Hey guys, I know that we are only supposed to run 87 on ours cars when stock with lower degree timing. Would it really hurt it to run one tank of premium with octane booster in it. I just wanted to mess around and see what it would do, plus I like the smell. Thanks guys lol I know its dumb.

 

It won't do anything, these motors make max power with 28 deg of timing and the ECM only puts in about 20 deg stock. So octane boost won't do anything but lighten your wallet.

Keep in mind Octane is just a rating of gasolines resistance to knock. 93 is less prone to pre-ignite than 87 is, the thing is that 87 will actually burn faster than 93. All of this is for nought because you'll never be able to feel the difference between the two let alone actually be able to get it to show up on a dyno.

 

so why does my car run better when i put 91 in it?

 

Wouldn't the hight octane gas be used for a hotter engine (at the piston)? That way you don't get pre-ignition and knock, right?

 

Same reason people feel better, after taking a sugar pill.

 

not true

it runs a lot better when i put 91 in it, not just because i think it does.

 

1987 owners manual, recommended fuel for 305 TPI was 91+ octane.

 

If your engine was in very poor shape, with a TON of build up on the valves, that could cause pre-ignition. Short of something being wrong, the car should actualy perform better on lower octane gas. With the wimpy 9.3:1 compression, very conservative timing, and heads that are fairly detination resistant. Lower octane gas, is actualy what you want to run.

Just think about what High Octane gas is. It is a gas that is less volitile. The timing and tune of the motor does not change based on the gas your run. So you actualy want to run the most volitile gas you can, without getting into a situation where you either detinate, or pre-ignite the charge.

The only time lower octane will perform worse, is if the knock sensor is pulling timing from the motor. This would only happen, if there is something wrong with the motor, causing it to pre-ignite the charge such as exesive carbon build up on the valves, causing a hot-spot.

 

TBI forum, hoss.

Is it safe to say that if you run your timing advanced (say 4*) the increase in octane will help? I have a sneaking suspicion that the computer will try to compensate for whatever value is placed on the timing.

 

It absolutly does not. The ONLY timing compensation that the computer does, is if it regesters knock, it pulls timing out for a set amount of time, then starts to taper the timing back in.

I ran stock, with 4*, and 87 octane all the time. My fastest drag stip times were with 4* advance, and 87 octane. I went faster there, than I did with 92 octane, or even 92 octane and more timing.

 

Are you saying that running with 4* advanced timing is a good thing or not? I think I am reading that running 4* advanced is OK and you can still run 87 octane... correct?

Lastly, I had my regular smog test recently and barely passed. A couple things could be at play there such as the fact that the car was cold, my cat is aging (and not very efficient), and I was running at 4* advanced timing. Of those thre factors, will putting the timing back down to 0* influence that any? I was thinking that replacing the cat would be better for my HC output.

Thanks Dew.

 

Timing will effect emmisions, What were you failing on though? Typically advancing the timing lowers the HC counts, but raises the NOx counts.

Yes, you can run 4* advance with 87 octane. (At least I could on my motor). My car ran the fastest, and best at that point. Every motor is diffrent, so what my motor likes, may not be what you motor likes.

 

Both my '91 TBI and '87 4bbl run so much better with high octane. With 87octane they sound like they need a valve job, and the computer retards the timing and kills the power. The only wat I could run 87 in my TBI car was to disconnect the wire that disables the ignition retard. Timing is set at 0*.

I understand what everybody says about octane ratings, and agree that I should be able to run 87octane, but experience says otherwise...

 

we need to get some dynos sheets up here with the different octane ratings

 

No one is going to spend the dyno time to test something that is common knowledge. If your vehicle runs better on higher octane you have a mechanical problem causing this. It's either a slipped balancer ring or some other kind of mis-adjusted timing. Or it's an abnormal buildup of carbon. I've even heard of fancy platinum or 4 prong spark plugs causing detonation because the plug retains enough heat to cause pre-ignition.

With low 9 to 1 compression 87 will do you fine, I was able to run 89 regulary with 9.8 to 1 compression with a really crappy squish height and very carboned up pistons. Even with 87 it only barely pulled a degree or two at WOT, and this is with 36 deg of total timing on old ineficent heads. A swirl port headed LO3 is going to tolerate 80 octane gas if they made it.

 

Well said. Swirl port heads are fast burn heads and do not necessitate a lot of initial advance. Lower octane fuel is more volatile (as Dewey mentioned) and combined with the fast burn characteristics of these heads makes for the ideal combo. As Bmonte mentioned platinum plugs can cause detonation. I ran the crap Bosche +4 plugs and had to use higher octane fuel to prevent detonation. I got rid of them and was able to reduce my initial advance back to 4° and go back to 87 octane.

 

I am currently runing 6° advance with 87 octane and am having no problems. I'll probably advance it even more in the future.

 

I wouldn't go any higher. If you do you are compromising the desirable traits of SP heads. These heads are not like any other. More advance does not work in our favor and does not add power. Even cammed motors with SP heads will only require around 28° of total timing.

 

Oh, okay well right on then. So why are there artcles around here saying that these engines like bewteen 6 and 10 degrees of inital advance? Is that not accurate, or maybe only with an increase in octane?

 

It is not accurate for the SP heads. Several of us found, that they like between 28*-30* of total advance at WOT. Anything telling you to run more timing, is people thinking it "feels faster" they will not have any numbers to back it up.