Iron or aluminum head?
Race gas vs pump gas on lower compression street/strip motor
- Thread starter B3422w5
- Start date
-
justinp61
Well-Known Member
- Joined
- May 15, 2009
- Messages
- 3,056
- Reaction score
- 4,039
Aluminum.
TT5.9mag
Two atmospheres are better than one
@yellow rose i looked all over rockets website also and couldn’t find any valuable information. Sonoco’s website had some good stuff and also VP’s website was pretty informative. But no one really gets specific with the relationship between burn rate and octane.
- Best explanation i have read. So the answer is, "it depends"
- Beyond Octane
- “High octane fuels burn slower.”
- “My motor doesn’t need all that octane so that fuel won’t do me any good.”
- “I need the highest octane so I can max out my timing.”
But before we ignore octane, it’s important to look at how octane is measured in the first place.
Octane numbers are measured using single cylinder engines that look more like something out of a Model T than any modern engine. These so-called “knock” engines are operated by trained technicians in labs under controlled conditions. Two tests are used – one for Research Octane Number (RON) and another for Motor Octane Number (MON). The RON testresults in a higher octane value than theMON test.
If you average RON and MON, you get an octane value called AKI, short for Anti-Knock Index. It’s often labeled (R+M)/2, reflecting the fact that it is the average, or midpoint, between RON and MON. AKI is used to rate retail pump fuels, and it is the most commonly used octane value for race fuels as well.
For example – Sunoco Ultra 93 typically has a RON of 98 and a MON of 88, thus yielding an AKI of (98+88)/2 = 93. If you look closely at the yellow octane labels at gas stations, you’ll see the (R+M)/2 description under the big octane numbers. We provide all the octane numbers for all our race fuels – RON, MON, and AKI. So while we may refer to fuels like SR18 as “118 octane” (that’s the AKI value), we also show the RON and MON numbers under “Technical Details”. For SR18, that would be 120 and 116 for RON and MON, respectively.
Many times you’ll hear that MON is more important than RON because the MON test is performed under higher temperature and engine speed conditions. While this may be true, the laboratory test conditions are not indicative of what real race engines – heck, even mild street/strip motors for that matter – see at the track. Also, some engines have shown a better correlation between horsepower and RON. So, a word to the wise: don’t get hung up on octane numbers.
For a great example of why octane is not the only fuel parameter to ponder, consider the engines used in Formula 1. These engines have compression ratios exceeding 18:1 and spin at RPMs pushing 20,000. Sounds like a candidate for 116 octane race gas, right? Nope – they use a 96 octane fuel!
It’s hard to find a wider variety of race cars than what shows up at your local drag strip. On any given weekend you can see all sizes and shapes of engines ranging from raspy 4-cylinder motors to booming big blocks, and with all kinds of power adders thrown in the mix. This is where looking beyond octane becomes real important. What works for your buddy’s Pro Stock car may not be the best choice for your other buddy’s turbocharged import.
Naturally aspirated race motors with large combustion chambers spinning at high RPMs really like high-octane, fast burning fuels. They need the octane to prevent uncontrolled combustion, and they need a fast-burning fuel so that the flame front can span the large bore of the combustion chamber quickly. If you’re not sure which fuel burns faster than others, one indicator is specific gravity. “Lighter” fuels – fuels with a lower specific gravity – tend to burn faster because fast burning hydrocarbons are themselves light. Look for a specific gravity close to 0.70 and you’ll likely find a fast burning fuel. Of course, consult with the fuel producer to verify your assumptions. You might be surprised to learn that some of the highest octane fuels may also be some of the fastest burning fuels!
You might also be surprised to know that fast burning fuels may not need as much timing as their slower burning counterparts. Many times we get calls from individuals who are dialing in new motors on a fast burning fuel but they’re using timing and jetting numbers from their old motor and fuel combo. “Retard the timing a couple degrees and see what happens” is not the suggestion they expect to hear! With high octane, fast burning fuels, it is easy to dial in too much timing. In such cases, the engine is not detonating, but it is past the point of optimum spark advance, so it’s just heating things up and making less power. A little less timing may really wake up the motor
yellow rose
Overnight Sensation
Yeah, after I posted that I went to the Rockett website too and couldn't find it. I was going to call them and see they could remember the tech bulletin or article and where I might find it. I haven't done that yet.
How often have people run pump swill of whatever octane that usually does the job but maybe had issues with its quality or purity causing tuning and running problems over time? More common than not I’d wager. You don’t get those possible issues with race fuel, at least that’s part of its purpose, it is blended to be consistent and the side benefit of having a longer “shelf life” That’s what I read decades ago. Those are a couple of lesser promoted attributes that are possibly overlooked by some that are IMO of more value and actually real vs. the power question.
Last edited:
yellow rose
Overnight Sensation
IIRC, the the lower grades of pump fuel are much less regulated and have many more "fillers" in them than the 92-93 octane pump gas.
That higher grade of pump fuel is much more regulated, and can't have a bunch of "fillers" in it.
Pump gas of all grades have more detergents and varnish inhibitors than race fuel. There are a few very good octane boosters that actually "condition" the fuel. By that, it means they use chemicals that attach themselves to the detergents and such and it makes some of these things combustible, and some of them it just makes them so they don't leave as much residue after combustion.
I use them only when I'm going to go to the drag strip. Just driving around I don't use them.
Years ago I thought I had a fuel issue and needed race fuel till an oldtimer told me to throw my champion plugs in the river and put Autolite plugs in it. Issues addressed and fixed.
weedburner
Well-Known Member
I was skeptical when I built my first low compression S/S nitrous engine on pump gas, but I've since built several and have become a big fan. Low operating costs give me a lot more seat time than my race gas buddies, that I see as the biggest benefit.
My current one is 4.03"x3.48" 9.36:1 static. Solid roller 251/261 114 LSA, flat tops with .084" quench. Dynamic varies quite a bit depending on the calculation method, but it pumps about 100psi on the starter. Set of 342cfm Brodix -11RI's w/ titanium valves from a 360 sprint car. Never been on a formal dyno, but I ran the spec's thru somebody's calculator at one point and it said 549hp NA @7473rpm. Down on NA torque vs a higher comp engine for sure, but how much torque do I really need driving around on the street when the car weighs 2525lbs with me in it? It gets 20mpg on the highway with locked timing, manual trans, 3.73 w/ 28" tires, no overdrive.
When I pull up to a spot where I want to stretch it's legs, all I have to do is reach behind the seat to open the valve, then flip the switch to arm the system. Currently spraying around 250 on Shell 93, plenty for the backwoods no-prep stuff that I like to do. That little low compression 355 really comes alive when I hit it with spray.
Grant
My current one is 4.03"x3.48" 9.36:1 static. Solid roller 251/261 114 LSA, flat tops with .084" quench. Dynamic varies quite a bit depending on the calculation method, but it pumps about 100psi on the starter. Set of 342cfm Brodix -11RI's w/ titanium valves from a 360 sprint car. Never been on a formal dyno, but I ran the spec's thru somebody's calculator at one point and it said 549hp NA @7473rpm. Down on NA torque vs a higher comp engine for sure, but how much torque do I really need driving around on the street when the car weighs 2525lbs with me in it? It gets 20mpg on the highway with locked timing, manual trans, 3.73 w/ 28" tires, no overdrive.
When I pull up to a spot where I want to stretch it's legs, all I have to do is reach behind the seat to open the valve, then flip the switch to arm the system. Currently spraying around 250 on Shell 93, plenty for the backwoods no-prep stuff that I like to do. That little low compression 355 really comes alive when I hit it with spray.
Grant
-
