this should answer your question about putting cheap gas in your car, I just hope we don't have another newbie ask this question next week.

http://www.dynospotracing.com/octane.htm

 

thanks maxwell that answer all.

 

May I suggest a sticky???

That's probably the best way of explaining what we're trying to get across in a simple picture.

 

Full throttle dyno chart, that doesn't answer the basic daily driver question of octain requirements and cost savings.

 

Any idea what octane the baseline fuel was?

 

I think it's pretty clear showing what lower octane fuel does to the powerband and HP loss.

If the engine keeps doing that over a prolonged period of time, it will deteriorate the life of the engine.
The chart states it's a GTI VR6 (for the example) which I'm assuming recommends premium fuel so I'd have to assume 91 or 93 depending on which part of the US they're in.

 

Great find Maxwell. The octane question is one the most annoying I have read on here. If someone wants to put in low octane, I would prefer they just do it and not discuss it here. Using the knock sensors to save your motor is just stupid IMO. I also don't believe you can save money. I accidently put regular in my car once and the mileage decreased along with the HP. the decrease in mileage could not be worth the few penny's saved. At the compression ratio our engines are at, anyone with engine building experience would tell you to use premium. Sure the engine will adjust because of the knock sensors, but before they can adjust they have to detect knocking. Any knocking is bad and can lead to holes in the tops of your pistons and a destroyed engine. Sticker this for the fools and let's move on.

 

Interesting thread. My '97 Prowler owner's manual states that it requires premium fuel also. I run regular fuel in it because it is the same engine that was supplied in the '97 Dodge intrepid (9.6/1 compression ratio and 214 HP 3.5 liter). Why Plymouth chose to recommend premium fuel in this engine, I don't know. Anyway, it runs just fine on regular and most experts will tell you that if your engine was designed to run regular, premium does nothing for it. The dyno curve shown in the first post is worthless as these are two different cars being run. Variation in engine break-in and driving styles will affect engine horsepower by large amounts. Just variation in engine machining tolerances will show measurable horsepower differences. If you want to verify horsepower losses on a motor, you need to run the same motor on both fuels. All that being said, I still run premium in my Crossfire because I'm not familar with this 3.2 liter motor and it may need premium fuel do to combustion chamber layout or cam timing or some other variable.

 

I agree. One would think that if they really wanted to prove the point, the only variable would be the octane of the fuel. While I also would never run anything less than premium in my Xfire, that chart is about useless and doesn't say much for the sponsoring site (or maybe it speaks volumes).

 

This issue has actually really bothered me as of late...I can't speak for base/limited owners, but I know a lot of the SRT6's were "leased" as company vehicels and mine in fact was a lease vehical. I'm wondering if the owners actually bothered using premium fuel and if not, what kind of damage it would have done to the motor? I bought mine with 37k on it so it's not like it was "new". Hmmm...

 

198,000 miles on my Dodge Dakota 4X4 running mostly regular unleaded fuel. (+/- 11,000 gallons) could never tell the difference between grades in this truck - 318 V8 Automatic - I haul bikes, moved myself & friends towing large trailers. 4X4 in the dunes etc...

I've replaced the starter, rear wheel bearings and axle rods, shocks, tires, brakes, belt, SP plugs, EGR valve, CAT and O2 sensor (smog issue) and one rotor and wheel - (tire blow out at 80mph) Two batteries (both exploded requiring me to discard my shorts - I thought the ex-wife was having her final revenge) Only gasket ever replaced was for the water pump. (been thru two) - and a rear seal on the tranny (leak)

The truck still hauls ****, romps thru the sand, burns a little oil now but a great little truck.

My Crossfire has never seen a drop of regular gas - Actually I wish I could get 93 or 94 octane.

In my experience, octane will affect performance but not engine life.

Can you save money and run low octane gas?

Sure!! at 11,000 gallons at .15c delta - that adds up to $1,650 over 13 years. Or about $127 a year, or $10.58 a month.

My Crossfire's worth an extra $10.58 a month.

My truck - can't tell the difference.

 

Even though the graph is for two different cars the author's point is to illustrate the ill effects of ANY car whose engine is using the knock sensor to compensate for lack of octane. Heck, that could be the BETTER engine of the two cars and if 91+ octane was used it COULD test better than the other car for all we know, but that wasn't the point of this article.

 

well he says that the difference in octane is 20hp, I'm only assuming they are similar engines, I agree it was poorly put together but that wasn't the point he was trying to make.

by using premium your saving money, because the engine is able to do more work over the same period of time, for the same volume of fuel added to the cars fuel tank. so in reallity your spending more money using cheap gas as compared to distance traveled using premium.

 

The MB SLK320 has a similar engine to ours...but the horsepower/torque ratings show that much difference. They didn't make any point, other than to give us dyno chart comparisons on two different cars. Like I said earlier, if octane rating is the point you want to make...then that should be the ONLY variable. But you and I can use that chart to argue our points until hades freezes and neither of us has proven anything and we have the documentation to back it up.

 

the MBZ slk320 is the exact same engine in the crossfire. Also bosch has two ecu maps, if you reflash your crossfire with the upgrade mapping, it will gain another 20 hp, all the while using premium as a constant fuel for both original and upgraded mapping.

are we done yet?

 

Any engine builder will tell you that at the compression ratio we have, you need premium. Sure our engines can adjust to regular by retarding the ignition, but for this to happen the knock sensors have to detect knocking. Knocking is not good. I usually get just a hair under 300 miles per tank. When I accidently filled up with regular, I was only able to go about 275. Losing the mileage didn't justify the savings either. You would have to live in an area with no hills and drive like a 90 year old woman to not have the mileage affected. Putting regular in the Crossfire is just as stupid as putting premium in a car that doesn't call for it. It is all about preventing pre-ignition. Excessive pre-ignition will melt holes in the tops of your pistons and detroy spark plugs. So who out there uses regular all the time? We all need to know in case you try to sell your Crossfire on this forum some day.

 

My understanding of Octane is the capacity to which a fuel can be compressed without igniting.
Higher octane gas is less likely to prematurely combust under pressure.
Power from the engine is derived from first compressing and then igniting the fuel.
Unless a motor has a high compression ratio you are quite possibly wasting your money by puting the highest octane rated fuel in it. An avg rated fuel might suffice depending on that ratio.
If the manufacturer recommends high octane then the motor was designed for it.

An example can be drawn by examining diesel fuel which has a very high octane rating.
The whole idea of a diesel motor is to compress the fuel to ignition point. We all understand the low RPM high torque derived from a diesel engine. A gasoline engine wont run on high octane diesel because it is incapable of compressing it to reach ignition.

It's not just a higher octane level that determines power. A low compression gas motor is incapable of deriving benefit from high octane gasoline because it does not reach a high compression threshold. The higher you can compress fuel before ignition, the more power you derive from it during ignition. Think of it as winding up a Jack in the box The more compression the lid can take before opening the more forceful the Jack-in-the -box exits.



Cheers!

 

I am...I should have known better than to have even responded to your post.

 

How and who can do the ECU reflash? That additional 20 hp wouldn't hurt (except probabliy the fuel mileage).

 

The dyno chart wasn't all that useless. Forget about the other car, I was amazed at how jagged the the chart was for the low octane car. Something is clrearly wrong with that car. The computer screwing with the timing is a very plausible as well as the the lost 20 HP. When you get an ECU remap you gain about 20 HP by gving the car more advance and leaning the fuel mixture. I can't wait for someone who has had a remap brag about using regular fuel.