Adrenaline_Z

LSA or LCA is the degrees between the intake and exhaust centerline. It does
not dictate when the intake valve closes.

Intake centerline is at times used to dial in camshafts, but even that cannot
tell you IVC because the duration my differ.

YOu need to use Intake Valve Closing (IVC) to determine the dynamic compression ratio.

hymey

Yes that is true, But if you have a 240 240 112+2 cam and the cc is too high and you need a later ICL with the same cam duration, By say 6 degrees you need to run a wider lobe sep. Wider lsa cams also have less overlap, the overlap effect works like a vacuum cleaner in certain rpm ranges so more cylinder pressure can be created with a tighter lobe sep through peak torque just because the running cyl pressure is so high.

For eg I run 25 deg through pk torque with a 232 234 112 I have a 46 deg IVC, with a 227 239 115 cam. in the same engine it has a 45.5 IVC it makes best tq at 28 degrees but it still wont knock with 32 degrees, the 112 cam will knock at 26.5 degrees, Even though the cranking compression is less with the 112 cam it takes less timing. Why? Because the extra overlap is scavenging and filling the cylinder better. cranking compression is a guide, Even going the same cam even slightly wider in lobe sep and a mere 2 deg later ICL can change things drastically, Overlap drops off running cylinder pressures drop off, all of a sudden the motor is taking more timing. This is normally what we do if the compression is to high, Even the same motor same parts can be different.

Adrenaline_Z

You are confusing cylinder pressure with dynamic compression according to
your last post.

Scavenging / overlap that you are tweaking improves cylinder filling which
increases the amount of charge presented to the chamber. This creates
a higher cylinder pressure.

When you degree a cam by shifting ICL by 2 degrees you are effectively
phasing all valve timing events...which directly alters dynamic compression
because the IVC event has moved.

Think of it this way:

A camshaft with the exact same lobe installed at the same intake center
will have the same DCR even if the lobe separation changes (110 to 112 LSA for example).

Overlap changes, but DCR does not in the above scenario.

DCR is just a mechanical ratio of when the intake valve closes based on the
amount of static compression available.

GrannySShifting

iTrader: (10)

IF (and thats a big if) the combination is right and fills the cylinder adequately, your not going to get much gain out of stretching it another point of compression. The margin for error can become so close (little lamer fuel one time, carbon buildup starts a pinging, a little edge here or there in the chamber)

If you have a small motor with a ton of cam in it it may want the upper end, but i dont think Id build buzzer for pump gas drive around duties.

When we built a LS motor originally at 13:1 dropped down to 11.5:1 saw virtually no loss in power, it was filling the cylinder. Even on high octane fuel the difference was little to none

Get the combo right and you dont have to rely on rolling the dice on compression, and youll have a tuning window with some margin. Just did a 408 LT1, hyd roller, forward facing intake made 466 thru a 5200 stall on our Mustang dyno. Decent CYlinder head and cam timing are close, not abunch of compression just 11.3:1

And your not going to want a dome in a chamber running on the edge of pump gas capabilities. Get the piston right to start

Carnaut

Unless you have some elaborate (& highly dialed-in) water-injection, other after-market anti-detonation device, or your later model engine assembly came with some detonation prevention mechanism(s) - of course you can NOT use 13:1 c/r(!)

I tried for many years to get away with 10.9/1 c/r, and after trying many after-market inventions I simply bit the bullet and mixed LL-100 AV fuel with pump 92 octane, and only then was I okay and actually got to enjoy my high(er) compression. If your local municipal airstrip won't sell you the (100) AV gas, then you'll have to pay the price via your local speed/off-road shop.

If you try to run high compression with pump gas your expensive motor will suffer (and not last long). Years ago someone built a 12.5/1 350 sb, it could pull the front of his Vega off the ground with good traction - but his motor was always noisy only lasted 8-10,000 miles (that was with 92-93 octane & so-called 'octane boost' - which only boosts your octane about .5 -.7 {½ to 3/4 octane, or "five to seven octane points"}

If you don't have access to better fuel don't use more than 9.3-9.5/1 c/r (!)

GrannySShifting

iTrader: (10)

Wow disregard everything this guy just said, most of them are 10.9:1 from the factory now bud

HioSSilver

iTrader: (8)

I want to no more about meth injection on n/a motors.

ramairws6

iTrader: (33)

No doubt, been running 11.8:1 all day long on 93 octane for 3 years with no issues..

Pro Stock John

iTrader: (34)

I ran 12:1 on pump gas, 422 cid LS setup, back in 2001-2002.

SBC's are different.

I'm thinking that maybe 12.5:1 on pump gas is doable, but I wonder what sort of compromises there would be with respect to timing advance...

If I build a new NA motor next year to run on 93, no meth etc., I'd probably go about 11.75:1.

ZMX

How so?

GrannySShifting

iTrader: (10)

Dont. If cyl head is good, there will be nothign to had except for complicatedness.

A very well thought out engine that moves air isnt going to car much about a hair more compression. Waste of time.

I have dropped a point made the combo happier and picked up bunch of power. Compression is what you use when you cant fill the cyl well, squeeze the **** out of it!

Pony Exp.305

iTrader: (6)

Same thing I posted few month ago. Interesting> I read the other E85 forum that those cars runs with 12.1-15.1 c/r on E85 and no problems...
Build 14.1 CR LS on E85 DD

Devils Mentor

iTrader: (20)

So for the most part 11.8:1 is the highest you really want to go for 93 pump and still be safe with a little cushion? 12:1 is MAX safe, but tuning MUST BE spot on... I like a little leeway so I guess it's 11.8:1

DM

LIL SS

iTrader: (9)

I am too


LSA has nothing to do with intake centerline. LSA however has everything to do with exhaust centerline.


Until you start to achieve more than 100% VE


You a smart guy, I know this from previous discussions with you and reading other things you write. Would you agree than an overly early EVO point is less likely to leave poluted air in the chamber? Generally speaking, one reason most cams ground for N20 have a good amount split with wider LSA's. Retains overlap to tune rpm band but opens the exhaust sooner to help evacuate the spent mixture. Early EVO's IMO are less likely to leave the cyl filled with spent gasses compared to a late EVO.





I don't think there is any specific static compression ratio that is "safe" I think as others are saying too, it is about the full combo. Everything must work together to achieve a harmonous motor that will make great power.

GrannySShifting

iTrader: (10)

No, i can build a motor that runs 13.5:1 on 93 octane pump gas no problem. And itll probably be a f'ing pig. but you could do it.

Fill the damn cylinder, dont get carried away with compression, and have at it. There are certain circumstances you turn to compression when you dont have other things in place, but in a perfect world, no

Adrenaline_Z

Generally speaking I will agree with this statement. Of course it will all depend
on RPM and what sort of pressures are occuring in the exhaust system; the
chamber; and intake system at that specific point in time the valve is open.

Sometimes reflections and odd pulse phasing in the motor will prevent gasses
from clearing out.

Also to clear things up:

LSA, or LCA is the value given between the exhaust centerline and intake centerline.

Volumetric efficiency is a cousin to cylinder pressure (IE: BMEP, IMEP, etc.)
DCR is just a mechanical ratio of when the intake valve closes in relation to
the total amount of Static compression.


IMEP = ((792000 x HP) / (Meff x CUIN x RPM)

Meff = mechanical efficiency = (-0.000009 x RPM) + 0.9188 (typical values)
Peak Pressure = (((IMEP / FIT) x (((CR - 1) x (y -1)) / ((CR^y) - CR))) + INT) x CR^y x 0.75

FIT = Correction Factor found in complex engine sim calculators.
CR = Compression ratio
y = Ratio of specific heats.
INT = Intake manifold pressure PSI (absolute)


BMEP = 150.8 x TORQUE (lb-ft) / DISPLACEMENT (ci)