Michigan BlckBrd

iTrader: (1)

Ok i know dumbest question ever, i know what Compression Ratio is i.e. ratio of max volume to min volume, but my question is how does this relate to anything?

I mean what would be the difference in a 500HP car with a compress ratio for 15:1 as opposed to a 500HP car that has a compression ratio of 10:1?

Is one better for forced induction builds?

Does one yield better low/high rpm horse power/torque?

Better acceleration? Gas mileage?

I am proud to live in a country where i won't get shot for asking such a dumb question!

hammertime

iTrader: (1)

I'll take a stab, and hope that those who correct me later are kind...

Yes, for a given fuel source, more pressure on the mixture when ignited will produce more power as the gases expand, and continue to provide more pressure on the piston as it travels down the bore. There are reasonable limits at either end of the spectrum, and a more practical explanation comes about when you consider dynamic compression. Static compression (SCR) is the volume ratio, as you stated, and nothing more.

Dynamic compression (DCR) takes valve timing into consideration, specifically the Intake Valve Closing Point, because you cannot compress anything until both valves are closed. In a nutshell, long duration cams and cams with wider lobe separations and/or later Intake CenterLine angles need more static compression create the same pressures as a shorter duration cam. There are calculators available on-line that will let you plug and chug numbers to see the difference. While the name dynamic implies that this number changes, you cannot change the DCR unless you have Variable Valve Timing of some sort.

Each full point of compression should theoretically yield 3-4% more power, within reason, and where not limited by detonation. Therefore, a 500HP engine that drops from 15:1 SCR to 10:1 SCR with no other changes would lose some 15-20% of its power, or 75-100HP.

Again, those numbers are based on theory, and nobody makes that kind of change without other major changes to the engine as a whole (i.e. you can't run 15:1 comp on pump gas, and you probably no longer need race gas at 10:1 comp). Another consideration, there are diminishing returns as compression gets higher and you approach the detonation point for the available fuel.

Yes, when you add more pressure to the system with any type of air mover, you still have to operate within the limits of the available fuel. You should lower compression some when using these type of power adders to compensate, or increase the octane of the fuel you are using.

These characteristics are primarily determined by the camshaft, heads, intake and exhaust systems on the engine. More compression is nearly always more power, as long as the fuel can support the pressure created without getting into detonation.

More power = more acceleration

If your engine is always restricted (think restrictor plate cup cars) more compression helps to get back some of the power lost. Picture a situation opposite that of forced induction, you have an engine that is starved for air, which therefore traps less pressure in the system when the Intake Valve does close. To make the most of the available air, more compression is desirable.

Adding compression is a way to enhance the efficiency of an engine. If you can make more power from the same amount of fuel, you should be able to reduce the amount of fuel required to maintain a steady speed on the road. The reality is, nobody seems to be able to make more power and save fuel. You would never raise your compression to 15:1 to get better gas mileage. That benefit is only realized until you open the throttle all the way and remove the restriction. At that point, you create a badly detonating engine operating well below the minimum required octane.

There is WAY more to it than that, but there is plenty here for the masses to pick apart.

conan

iTrader: (5)

If you had 2 346ci ls1's, one making 15:1 and the other making 10:1 it would be a hudge differance. So you could take a smaller engine with a high compression ratio and make the same power as a larger engine on a lower compression. Look at the LQ4 VS LQ9 both are a 6.0L with the same heads the LQ9 makes more power due to the flat top pistons (higher compression)VS the LQ4 with dished piston (lower compression).

Forced induction, the lower the compression the more boost you can run with some insurance against detonation. For a street car I would not want any thing lower than 9:1 compression. I'm running 9.4:1 compression and when I build another engine I'm going to shoot for 9.5-9.7:1 range so the low end ***** arent lost. Now you can run high compression in a forced induction app you just have to run a high octane fuel and meth would help too. Detonation control with higher octane fuel is the key.

Higher compression will always make more power.

Higher compression will have better acceleration and more power usually will never get better gas milage.

One big thing with a higher compression ratio in a N/A or FI application is you need more octane to bring it all together.

Paint_It_Black

iTrader: (1)

1) There would be no difference other than compression if they both have 500hp.
2) Depends. Typically lower compression is more boost friendly.
3) Depends on the camshaft, engine size, etc.
4) Depends on the valve events and head brake specific fuel consumption.

stevied916

iTrader: (30)

http://www.google.com/#hl=en&source=...07e9ccc08a197a


Google?

WhiteRhino

iTrader: (6)

Only thing i'd add is that a 15:1 motor will have more under the curve power. Versus a 10:1 motor with the same exact mods. Even though they both make a hypothetical 500hp.

ie; You don't make more top end or bottom end like with a cam. With more compression, you make both.

Kcssmaro

iTrader: (9)

I dont understand how you have lower comp. with a dished piston?? and higher with a flat or domed piston?

Can someone explain.

Paint_It_Black

iTrader: (1)

a dished piston adds volume at TDC, a flat keeps volume the same (as a stock LS), and a domed sticks up and takes away volume at TDC.

hammertime

iTrader: (1)

Remember that the compression ratio is the volume of the cylinder and combustion chamber at BDC divided by the volume of the combustion chamber at TDC. If you add volume to the TDC number in the form of larger combustion chambers in the heads or a dish in the piston top, the ratio changes accordingly.

Let's try to visualize the difference between an LQ4 6.0L and an LQ9 6.0L. The former has a dished piston, while the latter uses a flattop. I am glossing over several steps here, so that too much math doesn't get in the way of the obvious.

Cylinder Volume = 45.515 cubic inches
Combustion Volume for LQ4 = 5.4147 cubic inches
Combustion Volume for LQ9 = 5.1095 cubic inches

The difference between the two combustion volumes is only the dish volume in the piston, 0.3052 cubic inches or 5.00 cubic centimeters. Next, add the volumes, then divide the sum by the combustion volume to calculate the compression ratio for each. My numbers come out just a bit off the factory specs because of rounding errors, but you get the idea.

LQ4 Compression = (45.515 + 5.4147) / 5.4147 = 50.9297 / 5.4147 = 9.401:1

LQ9 Compression = (45.515 + 5.1095) / 5.1095 = 50.6245 / 5.1095 = 9.908:1

Now, back to that combustion volume. It is the sum of several different volumes, some very small. That volume includes your combustion chamber, the compressed gasket volume, any dish or dome in the piston, and the clearance volume (how far above/below the deck surface the piston is at TDC). Now hopefully you can see how even a small change like thicker head gaskets or valve reliefs in a flattop piston can change the effective compression ratio.