Hello All, After a pretty decent search I was surprised to see that this topic hasn't been covered save for people talking about running 100 in their cars and the debate as to whether or not it makes a huge difference. In a '95 F355 there are no knock sensors, thus the inability of the ecu to adjust for inappropriate fuel or harmful driving conditions that could cause detonation. I have been running about 94 Octane in my 355 since I've owned it. With a CR of 11:1 and given the performance built into the engine I feel as though anything less (specifically 91) is just not sufficient to run especially with the way I drive the car. Additionally, the engine/exhaust is pretty loud on these cars and I would find it tough to actually hear any pinging if/when it would occur. What are you guys running as far as fuel int he '95 models? My guess is that you could probably get away with 93, but I just blend for 94 at my local VP Racing fuel station. Has anyone done exhaustive research on this issue? I just can't see running 91 with or without knock sensors in these cars.
There is no debate. we know what octane does and it is not the end all be all for fuels. If you like VP go to their site and get a nice education on fuels. Also note how octane is calcualted. Once source stating 91 could be tha same as 93 but numerically different RON Vs. average yada yada yada i forget. Also the 2.7 355 is not blowing up anymore than the 5.2 355's which have the knock sensor. There are other ways to prevent the conditions that yield knock. A knock sensor was used with the 5.2 it was dumb easy. Finally, no one is tuning the 5.2 or 2.7 motronic while the 2.7 is potentially easier to tune. Our very own diy'er nodoubt got quite deep into the 2.7 but hit a wall looking at the way more difficult 5.2. If you can't home tune the ECU's you can capitalize on the small differences of 93 to 100 octane race gas.
What would be the highest octane you can run? VP seems to have quite a few above 100. http://www.smithtex.com/vpfuel.htm
I can tell you that when I run 106 race gas in the 95 F355 that :: A) there is no more power, and maybe a tad less B) there is a change in sound, taking the metalic scream out and putting in a more mellow yell. Sort of like it takes the edge off. All of this has to do with cyliinder pressures. After you fulfil the octane requirements of the combustion chamber shape (which includes the 11:1 compression ratio), power goes dow as one adds octane--yes DOWN. Power goes down because high octane gasoline does not have more energy per unit gasoline than lower octane gasolines, in fact most high octane gasolines have slightly less energy density (1%-2% level changes). In addition, cylinder pressure goes down since the high octane takes longer to burn the lower octane gasolines. This require more spark advance (which you don't have). My 95 F355 sniggles every so often on the 93 octane I put in the car. {A sniggle is an easily heard combustion event with just a hint of detonation.} Afterwards, the ECU does somethig to the engine maps to decrease the incidence of those sniggles. This is probably orchestrated by the O2 sensor in some way and done in the short term tables inside the ECU. But, you get the most power at the octane level where the car has the merest hint of detonation/pinging and sniggles are very close.
IMO knock sensors are a valuable asset for street cars knowing that fuel content is unknown and can vary considerably(i.e. not consistant). End of discussion...lol
Sorry guys, I'm talking more about the fact that guys are running 91 here in CA. I couldn't bring myself to run 91 and feel as though 94 is basically going to be the limit as far as any performance gains. Anyone running 91?? If so, scared??
I wouldn't be. Static compression ratio is pretty much meaningless, look up dynamic compression ratio and calculate it for the 355 and you'll see it's not too high. This is a product of the relatively high rpm that peak VE, and hence peak torque and the rpm where detonation risk is the highest, occurs at.
+1 ......... knock sensors on a street car with wide range of operating conditions is not only a great idea ... it is essential IMO ........ ........ as far as I know, 91 is the highest widely available gasoline here in CA ......... there are locations that sell higher grades sprinkled around ........ though I have not sought them out ..... so far I have only run 91 from chevron with a periodic extra techron additive ....... other than gaining the normal 'cool night' HP, I have not noticed anything negative ..... thus far .....
If higher octane fuel is not available, try a good octane booster. Torco is one I have used with good results. Effectively adds about $1/gallon to the cost of fuel. 32 oz can should be between $20 and $25 when buying by the case. Regards, Jerry
I put in 91 because I don't know any better and it was the highest at the Shell station. There are other gas stations that do offer 94 in my area so should I be putting 94 all the time instead? I feel like I did a bad thing here......
Lucky for me I have a station that offers 100/91/87. I just do a mix to bring it to 94 and always run that. At 11:1, I'm not going any lower. In most BMWs that I work on without knock sensors and a CR above 9.5:1, if you run anything lower than 91 they ping like crazy uphill. I have however, done zero research on this in regard to the dynamic CR on the 355. just not taking the chance as it's not like I'm filling it up weekly.
I have been running 91 in my Ferraris ever since 2000 with never an issue. If I were to do a track day, I might move it up to 93-95 average because that is what the manual specifies for optimal performance. All higher octane gets you is greater antiknock ratings, but unless the car can adjust for it, you get no benefit. Our 348's and 355's, short of being race modified (higher compression, higher revs, more advance), don't care as long as we use the minimum specified octane. Now if we run some boost, this might be more of a concern...
I use 95 octane fuel (Euro) which I believe is the equivalent to you 91 octane in the US. Its the one recommended in the users manual. When I track he car I use 98 octane (93 US octane I think) just for peace of mind. Anyways, there was once a research made by a car magazine here in Spain and most of the gas labeled as 95 were 96-97 octane in reality.
If we Californians were having pinging or knock as a result of low octane, wouldn't we be hammering ring lands and/or getting fuzz on spark plugs? I've seen neither on my car in spite of the cheap premium I've run a few times in the past and the (one time) I took it to red line.
Higher octane fuel will be more resistant to pre-ignition and detonation but doesn't necessarily mean a slower burn rate. (Many things effect the burn rate other than just the fuel of course). bobzdar- Good post. Another thing to consider is octane requirement goes down as your elevation from sea level goes up.
I run 91 race fuel in my 355 challenge no problem. I stopped with the 100 and 106 fuel after I found no increase in performance. IMHO, the higher octane does run cooler. What does make a diff is colder fuel. This increases the HP of the little 3.5 liter. That's why I need a fuel chiller for track days.
I have 2 points of reference on that: A BMW service bulletin my buddy got on his e46 M3 indicated BP and Shell kept the engines clean of deposits and the rest didn't. This was backed up by a friend that worked at Afton chemical where they do fuel and motor oil analysis - they used Citgo as control group as it has low/no detergent so is one of the worst performers (also used at Wawa, Sheetz etc). He also indicated BP and Shell were the best. So, I run BP or Shell in my 355...
Did they test Chevron? Don't put too much stock into anything BMW says. I've got TSBs on Oil Recommendations that pretty much say "this is the best because we have a contract with them"
