91 Versus 93 Octane on stock Gen IV's

[TrackAire]

Has anyone had a chance to test performance differences between 91 and 93 octane on a stock Gen IV? I'm not talking about the Mopar computer upgrade, just the stock programming.

Any dyno runs out there?

Thanks,
George

 

[Martin]

Every dyno result I've seen in California has been a bit lower than in states that have 'good' gas - so I'm pretty confident that there is a difference. I'm not sure if it's from the octane, or the stupid 'reformulated' gas in CA that has a lower BTU content per gallon. I'm thinking it's likely the BTU content...

 

[Tom F&L GoR]

If you're thinking about the ethanol, can't blame the BTU content like you would in a diesel because you're not fuel volume limited. The 02 sensor will know it's running leaner (from oxygen in the gasoline mixture) and enrichen it, and it is well within the range of the fuel system to do so. You "should" get same power.

 

[Newport Viper]

Here is alot of stuff I dont understand. Octane rating - Wikipedia, the free encyclopedia

 

[TrackAire]

I'm just curious what the added hp (and more importantly torque) would be if the car ran 93 octane with a stock computer. My understanding is the octane level (either up or down) will be read by the computer very quickly and adjusted to meet the parameters of what the knock sensors and programming can handle.

My "guess" is there's more torque to be gained than horsepower, especially down low. Then again, maybe the stock programming is optimized for 91 octane and anything above that may be detrimental. I wonder if the SRT engineers have any data on this?

Cheers,
George

 

[Martin]

If you're thinking about the ethanol, can't blame the BTU content like you would in a diesel because you're not fuel volume limited. The 02 sensor will know it's running leaner (from oxygen in the gasoline mixture) and enrichen it, and it is well within the range of the fuel system to do so. You "should" get same power.

I'd have to think about that a bit, and my mind is already fried for the day I'm not doubting you, but what I can't seem to wrap my mind around right now is how, at WOT and at approxmate stoichiometric combustion, the O2 sensors would know that the fuel has a lower BTU content and richen it up. There's only so much air available to support the combustion, so I can't figure out how the O2 sensors would play a role in this. It's Friday, and I'm sure a few beers from now it will be clear - but right now I'm scratching my head.

 

[sekhmet]

I'm just curious what the added hp (and more importantly torque) would be if the car ran 93 octane with a stock computer. My understanding is the octane level (either up or down) will be read by the computer very quickly and adjusted to meet the parameters of what the knock sensors and programming can handle.

From what I understand, you will not notice an increase in performance unless your car is knocking resulting in timing retardation from the lower octane.

The octane level is just a measurement of the fuels resistance to ignite from compression. 93, 91, and 87 octanes all have the same energy content.

 

[08Venom]

I thought I have read before,but maybe misunderstood,that the vehicles computer can advance the timing with higher octane fuel,because the resistance to detonation is higher e.g.91 vs 94.I thought one could pick up a few more horses,not a lot,but some.

 

[FastestBusaAround]

If you're not knocking, lower octane would give you more HP as there's less retard taking place.

 

[TrackAire]

FastBusa,

I'm not sure I'm understanding your comment. Do you mean since the lower octane has more BTU's it will create more energy?

I don't think I'll ever know if I'm knocking or not with the computer and knock sensors controlling the timing. Unless I go forced induction, I don't think I'll ever physically hear the car knock set up the way it is.

So, if the computer will retard the timing due to lower octane, does anyone know for a fact if it will advance the timing to take advantage of higher octane if it is available (higher than the recommended 91 octane)???

Is there a computer reader that can read the 08 and 09's while driving to see what the timing is doing under different loads with different octanes? With the variable camshaft and knock sensors, I'd think there is room for improvement if more than 91 octane is available (up to a point).

Cheers,
George

 

[FastestBusaAround]

I'm basing this on my experience with my Kawasaki ZX14 on the Dyno, which also uses knock sensing to retard timing in the event that it occurs. I checked with a few race tuners who confirmed to me that they always go for the lowest octane possible, to achieve more power, unless there's a knock issue. These bikes run with 12.5 compression and are more likely to knock under these circumstances, but I have to believe that the same holds true for our cars. I doubt that the system will advance timing in the same way that it retards it...but can't say for sure. Just my ,02

 

[TrackAire]

Ok that makes sense. Too much octane and you go the wrong way if the system isn't designed for it.

Since the Mopar race computer requires 93 octane to make a little more power, that's sort of what got me thinking about our stock setups on the 08-09's. I still don't think we'd make more hp in the top end, just more torque in the lower half of the rpm range if the computer let us take advantage of a couple more octane points.

Cheers,
George

 

[viper067]

Back in my college days, one of my friends was doing work for a major oil company. His testing was to find out if there was a MPG increase using higher octane fuel....obviously the company wanted to sell more higher octane fuel and this would be a good reason to push it on people. Unfortunately for them, it turned out that higher octane = lower MPG

go figure

 

[Chuck 98 RT/10]

If you're not knocking, lower octane would give you more HP as there's less retard taking place.

Interesting. I always theorized that unnecessarily high octane could be a performance detriment. Nice to see a confirmation.

 

[TrackAire]

I think your college friend is dead on..I've always been taught that the lower octane fuel had more BTU's...in my eyes more energy per gallon would mean more mpg. Hopefully Tom the oil guru might chime in to correct me if I'm wrong.

But...I'm sure when the fuel company tested the low vs high octane mpg deal, they had nothing close to the variable camshaft and potential computer control we have in the Gen IV's. Again, I'm no expert on this new camshaft technology, but if you had higher octane available and the ability to adjust the camshaft to create more cylinder pressure and keep it all in check with knock sensors, you might get more power. I'm really not concerned about better mpg, just more torque and hp.

Does anybody know if our camshaft within a camshaft has the ability to create more cylinder pressure if programed to do so? In a way it would be like a variable compression engine, if the octane and other variables are met, your power should go up......I think.

Cheers,
George

 

[GTS1997]

Wait, wait, wait......................


If I'm reading these posts right, lower Octane = more horse power?

 

[Chuck 98 RT/10]

Wait, wait, wait......................


If I'm reading these posts right, lower Octane = more horse power?

Depending on engine design, but yes. That is what we are discussing.

Tom, is there any difference in thermo conductivity between 87 and 93 octane fuels?

 

[TAILWAG]

I know the mopar PCM for 08's (and most likely other years as well) calls for a 93 octane + to work ideally. I haven't installed it in my car yet (I have headers and full cat back). If I happen to be in a bind and can only find a 91/92 octane fuel station, what would happen? Is the PCM not going to act to its full potential only?...or are there other issues involved?

Thanks.

 

[SilveRT8]

I know the mopar PCM for 08's (and most likely other years as well) calls for a 93 octane + to work ideally. I haven't installed it in my car yet (I have headers and full cat back). If I happen to be in a bind and can only find a 91/92 octane fuel station, what would happen? Is the PCM not going to act to its full potential only?...or are there other issues involved?

Thanks.

It was mentionned before by the Grailkeeper that the Mopar Controller will be fine on 91 oct although it's designed for best results with 93 oct.

 

[Tom F&L GoR]

Howdy , all. I'm reading through the posts so some answers may be repetitive again some more...

If the OEM PCM is designed for 91 octane (like Gen 1, 2... when knock sensors came in I don't know) then running 93 will not produce more power. Higher and higher octane may lose power because the rate of combustion (and therefore optimal timing) or volatility (and therefore in-cylinder air-fuel mixing) changes go beyond what the OEM PCM can adjust for.

Ethanol blended gasoline is from 0% to 10% EtOH, so has up to 3% ish percent oxygen in the fuel. The OEM PCM should easily adjust for this to produce the same power. Fuel economy will be worse on a per-gallon basis because 3% if that gallon is oxygen.

If there are knock sensors, then the PCM should adjust to max best torque if it can, until it thinks it's running too hot. Then depending on the algorithm, it'll do something to keep you from overheating the heads. If there are no knock sensors, over 91 wont' hurt, but won't help either.

Depending on what model year, I think some engines now look at O2 signals even at WOT. Those clever EPA scientists have been forcing OEMs to include more and more "off-cycle" emissions (i.e. making them part of the cycle.) But let's pretend they didn't... and have a beer... then the WOT fixed air-fuel ratio would be rich and the O2 would bring it towards stoichiometric. I don't believe oxygenated fuel would make a WOT rich condition turn into lean. The OEMs would have a lot of melted pistons as warranty returns.

While the octane rating is indeed the fuel's resistance to knock, the different octane ratings have slightly different BTU contents. In the good old days things like butane were added to the premium. It was cheap, high octane, volatile, and low BTU content. But regulations on tank vapors stopped that. Then heavy hydrocarbons were used in premium (you probably heard of toluene; there are many cousins) but again, the heavy HC cause emissions problems, so again they were limited. So these days I think all grades are pretty close to the same BTU/gallon, not considering the oxygenate addition.

If you are not knocking, a lower and lower octane would not change the power output because the timing isn't changing. A higher and higher octane would not change the power output either, because the timing isn't changing. The engine needs knock sensors to make a power difference.

I'm trying to think why a very high speed motorcycle engine would benefit from lower octane (in the absence of knock.) I think there is something else (oxygenate or not, volatility?) going on there.

Back in my college days, premium would have been lower MPG because of the volatile components added to make higher octane. (See butane above.) They were high octane, but low BTU. If you think about propane, it's the same - high octane, poor BTU, poor MPG.

George, you are on the right track. If the cam-in-cam system includes knock sensors, then you are right. Just go to an online auto parts site and see if you can order a knock sensor for a Gen IV. Then you'll know.

Chuck... thermal conductivity is like pink insulation (poor, it's a good insulator) and copper metal is good (heat transfer.) Anyway, if you've read this far, you'll see that while the gasolines of 20 years ago were different between premium and regular, today they are far less so because the allowed boiling range is limited. The volatile components are gone (similar reason why the gas cap has to be tight or the CEL comes on) and the heavy components are gone (because they cause HC emissions.)

Dunno if the '08 has a knock sensor and what would happen. 93 is not always available, so I would think it would adjust to allow 91 octane.

I guess I should visit the Gen III-IV side more often.

 

[Chuck 98 RT/10]

Chuck... thermal conductivity is like pink insulation (poor, it's a good insulator) and copper metal is good (heat transfer.) Anyway, if you've read this far, you'll see that while the gasolines of 20 years ago were different between premium and regular, today they are far less so because the allowed boiling range is limited. The volatile components are gone (similar reason why the gas cap has to be tight or the CEL comes on) and the heavy components are gone (because they cause HC emissions.)

Now I can't remember what my theory was. It had something to do with hotter fuel burning more efficiently than cold fuel. Now I don't remember how I phrased it. I knew I should have written it down.

 

[Chuck 98 RT/10]

I know what it was now. If a high octane fuel requires more compression to reach the temps that a low octane fuel reaches then the high octane fuel will not be as hot in a low compression engine as a lower octane fuel would be, therefor the high octane fuel would not burn as efficiently as the low octane fuel - in a low compression motor.

Just a thought. I don't know what I'm talking about.

 

[Tom F&L GoR]

Not too off the wall, but you are mixing diesel operation with gasoline. Compression in a diesel ignites the fuel, so a good diesel fuel needs lower compression (less heat) to ignite.

In a gasoline engine the heat generated during compression is used to convert the liquid fuel into vapor... and then it all waits for the spark plug to ignite it at a time that is most preferential to the crankshaft angle, connecting rod angle, etc, to create the push on the piston and torque on the crank. In either the low or high octane case, the fuel and air probably reach similar temperatures, and vary only slightly if the high octane fuel has higher latent heat of vaporization.

What that does not mean is higher boiling temperature. What it does mean is the energy required to turn liquid to vapor. Methanol boils at around 143F, but requires much more energy than gasoline does to convert to vapor. That is why alcohol on your skin feels so cold - it takes much more heat from your skin to turn to vapor than for instance, oil. (75F alcohol vs. 75F oil ... they are same temp, but alcohol feels colder because it's sucking heat out of you.) But for pump gasoline, the latent heat of vaporization of low vs. high octane is likely similar.

Where it gets complicated is in speed of combustion (once the spark lights the mixture, how long does it take to burn it all?) and cylinder wall temperature (people don't do this, but a hot cylinder wall will take less heat out of the combustion event and leave energy more for shaft power.) So it gets to be 4 million loose ends and trying to predict with certainty that all low octane fuels are better/worse than high octane (ignoring the advanced timing allowed with high octane) is an endless bench racing event. Unless the fuels are "race" custom blended and engines are specifically tuned for them, pump gasolines are not going to be different enough to matter to a street engine.

 

[ROCKET62]

When I attended the Zone Rendezvous in Columbus, leaving the 91 octane that we have here in Iowa, I can say for sure that my "Butt Dyno" tells me the Viper runs a lot better with the 93 or 94 I found on the road. It was a night and day difference on my '05 and if memory serves me correctly, 2005 was also the first year for knock sensors on the Viper.

 

[Dom426h]

this thread is ameture night with everyone beating around the bush.
Ill go ahead and join in adding some info without answering the op's question.

This statement is True:
A Gen4 engine will produce less power with a lower octane than 91since the timing will be ********.

ONE of the following statements is true:
A. A Gen4 engine will produce more power with a higher octane than 91 since the timing will be advanced.
B. A Gen4 engine will produce less power with a higher octane than 91 since the the timing cannot be advanced past the optimized point for 91 octane.


The Gen4 mopar performance ECU is optimized for 93. and it apparently is not capable of makeing more power with anything higher than 93...
http://forums.viperclub.org/srt10-srt10-coupe-discussions/629106-mopar-controller-98-100-octane.html

So with this info i am leaning towards B.