Re: Question about compression ratio and octane
[Re: Monte_Smith]
#2260750
02/27/17 10:57 PM
02/27/17 10:57 PM
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Joined: Aug 2003
Posts: 43,007 Bend,OR USA
Cab_Burge
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Bend,OR USA
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EVERYTHING is a factor. 11.1 motors can be just fine on pump gas and 9.1 motors can try and beat the heads off when you step on it. All depends on the parts, the combination and how the two come together.
So to answer your question, No, there is no "here is the number" where it needs to be better than pump gas Your local altitude and weather can and will also contribute to what your motors does on the local pump swill
Last edited by Cab_Burge; 02/27/17 10:57 PM.
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: Question about compression ratio and octane
[Re: 1970RT]
#2260841
02/28/17 01:08 AM
02/28/17 01:08 AM
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Joined: Jan 2003
Posts: 30,947 Oregon
AndyF
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I've read many times over the years about higher compression engines requiring higher octane fuel than pump gas has to offer so I was wondering, is there a specific level of compression that requires a jump from pump gas to some sort of higher octane gas or race fuel? Or, are there other factors of the engine build that would have a bearing on how much octane is needed? A good rule of thumb is that 12:1 or higher should be using race gas. You may or may not be able to break that rule depending on how many tricks you pull but the average bracket guy with a carb with 12:1 should be running something like C12 or VP101.
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Re: Question about compression ratio and octane
[Re: 1970RT]
#2260842
02/28/17 01:12 AM
02/28/17 01:12 AM
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Joined: Nov 2015
Posts: 870 Missouri
jwb123
super stock
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super stock
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Missouri
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The static compression ratio is not what determines if an engine will spark knock on pump gas. The dynamic compression ratio, Quench, piston design, etc. is what determines if an engine can use a specific octane fuel. Buy yourself a good computer based engine program and it will calculate those factors. I use performance trends http://performancetrends.com/Engine-Analyzer.htmIt will calculate dynamic compression, and give you cranking compression, a spark advance curve, for what ever octane fuel you put into it. Plus all kinds of other stuff. If you go through all the parameters it uses and learn what each one means you will learn a bunch about how engines operate and the factor that affect performance.
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Re: Question about compression ratio and octane
[Re: 1970RT]
#2260869
02/28/17 02:14 AM
02/28/17 02:14 AM
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Joined: Sep 2015
Posts: 1,096 Australia
ozymaxwedge
super stock
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super stock
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Australia
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Im at 12.5-1 with a 528 on the street but we have 98 here at the pump, I think it's just good enough for my combo.
1963 Plymouth Max Wedge 1971 Barracuda
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Re: Question about compression ratio and octane
[Re: ozymaxwedge]
#2261033
02/28/17 01:58 PM
02/28/17 01:58 PM
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Joined: Jul 2004
Posts: 19,317 State of confusion
Thumperdart
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I'm at 12.1.1 and drive on 91 sometimes and romp/race on VP-100 at the pump which they say is good to 14.1.1 w/alum. heads..........
72 Dart 470 n/a BB stroker street car `THUMPER`...Check me out on FB Dominic Thumper for videos and lots of carb pics......760-900-3895.....
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Re: Question about compression ratio and octane
[Re: 1970RT]
#2261095
02/28/17 03:51 PM
02/28/17 03:51 PM
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Joined: May 2008
Posts: 5,399 Aurora, Colorado
451Mopar
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master
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Posts: 5,399
Aurora, Colorado
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As mentioned, one of the biggest factor is Dynamic compression ratio. This is just re-factoring the compression ratio (engine stroke) from the point of intake closing ATDC. This has a direct affect to the cranking (low speed) compression pressure.
Because air pressure changes with altitude, the compression pressure will also decrease at higher altitudes.
Part of the theory is that as a gas, or air (a combination of gases) is compressed, it increases the temperature of the gas. So having a cooler starting inlet temperature can help reduce the chance of pinging.
Another part of the equation is the fuel itself. Gasoline is a combination of hydrocarbons which will vaporize and ignite at different temperatures. Also as fuel is mixing with the incoming air, it can have a cooling effect of the air/fuel mixture.
A harder part to quantify, is cylinder chamber design, size, spark plug location, chamber hot spots, ect. A large open chamber may allow the air/fuel to pre-ignite, where a smaller chamber reduces the chance of pre-ignition. A Quench design also creates a lot of motion of the air/fuel mixture.
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Re: Question about compression ratio and octane
[Re: dogdays]
#2261176
02/28/17 06:44 PM
02/28/17 06:44 PM
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Joined: Aug 2003
Posts: 43,007 Bend,OR USA
Cab_Burge
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Bend,OR USA
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Altitude plays a big role and the higher the altitude, the lower the cylinder pressure and lower the octane requirement. So Regular is 87 up to some altitude limit, maybe 5000 ft, then Regular will be 85 The other grades of gasoline follow.
R.
Good point
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: Question about compression ratio and octane
[Re: DaytonaTurbo]
#2261285
02/28/17 09:30 PM
02/28/17 09:30 PM
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Joined: Oct 2009
Posts: 1,267 North, Alabama
D-50
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pro stock
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North, Alabama
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My lowest cylinder is 195 psi and the highest is 201 psi. I have never run anything but Super unleaded (93 octane) in my small block. I run 35* timing, 10.9 comp.
1.33 60 ft,6.21 at 110.59 in the 1/8, pump gas small block,2950lbs,drag radials,mufflers and driven to track ...
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Re: Question about compression ratio and octane
[Re: DaytonaTurbo]
#2261355
02/28/17 11:21 PM
02/28/17 11:21 PM
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Joined: Jul 2004
Posts: 19,317 State of confusion
Thumperdart
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I Live Here
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170-175 cc at 12.1.1 comp............
72 Dart 470 n/a BB stroker street car `THUMPER`...Check me out on FB Dominic Thumper for videos and lots of carb pics......760-900-3895.....
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Re: Question about compression ratio and octane
[Re: DaytonaTurbo]
#2261841
03/01/17 08:57 PM
03/01/17 08:57 PM
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Joined: May 2008
Posts: 5,399 Aurora, Colorado
451Mopar
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What kind of cranking compression are you guys able to get away with on pump? I seem to recall reading ~180psi with quench and aluminum heads? It depends on how the engine is loaded. A RV/truck type engine (under load for long durations) you want to be in the lower pressure range, maybe 160 psi max. A drag strip car can get away with a lot higher compression on pump gas because of the gearing, high stall converter, and short run time (minimum heat buildup, and engine not operation in the lower RPM range). I would think 200 psi with a well sorted engine. For my street/strip car I usually shoot for 170 psi. If it is a true street car, error on the low side. Pump gas varies quit a bit, and sometimes the engine may run hot in summer heat.
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Re: Question about compression ratio and octane
[Re: 1970RT]
#2261850
03/01/17 09:08 PM
03/01/17 09:08 PM
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Joined: Mar 2008
Posts: 5,746 Ontario, Canada
Dodgem
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Ontario, Canada
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My engine was built to be 12.5 to 1 and I have never had anything but 94 in it??? I have not done a cranking compression yet. Stall was about 5500 should be 5000 this year??? 274/282 112 lsa installed at 108 574 cubes 64 cc indy heads I the think dish is 23.5CC. I have had the timing up to 39 and plugs look fine?? this boost compression thing says I should have 205 cranking at 800 feet http://www.rbracing-rsr.com/comprAdvHD.htmmy old engine analyzer 3.0 say 180 always found it closer than above????
Last edited by Dodgem; 03/01/17 09:39 PM.
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Re: Question about compression ratio and octane
[Re: 1970RT]
#2261875
03/01/17 10:13 PM
03/01/17 10:13 PM
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Joined: Jan 2005
Posts: 12,408 Kalispell Mt.
HotRodDave
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The new eagle hemi will hit 180 PSI and runs fine on the 87 octane although I don't know where the cam is sitting advance or retard wise with the engine off. My dads geo metro would pump 210 PSI stock and was heavily loaded all the time just to keep the car going down the highway, it ran better and got more MPG on the lowest grade than premium.
There is a whole lot more to PSI VS Octane that comes into play. My dakota with a 410 stroker and nearly 12 to 1 with iron heads needs a gallon of 110 octane added to about 18-20 gallons of premium to keep it quiet, I am in the process of lowering compression about 1/2 point so I hopefully don't need to put lead in my fuel and foul the O2Ss anymore. It would pump 220PSI and I have about every trick in the book in my engine to make it knock resistant.
I am not causing global warming, I am just trying to hold off a impending Ice Age!
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Re: Question about compression ratio and octane
[Re: ozymaxwedge]
#2262070
03/02/17 06:35 AM
03/02/17 06:35 AM
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Joined: Feb 2014
Posts: 234 Brisvegas, Australia
Alchemi
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Brisvegas, Australia
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Im at 12.5-1 with a 528 on the street but we have 98 here at the pump, I think it's just good enough for my combo. As Dogdays says its different in the us Our Aussie 98 is the same as their 92
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