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Old 06-03-2006, 04:13 PM   #1
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Default High octane gas in a N.A. car?

I would like to get some response from someone with experience in running high octane gas (93 or more) or octane boosters in a N.A.motor. In my years as a auto technician I was trained that most cars were designed to run on "cheap" gas. On vehichles with electronic spark timing control , at startup , the computer would advance the timing until it sensed a "knock" from the knock sensor , then retard it a few degrees to achieve an ideal setting for the timing according to the operating conditions.(altitude , gas , ambient temperature , engine temp. etc.) I was told that if a vehicle that was designed for 85-87 octane had , say , 95 octane , the computer would continue to advance the timing until it reached its max amount that it was supposed to need : then because it never sensed a knock (because of the high octane fuel) the computer would assume that the knock sensor was faulty , and set the timing back to a general "limp in" timing setting.This could actually cause a DECREASE in performance and efficiency. Is this so with our N.A. bonnevilles that do not call for high test gas?
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Old 06-03-2006, 07:33 PM   #2
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I know the PCM'* in L36 and L67 cars will only advance timing to a certain point (on L67 cars stock max advance is 17 degrees, L36 is maybe 25 degrees or so I'm not sure). If they don't see KR at that point they won't pull the timing, they just assume everything is going good. There may be some computers that do as you described about the limp mode, but this does not happen in our cars.

In my GP, I get about 5 degrees of KR if I run 87 octane gas. When I put 93 octane in it, the car gets no KR and the PCM advances the timing to max timing. I've never had a problem with it trying to pull the timing because it wasn't getting knock.

You should be find to run higher octane gas, but keep in mind it will only help until it gets rid of your KR, because once you get to that point the PCM won't add anymore timing and the extra octane will be slowing you down since it burns slower. So if you are going to run higher octane, you will want to find the octane that is just barely high enough to not give you any KR.

But in all honesty, I'd just run 87 octane and do some mild mods to the engine. For an N/A car higher octane gas is really unnecessary for a DD, and unless you're trying to get every possible hp out of the car that you can it just doesn't seem like a good idea price wise to run high octane gas. I only run premium in my GT when I go to the drag strip or the dyno. It still runs plenty strong on the street with 87 octane I just don't see the point to run higher when I can't feel the difference.
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Old 06-03-2006, 08:30 PM   #3
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In my old 89 Buick Lesabre (LN3 3800) I found optimum performance and MPG'* was at around 90-91 octane. Any more than that and it just didn't respond well to it.
I had a CAI, 180* t-stat and the MAF screen removed, 1999 L67 coil packs with 94 L27 ICM, but otherwise stock.
On my 95 L67 before the pulley, TR6 plugs (.054"), PEMs, and 3" exhaust it really liked 96-98 octane.
I've only run 93-94 octane since I did the above listed mods, but didn't really see much difference, but on the scanner showed another 1-2* advance (from 11-12* on 92 octane up to 14-15* advance with 94-95 octane).
Now today I saw 17.8* advance.
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Old 06-04-2006, 04:39 PM   #4
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Thanks for the inputs. I agree with you dbtk2 , as far as efficiency goes I might as well run 87 , but do you actually improve performance on the gt with high octane at the track? And how much? Also how do you get 14'* out of that car? Thats great!!
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Old 06-04-2006, 06:07 PM   #5
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Quote:
Originally Posted by bluebullet
Thanks for the inputs. I agree with you dbtk2 , as far as efficiency goes I might as well run 87 , but do you actually improve performance on the gt with high octane at the track? And how much? Also how do you get 14'* out of that car? Thats great!!
I've never run the GT at the track without 93 octane, but I'm sure based on my KR levels that it improves performance. The added 5+ degrees of timing will definately add power. Figure about 2hp per degree of timing and thats a 10hp gain.

As for the 14'*, the guy I bought the car from had a very good idea on exhaust setup for L36 cars and experimented with it on a few L36 cars. All he does is cut the exhaust right after the downpipe and back to the y in the exhaust and replace it with 2.25" pipe. This gets rid of the cat, u-bend, and resonator. Leaving the stock downpipe on, as restrictive as it is, actually seems to help the torque output down low. Not enough backpressure will hurt it obviously. Other than that the car has an open K&N air filter, an UD alt. pulley, and a 180 tstat. I took my passenger and rear seat out when I ran it (~70lb. weight savings), and I ran it at Norwalk which is pretty sticky. The good track prep, plus my minor weight reduction, and engine setup is what helped me pull off the 14.6. Plus with 209000 miles the bottom end has loosened up quite a bit and I'm sure that helps the power output a little as well.

With my ud water pump pulley that I just put on, plus the shift kit I have for it, and a few other minor mods I hope to get it a lot deeper into the 14'* by the end of the year, then comes the fun stuff.
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Old 06-04-2006, 09:45 PM   #6
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Default Re: High octane gas in a N.A. car?

Quote:
Originally Posted by bluebullet
I would like to get some response from someone with experience in running high octane gas (93 or more) or octane boosters in a N.A.motor.
I can only suggest not to use octane boosters, most of them if not all off the shelf are misleading. You have to read the fine print they say up to 8points but really they mean up to .8 of a point. It is much more cost effective to fill up with high(er) octane then it is to use a octane booster/ fuel additive off the shelf.

Ed
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Old 06-06-2006, 09:57 PM   #7
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Alright let me get this straight; are you guys saying that if I run 87 octane, my L36 will be at 5 degrees knock reduction? And if I fill up the tank with 91 octane, the PCM will actually advance the timing? Really?
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Old 06-06-2006, 11:22 PM   #8
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Really really.

The higher octane does not cause predetonation and pinging, therefore the knock sensors don't sense any knock/ping and advance the timing closer to the fullest programmed allowed amount.
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Old 06-07-2006, 04:35 PM   #9
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Quote:
Originally Posted by MACDRIVE
Alright let me get this straight; are you guys saying that if I run 87 octane, my L36 will be at 5 degrees knock reduction? And if I fill up the tank with 91 octane, the PCM will actually advance the timing? Really?
No, not exactly. I was stating that my particular L36 gets 5 degrees of KR on 87 octane. All cars are different, and they have different mods as well. So maybe yours gets no KR, maybe only a few degrees, or maybe a lot more than what I get on 87 octane. We don't know. The only way to find out is to scan it.

But in MY particular case, I get 5 degrees of KR on 87 octane. Meaning if I run premium, I'll gain 5 degrees of timing, and therefore gain around 10hp.
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