Ideal octane for a 420a

[Checkers]

Hey guys, I know it seems like a stupid question, but I can't seem to find anything definitive on the subject. I have a 99 Eclipse GS, and it has a 420a engine in it but not the 420a from the factory. I'm not sure what year the engine is from, because the previous owner told me it was from an '02, but that's a 3G and I don't think they have 420a's. Anyways, he told me he always used 91 octane to "keep the engine clean" so I've been using it so as not to shock the engine with a different octane. What octane should I be using, 87? And how do I safely transition if I need to?

 

[lino420a]

Bought my 420a with a bad engine, after i replaced it i tried to use 87 but would get bad gas mileage. Switched to 89 and fixed the problem. Try 89.

 

[hardcorandy]

engine maybe replaced in 02? I dont see a problem running low grade gas in your case. If you start hearing valve chatter on an uphill climb you may wanna up the grade of gas. IMO I think Sunoco has the best fuel.

 

[JusMX141]

I've owned my GS since 2002, used Super 93 when I first bought it....once gas prices went over $2/gallon I started using 87 and never looked back. Economy never suffered, and the car is still running great some 70k later.

 

[razrman8]

I always ran 91 in my talon 420a never had problems. Ran 91 cause I didnt ever want to be dissapointed. Now in my gsx i run straight 93. Thats all I have put in it since ive had it. Despite 93 octane being $3.04/gallon

 

[DSM1G90]

With a compression of 9.1 or so in a 420A NT motor, 85 or 87 is fine.

Don't waste your money with premimum fuels in any NT motor, just plain silly.

If one keeps the fuel filter changed out on a prevent maintence basis and system properly timed, many hundred of thousands of miles can be had running 85 or 87 regular fuel.

Like this 91 Eclipse next to my Laser profile pict: just kicked over 200k on it and it has the 1.8L and never has seen anything but 85 or 87 in it and it runs like a dream...

 

[born2tune]

40:1 gas/oil ratio .

nah, just joking but 87 is perfectly fine for it. because "octane" is nothing but a ignition delay component, so the higher it is i.e."103" the more octane chemical it has. it makes the gas ignite at a later time than "87" . 91+ high octane which is needed in high compression engines. because having 87 in a turbo car will cause the fuel to ignite before the compression of the engine cycle is completed, so having 93 or higher will increase the accuracy of ignition time for a perfect combustion cycle, having wrong kind of fuel can make your engine ping. but having high octane in one that doesnt need it, doesnt really affect anything but your pocket.

 

[talonesi95t]

when i first got my esi i tryed both 87 and 92 octane, found that i got way better mpg using 92. i would get 30-35 mpg with 92 and 87 was like 25 mpg.

 

[Checkers]

Ok, thanks everyone. I had read a lot about how it's dangerous to change dramatically, and also how since higher octane gas ignites at a later time, I might not get as much power as I want. It seems to be that lower octane will give me more power, but higher octane will have better mileage? Is this true? Also, is there a correlation between octane and engine health/cleanliness? I'm sorry I ask so many questions, but I really appreciate all the help. Thank you

 

[DSM1G90]

It seems to be that lower octane will give me more power, but higher octane will have better mileage? Is this true? Also, is there a correlation between octane and engine health/cleanliness?

As with all forums of all types, one member comes in and ask a question or two and other members gives the best answer that they can give and sometimes it's not an absolute answer due to the condition or situation of the one who asks the question.

Thus, as with spark plugs, oil, motor setups and now fuel recommendations, one has to find out for himself what works best for him - try all types of grades and find the one that works best for your certain application...and that is the best answer, even though it's not the absolute correct answer, one can give to you.

All I can say in my experiences is that using 91/93 is a foolish waste of time and money for a NT application where the lower grades works better and runs cleaner since it doesn't have all of the octane additives as the higher grade fuels for my NT applications.

In short, I'll never run premimum in an NT application if I don't need to. If I have to, then I have a serious problem with ignition timing, A/F mixture settings, carbon buildup inside the cylinder and other matters that needs major attending to so I can return the system back to the previous state where it ran effectively and efficient.

good luck in your find - DSM

 

[BK85]

I've been running 85 octane for 3 years now delivering pizza daily and never a knock or rattle in the 420a with 120k on it, and consistently get 30mpg.

BK85

 

[born2tune]

the cleanleness doesnt come from high or low octane, it comes from (ethanol content) most gas station mention {may contain up to 10% ethanol on all fuel grades} this is the cleaning agent, not octane. Now higher octane doesnt mean it also contains higher ethanol it also contains upto 10% (unless otherwise states of course).

there are some gas stations that state, 100% pure gas. which means 0% ethanol/cleaning agent in it. i personally stay away from it because my car has been fine with ethanol, but never experienced without it, so im good because i dont need to be experimenting with my engine right now .

but if you consider the theory of " perfect timing combustion with 87" then having a wrong mixture then may cause a combustion out of time, that in turn will create carbon build up in the wrong spots/or time.

 

[TotalEclipse21]

!! If your car calls for 87 use it. Don't waste money on 93 just because you THINK it makes it run better. Higher octane; the higher heat resistance, that is why forced induction engines take higher octane.

 

[crazyboy81]

87 is fine. I never have problems from it. 160k miles.

 

[gusu]

If you don't know about octane in the US we go by the AKI which is the Anti-Knock Index.

Basically, you don't have to worry about knock as much with a higher octane. Though you shouldn't have to worry about it on a 420a. Honestly anything other than regular is a waste. If you are ACTUALLY seeing better MPGs with it, it is ONLY because you are having less ethanol added to your gas.

Ethanol burns 33.3% faster than gasoline which means it gives you 33.3% less fuel economy but don't think you will get the full 33.3% less out of an entire tank. Since you are only getting UP TO 10% ethanol you should only see around 3.33%.

Also try to remember the Ethanol being added to your fuel IS extremely corrosive to rubber, and HAS been proven to be corrosive to piston rings. Though at 10% being the maximum added, you shouldn't see too much damage done. Though it WILL eventually happen.

So if you want to prolong the life of your rings and your fuel system I'd go with a lesser ethanol blend, though you shouldn't honestly ever see any difference.

Which is why I said in the first place anything over regular gas is a waste of money. So if you are into wasting money on something useless why not send that useless money to me for this post?

 

[Fotowntalon]

If i were you i'd be running 87. I ran 87 all the way up till I got my new ecu, then had to switch to 93, but that was only because of the timing advance, if you are stock or lightly modded 87 won't hurt anything

 

[Checkers]

Ok, thanks everyone. I'm going to try out 87 and 89 just to check, but it won't hurt my engine, right? Like, the shock from the change won't hurt anything, correct?

 

[Fotowntalon]

You make it sound like fuel flows through the whole engine... No it won't hurt anything.

 

[Silent3]

I usually put in the best gas I can afford. If its 89 its 89.. if its 91 its 91. higher the better.

Gonna burn cleaner the higher the octane

 

[DSM1G90]

Gonna burn cleaner the higher the octane

BZZT! "Wrong answer Hans.."

.. ain't that supposed to be the opposite since higher octane fuels have more additives whereas lower octane has lesser additives and is allowed to burn hotter, quicker and cleaner-why you don't want to shove that stuff in a high compression situation?

If you really want to run "pure gas" with no additives, try some "white gas" - that stuff will really run super clean, super hot and and ping like crazy since it was made for very low compression usage like 5.0:1. Thus, not for use in our present motors.

-DSM

 

[born2tune]

BZZT! "Wrong answer Hans.."

.. ain't that supposed to be the opposite since higher octane fuels have more additives whereas lower octane has lesser additives and is allowed to burn hotter, quicker and cleaner-why you don't want to shove that stuff in a high compression situation?

If you really want to run "pure gas" with no additives, try some "white gas" - that stuff will really run super clean, super hot and and ping like crazy since it was made for very low compression usage like 5.0:1. Thus, not for use in our present motors.

-DSM

BOOM! HEADSHOT!!!

 

[Black Widow97]

I ran 89-90 in my old n/t.

 

[JKRacing]

My mother who owned my car before me ran 87 in the 420a NA and she said the gas mileage was junk. I ran it on 87 when i first got it and took it for a cruise with a local car club I am part of and it ran like a bag of c**p through the whole cruise. Bad acceleration, and at high rpms/speeds i thought the motor was going to fall out of the car. I changed to 89 and it seemed to be better. My 420a loved 91 so I started using it. It ran like a dream on cruises with the club I have now changed to a 50/50 mix of 89/91 and it seems to have not changed too much from full 91 other then it being slightly cheaper. Best bet is sticking with 89 or the 50/50 I use IMHO.

 

[Alex7747]

^ you do realize the date it was posted, right?

 

[DSM1G90]

Ok, going to add to this two year old post: Why it runs like crap is due to carbon buildup which raises compression and why you're increasing the octane numbers.

My mother who owned my car before me ran 87 in the 420a NA

You can thank your mother for not punching on the gas pedal from time to time and I bet the car was used around town and never on the highway/freeway. Thus, with her easy driving, the motor is so junked up with carbon that it's unreal.

And do you realize that higher octane fuels don't burn as hot as do lower octane fuels due to all of the extra additives added in the fuel.

Get a tune, plugs and belt change (old one is prob all stretched out causing cam timing issues), and then blow all of the carbon out with good hard accelerations (like get on the freeway, jam the shifter in 3rd and hammer on the gas pedal until almost redline. I bet you'll leave a trail of black sooty smoke behind you from all the carbon that has built up in the chambers) and I bet that DODGE motor will run tonnage better on lower octane.

EDIT: To add - how many miles is on this 420A motor anyway? If quite a bit, I bet the CAT is getting plugged up and this will rob power from any motor and adding higher octane fuels will help very little.

Have your CAT checked out.

Done - DSM