I was at the drag races a while back and I was talking to a guy about what kind of gas he runs in his car..He said that he usely puts about 5 gallons of track gas in when he runs his car,but on the streets he will run 93 octane at fill up,and when it gets below the half way mark he will top it off with 83 octane untill it gets down to about 1/4 tank , then he will add 93 octane again..
He said it works for him ,so i thought it might work for me.
(I Dont drive my car much, im running a 350 sb with a 560 hyd roller cam with a tunnelram with 2-450 cfm holly carbs.. with flat-top piston) so i tried his idea,at first i couldnt tell nothing,but after about a couple of weeks i notice it didnt run as good. at first i thought it had something to do with some changes i did to the carbs. i went down in jets sizes from #60 to #58, because of a little black smoke i had when i hit the gas, But now im thinking that had nothing to do with it, it has to be the gas... and plus, when i shut it down,it spits and sputters at times.. I was trying to save some money but instead I lost.. What do ya'll think???am i right??
im going to run this gas out and refill with 93 octane so my engine can get well,and i also know that changes in the weather will affect how it runs but im pointing more to the gas i was running.. I really havent had no problem with it untill i tried this bright idea.

any feed back would be great..
Robbie

Gasoline grade has absolutely nothing to do with how good or bad your engine runs. The ONLY reason for 'higher grade' gasoline is to prevent detonation. Use the lowest (cheapest) grade possible in your engine that doesn't ping. Buying higher octane than that is just making Dick Chaney (if you subscribe to conspiracy theories) richer and does diddly squat for your engine.

well you might be right on low compression engines,but my engine is around 10-1 and if i remember right ,our performance machine shop that deals with racing engines told me i will need to run high octane gas because of my compression,but also said it would run on the cheap gas too but not as good.. and like i said in my first post it wants to ping after i shut it off now ,and it has never done that before when i ran the 93 octane.. I have ran the 93 sence i built the engine..
and another thing i thought of too, i might have gotton bad gas too. it could happen.. thanks.
Robbie

perhaps you have too much of a shot from the accelerator pumps (black smoke)
where is your timing set at (initial and total mechanical)?
how much cyl. pressure are you running on a compression test?

Run-on is caused by too much advance. Back it down a degree or two and see how it runs.

Im running a msd #8360 dist. I have the total advance set at 36.
on my carbs i have the 50 cc acc pumps, and #31 squritters, with 5.5 power valves. . im going to back the timing down to see what happens..

Rhansen i will check a couple cyl to see what compression i have.
I had to put the 50cc on because i had a bad bog with the 30cc that came with the carbs.
thanks robbie

If its running after you turn it off, from my understanding, its the fuel grade. There's no spark at that point, so its just self igniting due to the compression ratio and engine temp combined with low fuel grade.

I'm not certain about fuel grade making the car run poorly. Maybe if its igniting just before the spark or something, but I think you'd hear that. Increased fuel grade however CAN increase performance a little on newer engines with knock sensors, because the computer can advance the timing farther due to the high fuel grade.

[color=lightblue][size=3]Doc Here

Check your Jet adjustments.. (Black Smoke) For proper Settings

Check your Curb idle , Should Be round 950 , high idle on rich mix will cause runon also...

Timing may be a bit advanced, but I would look there last.. if it's not pinging and overheating...

Doc
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From what I understand, your higher grade gas isn't better or higher quality gas, it just has additives in it that slows down the combustion process. It burns slower. 87 octane fuel burns faster and that is where you get the ping as the flame burns too quickly. I have to run premium (91 octane) fuel in my Turbo Tbird or it will ping, especially bad in summer months.
You may need an idle solenoid on the carb to drop your idle when you shut it off. Then you can run the idle at 950 with the solenoid hooked up and set it to 600 or less with the solenoid disconnected so it will shut the throttle plates when you turn the key off.

When ya"ll say curb idle, are you talking about were it idels at in gear?? our in park?? and also,should i drop down in jet sizes too .or should i not worrie about it,meaning that when i hit the gas,there aint alot of black smoke,just a puff???? Its running better now..
Robbie

I don't think the burn speed directly affects detonation. Generally, the faster the burn the better. Unfortunately, as burn rate increases the ability to self ignite also increases, which is what knocks. The idea, at least on engines with no knock sensors (otherwise it gets grey) is to get the fastest burning fuel (low octane) that also prevents knock (higher octane) under the engine loads (approximately read: temperature, but also including ignition advance and such).

That's what I've read anyways.

?? Burn speed ABSOLUTELY affects detonation! Detonation (also known as pre-ignition, or "self igniting" as you said) is when the charge "detonates" before it gets to the top of the stroke. The only way to get more performance out of an engine is to get more fuel and air into the chamber, and more heat to burn it. This limits the use of low octane fuel. Given that higher octane fuel burns more slowly, the charge does not "detonate" when more heat (compression is directly related to heat, heat is what makes power) is applied. Nitromethane is used in top fuel cars because it burns much more slowly than gasoline does, which allows more compression and heat to be used.

K

Pre ignition and detonation aren't the same things.

Pre-ignition occurs when the plug fires the intake charge to early and the piston is still rising up the bore. The sudden increase of combustion pressures creates a knock or a uncontrolled burn due to inefficient quench and turbulence, amongst others.

Detonation occurs when a carbon deposit or over heating of valves, too high head temps and sharp object's act as as a "glow plug" and start the flame front on one side of the bore and as spark plug start's the flame front on the other side of the bore the collide and produce enough vibration it is heard as a knock ping.

Both can cause the symptoms you are describing is happening to you. Control the burn with higher octane or start the combustion process later in the cycle(retard timing).

I stand corrected! So its the cause that designates the term, I C. In any case, the octane of the fuel will save you from either cause of the charge burning too early, correct?

K

johnsongrass1 & killerformula,

I humbly submit that both of you are wrong ,with the following explanation....
Neither of you cited any sources for your statements, so I looked for one. Being an aircraft mechanic, I used the FAA Aircraft Technical dictionary as my source..

It defines Detonation as: "An almost instantaneous release of heat energy from fuel in an aircraft engine caused by the fuel-air mixture reaching its critical pressure & temperature. It is an explosion rather than a smooth burning process."

preignition: "Ignition occurring in the cylinder before the time of normal ignition. Often caused by a local hot spot in the combustion chamber igniting the fuel-air mixture."

killerformula, I'd say your last statement was true..
JA