As the title reads. Pros and Cons of High Compression. 11:1

 

The more you can compress the mixture, the more force you will have.

The problems comes when the quality of fuel (the octane rating) isnt enough to keep the mixture from pre-igniting under heat and pressure.

 

what if say your running 12.5:1 ---13??? What octane do you recommend?

 

The key to compression is that heat makes horsepower. The more heat you can generate in the cylinder, without destroying parts, the more intense the explosion is which in turns provides the force to move the piston. This is why superchargers and NOS make power. If you are able to increase the volume stuffed in a confined area the more the compression ratio will be.

You also have to take into consideration that two much compression has it's issues. To run 12.5-13:1 compression will definately need fuel such as VP C10. It is 100 octane and is unleaded, which won't effect the O2 sensors. You also will have to watch you spark advance as well. Should keep it in the 22-24 degree range.

 

What octane for 13:1? Well, that's hard to say. Some factors in this...

Camshaft design
Engine temperature (especially the heads)
Piston design
Quench area
Piston velocity
Intake air temperature
Fuel/air mixture (a rich mixture will allow you to get away with a lower octane)

But generally speaking I would think that 100 MOTOR octane (there's three means of measuring octane and "motor" octane is the one that you should be concerned with) should be fine with 13:1 even without lowering the timing advance below 27 degrees. I've been using 99 motor octane (Motorsport 103) with 12.2:1 C/R, a mixture set on the lean side, and the timing advance at 31 degrees. No ping, no KR.

BTW, C10 has a motor octane of 96 and is non-oxygenated. For a little more power use an oxygenated fuel like Motorsport 103, CSP (motor octane 96.6) or for really high C/R use HTG (motor octane 106.)

 

i have been told that you gain/loose about 10% for every full point (like from 10:1 to 11:1) of compression you change.

so say on a stock motor changing from stock to the above should net you around 30hp from the compression change alone (flyhwheel) based on a 300hp (flywheel) engine.

like my engine.. from the compression drop i am down about 40 flywheel hp (10.1 to 8.75).. which is why I dynoed 301 with most boltons instead of the 340 or so that guys in my area dyno.

anyone know what the 'motor' octane is on say crown 93 research octane?

 

i have been told that you gain/loose about 10% for every full point (like from 10:1 to 11:1) of compression you change.

Nah, it's nowhere near that much. More like %1.5 per point. Also realize that this goes down as C/R is increased. You'll see a MUCH bigger gain going from 10:1-11:1 than by going from 15:1-16.1.

"Pump" octane is motor octane and research octane averaged together (R+M/2). So, 93 octane at the pump would be about 87-88 motor octane.

 

What octane level also depends on your altitude. Here in the milehigh city, I am running 12:1 compression on 91 crapgas. When I have gone to lower altitudes, Ennis Tx, I had to run 100 on the street and 104 at the track.

 

So a 13.5:1 motor would be fine on say C16? <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" />

 

Where to buy these higher octane gases? MTI sells what kind og octane? its like $5 a gallon or so? <img border="0" title="" alt="[Confused]" src="images/icons/confused.gif" />

 

just go to the liquor store and buy a bottle of everclear and run your car on that. (its been done before) <img border="0" title="" alt="[Big Grin]" src="gr_grin.gif" />

 

Here
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Information:

The change in horsepower due to the change in compression ratio is relative but not directly proportional. That is to say that a change from 8:1 to 9:1 will give you a larger increase than would the change from 13:1 to 14:1. I seem to recall for every one point change around 7:1, that is the change from 7:1 to 8:1, would be slightly more than a 3% power increase. Once you get up to around 13:1, that same one point change is only good for about a 1.5% power increase.

The rule of thumb for the compression ratios run in most street engines is: for every point change in the compression ratio your power output will change by 2%. Using this rule of thumb on an engine that produces 400 hp, every 1 point change in compression ratio will result in approx. a 8hp change in output.

One thing that you have to remember is that this is a static model. The only variable changing is the compression ratio. Most of the time when a compression ratio change is made, significant other factors are changed which can significantly affect power output.

On avaliable pump gasoline it probably could be argued that your power might actually increase. This would be true if your compression ratio were high enough to force the use of a retarded timing curve (due to pre-ignition).

The TRW L2323F forged piston which is rated at 10.25:1 comes out much less if you were to actually compute the mechanical compression ratio. Two factors that reduce the mechanical ratio are the as follows:

1. The steel shim head gaskets Olds used had a compressed thickness of only .017". The common Felpro head gasket is .043".

2. When you have a valve job performed, the valves have their margin reduced, reducing their slight protrusion into the combustion chamber. Valve seats are slightly recessed into the head by the grinder. Both of these increase the head's combustion chamber. If any or all seats are replaced, this could go either way.

These two factors can increase your combustion chamber volumn by 5 to 7 cc's. This is enough to significantly change the compression ratio.

Consider the use of the car when determining the compression ratio. Your camshaft profile probably has the most significant impact on what mechanical compression ratio you should run. A longer duration camshaft will allow you to use higher mechanical compression ratio pistons because it lowers the effective compression ratio by keeping one or both valves open slightly into the compression stroke.

Be careful about the effects of production tolerances on compression ratio. Simply selecting a set of pistons labeled as 9.0:1 is not enough - you have to take into account the real combustion chamber volume, head gasket thickness, piston dish volume, and piston deck height. In reality, these dimensions are usually on the large side, resulting in less than the advertised compression ratio. While this is certainly safe from a detonation standpoint, it is not particularly healthy for performance.

The bottom line is that during any quality engine rebuild, it pays to take the time to check all of the factory dimensions. You may decide that it's not worth the time and expense to correct discrepancies, however at least you'll make that decision consciously.

[ Thanks to GABowles, Joe Padavano for this informatiton

 

How much power am I giving up by running 9.8:1 compression vs 11.8:1. This is a 388 with 6.0L heads that have HUGE chambers...76.4cc.

 

I'll simplify this, high compression = more power, this is good. they only bad thing is your fuel requirments will go up, or you will have to decrease some timing, which is no big deal, if you have LS1edit.

 

if going from say stock to 11:1 compression is only worth 8 hp.. why bother with the hassle?

you can get 8hp from lots of other places without sacrificing the comforts (gas, etc) of stock compression.

 

i have been told that a higher compression ratio gives a better throttle response...but im not sure how true that is...correct me if im wrong.

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by restoman45:
<strong>i have been told that a higher compression ratio gives a better throttle response...but im not sure how true that is...correct me if im wrong.</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Yes high compression gives better throttle response, power band is expanded on both ends also.Gas mileage is better also. <img border="0" title="" alt="[Big Grin]" src="gr_grin.gif" />