compression ratio

[Foreman72]

hey what are your engines @? particularly SBs...if i have 10:1 should i shoot for 91 octane or higher...what if i shoot for 9.5:1? any change?

thanks for the help gents

[firefighter3931]

Lots of different factors will influence detonation resistance....but generally 9.5:1 works fine with 93 octane.


Ron

[Foreman72]

is there any sort of explanation anyone can give me to better explain the relationship between compression ratio to octane? the lower the compression the higher octane needed right? or do i have that backwards

[Ghoste]

You have it backwards, the higher the cylinder pressure, the greater octane needed to prevent detonation.

[Foreman72]

thats what i thought...as soon as i hit post i was like no thats not right...ok cool...so what are some of you other SBs getting for compression...if i can run 10:1 and use 91 then i'll be ok...just can't wait to see the gauge drop as i drive

[flyinlow]

The simple part of compression is that more is better . Ask anyone with a Diesel  (20:1 CR)    However.........


What CR you can tolerate varies with several factors:

What gas do you use?  In Ohio we can get Sunoco 94 Ultra . Some states can only get 91 octane.

Effective CR.....Static CR modified by the specs of your camshaft. Big cams reduce the peak pre ignition cylinder pressure at lower engine speeds.

ignition timing

air fuel ratio

deposit build up in the engine

engine material.... aluminum heads can run higher CR , but lose some of the gain to heat loss. ( your radiator will have to work harder)

A stock iron head 318 with mild cam would work with a 9.5 CR on pump gas from the experience I have had with them.


Octane rating is the ratio of isooctane (good stuff ) to isoheptane (cheep stuff) .  100 octane is 100% isooctane or a mix of isoctane and isoheptane with addatives to enhance the gasolines anti - nock rating. They used to use tetraethal lead to bump the octane number. 115 octane "purple passion" av gas  had so much lead in it , it would lead foul the spark plugs at low power setting. This fuel had 115% better anti knock rating then pure iso octane. It was used in High boost turbo supercharged aircraft engines.

In a piston engine you compress the air/fuel mixture. Then ignite it. Hopefully the flame front propogates smoothly across the combustion camber rasing the pressure applied smoothly to the top of the piston.

The tempurature of the air/fuel mixture that has not yet burned rises as the flame front crosses the combustion camber because it is seeing an increase in pressure.

If the temp/pressure is to high, the remaining unburned air/fuel mixture suddenly flashes all at once. This causes a spike in pressure which hammers on the piston/rods/bearings crank/block and makes a knocking sound.

Higher octane  has higher anti knock ratings. You can run more CR

It has been 3 decades since I read about this . Hope I remembered it right.

Craig

[Foreman72]

sweet man...thanks a ton... 

[PocketThunder]

The simple part of compression is that more is better . Ask anyone with a Diesel  (20:1 CR)    However.........


What CR you can tolerate varies with several factors:

What gas do you use?  In Ohio we can get Sunoco 94 Ultra . Some states can only get 91 octane.

Effective CR.....Static CR modified by the specs of your camshaft. Big cams reduce the peak pre ignition cylinder pressure at lower engine speeds.

ignition timing

air fuel ratio

deposit build up in the engine

engine material.... aluminum heads can run higher CR , but lose some of the gain to heat loss. ( your radiator will have to work harder)

A stock iron head 318 with mild cam would work with a 9.5 CR on pump gas from the experience I have had with them.


Octane rating is the ratio of isooctane (good stuff ) to isoheptane (cheep stuff) .  100 octane is 100% isooctane or a mix of isoctane and isoheptane with addatives to enhance the gasolines anti - nock rating. They used to use tetraethal lead to bump the octane number. 115 octane "purple passion" av gas  had so much lead in it , it would lead foul the spark plugs at low power setting. This fuel had 115% better anti knock rating then pure iso octane. It was used in High boost turbo supercharged aircraft engines.

In a piston engine you compress the air/fuel mixture. Then ignite it. Hopefully the flame front propogates smoothly across the combustion camber rasing the pressure applied smoothly to the top of the piston.

The tempurature of the air/fuel mixture that has not yet burned rises as the flame front crosses the combustion camber because it is seeing an increase in pressure.

If the temp/pressure is to high, the remaining unburned air/fuel mixture suddenly flashes all at once. This causes a spike in pressure which hammers on the piston/rods/bearings crank/block and makes a knocking sound.

Higher octane  has higher anti knock ratings. You can run more CR

It has been 3 decades since I read about this . Hope I remembered it right.

Craig

So if i have a really good radiator to keep the temperature in the heads down, i can run a higher compression ratio on 93 octane gas and not have knocking?

[b5blue]

Oct is resistance to combustion. Reg burns easier. For a young gun you need to ask yourself "do I want to pay for super for the life of my motor?" To build a higher compression motor is a commitment to paying more per mile to get more power. If your going to drive daily this gets old quick as the extra 30 cents a gallon adds up, and mileage is not good ether.  I suggest a 9 to one build with a mild cam for you Eric. You have allot on your plate now, school is first, repairs to the car are going to cost allot of time and money you don't have yet. It's not worth the extra $$$ to shave 1/2 a sec. on a 1/8th mile timeslip or need a high stall converter and all the extra beef needed to compensate for. Remember You can "power up" later. Stay in the middle of the road for now.   

[HPP]

Go play with this some and you'll see some interesting results.

http://www.rbracing-rsr.com/comprAdvHD.htm

Using this I found out my 10.5:1 engine with my cam at my altitude was only 154 psi, which I verified with a compression guage, and resulted in a 7.82:1 dynamic ratio. Kinda low. I need a mucho smaller cam or need to live 6000 ft lower to have a true 9:1 engine.

[c00nhunterjoe]

static compression tests are misleading when it comes to seeing about your compression ratio, the cam specs can wreak havoc on your mind when looking at that gauge.

[alcusswhen]

is there any sort of explanation anyone can give me to better explain the relationship between compression ratio to octane? the lower the compression the higher octane needed right? or do i have that backwards

You have it backwards. I'm running a measured 9:4 to 1 cr in my 318/391 stroker and I run 83 octane without pinging.
Now if I really wanted to run the car hard I'd bump up the timing and add a octane booster. Every engine likes something different the next guy may not be able to get away with 83 he may need 91.

[SeattleCharger]