hmmmm... with 91 octane and adding alky, so whats the final octane?

[Quick6'n'-K.C.]

i know thats not worded right, but since alot of us have 91 octane here and run alky, what would the final octane be?

i know 2.5 gallons of xylene to the full tank to make about 99 octane and i could run 19 psi to 20 psi of boost and thats about it-

change back to 91 octane, and run denatured alky, i could run 24° timing and 24 psi of boost-


i was reading in another tech post that a member was saying "alky is really the equvalent of about 100 octane fuel"

yet i can run 4 more psi of boost, so that makes me think its closer to 108 octane than 100 right??

i cant see some of my friends running 120MPH with 24° timing 23 psi of boost only on 100 octane


just a friendly question

THANKS
BW

[1badTTA]

You really answered your own question

Alky injection will not give a definitive octane number due to the added benefit of its cooling properties. The closest you can come to getting a real number is to see what you can run with Alky and then try to duplicate it with only a certain octane fuel.

I would definately agree that Alky injection is better than 100 octane. There are numerous people who have run about the same numbers (give or take a little) with Alky as with 110 octane, and slightly slower than running C16 (116 octane?) so your saying 108-110 octane would be a pretty good assumption.

Also alot of people running 108 or 110 octane chips with Alky with no problems.

[Roger]

Back in the old days - WWII, the military was using alky in reciprocating A/C power plants. The military was in the habit of using a dual octane rating on A/C fuels.

A few I can recall were 100/130 & 115/145. The 2 different ratings were without & with alky. I'm aware of the military using a sofisticated method of utilizing the alky, but I see a difference of 30 octane points between without & with alky.

I'm aware of the many different alky setups for the TR & some of them are very well engineered, so I see a possibility of up to a 30 point difference in octane rarings.

I don't know all the reasons as to why alky can make up to a 30 point difference in octane ratings, but I know the military was able to rate A/C fuel with this much difference.

2.5 gallons of xylene into a nearly full tank of pump gas IS NOT going to net you a 100 octane blend! You need to use a 1/3 ratio mix with 93 octane pump gas to get close & anything over this ratio is really not worth the extra expense (driveability starts falling off) :

117 + (93 * 3) / 4 = 99 octane final blend.

Making the assumption (scary thing huh!?) that you were filling the tank by adding 2.5 gallons of xylene, you were putting it on top of ~15.5 gallons of pump gas:

(117 * 2.5) + (93 * 15.5) / 18 = 96.3 octane final mix

And if you were running 91 octane pump gas:

(117 * 2.5) + (91 * 15.5) / 18 = 94.6 octane final blend

This is why you were only able to run 19~20 lbs of boost. In my car with a 99 octane xylene blend & mild 20° chip I can run 22~23 lbs of boost no problem. (to the tune of a solid string of 11.4's @ ~119.5mph)

1badTTA: BINGO!! You can come up with an "effective" octane number by comparing how an alky system performs against race gas on the same tune, but there really isn't a way to get a "true" octane number for one.

Roger: Those numbers the govt. was getting was on a supercharged octane engine as used for aviation fuels and are rated on a totally different scale than automotive fuel octane engines, SOOOooo it's like comparing apples to oranges. Unfortunately, as far as I know NONE of the supercharged engines that were set up to test alky inj. systems are in operation anymore & there have never been any automotive octane engines set up to do this.

Doug C.

[Quick6'n'-K.C.]

very good explanation of how the xylene mix works, but 2.5 gallons of xylene to the tank only gets the car an additional 3.5 octane?

your math seems right, but for some reason the car just runs better? maybe my gas tank is smaller and holds less fuel

just kiddin, so what are your views of a car able to run 121 in the 1/4 with 91 octane and a .035 nitrous jet running denatured alky and 24° timing up to 83 MPH?

seems like more than 100 octane to get those numbers right?

car and driver= 3640

Since I don't have any personal experience with running an alky system PLEASE feel free to take this with a grain of salt....

Strictly based on what I've observed here on the board & how cars in the local Buick club perform on alky setups versus Homebrew blends (ET / chip timing / boost) I think that as in all things, it all boils down to the car's combo.

On stock / near stock cars with the original intercooler etc...(say a low 13 to mid 12 combo) the alky systems seem to run compareably to a car with the same combo running a 96~98 octane xylene blend.

On "moderate" cars with say a duttweiler neck & te-44 turbo (low 12 / high 11 combo) the alky guys & homebrew guys seem to be running neck & neck.

Where the alky seems to pull ahead a bit is on cars with more "serious" upgrades (mid 11's & faster.) Alky system cars seem to perform more in the range of what a car with the same combo could run on ~104 octane race gas.

Hope this makes sense! It's been a LONG couple of weeks!!!

Doug C.

[Quick6'n'-K.C.]

combo and MPH-

i will post back this saturday when we get to the track again,
pushing past 121 in the 1/4 on 91 octane and alky seems to be a limit right now... i cant see it doing it on 96 octane, or 100 either right?

your saying that xylene and alky users are neck and neck, yet when guys add more mods they go faster right?
well, i do see your point, but i think MPH reflects how much horsepower the car has, not the ET... ET is only how well the car is applied...

from your posts you know alot about these turbo buicks, glad to have you aboard

thanks for the post

BW