Originally Posted by
kennybell.net
What parameters determine how much boost my engine can run on 91, 92, 93 or 94 octane?
Good question. One of the top 3 most commonly asked.
COMPRESSION RATIO - Lowering the compression ratio allows the engine to run more boost with the same octane. For example, 8.0:1 vs 9.5:1 can tolerate another 3 psi. 3 psi gives 38HP. The 1.5 lower CR loses 3% (2% per point) or around 9HP in a 300HP engine. That nets 29HP (38-9=29). Now you know why OEM's use low compression ratios on supercharged engines.
AIR CHARGE TEMP - The supercharger itself determines how much boost the engine will accept. Some superchargers require an intercooler. Others, such as the Kenne Bell, don't need intercoolers because the discharge air temp is cooler. 1 psi boost increases supercharger air discharge temperature 10°-20°, depending on supercharger type. A 6 psi "rated" kit usually requires 8 psi "total" boost (+2 psi to overcome inlet and outlet losses). Consider two 8 psi superchargers: 8x10=80°, 8x20°=160°. The 160° supercharger should use an intercooler. An air to water intercooler will lower that 160° to around 80° (160°-80°=80°) or the 80° temperature of a more efficient supercharger without an intercooler.
BOOST - Both higher compression ratios and boost increase cylinder pressure and demand higher octane fuels to eliminate detonation. Fuel octane determines the amount of boost an engine will accept. Kenne Bell runs 6-7 psi on all our Ford, GM, Chrysler and most other kits with our Twin Screw superchargers. 1.5 octane will support one psi of boost. Example: If fuel octane is 91 or you add a can of NOS Octane Booster you can typically run 8 psi. Intercooled kits can typically run 3-4 more psi.
IGNITION TIMING - Retarding ignition timing will allow the engine to run more boost but not without a penalty. 4° retard=16HP.
AIR FUEL RATIO - You can't make horsepower without fuel. Richer air fuel ratios reduce power but permit higher boost levels on a given fuel octane. Leaner ratios make more power but need lower boost so a lean mixture with high octane makes the most HP. We thoroughly understand all the parameters of supercharging. Supercharger kit tuning demands ignition timing, air fuel ratio, fuel octane, boost, compression ratio, discharge temperature and supercharger parasitic losses all be carefully considered in the design.
What causes detonation? 1. Lean fuel mixture, 2. low octane fuel, 3. excess boost, 4. lack of fuel, 5. advanced ignition timing, 6. vacuum leak, 7. "hot" spark plugs, 8. overheated engine, 9. excessive inlet temperature (underhood filter) and incorrect readings from cheapie non calibratable wide band $35 VW Jetta oxygen sensors.
Why is knock protection so important? Knock (detonation) can destroy your engine if it is not negated. Suppose your wife or a friend unknowingly fills your tank with 87, or you get a "bad batch" of gas, the engine overheats, someone installs a pulley that raises supercharger boost, the wrong spark plug heat range is installed (too hot) or a vacuum leak develops and the engine leans out. That's when an engine needs knock protection. No one in this industry has more experience than Kenne Bell with knock protection. It's also why we prefer "tuners" leave our kit calibration as is. The knock sensors are "sensitized" to distinguish knock from supercharger noise.
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Guide to Boost, Octane, and CR
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