fuel and boost

greenpony02 · **fuel and boost**

My car is stock and I�m seriously considering turbo charging it after I get some money here in the near future. I�m thinking of running between 4-5 pounds of boost. My question is what octane fuel will I have to run? Is 87 still OK or will I need to run premium?

timpryor · 04-14-2005, 04:03 PM

premo

greenpony02 · 04-14-2005, 06:48 PM

Ok that's what i figured. That then brings another question to mind. Why? I know higher octane is used in higher compression engines or engines with advanced timing to avoid detination, but what does the turbo charger do to create the need for higher octane. I can see needing a larger fuel pump...more air = more gas (maybe) but i don't get the octane thing

timpryor · 04-14-2005, 08:57 PM

i dont know, ALL i know is if you want to keep your current engine you better run premium.

greenpony02 · 04-14-2005, 09:20 PM

yeah like I said I figured that and I planned on it anyway. I guess I just I have an inquiring mind. If anyone knows the reason I'm still curious...

Thomas91169 · 04-22-2005, 03:00 AM

higher octane allows you to run more boost.

im tuned for 20psi with 91 octane. if i were to put in 100 i could safely do 24-25psi. add some 116(race gas) in and richen the tune a bit and i "could" run 30psi without detonation/timing retardation, but there are limits to the stock 4g63.

Tbird232ci · 04-22-2005, 08:50 AM

a turbo/supercharger is essentially making your engine "higher compression", because its compressing air into the cylinders

Taken from http://www.inductionmotorsports.com/...html?tech.html

"What's better, low compression and more boost or high compression and less boost?"

There are certainly reasons to try to raise compression ratio, namely when off-boost performance matters, like on a street car, or when using a very small displacement motor. but when talking purely about on-boost power potential, compression just doesn't make any sense.
People have tested the power effects of raising compression for decades, and the most optimistic results are about 3% more power with an additional point of compression (going from 9:1 to 10:1, for example). All combinations will be limited by detonation at some boost and timing threshold, regardless of the fuel used. The decrease in compression allows you to run more boost, which introduces more oxygen into the cylinder. Raising the boost from 14psi to 15psi (just a 1psi increase) adds an additional 3.4% of oxygen. So right there, you are already past the break even mark of losing a point of compression. And obviously, lowering the compression a full point allows you to run much more than 1 additional psi of boost. In other words, you always pick up more power by adding boost and lowering compression, because power potential is based primarily on your ability to burn fuel, and that is directly proportional to the amount of oxygen that you have in the cylinder. Raising compression doesn't change the amount of oxygen/fuel in the cylinder, it just squeezes it a bit more.

So the big question becomes, how much boost do we gain for X amount of compression? The best method we have found is to calculate the effective compression ratio (ECR) with boost. The problem is that most people use an incorrect formula that says that 14.7psi of boost on a 8.5:1 motor is a 17:1 ECR. So how in the world do people get away with this combination on pump gas? You can't even idle down the street on pump gas on a true 17:1 compression motor. Here's the real formula to use:

sqrt((boost+14.7)/14.7) * CR = ECR

sqrt = square root
boost = psi of boost
CR = static compression ratio of the motor
ECR = effective compression ratio

So our above example gives an ECR of 12.0:1. This makes perfect sense, because 12:1 is considered to be the max safe limit with aluminum heads on pump gas, and 15psi is about as much boost as you can safely run before you at least start losing a significant amount of timing to knock. Of course every motor is different, and no formula is going to be perfect for all combinations, but this one is vastly better than the standard formula (which leaves out the square root).

So now we can target a certain ECR, say 12.0:1. We see that at 8.5:1 CR we can run 14.7psi of boost. But at 7.5:1 we can run 23psi of boost (and still maintain the 12.0:1 ECR). We only gave up 1 point of compression (3% max power) and yet we gained 28% more oxygen (28% more power potential). Suddenly it's quite obvious why top fuel is running 5:1 compression, that's where all the power is!!

8.5:1 turns out to be a real good all around number for on and off boost performance. Many "performance" NA motors are only 9.0:1 so we're not far off of that, and yet we're low enough to run 30+ psi without problems (provided that a proper fuel is used).

greenpony02 · 04-22-2005, 10:11 AM

thanks for the info

I had a feeling i wound essentially be causeing higher compression but now i know for sure

Tbird232ci · 04-22-2005, 10:24 AM

any time

**Brent** · 04-22-2005, 02:14 PM


Quote: Originally Posted by timpryor i dont know, ALL i know is if you want to keep your current engine you better run premium.

You don't know yet you told him premium

timpryor · 04-22-2005, 03:56 PM


Quote: Originally Posted by Brent You don't know yet you told him premium

no i told him i dont know he exact reasons

Monkey · 04-22-2005, 11:30 PM

you can run 87. Just retard the timing.

I just scanned the big tech post from Tbird but the reason you need more octane with boost is because of heat. The compressed air causes a ton of heat. Hot, compressed air getting compressed again (in the combustion chamber by the piston) gets even hotter. Getting too hot with low octane fuel causes spontaneous detonation. That is bad. However, if you mix a higher octane fuel in the mixture, it takes more heat and spark to ignite. This makes it harder for the hot air to spontaneously combust because it needs an outside source (spark plug) to initiate the reaction. -- Proper procedure in the engine. Like Thomas mentioned, the higher the octane, the higher the boost you can run. Race gas burns really slow compared to 87 octane so you can induce more heat and more pressure without making it combust.

Oh, and Thomas -- I think the turbo would lose efficiency before 30psi and the turbo is you weak link on high boost, not the 4G motor. I have a bigger turbo and on race gas, my efficiency map shows it's only good to about 28psi. I would be willing to bet yours runs out of steam at 25psi before it just blows hotter air.