Why lower the compression ratio?

[Checkered-Member]

many lower their compression level to run more boost, but you HAVE to run more boost to compensate for “weaker” explosions in the combustion chamber.

To me it makes more sense to raise the compression therefore run less boost but still get the same amount of horsepower…am I missing something here?

I get the point is to strengthen the internals with forged this and that, but why run lower compression then higher? Wouldn’t the end result be the same?

[TheOne]

you could run more psi and not get as much detonation as having higher compression.(higher comp at high psi = detonation, those some people here keep their 9:1 compression on the ka)

[Checkered-Member]

You miss the point, at a higher compression ratio to achive a HP goal you will have to run less PSI, less PSI = less detonation.

[Nismo_Freak]

It's simple, you will never make more power by raising the compression ratio in an unlimited forced induction engine.

Reason being is that you are replacing cylinder volume with aluminum (piston material) rather than more air/fuel mixture. While the static compression is higher and more efficient with a high compression piston, the dynamic pressures are less efficient, and thus the engine is thermodynamically less efficient.

The only instance where raising compression would be ideal is when you are limiting yourself to a set power region (typically moderated) and you are producing that power with no knock. You could then raise the static compression a slight amount and produce a wider powerband.

[fiznat]

The issue is, as it seems to be with most any turbo discussion, heat. Assuming a given intercooler (at a given efficency), a higher compression ratio will increase heat in the cyl much more than an increase in boost. Lowering the compression ratio and adding boost will result in lower temps as compared to raisng the compression rato and lowering the boost. You can achieve the same power going in either direction, but obviously the route that produces the least heat (thus less detonation, autoignition etc) is preferable.

The tolerance for how high of a compression ratio you can go depends heavly on the temperature of your intake charge. Basically, you can stand the extra heat from the compression if your intake charge is already extrememly cool. This means a very efficient intercooler, a well sized turbo, and perhaps some sort of alochol/water injection to also cool the charge.

[sanioll]

Haha, every time I read stuff here, you guys are thinking way too hard.

1. If you increase compression, you will burn the fuel more efficiently and make use of its energy better.

2. If you increase compression by replacing it with metal(forged aluminium), bigger pistons, then you might get a detonation.

3. You have to lower the compression and run higher boost to AVOID DETONATION.

WHY DONT YOU GET DETONATION AT THE SAME COMPRESSION, WITH LOW PISTON AND HIGH BOOST??--beause fuel in the air/fuel mixture absorbs the heat, and thus lowers the overall temperature in the cylinder. Thats why you do not get a pre-spark detonation.

WHY DO YOU GET A DETONATION WHEN COMPRESSION IS HIGH, WITH BIGGER PISTONS??--in this case, metal replaces that empty space. And metal's thermal conductivity is way lower than the luquid's (fuel). So in this case you DO GET A DETONATION.

*Theres no metal that can withstand detonation. Temps in the core are way too high to melt anything.

kabish??

[rn240sx]

many lower their compression level to run more boost, but you HAVE to run more boost to compensate for “weaker” explosions in the combustion chamber.

To me it makes more sense to raise the compression therefore run less boost but still get the same amount of horsepower…am I missing something here?

I get the point is to strengthen the internals with forged this and that, but why run lower compression then higher? Wouldn’t the end result be the same?

When u RAISE the compression, u make the combustion chamber SMALLER. This is more commonly used for high hp N/A motors...

When u LOWER the compression, u make the combustion chamber LARGER. This is more commonly used for turbo applications...

Now, here is what happens. When u run a high compression motor ( say 10:1) the chamber is very small in comparisson to a (9.5:1) compression motor. When u turbo charge this HIGH COMPRESSION motor, u are forcing A LOT more air and fuel into it, when u do this, you will get more POWER, but at the same time the temps will be very high and the chances of detonation just amplified.... Why, cause ur trying to force all this air and fuel into such a small area... Not very much room to breathe per say...

In a low compression motor, the combustion chamber is LARGER... Meaning u have more area to play with. If u have more area, the heat is not as much of a concern and the chances of detonation have just went down...( to an extent, but thats a whole OTHER story )

Now when it comes to forged internals... That has nothing to do with compression ( well i shouldnt say nothing, but forged and compression are different ) Forged internals just means that the parts are STRONGER than oem and can take more abuse... But in no way are they bullet proof. you can buy forged pistons that will lower or raise the compression as well as buying a headgasket that can also raise and lower the compression..

They will just buy u more time if something goes wrong and u have enough time to notice and pull back..

I hope i explained this well enough, if i missed anything or explained it wrong, please let me know and try not to (BURN) me...

ThanksRobert

[rn240sx]

Theres no metal that can withstand detonation. Temps in the core are way too high to melt anything.

kabish??

Actually there is "UNOPTANUIM" which is a form of metal which gets STRONGER as it is applied to heat...

Didnt u watch " The Core"

[fiznat]

I swear thats what I said... its all about heat!

[sanioll]

Actually there is "UNOPTANUIM" which is a form of metal which gets STRONGER as it is applied to heat...

Didnt u watch " The Core"

we've been watching too many movies in mom's basement, haven't we?

[rn240sx]

we've been watching too many movies in mom's basement, haven't we?

LMAO..!!!

[Checkered-Member]

You guys are not following my train of though; if my compression is higher I can force less air into the chamber to get the same amount of horsepower.

An example: lets say I want to make 250whp on stock compression, to get that number people have been running 9-10psi

Now, with lower compression to get the same number I have to run 11-12psi

With higher compression I have to run 7-8psi

If I run a lower boost level, I have to compress the air less, so the air will be cooler. The pressure inside the turbo manifold will be lower (because the wastegate will open sooner) and therefore less backpressure. And I have to use less fuel, so my MPG will be higher.

Am I making sense?

[sanioll]

An example: lets say I want to make 250whp on stock compression, to get that number people have been running 9-10psi

With higher compression I have to run 7-8psi

if you pass the threshold, you will get a detonation. On stock compression, air/fuel mixture gets hot. Now youre forcing more air/fuel mixture at extra 7-10 psi, this will increase the temperature inside the cylinder. YES, THE EXTRA FUEL THAT YOU'RE FORCING THRU, WILL ABSORB SOME OF THAT HEAT, BUT IT MAY NOT BE ENOUGH. SO MORE HEAT, AND YOU WILL EXPLODE THE MIXTURE, BEFORE THE SPARK PLUG. WHILE ITS BURNING, YOURE FORCING THE CYLINDER EVEN HIGHER, COMPRESSING THE EXPLODING MIXTURE EVEN HIGHER. DO YOU HAVE ANY IDEA HOW HIGH TEMP GETS?

Now, with lower compression to get the same number I have to run 11-12psi

now you have to run at higher boost, your intake temps after the turbo will be much higher than 7-10 psi. But now, you have more space in your cylinder, and you actually forcing more fuel. This extra amount of fuel absorbs the extra heat, and lowers the temps in the cylinder. Basically, extra fuel has more heat holding capacity, and it absorbs it much faster, split of a second, before the spark. (metal cannot do that) So YOU DO NOT GET A PRE-SPARK DETONATION.

THIS IS WHERE YOU ABSOLUTELY, HAVE TO LOWER THE INTAKE TEMPERATURES AFTER THE TURBO. USE OF HEAT EXCHANGER(a.k.a. intercooler) is very practical.


Modified by sanioll at 6:48 PM 6/4/2005

[Checkered-Member]

I can look this up, but dont have time or interest. I believe every time you increase the compression ratio of a gas, it's temperature doesn't increase at a set rate. This is why it gets way hotter at high compression.

That’s what I’m looking for, if the relationship is not linear I would like to see evidence instead of opinions.

if someone could find this, I searched around but came out empty.

[sanioll]

try searching for ideal gas law and go from there. we learn this stuff in high school, right.

well, maybe I will search for it, after i'm done with some stuff. I'm at work right now.

[sanioll]

okay i did some quick reading, and assuming that i read it properly(didn't look into main formula enough) it is linear.

V1/T1 = V2/T2

V1=ORIGIONAL VOLUME, V2=FINAL VOLUME, T1 ORI TEMP, T2 FINAL TEMP.

that may not be the formula to look for, but logic tells me, if you compress .6 liters to .6/8, then .6 liters are contained in 1/8 space of origional cylinder. to find the temp change, then you have to multiply .6*8, which would give you the volume of the full size. then just plug it in, and you get the temperature in kelvin of 2400 K - 273 = 2127 Celcius. So i dont know, dont think this is what youre looking for. at 2127, that fuel will explode, hehe.

see these sites, see ideal gas law. formula is linear, so increase in temp has to be linear too.

http://members.aol.com/profchm/charles.html

http://hyperphysics.phy-astr.g...ml#c4

http://chemed.chem.purdue.edu/....html

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having said all this, it all comes down to amount of liquid that can absorb the heat. So you need more fuel to absorb the heat in the combustion chamber. you can run higher compression, insanely rich. complete waste. THIS WHY PEOPLE GET WATER INJECTION INTO THEIR INTAKES, WATER ABSORBS THERMAL ENERGY. YES YOU CAN INCREASE BOOST A BIT WITH WATER INJECTION, BUT MOST OF THE TIME WATER INJECTION IS A BAD EXCUSE TO POORLY TUNED FUEL SYSTEM.

or

you can run low compression, high boost. here you are pretty much having the same pressure in the cylinder as above, but now you have more liquid/fuel to absorb the heat.---------------------------------------------------------------------------------------

I say stick with what people been doing for years. low compression, high boost, decent air/fuel ratio.
Modified by sanioll at 8:55 PM 6/4/2005

[Checkered-Member]

I say stick with what people been doing for years. low compression, high boost, decent air/fuel ratio.

i seem to have a hard time following what everyone else does...if that was true i would drive a tan minivan and spend money on starbucks and cigarettes…NEVER.

So it is linear…good…good

I already mentioned the advantages of running lower boost…lets review

Less air to compress = colder air = more efficient setupWastegate opens early = less backpressure = more efficient setup Smaller air/fuel mixture = better mileage = more efficient setup

[sanioll]

Smaller air/fuel mixture = better mileage = more efficient setup

forget about smaller air/fuel mixture and forget about better mileage.

You dont drive on pedal to the metal all the time. Your gas mileage will suffer about 10% for daily use, non-boost daily highway travel. When you step on it, efficiency matters.

For our cheap 240sx, fuel effciency must not matter. You're not trying to win 24 hour race here. What are you trying to make less gas stops??

the gain from your setup is very little, and very risky. I guarantee, you will switch to standard way, after blown engine.

[Red-KAT]

I already mentioned the advantages of running lower boost…lets review

Less air to compress = colder air = more efficient setupWastegate opens early = less backpressure = more efficient setupSmaller air/fuel mixture = better mileage = more efficient setup

Just shoot some NOS on yer IC and call it a day.

Or pick the correct turbo for the pressure you will be running. That could help. :P

[klattr1]

the way ive always looked at it is that it really depends on how much you are willing to boost in order to decide which compression ratio you want and the application as well.

lets say Bob builds his motor at 8.5:1 compression ratio and boosts only to 20 psi and makes 375 rwhp with race gas.

well Bob might figure out that he could prolly utilize a 9:1 or even stock (9.5:1 but forged) compression ratio and still have a long lasting motor assuming the tune is good. then he boosts to 20 psi and makes 390 rwhp instead.

ive always told people if they are willing to boost to the 25-30 psi range, then stick with 8.5:1. if they boost 20-25 psi, then 9:1. if they stay under 20 psi, then 9.5:1.

ive even toyed with the idea of going a lil higher in the compression ratio (either 9:1 or 9.5:1) because my car is no longer a daily driver, the consequence/risk factor is different, i generally dont boost over 25 psi and I will prolly run race gas all the time.

[Checkered-Member]

Right now I'm looking at .040 overbore 10.5:1 JE Pistons, Pauter Rods. 370CC injectors and a piggyback...to increase engine efficiency: generic IC, JWT cams, head work and ported intake mani.

7-8 psi with a T3/T4 BB if I can get my hands on one.

[klattr1]

as long as you know that 7-8 psi is all you will boost, then it might work fine. you might make like 250-260 whp at that boost level but not have as much HP potential without detonating as a 9:1 setup which could be run to 350-400 whp no problem.

if having the most power at 7-8 psi is your concern, the try it out. otherwise, if you are looking for more power, then go lower compression and boost more.

some people get in this "efficiency trap" in which they'll never be satisfied with the parts they have on the motor and will always change out parts and keep it at the same boost level as before looking for more power that way. whereas you could maintain that previous setup and just make the power at a lil higher boost level.

its all about goals i suppose.

[WDRacing]

Higher compression will NEVER make more power then lowering the compression and adding boot. Simple physics will always prevail. Here's a quote from Kenne Bell. He's been doing this ALONG time.

"What parameters determine how much boost my engine can run on 92 octane? Good question. One of the top 3 most commonly asked.COMPRESSION RATIO - Lowering the compression ratio allows the engine to run more boost with the same octane. For example, 8.0:1 vs 9.5:1 can tolerate another 3 psi. 3 psi gives 38HP. The 1.5 lower CR loses 3% (2% per point) or around 9HP in a 300HP engine. That nets 29HP (38-9=29). Now you know why OEM's use low compression ratios on supercharged engines.AIR CHARGE TEMP - The supercharger itself determines how much boost the engine will accept. Some superchargers require an intercooler. Others, such as the Kenne Bell, don't need intercoolers because the discharge air temp is cooler. 1 psi boost increases supercharger air discharge temperature 10°-20°, depending on supercharger type. A 6 psi "rated" kit usually requires 8 psi "total" boost (+2 psi to overcome inlet and outlet losses). Consider two 8 psi superchargers: 8x10=80°, 8x20°=160°. The 160° supercharger should use an intercooler. An air to water intercooler will lower that 160° to around 80° (160°-80°=80°) or the 80° temperature of a more efficient supercharger without an intercooler.BOOST - Both higher compression ratios and boost increase cylinder pressure and demand higher octane fuels to eliminate detonation. Fuel octane determines the amount of boost an engine will accept. Kenne Bell runs 6-7 psi on all our Ford, GM, Chrysler and most other kits with our Twin Screw superchargers. One octane will support one psi of boost. Example: If fuel octane is 94 or you add a can of NOS Octane Booster you can typically run 8 psi.IGNITION TIMING - Retarding ignition timing will allow the engine to run more boost but not without a penalty. 4° retard=16HP.AIR FUEL RATIO - You can't make horsepower without fuel. Richer air fuel ratios reduce power but permit higher boost levels on a given fuel octane. Leaner ratios make more power but need lower boost so a lean mixture with high octane makes the most HP. We thoroughly understand all the parameters of supercharging. Supercharger kit tuning demands ignition timing, air fuel ratio, fuel octane, boost, compression ratio, discharge temperature and supercharger parasitic losses all be carefully considered in the design.What causes detonation? 1. Lean fuel mixture, 2. low octane fuel, 3. excess boost, 4. lack of fuel, 5. advanced ignition timing, 6. vacuum leak, 7. "hot" spark plugs, 8. overheated engine, 9. excessive inlet temperature (underhood filter).Why does Kenne Bell use a 6 rib belt to drive the supercharger and others use an 8 rib belt? Because our Twin Screw supercharger has less parasitic loss than others and therefore requires less HP to drive - which means the engine develops more HP with a Kenne Bell.Do I need an adjustable fuel pressure regulator? Unless you're playing with an '86-'93 Mustang, don't bother. What is the most ridiculous aftermarket product? Hands down, it's the "little intake fans" and the "throttle body spacers" with the bore grooves. Right there on the list has to be those "underhood exposed filters" that suck in 200° underhood air instead of ambient (70°-100° air from the fenderwell). 10° air temp rise is -1% HP. That's how dynos are calibrated for temperature changes. Does anyone really believe that all those OEM manufacturers designed expensive plastic inlet systems - for the last 20 years - that pull cool air from the fenderwell, hood or grill because they didn't have anything else to spend their money on? Then there's those "factory calibrated" mass air meters that do not use a chip or re-calibrated processor with oversize injectors. They unwittingly reduce the voltage signal to the mass air meter to compensate for the larger injectors at WOT but they neglect all the driveability issues. The result is over advancing timing, lean mixtures, detonation, surging, bucking etc."

That should cover most related issues with both high compression and heat involved with boost...

WD

[TrunkMonkey]

let me stick my 2 cents in...

Less air to compress = colder air = more efficient setup

no. remember, regardless of what you do to your compression ratio, your engine is still going to pump the same amount of air. what creates the heat is the density of the air/fuel mixture when it is ignited. the more you squeeze the mixture, the more energy will be created when it is ignited thus the more heat created.

Quote »Wastegate opens early = less backpressure = more efficient setup[/quote]when the wastegate opens doesn't determine the amount of backpressure in the system. as a matter of fact, this is the last thing you should worry about. there are much more effective ways of controlling backpressure.

-demetrius

[nissanfanatic]

One of four things can get you around detonation at a certian power level.

1)lower compression pistons

2)retarded timing

3)higer octane fuel

4)charge cooling (water/alky injection)

You can't take the timing back too far becaue you want PCP(peak cylinder pressure) to occur around 20 ATDC(after top dead center). The whole idea is detonation avoidance. Cooler air&fuel and/or a later fire.

Good reads on the subject all from SDSEFI

Fuel Octane vs. HP

Ignition and Combustion This one has blots of info on the subject, but still a very good read.

Intelligent Engine Modifications

Pushing it Too Far

Well have fun.

[Nismo_Freak]

The issue is, as it seems to be with most any turbo discussion, heat. Assuming a given intercooler (at a given efficency), a higher compression ratio will increase heat in the cyl much more than an increase in boost. Lowering the compression ratio and adding boost will result in lower temps as compared to raisng the compression rato and lowering the boost. You can achieve the same power going in either direction, but obviously the route that produces the least heat (thus less detonation, autoignition etc) is preferable.

The tolerance for how high of a compression ratio you can go depends heavly on the temperature of your intake charge. Basically, you can stand the extra heat from the compression if your intake charge is already extrememly cool. This means a very efficient intercooler, a well sized turbo, and perhaps some sort of alochol/water injection to also cool the charge.

Good points.

Don't forget you are decreasing volume and raising pressure when you go with a higher compression ratio. You also are increasing the thermodynamic efficiency of the combustion, so more of the generated energy is expelled through mechanical means (pressure) and less is generated as latent heat of combustion. On top of that your inlet charge temps will be slightly cooler, and the fuel will also be cooler due to lower operational fuel pressures. Everything comes together to help lower the overall effect of the compression increase. Hell even the EGT's will be lower with a higher compression engine, so you have lesser heating of the coolant.

Otto Cycle Diagram



As you can see the effective heating is minimal within compressing the mixture further because you have a greater decrease in volume compared to pressure. It is the pressurization that increases heat due to incidents of molecular friction.

P1 * V1 / P2 * V2 = T1 / T2

Thermal Efficiency Table



As you can tell we are gaining roughly 3.2% efficiency in the combustion process which can be loosely translated into more power. However you will notice that the trend is for less and less efficiency as the compression increases. This is because you are closing the gap between the reactants less and less. Now, this is all static compression. When you look at a boosted engine you are producing dynamic compression ratios upwards of 14:1, where a single point of compression yeilds a meager gain in efficiency.

[Nismo_Freak]

So it is linear…good…good

No, it is not linear.

Look at my otto diagram and understand this.

PV = nRT

nR is fixed, and as a result PV does not have a constant linear relationship.