Octane ratings?

[Soylent_Green]

What are the required octane ratings for different comprerssion ratios? My motor is almost 10 to 1 , do I need to pony up for the 93? or higher?

#-o I should know this already.

[1980_Cj7]

You need to figure out your dynamic compression ratio, which will be considerably lower than the static compression ratio. It is affected by the valve events determined by your cam. The later your intake closes, the more compression can bleed off.

Keith Black Silvolite pistons web page http://kb-silvolite.com/index2.php has some real good techincal articles and calculators that you plug your specs into to calculate your dynamic CR. I have been told by their techincal experts that you have to stay below 8 to 1 DYNAMIC CR if you want to run pump gas, even 93 octane. You can go a bit higher with aluminum heads.

[jeepsr4ever]

Yes you should run at 93 octane. If you are hearing pinging then you should bump up the octane.

[82Waggy]

From the KB site:

COMPRESSION RATIO -vs- COMPRESSION PRESSURE

Compression Ratio as a term sounds very descriptive. Compression ratio by itself, however, is like torque without RPM or tire diameter without a gear ratio. Compression ratio is only useful when other factors accompany it. Compression pressure is what the engine actually sees. High compression pressure increases the tendency toward detonation, while low compression pressure reduces performance and economy. Compression pressure varies in an engine every time the throttle is moved. Valve size, engine RPM, cylinder head, manifold and cam design, carburetor size, altitude, fuel engine/air temperature and compression ratio all combine to determine compression pressure. Supercharging and turbocharging can drastically alter compression pressures.

The goal of most performance engine designs is to utilize the highest possible compression pressure without causing detonation or a detonation-related failure. A full understanding of the interrelationship between compression ratio, compression pressure, and detonation is essential if engine performance is to be optimized. Understanding compression pressure is especially important to the engine builder that builds to a rule book that specifies a fixed compression ratio. The rule book engine may be restricted to a 9:1 ratio, but is usually not restricted to a specific compression pressure. Optimized air flow and cam timing can make a 9:1 engine act like a 10:1 engine. Restrictor plate or limited size carburetor engines can often run compression ratios impractical for unlimited engines. A 15:1 engine breathing through a restrictor plate may see less compression pressure than an 11:l unrestricted engine. The restrictor plate reduces the air to the cylinder and limits the compression pressure and lowers the octane requirements of the engine at higher RPM.

At one time compression pressure above a true 8:1 was considered impractical. It still is in many cases with today's gas. The heat of compression, plus residual cylinder head and piston heat, initiated detonation when 8:1 was exceeded. Some of the 60's 11:1 factory compression ratio engines were 11:1 in ratio, but only 8:1 in compression pressure. The pressure was reduced by closing the intake valve late. The late closing, long duration intake caused the engine to back pump the air/fuel mix into the intake manifold at speeds below 4500 RPM. The long intake duration prevented excess compresson pressure up to 4500 RPM and improved high RPM operation. Above 4500 RPM detonation was not a serious problem because the air/fuel mix entering the cylinder was in a high state of activity and the higher RPM limited cylinder pressure due to the short time available for cylinder filling.

The following compression guide should be considered realistic for sea level operation. Cam timing and special applications can move the recommendations around some, but in most cases the following recommendations work.

PUMP GAS (regular)
8.5:1-Non-quench 2 valve head road use standard sedan, without knock sensor.

8.5:1- Quench head engine for tow service, motor home and truck with torque cam.

9.0:1- Street engine with proper .040" quench, 200¬? @ .050" lift cam, iron head, sea level operation.

9.5:1- Same as 9:1 except aluminum head used. Light vehicle and no towing.

10:1- Used and built as the 9.5:1 engine with more than 220¬? @ .050" lift cam.

10:1- 4" and smaller bore, high RPM cam, cold plugs, good fuel distribution, full power limited to 30 seconds W.O.T.

[1980_Cj7]

What are the required octane ratings for different comprerssion ratios? My motor is almost 10 to 1 , do I need to pony up for the 93? or higher?

#-o I should know this already.

Speaking of "pony", yes, you are probably going to have to get your "pony" two more legs, ha, ha.

[rollen dean montoya]

try 89 to 91 octane 1st.

[Soylent_Green]

Speaking of "pony", yes, you are probably going to have to get your "pony" two more legs, ha, ha.


That wasn't in my build-up budget.

:-|

The horse will have to go without.

8)