THE CR VS. OCTANE DEBATE

[MandarinaRacing]

6000 Hp on PUMP GAS

I believe I've read this a million times on performance mags...what's the deal? Is this true? I know that detonation resistance depends not only on octane, but also on chamber design, spark plug placement, piston design maybe some other things. But it's really difficult to make an educated guess on how far to push the envelope on compression ratio. For our little sixes CR is a big thing, to me is another way to get more Hp from such a small "package" . After giving it a LOT of thought I've made the decision on running something like 11:1 even if I have to use race gas with my premium gas. I don't plan to use my car as a daily driver, but I know that even so detonation would be a concern, especially on a tropical climate . Anyway just somo food for thought !

Alex

 

[MustangSix]

11:1 on pump gas is possible, but only if you get the right combination of engine assembly, fuel curve, cam, and ignition to work in concert.

11:1 represents a static compression ratio, but figure that becomes critical is Dynamic Compression Ratio. It's a lot harder to figure because it is "dynamic" and changes according to engine speed, load, and VE.

Some examples:

A lightly loaded engine at part throttle cruise may be operating at 30% VE because of the restriction of the throttle plate. Since the engine is drawing so little fuel and air in, the cylinder pressure is very low and no pinging occurs.

An engine with a lot of overlap will generate lower dynamic cylinder pressures because so much of the intake charge is being blown out the exhaust. The engine makes more power at higher speeds because there is less intake charge dilution, but cylinder pressure may be lower.

An engine built with a lot of quench area will cool the air fuel mixture by bringing into extremely close proximity to the head as the compression stroke occurs. This cooling leads to lowered dynamic cylinder pressure. the quench area also enhances turbulence as it squishes the mixture together into the chamber.

Ignition that occurs earlier in the combustion cycle tend to raise cylinder pressure. Retarding th time reduces it. Generally, advancing the timing to optimize cylinder pressure increases power, however and engine with too much compression will lose power as the mixture preignites and the piston is hammered by the resulting shock of the advancing flame front.

To reliably run 11:1 on pump fuel you'll have to get a cam with sufficient overlap, run zero deck or better to get maximum quench, and work the ignition curve.

 

[rbohm]

8) one other thing that helps detonation resistance is a better rod length to stroke ratio. with the longer rods you can reduce ignition timing and still make the same power.

 

[Anonymous]

That's the reason why the next MAXIMUM EFFORT engine that I build will run ETHANOL.
Mix it with 5% acetone and 10% gasoline and you have a great racing fuel for $1.50 a gallon.

Anyone up for 10 to 1 compresion plus 20 pounds of boost?

I've always heard the engine builders say that high RPM is much harder on an engine than HIGH BOOST. One of these days I'll have to find out.

John

 

[Anonymous]

I've always heard the engine builders say that high RPM is much harder on an engine than HIGH BOOST.

True. If one increases the amount of air injested during the intake stroke by 100% (15 psi boost), Pz (max cylinder pressure) increases only about 25%. Yet if one doubles the RPM, the mechanical forces on the piston increase by 400% (Ek = mv2 /2, where m is mass and v is velocity).