octane requirements

[jimmag]

Guys,being fairly new to racing GN's I have a question that might be "stupid". Is there a formula to figure out exactly what octane is necessary for a specific motor.In my limited experience I would expect some math that would use ,compression ratio ,boost,timing ,intake air temp,etc to establish an octane requirement.
When I ask experienced people much smarter than me ,I get different answers,all based on something like,"Ive seen cars blow head gaskets using that". "I wouldn't use less than 104, no alky or 93 plus alky is fine",etc,etc.
My car is probably a good example. i have been told ,"use 104 ,wouldn't use less than 110 , better use 116"
Seems to be a lot of seat of the pants or experiential answers. Not that this is necessarily bad and maybe it's me and I'm the dope but WTF . With all the "science " in racing and motors and the addition of computers ,at the risk of being hammered on this board ,I have to ask ,"is there a specific formula for estimating octane requirements for our GN's.Jim Maguire

 

[earlbrown]

yep. the formula is to start at a baseline level with no knock. Add boost until you see knock..... that is the point where you need more octane.


Anyone that says a definite number about anything is going to be wrong 99% of the time (even a broken clock is right twice a day).

No two instances are ever the same.


To answer your question more scientifically, if there were one formula that had all the variables, to indicate exact octane requirements, it would be ten miles long. There are MANY MANY variables that determine detonation resistance in an engine. Quite a few of them have nothing to do with the car.

 

[Amelio]

Earl said it pretty damn good......


From your sig you have a nice build and from your time posted it sounds like the car fly's....I also see it has a Fast system.....are you doing the tuning? If you are you should be able to answer your own question......I do not have a Fast system I run a stock ecu with a Power logger and review my runs...but I am pretty sure the fast system can do the same and much much more.

How hard are you squeezing the car for that 9.98? If its a clean run with no kr and your A/R looks good than your fine

I guess my point is or its more curiosity is that......to me you have a rookie question......... but you have a good build and a fast car....you should have the answer to your question already.....

 

[jimmag]

Amelio,yea ,I am a rookie ,that why I tried to preface my statements with I gotten a lot of experiental answers ,I was hoping for a rote scientific answer and I guess it doesn't exist .Thanks for your feedback.It's a process not an event and Im still learning,jim maguire

 

[Amelio]

Hey Jim,

as Earl stated there is no real formula its based on the tuning......as you increase your timing/boost you will find the limits of whatever fuel you are running.

If you start with 110oct just keep stepping up your boost or timing depending on what your goals are until its maxed out then step up the octane.......I do know if you run more octane than is required it will actually slow you down.

Who tunes your car?.....a 9.98 @137 sure does not sound like a rookie tuner

 

[Reggie West]

Amelio,yea ,I am a rookie ,that why I tried to preface my statements with I gotten a lot of experiental answers ,I was hoping for a rote scientific answer and I guess it doesn't exist .Thanks for your feedback.It's a process not an event and Im still learning,jim maguire

I cant give you a math formula but I can say that the fuel needed will depend on what you are doing. 91 pump gas and an alky kit will work fine on the street but you will need to tune it differently. There are guys out there using pump gas and alky on the strip and they are running high 9s doing it. Your tune needs to be spot on to make it work. When you use an alky kit, you will need only one fuel pump and the alky pump is your second. Meaning you literally need to keep the second in tank pump from coming on when using the alky. If you dont you will flood the motor to the point it dies because the plugs cant light off the amount of fuel you are throwing at it. Or you just push out a head gasket. Ask me how I know.

Race gas is what I would use for the strip. Reasons? You can run as much timing as you like with race gas (meaning C16) and boost. Typically you are trying to max out boost when racing meaning 25 to 30 psi. Thats a lot of cylinder pressure and you need a fuel that will not detonate on you. That is cheap insurance, even at $8 to $9 a gallon or more. Your double pumper set up doubles the VOLUME of fuel when both are activated so you end up trimming ( subtracting ) fuel out of the VE table to get the right AF ratios to make max power. Look at your VE table and you should see where the VE numbers are being reduced around 10 psi or so( 69 kpa ) and above. That is the point where your second fuel pump comes on.

Fuel for a turbocharged car has different requirements. The fuel has to withstand greater pressures, 16 to 1 under boost, and temperatures. It's not uncommon to see Exhaust Gas Temperatures of 1600 to 1750 degrees coming out of a turbo. The charge temperatures coming into a turbo motor are greater because you are compressing the air too. A NA motor is sucking in as much air as it can. A turbo charged motor is stuffing air into the cylinders.

Hope this helps

 

[Red C5]

I had a FAST on my car too. Basically had different tunes for different amounts of boost, 91 octane 15psi, 100 octane 19 psi, C16 35 psi. All I would do is change the boost controller setting based on what gas I had in the car.

 

[bison]

yep. the formula is to start at a baseline level with no knock. Add boost until you see knock..... .

I do not agree with this "formula" and would not do this if you value youre expensive engine. Staying ahead of the curve is most important When making 120+hp per hole you cant have any detonation ever or its a shit show. More octane is not necessarily better if not needed though. That is true. There truly is no formula. The higher the cylinder pressure and the more heat the more octane required. Its easier to raise rpm drop cylinder pressure a little and make more power safely than to increase the cylinder pressure and rely on it to move the car.

 

[jimmag]

Amelio and the rest of the guys,good info and thats my point . A lot of varying opinions .Amelio ,Otto Pernek originally set my car up then he kind of evaporated thenI had Ted Jannetty do the tune Im running now .Not sure if I should just leave it for now .While Ted Is not a "GN Guy" he is a great mechanic and runs a good shop.His shop built our 2010 SS Camaro ,were running 9.80 and came in second at last years Camaro Fest in Indiana. Ted says its an internal combustion engine and the same science applies to a Buick V6 or a small block!
As far as me tuning it ,fk no ! Last year I adjusted the boost.Thought how hard can it be? Ran like hell until the windshield had water spots on it and I got steam bath when I opened the hood ill never do that again but Im a rookie and I am learning,jimmaguire

 

[earlbrown]

I do not agree with this "formula" and would not do this if you value youre expensive engine.

We both agree. That 'formula' wasn't for tuning, it was to find the OP's octane limit. I guess I could have said add boost slowly until you see knock (then back off)

 

[Amelio]

Ok Jim that makes sense I have heard about Otto's tuning abilities.......sounds like you got a screaming camero too...very cool...

I would not suggest just turning up the boost (your right its not hard that is the easy part) unless you know how to monitor your pass and can interpret whats going on......on a car that makes 9 sec power that is a recipe for disaster!

I agree with both Bison and Earl.....I am by no means a pro tuner but I got a pretty good handle on it........I like to increase my performance until I see my fuel ratio get to a point where I feel KR is at the door......I never usually tune until I see it........once I get that threshold I then make a change to my fueling whether its increasing the fuel or adding octane.....I then bump up the power whether its boost or timing and keep going until I am at the fuel threshold again and the process starts over.

I don't know if that is the best way, again I am not a pro but it works for me....my car does not see kr and I have had the car run pretty damn good just on pump fuel with this method.

 

[Razor]

Other variables like carbon in the combustion chamber, cylinder head design, quench, compression ratio, etc.

There is no silver bullet one fuel/one tune/ etc that works for all. The best scenario is work up on the tuning starting low and working up.. and go from there keeping variables down. Successful racers follow this versus turn it up to 35 psi and see what happens. The alcohol stuff is tuned this way.. 1 psi at a time.