why have lower engine compression?

Question

i have a 461ci engine with 8.5:1 compression. i always thought that lower compression was better but now i am reading that higher compression is better. my question is will a supercharger change my compression and can i raise my compression and add a supercharger?

Answer 1

A supercharger will not change your compression. You will get higher
rpm, Higher compression will give you more power at low rpm.

bedst regards

Answer 2

Compression ratios govern the pressure the charge in the cylinder reaches when compressed. The higher the octane rating of a fuel, the higher this pressure can be without the fuel igniting on it's own before the plug sparks. On a modern engine with pump fuel, somewhere in the region of 9.5:1 - 10:1 is about right. Higher compression gives a higher efficiency, basically.

A supercharger will not change the compression ratio as such, but it will mean that the air entering the engine has already been compressed a certain amount, so gives the same EFFECT as raising the compression ratio. You could both raise your compression ratio and fit a supercharger, but it'd probably be a lot better to just fit a supercharger and run it at higher pressure.

I'd recommend getting hold of a decent book on basic tuning, and one on super/turbo charging, that should give you all the information you need to make a decision. I've heard good things about the ones publiched by Speedpro.

Answer 3

Lower compression allows use of less expensive gasoline with a lower octane rating, and makes the car better suited for artificially increasing compression via supercharging or turbocharging. Cars with 8.5:1 should run fine on 87 or 88 octane, while 10.5:1 cars typically need 91 octane to avoid igniting the gas too fast, known as "knocking" or "detonation".

The other poster was correct in that air pressure at sea level is around 14psi, but with a supercharger with a modest 7 psi of boost, for example, the intake air is pressurized to a sum of 21 psi. This gives a theoretical 50% more air than normal, which in practice results in approximately 40% more power if designed well.

Adding 50% more air and gas to a cylinder that already has high compression results in even higher peak combustion chamber pressures and higher power, increasing the risk of detonation as well as increasing wear to the engine.

Answer 4

Hello

Lower compression allows the use of lower octane gas. Compression ratio (cr) is the cylinder volume divided by the head volume. 93 octane fuel will tolerate 9.5 to 10.5 cr DEPENDING on your set up. Higher cr means a bigger bang (high pressure) and more power.

Unlike a naturally aspirated engine which draws in fuel and air, only; A turbo or supercharged engine forces fuel/air into the cylinder increasing it's volume thus increases your cr.

Shaving your heads will reduce chamber volume thus increasing cr.

If you cr is to high for the fuel the pressure will force it to ignite before the spark, during the compression stroke, this must be controlled or avoided.

Decide how you plan to use this car then talk to a few engine builders. Look for a forum of people who own a similar model and get some advice.

Keep the shiny side up!!!!

Answer 5

Yes it will, thats exactly what they are made for. In fact if you are going to run a supoercharger or a turbo, you will need a lower compression setup around 8:1 (normally aspirated) depending on the amount of boost your resultant compression will be around 10.5 to 11:1 Think about it, amibient pressure at sea level is about 14.5 PSI, for a normally aspirated engine (without a turbo or supercharger) that is the MAXIMUM amount of manifold pressure you will ever see.. Now lets talk about what makes horsepower.. there are ONLY two things that can create more horsepower... Power Strokes per Minute, and Manifold Pressure.... There are a variety of modifications that can be done, but all of them affect one or both of these. to get more manifold pressure above and beyond ambient, you can add a blower, or turbo.

When adding a huffer like that on an engine, you must also take into consideration how well built the engine is. Stronger head gaskets are required, a different camshaft specification is generally required as well. You should check to see if your pistons and rings will handle the extra pressure as well. Cast pistons will not hold up to the punishment, Forged pistons are a much better idea.

Ok after reading some of the other entries here if you want to get technical, compression ratio is the ratio of volume of the cylinder at Bottom Dead Center, vs the Volume at Top Dead Center, and is calculated based on Bore, Stroke, and Chamber volume. This doesnt change with the addition of a blower, only machining, changing the pistion dome, bore or stroke can change this value. What I was referring to is working compression, with blown engine your charge pressure is increased giving you all appearances of having a higher compression ratio.

Answer 6

Are people looking at it to determine if it has low compression? You need to have a compression check performed like with a compression Gage. Do it dry or without oil and also do it wet by putting a about 1/2 ounce of oil in each cylinder before cranking engine to get the reading. Compare your results to the maintenance manual. They should be within limits and not vary from each other by more than about 10-15 %. You can also put a vacuum gage on the engine while running. The automotive vacuum gages have all the things that can be wrong printed on the face of the gage. Very easy test to perform and will tell you what if anything is wrong. Also do all maintenance specified in the manual for your car like adjusting valves and so on. As far as it not starting up when hot that could be an electrical problem like the starter going bad.

Answer 7

NO, a supercharger ( any object used to increase airflow ,including Roots blower,Turbo or Nitros) will NOT change your compression but it will alter your cylinder pressure on the second part of your question ,YES ,you can raise your compression ratio and add a supercharger ,BUT it is NOT recommended as it will be harder on the engine and will be a less efficent use of your power adder- rule of thumb is HIGH BOOST needs LOW COMPRESSION - HIGH COMPRESSION is used most effectivly on naturally asparated (non-supercharged) engines. Some of the little tuner engines only have 6 to 1 CR but run 40 pounds of boost for 700hp -its all about how much air and fuel you can pack into the cylinder !

Answer 8

lower compression will reduce emissions. that is the main reason its done. not lower octane fuel, or any other reason.(although there is truth to that) it will lower compression due to the fact that ign timing can be changed (usually advanced) to gain more burn time, thus getting a more complete burn, thus lowering emissions. in this day and age of economy/emissions, this is a easy way to get some economy out of a big engine.