Boost on CR
T66
10-12-2005, 11:31 AM
Hi Guys and Girls. I’m going to Turbo a 2 L 16v Mazda motor with a CR of 10:0:1. Would I be able to boost 15psi with sufficient fuelling (4 times 650cc injectors Pump gas) or should I lower the CR? If not, what kind of boost am I looking at on the above mentioned CR?
T66
10-12-2005, 01:52 PM
Anybody pleeeeeease!
Polygon
10-12-2005, 02:00 PM
I would suggest taking the CR down to between 8:1 - 9:1 if this is going to be a street car. With that much boost your tuning will have to be near perfect. You're just playing with fire and 91 octane may just not cut it. Also, if you get a bad batch of fuel at the pump not even the tuning will save you. As soon as you hit full boost you will most like bust you pistons, or worse, and end up with a set of ash trays.
Then again you could bring the boost down on the street. However, 15 PSI at 10:1 on 91 octane equates to ash trays. I would say that you might be alright down towarads 10 PSI.
Then again you could bring the boost down on the street. However, 15 PSI at 10:1 on 91 octane equates to ash trays. I would say that you might be alright down towarads 10 PSI.
beyondloadedSE
10-12-2005, 02:11 PM
psi means nothing...its all about cfm.
T66
10-12-2005, 02:15 PM
Thank you.
Polygon
10-12-2005, 03:05 PM
psi means nothing...its all about cfm.
Well of course, but PSI is better understood by most people.
Well of course, but PSI is better understood by most people.
nissanfanatic
10-12-2005, 09:03 PM
No PSI does mean something when it comes to octane/CR selection.
What causes detonation/preignition on boosted cars running lower octane fuel(and higher octane if you run some crazy boost) is intake air temperature. 15psi is still 15psi of heat from enthalpy(s). Efficiency of the turbo will come into play as well. Take the heat created from compressing air to 15psi and divide it by the percentage of efficiency to get actual compressor outlet temperature.
Static and dynamic compression ratio are key to determining what type of fuel you can safely run. Typically you want to stay below 18-19:1 dynamic CR on 93 octane. Note that you will need to run extremely retarded timing which is really dumb IMO. *<20 is just not the way to go. *This is all assuming you have an efficient exhaust system along with an efficient air to air itnercooler. water to air/ water injection setups can def push more on 93 octane. Why? Cooler IATs.
Lower CRs allow for more boost on pump gas due to less compression heat as well. It all revolves around controlling combustion chamber temperature. Fuel burns quicker in a hotter enviroment. But you will notice that on the exact same size engine with two different CR, torque will be damn near the same when it comes time to switch to higher octane fuel. AKA the knock limit. PCP achievable before knock occurs from extreme pressure/temp causing pre-ignition. The fuel begins to burn before spark is applied due to very high pressure/enthalpy(s) temperature byproduct. This is how diesels run.LoL
What causes detonation/preignition on boosted cars running lower octane fuel(and higher octane if you run some crazy boost) is intake air temperature. 15psi is still 15psi of heat from enthalpy(s). Efficiency of the turbo will come into play as well. Take the heat created from compressing air to 15psi and divide it by the percentage of efficiency to get actual compressor outlet temperature.
Static and dynamic compression ratio are key to determining what type of fuel you can safely run. Typically you want to stay below 18-19:1 dynamic CR on 93 octane. Note that you will need to run extremely retarded timing which is really dumb IMO. *<20 is just not the way to go. *This is all assuming you have an efficient exhaust system along with an efficient air to air itnercooler. water to air/ water injection setups can def push more on 93 octane. Why? Cooler IATs.
Lower CRs allow for more boost on pump gas due to less compression heat as well. It all revolves around controlling combustion chamber temperature. Fuel burns quicker in a hotter enviroment. But you will notice that on the exact same size engine with two different CR, torque will be damn near the same when it comes time to switch to higher octane fuel. AKA the knock limit. PCP achievable before knock occurs from extreme pressure/temp causing pre-ignition. The fuel begins to burn before spark is applied due to very high pressure/enthalpy(s) temperature byproduct. This is how diesels run.LoL
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