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 More octane, more timing = more power?

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rauldelp

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Join date : 2015-05-27
Age : 34
Location : Hollywood, Florida, USA

PostSubject: More octane, more timing = more power?   Tue Jan 16, 2018 12:39 am

I'm sure this has been asked before but I can't seem to find it specific to the Tbi GM cars from 87-93.

I'm having fun tuning my L05 equipped Roady, but I notice that just when the timing seems to start really helping part throttle acceleration, (+4 - +5 from factory) the knock counts start up. Not a lot, but I think we're shooting for 0 knock.

I usually run 87 octane with ethanol, but this morning I filled up with 93 with the same 10% octane. After a couple miles I did a pull and it really seemed to like it. Almost 0 knock, and the pants dyno is telling me I did something right.

Is this normal for these cars? I'm looking forward to seeing how it does mileage-wise. It feels like it takes almost no throttle on the highway to speed up vs the stock timing ( which knocks a little anyway).

Could my knock sensor be too sensitive? (Is that a thing that could happen?)

I may end up having 2 tunes, one for daily driving, and one for fun times!

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Fred Kiehl

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Location : Largo, FL 33774

PostSubject: Re: More octane, more timing = more power?   Tue Jan 16, 2018 10:33 am

To get 0 knock, you may have to move your timing back a little. Higher octane allows the use of more timing, but there is a limit for a given octane. Alcohol vs. no alcohol is not an issue, it is the octane rating that counts. You could further test your theory with some octane additive (figure out what octane you mix to). You can then gauge what octane you need to get to 0 knocks. The way you calculate octane without draining the tank is to figure 22 gal full, put the desired octane additive for 22 gals. in the tank and fill it. To check another octane, run a couple of tanks of gas through it, and do another mix. If you want higher octane, run a couple of tanks of 93 through it, and then do the mix.
You may also put too much timing in it, and hurt the HP. There is a limit to the timing you can use. Check the total timing advance, and 35~36 deg. is about the upper limit.
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rauldelp

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PostSubject: Re: More octane, more timing = more power?   Tue Jan 16, 2018 8:35 pm

I get that 35-36 is the upper limit, but right now I'm needing premium to get 23 with no knock. If I adjust it back to factory (15 degrees all in) it still knocks with 87. And it feels lazy.

I'll keep mucking with it. I was hoping it was just a "thing" with the Buick L05 motors that they liked 89 octane or something.

I just got the tdc stop tool (the fancy comp cams bolt you put in the spark plug hole to help determine tdc if my harmonic balancer is slipped) so maybe the balancer 0 is actually +10 or something. I don't think so though. It may be +3-4 but I was using a wire so I dunno how accurate that was.
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Fred Kiehl

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PostSubject: Re: More octane, more timing = more power?   Tue Jan 16, 2018 10:59 pm

To set the timing, you must unplug the connector on the passenger's side valve cover. Then you set it to 0 with the timing light, at idle. I have never had a harmonic balancer slip. According to the FSM, they are keyed on the LO3/5. After setting the timing with the connector unplugged, plug it back in, and then read your timing. I believe the original maximum timing is much more than 15 deg., but I do not know what it is.

I would trust the damper to find TDC when it is at 0 on the scale. There are two TDCs. One on the top of the intake stroke, and one on the top of the compression stroke. The damper does not care which TDC you use for checking it.

It is possible that the computer does not like the timing advance for the maximum amount of fuel you are getting. If that is the case, you can raise the fuel pressure to achieve more fuel flow with a shim under the spring in the regulator. If you are running lean, it will possibly detonate as well. The O2 sensor can only request fuel. If there is not enough, the mixture will go slightly lean. It may still be within the O2 sensor's ability to measure mixture, but it may still be lean enough to detonate (knock) at the timing you selected. The knock sensor then causes the computer to reduce the timing until it does not knock.

My guess it that maximum timing should be achieved at about 2500-3000 rpm.
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