Why more compression?

[Anonymous]

We always talk about getting more or less compression?

Currently I am asking for my engine to run at 9.6:1 with a higher performance cam, and I was told you can get away with running pump gas as long as you stay close to the octane rating. I also read a recent post about ratings on gasoline being a little higher than the pump says...


so my question is two parts....

1. Why do we want to increase the ratio on a non-turbo engine and what effect does it have?
2. What is the max CR you could safely run in 93 octane? will I be OK with 9.6?

Thanks

 

[Anlushac11]

8)

As you know a gasoline engine draws fuel and air into the combustion chamber and burns it with explosive force.

this force has no route of escape so something has to give and its path of least resistance is to push the piston down. Multiply this times 5 in the correct order and you have crank rotation. This can be measured in horspower and torque.

The greater the compression ratio the more force exerted and the more power. On average a 1 point increase in compression will net about 10 horsepower. Ergo 8.5:1 to 9.5:1 will yield about 10hp.

Typically raising compression raises power but also increases emmisions. Newer model cylinder heads are designed to burn the mixture more completely and this in turn reduces emmisions. Thats why you see newer engines with higher compression ratios again. That and computer comtrolled fuel injections that maintains pinpoint accuracy to reduce emmisions.

Generally octane with decimal one place from left roughly equals compression ratio. 93 Octane should run about 9.3:1 or 9.5:1

 

[65coupei6]

Anlushac11 basically answered your question.

More compression = more hp.

And yes generally the compression dictates what grade gas you run. But, that is general. I have a 9.6:1 and am using 91 octane which is the highest I can get in Cali at the pump. I have no pinging or detonation.

But pinging / detonation also has to do with plug heat range, carb mixture, and timing. If you get all of the above correct I do not see any reason why you can not run a higher compression.

 

[rbohm]

8) a bit of math for you to help clear the water. take two engines, one with 10:1 compression and one with 8:1 compression. atmospheric pressure at sea level is 14.7psi, but to keep the math simple i will round to 15. we will also assume the cylinder fills 100% again to keep the math simple. at max compression the 10:1 engine is compressing the a/f mix to 150psi where as the 8:1 engine is hitting 120psi. when the cylinder fires the combustion pressure rises by a factor of 10, so the 10:1 engine has a max cylinder pressure of 1500psi and the 8:1 engine is at 1200 psi. to figure the net pressure on the piston subtract the initial compression pressure. the 10:1 engine is 1350psi and the 8:1 engine is 1080psi. the additional 270psi on the piston is more work the cylinder with 10:1 compression is doing over the 8:1 engine. this means more power and better fuel efficiency as you use less throttle pressure to maintain the same rpm. again remember the numbers are rounded off and certain assumptions are made, but the difference is still there percentage wise.

 

[CZLN6]

Howdy All:

Wow rbohm! That was one of the best was descriptions of compression ratio I've ever heard. I hope you don't mind if I use it next time one of the grasshoppers needs a clearifying lesson.

A couple of other factors which will effect CR;

Less quench, aka deck clearance=deck height (the distance from the top of the piston to the deck of the block) + the compressed thickness of the head gasket. On the typical '66 200 with a deck height of .025" and a OEM steel shim head gasket at .025" equals .050" of deck clearance. Ideally, for performance, knock resistance and maximum turbulence for better combustion efficiency you'd like no more than .035" quench. This tightness pushes fresh A/F mixture out of the crevice between the deck of the head and the top of the piston into the combustion chamber and the piston dish with vigor, causing turbulence and promoting a better more complete burn.

Smooth and polish the inside of the combustion chamber. This eliminates hot spots and glow plugs, and minimizes carbon build up and CR creep. Polished surfaces reflect heat to where it is supposed to be.

Cam timing. More cam duration allows a higher static CR. Earlier intake valve opening and earlier exhaust valve opening reduces cylinder pressure at lower rpms.

Higher elevation. You can add a .10 to the octane generality for each 1,000 feet above sea level. For example, 87 octane, 8.7:1 CR in(sea level) Long Beach or Miami would increase by .5 to 9.2:1 for (5,000 ft above sea level) Denver or Idaho Falls.

Ambient temperature and humidity. Higher temp and lower humidity = less CR. Lower temp and higher humidity allows a higher CR.

Dished pistons. For a number of reasons dished pistons tend to make more power and are able to tolerate a higher CR then a flat-topped or domed piston. Ideally, we'd like a piston with a dish that mirrored the shape of the combustion chamber, thus maintaining a higher ratio of quench to bore. Chevy sixes have a "D" shaped dished piston. Why can't we?

Hotter, fatter spark. A hotter and larger spark creates a better flame kernal and, consequently, a flame front, or burn. Thus less likely for spontanious ignition.

And, factors like A/F ratio, engine temperature and ignition must be carefully managed for best results in any engine.

Adios, David

 

[BB6Mustang]

Jimbo65, Increasing the CR will indeed give you a little more power from idle to redline, but it will also significantly raise the engines thermodynamic effeciency, and therfore give you much better gas mileage.

As to your second question, yes you should be able to run a 9.6 CR depending on your cam. Cams with longer durations lower cylinder pressure because they stay open longer on the compression stroke. So radical cams need a higher CR to keep cylinder pressure the same as an engine with a smaller cam. Many other things also affect CR, but they have already been pointer out.

 

[rbohm]

8) david you can use that explanation anytime you like. i learned from my high performance engines instructor in college. his name is denny wykoff and he has been in a few enthusiast publications recently about a cam selection program he developed. one reason why dished pistons make more power than flattop or domed pistons is they introduce more turbulance in the combustion process than a flattop, and dont interfere with the flame front like a domed piston.

 

[Anonymous]

Thanks a lot for the information, it is well received...

Here are my cam specs...

I think 9.6:1 will work nice in Atlanta GA...

270/280
204/214 @ .050 .420"/.442"
110* L/A


Thanks again....