View Full Version : octane boosters..

06-27-2005, 12:47 AM
was reading the fuel area of the tech section, about octane boosters, and all this info on toluene. the testimonies, were people, that were freakin out that their cars run WAY better on this higher octane gas they were makin themselves. as i read on, the final octanes, of these peoples gas was at like 93. thats aint $hit. i run 93 all the time from the gas station, and my car aint all that. in fact, i cant tell a difference between the lowest, and the highest ocatanes at my local station at all. my 1st question is, does ANY engine benefit from higher octane gas? (over 92 octane) or just turbo and supercharged ones. and is there a chart on here, that discusses the appropriate octane per pound of boost over the stock 15lbs? like if i were to turn her up to 17, what octane would i need to support that and so on etc...
secondly, if i never turn up the boost, but jump to say 95 octane, am i wasting time? would i even help anything out with higher octane at all? or should i just stick with the standard stuff.

06-27-2005, 03:10 AM
my 84 hotair LOVES E85 fuel- 100+ octane, 85% ethanol, less than $1.70 a gallon, and available all over the place around here.
it audibly pings at 15psi on 93 octane but seems to run pretty good, but doesn't ping at all with half a tank of E85 mixed with half a tank of 87 octane swill- and it makes a couple of pounds more boost (17 or so) and pulls a LOT harder- it feels like a 50hp increase in my "butt dyno".
just something to think about...

06-27-2005, 03:21 AM
where do you get the ethanol?

06-27-2005, 12:04 PM
in that fuel section, it was talking about how ethanol isnt that good for your fuel system. you said your car pings at 15 pounds on 93 octane? isnt 15 lbs stock? and your cars pinging? what did you do to your car? or did the hot air cars come with less stock boost. im guessing, because you have no intercooler?

06-27-2005, 09:17 PM
I am presuming that you know to make the necessary changes to allow for high Ethanol content in your fuel. You need to be careful when running more than about 10% of Ethanol without increasing your fuel/air ratio and changing the spark timing, as compared to Gasoline, especially on a boosted engine. Running high amounts of Ethanol without the necessary adjustments can lead to melted pistons or overheated catalysts. The problem with Ethanol is that it changes more than just the octane (9:1 is stoich compared to 14.7:1 for Gasoline).

You can certainly adjust the calibration for E85, or any other ratio of Ethanol to Gasoline you'd like, but you'll need to keep that ratio nearly constant after the proper adjustments are made or readjust to match the actual fuel as it varies. I can provide a formula to calculate this compensation for anyone that is interested. However, the formula is just a guideline and it is not a substitute for the proper instrumentation and know-how.

I can run about 25 PSI of boost on 110 octane gasoline and about 17 PSI on 94 octane on my 87 TR, but that is with near optimum spark timing and a few other important mods. The 84/85 cars had much less boost from the factory (about 9 pounds?) where the factory setting for the 86/87 cars was about 14 PSI. Fuel quality and content has also changed drastically since then, artificially raising the effective octane and fuel flow requirements for the same level of performance.

Whether you can benefit from more octane on a naturally aspirated engine is a function of your compression ratio and several other factors. Most modern non-boosted engines usually don't benefit from much more than about 94 octane.

Unless the control system has a working knock sensor system and reasonable automatic compensation for octane, increasing the octane usually won't help unless you manually compensate with spark timing adjustments or unless the engine was knocking to begin with. A factory knock retard system doesn't compensate for any necessary change in air/fuel ratio due to use of non-gasoline fuels.

Just my 2 cents....

06-28-2005, 12:07 AM
ahh i see then. just had to ask about the octane for normal engines, as my little brother swares up and down, that on his 85 dodge pickup, with like 170000 miles on it, on a slant 6, that he put a little bottle of octane booster in it, and it made it noticably peppier. i told him that it was most likely, someting that bottle had in it, that actually cleaned up a component in your carb. as octane doesnt really affect your engine that much. not with alll those miles on it anyways. but as for my boost pressure. how do i know what octane to match my engine, when i start goin above 15? i need to know, all the proper octane ratings i need to be at, for every pound of boost over stock, so that i know whats goin on, and how much to turn her up.

06-29-2005, 08:31 PM
If your brother's truck was "pinging" (spark knocking) before the octane booster, he might have seen some actual performance improvement with the booster. With that many miles, if the rings are still good, there might actually be a small increase in compression ratio due to carbon buildup in the combustion chamber. The carbon can also cause preignition, which the booster might help. However, you didn't mention what kind of octane booster he used. Most so-called octane boosters don't do very much (if anything measureable) to the octane, so I tend to agree with your assessment.

I don't know of any published guideline for octane vs. boost on a TR, as there are many variables such as the amount of ignition advance you run. You need to have a good way of knowing that knock is occuring as a starting point. If the exhaust is loud, this might be impossible without some additional instrumentation (there are lots of instruments available). Without any knock indicating instruments then you need to have some good "listening skills" to be able to hear it when it happens and a fast foot to stop it once it starts... or risk catastrophy. Such listening skills usually fall into the same category as the "butt-dyno" your brother used to measure the performance of the octane booster.

On a boosted car it is always better to err on the side of too much octane. If I had to give a conservative "rule-of-thumb", presuming the air-fuel ratio is correct, I'd say to use around 94 octane for 14 psi and then about 2 octane points for each additional pound of boost.