Thread: Boost and Compression
12-10-2009, 11:17 AM #1
Boost and Compression
Posted this link on the Yami section also. Its a great reference chart for building an engine.
Now you can see that 93 octane doesnt get you very far, SAFELY..
12-10-2009, 11:28 AM #2
12-10-2009, 11:29 AM #3
12-10-2009, 11:38 AM #4
That's a very cool chart! Good guideline.
Looking at the chart would that mean "roughly" that running 8lbs of boost on a 8.0:1 compression would yield you an essentially 12.4:1 compression motor.
All else equal would this translate into the same output as running 12.4:1 naturally aspirated?
Also makes me wonder why we're not detonating ! I know we're running 14+ pounds of boost on pump gas.
12-10-2009, 11:40 AM #5
12-10-2009, 11:48 AM #6
Not applicable to our skis ... we run centrifugal SCs which run much cooler than roots SCs, especially at higher boost levels, plus we are running BIG water to air intercoolers, which has a huge impact on what kind of boost you can run. Check the info below as listed on Procharger website ... shows it is possible to run 15 - 18 lbs boost with a centrifugal SC and very good intercooling, on pump gas with our 8.5:1 engines.
Reliable High Performance
Clearly, the only type of performance that matters is reliable performance, and detonation is the single biggest threat to engine reliability. The boost range for reliable performance, without detonation, can be determined by looking at the type of supercharger technology being considered, and the compression ratio of the motor. With a lower compression ratio, an engine can safely handle more boost, everything else being equal. Similarly, if the temperature of the compressed air is lower, an engine will have a much higher detonation threshold (the point at which fuel ignites without a spark), and will be able to safely handle more boost. The chart below helps to illustrate how the overall efficiency of the entire supercharger system can be increased by both leading edge supercharger efficiency and the use of intercooling. Simply locate an engine's compression ratio at the bottom of the chart and trace upwards to determine the maximum reliable boost level. The amount of heat produced (adiabatic effiency) by each supercharging technology is what determines the boost limitation. While gear-driven centrifugal is clearly the superior supercharger technology, it is also clear that the biggest benefit comes from intercooling. These calculations assume moderate timing, 92 octane pump gas, and a good supply of fuel to the cylinders. As mentioned previously, detonation is the single biggest threat to engine reliability. It is heat and detonation that cause blown head gaskets and burned pistons, not boost. Achieving maximum performance from a given engine while avoiding detonation requires the right combination of intake air temperature, timing and fuel quality. For example, without intercooling a stock 5.0 with 9.5:1 compression ratio can only hold 5-6 psi of boost before detonation becomes a problem. The only way to safely run more than 6 psi of boost and still make a meaningful increase in power without an intercooler is by using racing fuel to avoid detonation. Many companies also employ "band-aids" such as ignition retard and larger injectors to run 8-9 psi on pump gas, but the resulting increase in performance is only marginal (since both of these band-aids suboptimize) and detonation is frequently still a problem.
Last edited by Vern; 12-10-2009 at 11:50 AM. Reason: fixed
12-10-2009, 11:55 AM #7
12-10-2009, 12:17 PM #8
Its the one area we have an advantage over cars ... we have so much cool lake water already pressurized by our pumps, its perfect for serious intercooling which is a huge factor in allowing us to run pretty high boost.
12-10-2009, 01:28 PM #9
- Join Date
- Nov 2006
- Delray Beach, Florida
With lake water, even what would be considered a "BAD" intercooler will yield charge temps of 145 - 155F at 18 to 20 psi. This would be considered very good in most air to air car applications.
A Good or Great charge cooler in one of these boats can easily yield charge temps just a few degrees over water temperature, water temp is generally lower than ambient air temp, so it is quite common to see charge temps at boost levels above 20psi that are lower than ambient !! NEVER going to see that in a car.. Unless you spray the charge cooler with Nitrous or pack it with dry ice for a few runs...
Cold charge temps are just another reason we can run radical timing maps, and lean mixtures while producing big power numbers..
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