My90Iroc

iTrader: (26)

I'm doing some research about dynamic compression and in my searches it seems a dynamic compression of 9.1:1 or lower is needed for pump gas (93). According to the calculator I'm using it seems an F13 with 5.3 heads will yield too high a dynamic compression, yet I see combinations very comparable to this a lot on this board. Tell me if I'm doing or inputting something wrong.
http://www.kb-silvolite.com/calc.php?action=comp

Here are my input values:
Cyl. head cc: 61.15 (stock 5.3?)
piston head cc: 0 (flat top)
gasket: .053 (Fel-Pro MLS per jegs.com)
gask. bore: 3.945 (Fel-Pro MLS per jegs.com)
Cyl. bore: 3.898
Deck Clear.: -.008
Stroke: 3.622
Rod Length: 6.098
IC ABDC: 58 (43 for F13 + 15 per instructions on calculator)

Results: Static Comp. 11.092 Dynamic Comp. 9.267

Let me know what you guys think. Thanks.
__________________

MadBill

For a double post, a double reply...: https://ls1tech.com/forums/generation-iii-internal-engine/409577-dynamic-compression-question.html

DAPSUPRSLO

iTrader: (14)

Hey MadBill, how accurate do you think that calculator is? According to that my buddy with his 12.8 to 1 SCR sb2 motor can run pump gas, if 9 to 1 DCR is truly the magic pump gas or no pump gas number. His cam doesn't close the intake valve till 69 ABDC though. If that is true I suggested he may want to advance that cam a little to bring the DCR up a hair. Thanks.

MadBill

The calculator came out exactly the same as my full-scale geometry diagram of an engine, so that part is correct. I think the part of the concept people have trouble with is the fact that detonation is mostly a low speed phenomenon, so that at high RPM, when the cam comes into tune and the V.E./B.M.E.P shoots up, the now higher-than-'mechanical' CR still isn't a problem. With aluminum fast burn chambers and good knock sensors, LSx engines should be far better off than the average iron head SBC for which the 9:1 DCR limit is suggested.
I've been working on specs for a 7 litre engine for a road race Z06, and with the cam Dynomation 'wants', it needs over 14:1 to reach 9:1 DCR!

DAPSUPRSLO

iTrader: (14)

Ok good deal. One more question though, when the cam comes into the optimal VE/BMEP and actual cylinder compression becomes higher can one run into problems with detonation cause of the higher cylinder pressures or does the speed at which things occur at those rpms limit the time for detonation to occur to a point where it's not an issue? Sorry for all the questions, inquiring minds want to know.

treyZ28

BMEP is always the issue when ion, detonation, air flow or power are in question

BMEP is actually what will cause detonation. you can have 19:1 dcr but if your bmep is low- you wont knock. Its the pressure and heat etc that cause detonation- not the ratio between initial and final volume of the cylinder.

If you look up some charts- to find max spark timing they will use a constant MEP, not compression ratio.

DAPSUPRSLO

iTrader: (14)

Thanks. Man, this stuff is getting confusing, ha ha. So in general a motor which fills the cylinder much more effectively (i.e. a sb2 headed monster) will produce more MEP and hence be more susceptible to knock at wot. Since these motors do not run very well at lower rpms at cruz, or in other words don't generate high MEP values at lower rpms at cruz, then they should have no issues with nock at lower rpms at cruz, this all in theory ofcourse.

treyZ28

sure. why do you think supercharged cars will knock

why do you think you can have a motor drive find and knock under WOT.

DAPSUPRSLO

iTrader: (14)

Good point! I think it's starting to make sense now.

treyZ28

it will gradually become more intuitive. and then more complicated stuff will confuse you and some arrogant jerk will tell you it will be intuitive

DAPSUPRSLO

iTrader: (14)

How funny, i've run into that time and time again. Not everyone can be born and immediately know all the answers to everything (important to remember this when on the side of the teacher and as the student). It's part of the fun in finding this stuff out though, lol.

MadBill

The saving grace is the 'ignition delay' (think of it as cooking time) between achieving the conditions of heat and compression needed to detonate the remaining mixture and the occurance of the actual explosion. At high RPM, this interval is generally great enough that the normal flame front comes through first.

DAPSUPRSLO

iTrader: (14)

Thanks MadBill

TAQuickness

iTrader: (28)

I'm trying my best to follow along, it's been a while since i've slept, VE = Valve Events or Volumetric Effeciency? & BMEP = ?

treyZ28

Id say volumetric, see how its coupled with BMEP

TAQuickness

iTrader: (28)

Starting to make a little sense now... k

MadBill

yes, V.E. is Volumetric Efficiency, the percentage of a full cylinder's worth of fuel/air the engine takes in during each cylinder's induction stroke and B.M.E.P. is Brake Mean Effective Pressure, which is a shorthand term for relative power. It means the average, steady pressure, which if applied to the pistons through the entire power stroke, in the absence of any intake or exhaust pumping work or engine friction, would result in the observed flywheel power.

TAQuickness

iTrader: (28)

Thanks MadBill. I have a fairly good grip on VE, but the BMEP is a new term to me. Sounds interesting. Your sig is noteworthy