Guest bang847 Posted October 8, 2002 Share Posted October 8, 2002 Hey guys I had a question that popped into my mind.. when a N/A car has a compression ratio of 12:1 it cannot even run on pump gas. while a low compression turbo car that eventually has a higher compression ratio due to the forced induction can run normally on pump gas... dosent the end result after turbo effectively have a higher compression ratio than that of the high compression car? Quote Link to comment Share on other sites More sharing options...
Lockjaw Posted October 8, 2002 Share Posted October 8, 2002 Thats my story and I am sticking to it. Quote Link to comment Share on other sites More sharing options...
SleeperZ Posted October 8, 2002 Share Posted October 8, 2002 And the answer is...... No. The reason you can't run pump gas on 12:1 compression is the temperature rise for a 12:1 compression - on a turbo with 8:1 the temperature rise is a whole lot less even though you started with lots more mass under boost pressure. The "effective" compression ratio of a turbo is just that, an "effective" compression ratio. It doesn't have all the qualities of a high compression motor, just the power giving qualities Quote Link to comment Share on other sites More sharing options...
240Z Turbo Posted October 8, 2002 Share Posted October 8, 2002 You are speaking of dynamic compression and yes a turbo motor can meet or exceed a 12:1 dynamic compression ratio. However, turbo setups add more fuel in the 11.5:1 range on pump gas and we also retard the timing significantly to reduce the possibility of detonation. Quote Link to comment Share on other sites More sharing options...
TimZ Posted October 13, 2002 Share Posted October 13, 2002 Originally posted by 240Z Turbo:You are speaking of dynamic compression and yes a turbo motor can meet or exceed a 12:1 dynamic compression ratio. However, turbo setups add more fuel in the 11.5:1 range on pump gas and we also retard the timing significantly to reduce the possibility of detonation. Sorry James, but it isn't dynamic compression ratio, either. Dynamic compression ratio only deals with the effect that the valve timing/intake tuning/etc has on the engine's ability to fill the cylinder at a given rpm (i.e, volumetric efficiency, which also is independent of boost, no matter what people might say). For instance, a long overlap cam will generally give low dynamic compression at low rpm, because there is time for the fuel/air charge to leak back out before the intake valve closes. Therefore, the effective volume at low rpm is less at the beginning of the compression stroke, which results in a lower dynamic CR. At high rpm, there is no time for this to happen, so the dynamic compression is higher. Strictly speaking, compression ratio specifically deals with the efficiency of the combustion cycle, and can only add so much. Static CR is defined as the volume at the bottom of the stroke divided by the volume at the top of the stroke. Dynamic CR is the effective volume at the beginning of the compression stroke divided by the volume at the top. The difference between Dynamic and Static CR is that Dynamic CR changes with RPM, and Static does not. Boost pressure will not change these relationships. Boost pressure deals with the amount of fuel/air that is combusted. Nathan has it correct - the detonation problem is due to the tempurature rise during the compression stroke. The temperature at the end of the compression stroke depends on two things - the temperature of the fuel/air charge at the beginning of the stroke, and the compression ratio. Basically, the temp rise due to CR is added to the starting temp. If the end temperature is high enough to ignite the fuel/air charge, then detonation occurs Hopefully it's becoming clear that the turbo effects the starting temperature only. If you have a very efficient intercooler, you can do a pretty effective job of controlling this (at least to a far greater extent that you can with cr). Sorry to launch a big diatribe on this - seems like I'm making this post about once every year or so... Quote Link to comment Share on other sites More sharing options...
DavyZ Posted October 13, 2002 Share Posted October 13, 2002 Most turbocharged engines are electronically fuel injected. EFI makes a big difference in being able to run higher compression ratios with less detonation for the reasons listed above--this applies to an N/A motor as well as turbo. I'm getting tons of great info out of Fuel Injection by Jeff Hartman of the Powerpro Series. I strongly suggest this for any EFI questions you have. After I read this entire book, I'm going to read Maximum Boost by Corky Bell. Davy Quote Link to comment Share on other sites More sharing options...
TimZ Posted October 13, 2002 Share Posted October 13, 2002 Originally posted by 240Z Turbo:there are several calculators on the net to determine your dynamic compression ratio and yes, it is based on cam specifications, but also on the amount of boost you are running. Here is one calculator, but you can find many more: Dynamic Compression calculator Yes, I know people have put calculators out there to generate numbers for this, but they are not based on sound theory, given the physics of the Otto cycle (this is the thermodynamic principle that your 4-stroke engine is based on). The calculator that you linked to is flawed in this respect, and also in the fact that it does not consider RPM at all. Probably useful for calculating cylinder pressure at cranking so that you know what your compression gauge should read during a compression check, but not very useful for much else, IMHO. ...Can you tell this is a pet peeve of mine? Quote Link to comment Share on other sites More sharing options...
240Z Turbo Posted October 14, 2002 Share Posted October 14, 2002 ...Can you tell this is a pet peeve of mine? Yes, Tim had me pull out the physics book and thermo book to read up on the Otto cycle. I am determined to show you my side of the story. Just need to figure out what exactly dynamic compression means???? Quote Link to comment Share on other sites More sharing options...
TimZ Posted October 14, 2002 Share Posted October 14, 2002 Originally posted by 240Z Turbo: Just need to figure out what exactly dynamic compression means???? Heh Heh Heh...Good luck with that... Quote Link to comment Share on other sites More sharing options...
240Z Turbo Posted October 14, 2002 Share Posted October 14, 2002 there are several calculators on the net to determine your dynamic compression ratio and yes, it is based on cam specifications, but also on the amount of boost you are running. Here is one calculator, but you can find many more: Dynamic Compression calculator Quote Link to comment Share on other sites More sharing options...
jgkurz Posted October 18, 2002 Share Posted October 18, 2002 I looked up the web site that James referred to and got stuck on the "Inlet Valve Closes ABDC" blank. Does anyone know what this setting is for an L28ET? Thanks...... Quote Link to comment Share on other sites More sharing options...
Lockjaw Posted October 18, 2002 Share Posted October 18, 2002 Maximum Boost is kind of boring. A good reference guide, but boring. Quote Link to comment Share on other sites More sharing options...
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