the compression ratio, no one has cc'ed a stock 390 piston, my dad will be doing that hopefully within a day or two.

this will be my dd for about another year ish, with car pooling, so im not too worried about gas mileage, but i live in the middle of know-where. we used to have to drive into town to get diesel for our truck and tractor. i dont want to do that on premium gas for my truck. although that zf5 im swapping in will help.

also if the stock closes at 70 degrees, why does the comp 252 close at 52? more compression means more torque?

 

You're still using duration at .050". Advertised for that cam is 270/280°, maybe?

 

that is what the cam card said it closed at, my bad. im not certain how to calculate that. ill look into that calculation more today.

 

The theory is that earlier intake closing, and the higher dynamic compression, should make more torque at lower rpm. There's more to cam design than intake closing point but it pretty much determines an engine's power curve.

 

Which is why it's so damn hard to choose a cam.

 

[quote=BaronVonAutomatc;14277848]"Both of those cams have less duration and less overlap that the stock cam..."



Uhhhh, no, that is not quite true. The oem cam is .192* duration @ .050. That Howard cam is what, like 210* @ .050, which is quite a bit more duration.

 

True, but what are the advertised durations on the OEM vs. that Howard cam?

 

Advertized is 268 on stock and 263 on the howards and 280/289 on the rv cam

@.050
Howards is 209
Rv is 204/214

 

"Advertised durations" like a lot of other advertised claims, are very much worthless, meaningless. Best use the .050 point for comparing specs.

 

Advertised duration tells a lot about the cam like how shallow/steep its ramps are and most importantly when the intake valve actually closes fully. Duration at .050" is worthless when calculating DCR because there's no way to know at what point the valve is closed and compressing the A/F can begin. Degreeing the cam is the only way to be 100% sure when the valve events are happening, though.

 

"Degreeing the cam is the only way to be 100% sure when the valve events are happening, though"

X2