bumping timing advance with 100 octane

[berettafan]

2.4 w/ webers and supposedly 2.2S pistons. single plug.

what's the best way to determine just how far i can advance my timing w/ 100 octane race gas? i am currently running 32 deg. advance w/ 93 and i can't hear anything bad going on in the motor but i wonder am i missing out on more power?

do i need dyno time w/ a a/f meter and other techno goodies?

[911pcars]

Originally Posted by berettafan

2.4 w/ webers and supposedly 2.2S pistons. single plug.

what's the best way to determine just how far i can advance my timing w/ 100 octane race gas? i am currently running 32 deg. advance w/ 93 and i can't hear anything bad going on in the motor but i wonder am i missing out on more power?

do i need dyno time w/ a a/f meter and other techno goodies?

I would say yes to the dyno time along with A/F and EGT meters.

Detonation is often not audible at high rpm. I'm sure you can do without melted pistons.

Sherwood

[berettafan]

Sherwood would ambient temp have an effect? should i do this on the hottest day?

[longhornchris04]

Depending on your compression (i don't know the 2.2S specs), you may not get anything out of more timing, in fact you may lose power. Short version, w/o sufficient compression, extra octane means nothing no matter how much timing you throw in.

See my post over in the 911 Tech forum (9.5 vs. 10.5) for details.

Ideally your compression and timing are matched to the best octane you have. Reducing the timing is a way of compensating for the CR being to high. Now, this isn't necessarily a bad thing, as the Dynamic CR, engine temp, and a few other factors play into detonation. Its quite possible to have an engine w/ a dynamic CR that needs reduced timing through part of the rev range. In this case, you would get benefits in that part of the range, but not where the DCR isn't as high. DCR is determined by camshaft and other engine dynamics. There are no "easy" ways to calculate it - tuning with a knock sensor, A/F, EGT, etc is the safest bet. To take the most advantage out of this, electronic ignition w/ MAP, crank, and knock sensors are needed, as is a lot of tuning. With a traditional distributor, your only real option is to adjust the static advance, so you may not be able to get much.

As to the ambient temp question - yes, but mostly because w/ our air cooled motors your head temp is more tightly coupled to air temp. The higher the temp, the more likely for detonation to occur.

[Steve@Rennsport]

You've received a ton of excellent and sage advice here,.....

I'd kindly suggest that you should not rely on your ears as the defacto "knock-sensors" to determine maximum timing values as that will get VERY expensive.

Thresholds of detonation are not static; they are dynamic and change with air temperature, engine load, fuel mixture, and cylinder head temperature. Given the variations found in pump gasolines, one should not attempt maximizing peak timing numbers as those figures would only be valid on that day, using that batch of fuel.

Fortunately, today's engine configurations are all well known so its not difficult to establish good timing numbers, based on your engine and what fuel you use unless yoiu have some custom head modifications.

Remember, when in doubt, "Less is More",...

[berettafan]

thanks guys, two fantastic threads on this subject which i'm sure i'll be referring to over and over again!

[spuggy]

Originally Posted by berettafan

2.4 w/ webers and supposedly 2.2S pistons. single plug.

what's the best way to determine just how far i can advance my timing w/ 100 octane race gas? i am currently running 32 deg. advance w/ 93 and i can't hear anything bad going on in the motor but i wonder am i missing out on more power?

You could try knock sensing with per-cylinder ignition retard. Might save your motor on a hot day or a bad tank of gas, if it's setup borderline for a cooler day/good gas:

http://www.jandssafeguard.com/

Yup, the guy who designed and builds it does say that it works fine on air-cooled 911's and 930's.

In fact, he's a member of this board.

http://forums.pelicanparts.com/911-930-turbo-super-charging-forum/405586-knock-sensor-post4763328.html

Every time someone trots out "knock sensing doesn't work on air-cooled 911's", I think of the 993 and the factory-fitted knock senors.. Today's $50 cell phone has far more CPU power and memory than an 80's Bosch DME.

Been meaning to buy one and book dyno time for a while now....

[Alan L]

Well, you have pretty well got it all already. But I would +1 on not using your ears to determine knock - especially on a noisy aircooled 911. I've driven them for over 10 yrs and would not swear on being to pick a detonation before my engine melted.
You would need some real numbers to base your timing on.
Regards
Alan