Question about compression ratio and octane

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?

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

Even the variance of the local pump swill......

Todays technology lets you build an eng to run fine on pump when you build an eng to take advantage of technology. Build good quench in the eng and aluminum heads help as running a cam to keep the cyl pressure low enough for pump. Course todays pump gas is very crappy but I see guys running 11.0 and more comp running on pump. But like Monte said even a 9.0 eng can ping and run bad if not built right. The combustion chamber designs are also much better on the aluminum heads of today. I run 10.6 comp ratio and have nice quench built in my eng. I use modern aluminum heads and with 36 total timing runs fine on 92 pump. Your best bet is to get with a good eng builder and work out your combo of what you want and what will work with the eng you want to build. Then build it to work on the pump in your area. Ron

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.

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.htm

It 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.

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.

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..........

I was always a big fan of 12.1 compression, run 112. 14.1 compression, run 114. I actually have 13.? compression and have been running 114 since I switched to the Indy heads. I got a barrel of VP110 that I have been running and seems to be fine. VP is great fuel though and my car seems to like it and Sonoco the best. They have Sonoco 112, 114 and 116 at the pump at Sonoma Raceway as well as 93, 98, 100 unleaded. I heard that the 114 is just a blend of the 112 and 116 so you don't really know where it's at so I just get the 116 because it's the same price. I recently put some 110 in my Dart with the frankenstein motor and it is WAAAAAY happier. The unleaded we get in CA is crap. If you have any decent investment in your HP engine I would stay away from unleaded or mix 5 gallons of 110 in it like I did.

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.

The US uses a weird octane rating system. It's the average of the Research Octane Number and the Motor Octane Number. Back in the day, the number on the pump was the RON. RON and MON are usually from 8 to 10 numbers different, so to approximate the US rating from a Research Octane Number you usually subtract 4 or 5. That puts the 98 RON down to 92 - 93 by the US.

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

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?

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.

170-175 cc at 12.1.1 comp............

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.

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.htm

my old engine analyzer 3.0 say 180 always found it closer than above????

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.

As Dogdays says its different in the us

Our Aussie 98 is the same as their 92

That's what you gotta look at. A cranking compression of 170 - 180 will usually safely live on 92 pump gas. Over 180 and you better have all your in a row.

Another important factor when pushing the limits is VE, my stroker SB is probably not getting a real high VE with smallish magnum heads but my PSI is real high. If it was flowing real awesome as the RPM went up I would probably have much higher PSI from the inertia of the gas flowing in the cylinder that I would have big issues. Thats my theory, someone smarter has my permission to debunk it.

for sure. Dyno #'s.