FastBack

What octane are you guys using for your 305TPI? i have been using the highest grade, like 93 maybe? i was wondering if using 87 would hurt it at all. i know the manual recommend 93 high grade but i was wondering if anyone thought different about this. i don't wanna be paying 2.05/gal if i don't have to. thanks!

vwdave

I dont have a 305 TPI, but here in florida, pretty fresh 350 TPI, beating the hell out of it, I run 87 and get slight knock up top in 3rd gear. So, I jumped to 89.

89Warbird

If you can hear it knock you have problems. The computer will automatically pull some advance out of the spark curve whenever detonatoin is detected by the knock sensor. If it is so bad it can't pull enough advance out then that's a problem. Get a scanner hook it up and find out when or if your car is pulling out any spark advance. Keep going up in octane until it stops or you can't get a higher octane.

LAFireboyd

iTrader: (3)

Do you want your engine to run adequately, or do you want it to perform at its peek? The answer seems simple to me, especially when the difference from one grade to the next is only $1.50/full tank. And you're likely to see an extra mile/gallon with 93, which will make-up for the difference.

Be happy you CAN get 93. Not all of us are so lucky.

onebinky

iTrader: (1)

It all depends on your compression ratio, your head/block metalurgy, condition of your engine, and the quality of the gas in your area.

I have a 305 iron/iron with about 9.6:1 compression with decent quality gas. Even though it's 10% methanol mix in illinois, I can still run 87 octane without knocking.

Octane is a burn retardant. That said, you want to run the lowest octane fuel you can get away with, WITHOUT detonation. You want to get the quickest most intense burn possible. If you start to get knock, it will kill performance, damage the motor, not to mention the fact that the ECM will start to pull spark out until the detonation quits. Also bad for performance

onebinky

iTrader: (1)

It all depends on your compression ratio, your head/block metalurgy, condition of your engine, and the quality of the gas in your area.

I have a 305 iron/iron with about 9.6:1 compression with decent quality gas. Even though it's 10% methanol mix in illinois, I can still run 87 octane without knocking.

Octane is a burn retardant. That said, you want to run the lowest octane fuel you can get away with, WITHOUT detonation. You want to get the quickest most intense burn possible. If you start to get knock, it will kill performance, damage the motor, not to mention the fact that the ECM will start to pull spark out until the detonation quits. Also bad for performance

vwdave

Thats what I did. I pick it up in the scanner.

FastBack

so i just need one of those cheap scanners from autozone? but won't they only find problems if the SES light is on? i didn't think so but that is what i was told. anyways, if i scan it, and get a code that says my spark is being held back i should move up a grade and see if that helps? thanks again

White91GTA

i only run 93 octane in mine.

91GTABird

Can someone give a detailed explanation of what detonation actually is? Iv'e searched and cant get the mechanical explanation of it of what makes the knocks and pings. Sorry Fastback, figured since where on the subject.

FastBack

hey don't worry about it, this seems like a good place to discuss it. detonation is when the spark ignites the air/fuel mix. this happens at TDC or top dead center. when the piston is at the top most part of the compression chamber and the air/fuel mix is at its most compressed stage. as for the knocks and pings are concerned, here is what i have always believed, someone correct me if i'm wrong. if the mix is not ignited or detonated at TDC a knock or ping will occur because the piston is not being forced down when it should be. idk if this is totally right, some one correct me because now i am curious. i never really thought about it that much. hope i can help. thanks

onebinky

iTrader: (1)

Detonation is NOT when the spark ignites the mixture. That is the regular combustion process taking place.

Detonation is when the tempuratures of the cylinder ignite the fuel mixture during the normal combustion process. The causes of this could be high compression, excessive carbon build up, or excessive ignition timing. This causes an uncontrolled explosion instead of a controlled burn. That is the knocking and pinging you hear, it's very hard on internal engine parts.

Preignition is a slightly different animal. Preignition is when carbon deposits in the combustion chamber ignite the A/F mixture BEFORE the spark plug does. So when the spark plug does fire, it gets a second flame front going in the cylinder. The two flame fronts collide, and cause an explosion as well. The results between the two forms of detonation have the same results. Cracked engine parts and burned pistons.

89Warbird

Okay, detonation is when you get two flamefronts and they collide in the combustion chamber. The spark is sent as the piston is traveling up Before Top Dead Center (BTDC). At idle usually 6 to 12 degrees at redline about 30 to 40 degrees depending on the engine and tune. Detonation is an uncontrolled explosion, not combustion which causes you to lose power because the mixture is ignited by a hot spot in the combustion chamber or the top of the piston creating two distinctly different pressure waves the one from detonation being vastly larger causing a pressure spike in the combustion chamber that collides with the other pressure wave and the piston, chamber, cylinder et all. This pounds the rings, ring lands and rod bearings and makes the pinging sound.

Pre-ignition happens when the mixture is ignited by a hotspot prior the the spark event. This is very, very bad and causes the same problems as detonation only usually more severe.

You cannot tune with the SES light.

onebinky

iTrader: (1)

Couldn't have said it better myself

FastBack

thanks for the explanation, good to know that i was wrong, but at least now i know. thanks

91GTABird

Thanks for all your explanations.

I always had the idea of that b/c you get detonation mainly after the engine has warmed up. But what i dont understand is (2) things.

(1) If the a/f mixture ignites Before Top Dead Center, then why wouldnt that cause the the pistons, rods, and crank to go back down (backwards) and the motor try and run backwards?

(2) If your ignition sparks at 12*, is that 12 degrees of crankshaft rotation After Top Dead Center? If so how does retarding the ignition get rid of detonation?

TheMysticWizard

iTrader: (1)

It takes time for the spark to get from the coil, through the distributor, through the spark plug wire and to the plug. It's not much time, but as you can see, a few degrees is what it takes.

No, it's BTDC, but by the time the spark gets there, it's close to TDC, if you have too much timing, the spark is actually getting to the cylinder BTDC, thus you have ping/knock. Retarding the timing, (because most of us are BTDC as our initial timing) gets the spark to happen closer to actual TDC, thus the with the time it takes for the spark to travel plus the timing setting assures the spark arrives at the right time.

Hope I understood it correctly when I learned it, and I hope I have relayed it to you, so you understand also.

BTW I run 89 Octane all the time now, I do get occasional ping/knock, I used to run 87 all the time, but I did some data logging with 87 and saw just how much my timing was getting retarded. So now it's 89 all the time, unless I'm going on a long trip, and I have the money for the 91.. or if I'm going to the track.

gatorcola

I was searching and found this thread. After reading the quote from above - I just had to bump it up.

1fastam

iTrader: (1)

yeah wish it was only $2.05. makes you wish for younger days(oh wait, that was only a month ago) haha, later jimmy

Theking

iTrader: (1)

In Sweden we only have 95 and 98octane, what shall I use ?
Right now im running on 95octance but someone told me that 98 is better..

What do you guys think about that ?

The price for 1Liter (0.2639gallons) 95 Octane is : $2.21
The price for 1Liter (0.2639gallons) 98 Octane is : $2.27