[sasilverbullet]

What is this part circled in red? It's right underneath the throttle body.



   



When I turn the key to position 2, it runs, even though the engine hasn't started.

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[Greimann]

That is the idle stabilizer valve. It opens and closes to keep the idle .... well .... stable. The computer wants 840 rpm exactly and this valve is the device that keeps it there.

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[sasilverbullet]

So if my idle moves around some, this is the culprit?

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[Greimann]

If the idle fluctuates wildly, then this could be a source of a problem, otherwise a small idle hunt shows that it is working properly and there is something else causing the idle to destabilize. I know before I did the performance chip and adjustable fuel pressure regulator, the idle had a small hunt. I attributed it to a lean surge, or some other condition that fought the idle circuit. Now it is dead stable.

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[sasilverbullet]

So what exactly do you think made it hunt?

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[Greimann]

I think it was just too lean a mixture at idle for the best stability. The chip made the idle noticeably more stable.

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[apex]

Every few seconds my RPM drops a couple hundred - minor - not "wildly" as Dave puts it. It's always done this and always bugged me but I've never really known where to start the hunt. I have read that this may be not uncommon with our cars?

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[rustech]

Mine hunts within +or- 50rpm of the 840rpm idle which gives a range of 100rpm. If your range is 200rpm, that is a bit abnormal.



...but yes, a hunt does seem normal with every car I've driven, and mine still does it with my new chip.

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[Greimann]

I had that too and I remember it going away after dosing up with 5 gals of 100 octane. The computer will give more timing advance with better gas. Idle hunt is typically timing and mixturte related. The computer is constantly fiddling with these settings to obtain the best emission profile. Perhaps at the ragged edge of the range, the idle quality suffers.

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[LivingDust]

Where are you guys finding 100 Octane gas? Do you buy it in cans? Or are you pumping it? Lots of you have reported good things after doing it, and I would like to give it a try.

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[flash]

we get it at the pump - i have a list of all the stations in california, but that's it



be careful - it's pricey - also, more than a 50/50 mix is a waste of money - the car doesn't run any better with more than that - if you mix with 91, that equates to about 95 octane (RON+MON/2) - a 5 gallon per tank mix gives you about 93 octane - these figures would change if you are mixing with something other than 91



the rule of thumb is that a 1% raise in octane will give you a 1% raise in power, as long as you have the compression and timing management to handle it - this means that going from 91 to 95 octane would then be about 10 horsepower - i'm not sure it's really that high, but most people can feel the difference on a stock car

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[Greimann]

[quote name='LivingDust' date='Aug 1 2005, 05:27 PM']Where are you guys finding 100 Octane gas?  Do you buy it in cans?  Or are you pumping it?  Lots of you have reported good things after doing it, and I would like to give it a try.

[right][post="8228"]<{POST_SNAPBACK}>[/post][/right][/quote]

Unocal 76 is the only chain that sells it in Socal, and those that do are few and far between. Maybe only two in Orange county. You have to really sniff them out.

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[LivingDust]

[quote name='flash' date='Aug 1 2005, 07:35 PM']- if you mix with 91, that equates to about 95 octane (RON+MON/2) - a 5 gallon per tank mix gives you about 93 octane - these figures would change if you are mixing with something other than 91

[right][post="8229"]<{POST_SNAPBACK}>[/post][/right][/quote]



Do you guys in SoCal not get 93 octane (RON+MON)/2 at all normal stations? Just about every station here in Texas has a premium pump which is 93 octane. If you guys are mixing race fuels to get up to a mix of about 93, then there is probably no point for me. It seems that some owners are finding that at least some of our cars run a bit better at 95 octane. But that could be because you are starting from fuel that is 91, and Porsche recommends a minimum of 93...



[quote name='flash' date='Aug 1 2005, 07:35 PM']the rule of thumb is that a 1% raise in octane will give you a 1% raise in power, as long as you have the compression and timing management to handle it - this means that going from 91 to 95 octane would then be about 10 horsepower - i'm not sure it's really that high, but most people can feel the difference on a stock car

[right][post="8229"]<{POST_SNAPBACK}>[/post][/right][/quote]



My understanding is that the higher octane fuels can be compressed further without spontaneous ignition. Knocking and pinging occur because the fuel ignites prematurely. (It is kind of ironic that people use the phrase "high octane" to describe a volatile situation when it is low octane that is more volatile...) Knock sensors can hide an octane problem by delaying ignition, but while the knock may be gone, the fuel is not being ignited at the optimum point in the cycle.



But once fuel has a high enough octane ratio to assure against premature ignition, there is no other real advantage to the higher octane. Any gain in HP is not from the fuel exploding with more energy, but only that the fuel is exploding at the right time for maximum effect. Correct?



In other word high octane fuels to not increase HP, but rather lower than optimum octane fuels rob HP from high-compression engines that need fuel that won't sontaniously ignite when compressed as much as the engine is going to compress it.



If there is any of this I have wrong, please let me know.



[quote name='Greimann' date='Aug 1 2005, 09:09 PM']Unocal 76 is the only chain that sells it in Socal, and those that do are few and far between. Maybe only two in Orange county. You have to really sniff them out.

[right][post="8242"]<{POST_SNAPBACK}>[/post][/right][/quote]



Lucky for me a lot of people race here in TX. A quick post to my local club and I found 3 or 4 places right here in Austin where I can get unleaded race fuels.



The only reason I am tempted to mix a bit of higher octane gas is because I have read several posts now where one or more mysterious miscellaneous problems went away by doing so. (Idle hunting, poor throttle response from idle etc.)

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[Greimann]

Do you guys in SoCal not get 93 octane (RON+MON)/2 at all normal stations?

Nope. The best we can get is 91.



They took away 93 octane 4 or 5 years ago and for the sports car crowd...Their world crumbled ...And only those mobile enough to scavenge, brutal enough to pillag would survive. At last, the vermin had inherited the earth. And in this maelstrom of decay, ordinary men were battered and crushed......men like the Warrior Max....



Woah where did that come from? <img src="/forum/images/smilies/968/smile.gif" class="smilie" alt="" />





Your other statement about the technicalities of octane are spot on.

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