Moral of that blabber, your idea indeed is true, but you must take into account heat generation from higher cylinder pressure. Therefore premature ignition (detonation, pinging, knock, whatever you call it) happens much easier at higher compression ratios. If you run really good fuel, you could do that all day.

Ideally, 12:1 is the most efficient a gasoline ICE compression ratio you want. We lower compression ratios so that we can run higher cylinder volume pressures, aka increase boost.

Hope this helps. I just ranted real fast but search online (NOT wikapedia, theyre ****ing morons) and you will learn a lot. It all makes complete sense when it comes down to it. Read up about the Rankin cycle (steam engines/turbines) and the Brayton cycle (gas turbines), those are REALLY badass.

EDIT: I didnt read your other post, i appoligize. That is indeed correct, higher comp ratio with lower boost would technically be better. Most poeple dont know or think about that. The only bad part about that is that you would limit yourself to allowable internal cylinder pressures. If you are designing a motor for certain parameters, higher compression FTW!

Modified by neonbomb at 9:17 AM 6/27/2008

1990 z32 300zx Stage 4++ *RIP*
2002 Subaru GD Impreza WRX Sold

You must KNOW VE at every rpm then multiply by real CR [not published] to learn the BMEP.
Measured CR often is much lower than published CR.

All traced back to the single cylinder gasoline test engine where real CR can be varied from 6 to 15 to measure knock index on fuels. Another useless old machine that bears little usefulness with 32 valve penta roof aluminum heads.

PSI in plenum does not tell you real psi in cylinder, as in how many grams of air are compressed into what space.

So it's not a design of effeciency, but a design of infrastructure.

Here is my question:
If both engines are running on the same octane levels as well as cam profile...
Plus these engines were both on the brink of pre-ignition

Would lowering "CR" inorder to run more boost (while still being on the brink of pre-ignition) result in one engine making more power then the other?

Or will the changes negate each other?

Quote, originally posted by jrsb202001 »

if ur 240sx doesn't have a unique sound (ie. engine swap) or a sick body kit and a paint job then ur ride is "just another 240sx"

Sending into cylinder superheated air from super/turbo charger or eventually heating air before spark from piston compression.

Preheated air vs air heated just before ignition.

You would need to accurately measure cylinder head, valves, and piston crown temperatures in sub millisecond intervals to get your answer. Wheter one method is better than the other.

Notice that supercharged engines run richer than necessary to cool combustion. [to slow flame speed]. This extra fuel takes up space that could be used by air. Inert gases always decrease flame speed and air is 75% inert.

Study direct injection systems and patents

http://www.motorcycle.com/how-....html
http://www.erc.wisc.edu/modeli...r.pdf

"Consideration #1
Heat from compression by a supercharger or turbo can be removed (for the most part) through use of an intercooler. Heat from compression within the cylinder cannot. Also, the cylinder pressure at the end of the compression stroke (prior to ignition) goes up exponentially with an increase in static compression ratio, versus a linear increase with boost pressure. Therefore, increasing the static CR is going to unavoidably push you closer to the knock limit for a given fuel. In other words, the octane requirement goes up more by increasing the static CR than it does by increasing boost.

For example, increasing the static CR from 8.5 to 9.5 increases the temperature within the cylinder at the end of the compression stroke (but before ignition) by ~63°F, (assuming IAT2 = 130°F and ideal adiabatic compression with γ = Cp/Cv = 1.4. I won’t bore anyone with equations. The situation doesn’t change much even if IAT2 were only, say, 100°F. In that case, the increase in temp at the end of the compression stroke goes up by ~60°F for the same increase in static CR). Also, the pressure at the end of the compression stroke (before ignition) goes up by ~97 psi from 574 psi to 671 psi, assuming atmospheric and boost pressures of 14.7 and 14 psi, respectively. On the other hand, increasing the boost pressure from 14 to 15 psi increases the outlet temp of the compressor by only ~11°F, assuming AE=60% and IAT1 = 90°F. And by further assuming an intercooler efficiency of 80%, the increase in IAT2 is only ~2°F. Hence, the increase in temp at the end of the compression stroke will hardly change at all. Also, the increase in cylinder pressure at the end of the compression stroke only goes up by ~18 psi (from 516 to 534 psi) with this increase in boost pressure.

So summarizing the effects of increased temp and pressure at the end of the compression stroke for the two cases:
Increased CR from 8.5 to 9.5: ΔT = ~63°F and ΔP = ~97 psi
Increased boost from 14 to 15 psi: ΔT = ~2.4°F and ΔP = ~18 psi

A higher temp and pressure increase the likelihood of deadly preignition for a given octane fuel. And for those astute observers that know the physics I’ve applied, yes, although I’ve idealized things to keep it simple, (by not including effects such as heat loss thru the cylinder walls during the compression stroke or ignition and valve timing in the calculations), I’m sure they’ll also recognize that this doesn’t change the conclusion.

Consideration #2
Power is increased by two completely different mechanisms for the two approaches. Increasing the static compression ratio increases power via an increase in thermal-conversion efficiency. Increasing boost pressure increases power via an increase in mass-air flow rate. There’s less gain in thermal-conversion efficiency (and hence power) via an increased static CR compared to the power gain by increasing the mass-air flow rate via an increase in boost pressure. For example, increasing the static CR from 8.5 to 9.5 results in an increase in thermal-conversion efficiency (for an ideal Otto cycle) of about 3.2%. On the other hand, increasing the boost pressure from just 14 psi to 15 psi, increases the mass-air flow rate by about 3.5%. If boost pressure is increased by 2 psi, (from 14 to 16 psi), the increase in mass-air flow rate will now be more than twice that compared to the increase in thermal-conversion efficiency, (~7% vs ~3.2%), and ΔT and ΔP still won’t be as great as they are when increasing the static CR from 8.5 to 9.5. Therefore, not only can it be “safer” from the knock point of view, but a little more power is gained as well, (relatively speaking that is)."

Thank you

My (less technical detail, but still valid) take is that higher static CR provides better off-boost torque, less boost required for same torque through the rev range, and slightly better efficiency. The only downside is that you hit the knock limit at a lower peak power value.

Guys who are after maximum peak torque and power will run lower static CR.

The 'thermodynamics for dummys' reason for this is as follows:
1-Compressing the charge either at the turbo or in the cylinder increases temperature.
2 - You don't get a chance to cool the charge in the cylinder. (Unless you're injecting water or meth...)
3 - For a cylinder full of air and fuel at a given pressure, knock is a bigger problem when temperature is higher.

I personally like lot's of area under the torque curve and will give up peak torque for a broader torque curve. I've selected a static CR that delivers decent off-boost performance and leaves enough room at the top end to make the peak power I want on the available gas.

My numbers are probably not that meaningful here as I'm running an RB26, but the priciples hold for all engines.

Cheers,
Dan

Q45tech – If you refer back to the original thread where that information you copied and posted actually came from, you’ll find that I’ve added a Part II, which goes into more detail. It may be of interest to you and others. I hope you find it useful. Here’s the link:
http://www.modularfords.com/fo...51059

Drop by again.

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