OK, heres a question for you all you tuning techo's

bugeyes · 03-19-2005, 08:05 AM

What ultimately limits the maximum boost?

Lets say theoretically the engine holds together. If you keep winding up the boost you normally induce detonation, this can be reduced by adding fuel or retarding timing.
Retarding timing tends to increase egt's, running too rich can also send the egt's high. Do you get to a point where you can no longer retard the timing any further or add more fuel due to high egt's. Therefore leaving no other way of getting rid of detonation (unless you go for water injection which slows the combustion of fuel and can reduce detonation). Is this then the ultimate maximum boost of the engine ?

Any tuners explain this?

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jblaine · 03-19-2005, 09:46 AM

You have it right. Octane is your limiting factor in a theoretical/ideal world where your engine can handle the power and your turbo can make the boost. Your only other option is to reduce the compression ratio of the engine which will buy you some more room.

SaabTuner · 03-19-2005, 10:17 PM

Quote:

Originally Posted by jblaine

You have it right. Octane is your limiting factor in a theoretical/ideal world where your engine can handle the power and your turbo can make the boost. Your only other option is to reduce the compression ratio of the engine which will buy you some more room.

I know this isn't quite what you're looking for, but it might still be helpfull.

There is also "mechanical octane", which is the tendancy of any given part configuration to resist detonation. Pistons with good quench, and oil-cooling jets for the pistons ... along with cooler coolant temps etc etc etc ... all allow you get to more power on the same octane rating.

The other option, which I have not seen any Subaru engine-builders employ, is making the exhaust parts, such as the exhaust valves and turbine-wheel, out of Inconel. The reason this can allow more power is that you can pull more and more timing and simply run more boost or a higher compression ratio. My Saab, as an example, runs an EGT of well over 1850*F completely stock with no ill effects.

Does this allow it to get more power on crappy Cali 91 octane? Absolutely!

Ultimately, the limit is governed by octane on one side and EGT on the other. If you raise the limit of either, you can tune in some extra power.

It's just a lot easier to dump in race fuel than it is to instal Inconel exhaust valves.

Adrian~

hippy · 03-19-2005, 11:04 PM

More fuel makes the egts and in cylinder temps cooler. The extra fuel(beyond 12.5 to 1 or whatever) never gets used because there isn't enough air to create the chemical reaction needed for it to burn. It just absorbs some heat from the combustion along with slowing down the burn, and gets pushed out the exhaust.

The more fuel ya use(beyond 12.5 to 1 or whatever), the higher the chances of misfiring. This is cause more fuel makes it harder for the plugs to light the a/f mixture. Kinda like trying to poke a knife through a piece of plastic. The knife being the spark, and the fuel being the plastic. The thicker the plastic(the more fuel beyond 12.5 to 1 or whatever), the harder it is for the knife(the spark) to make it through. The higher the boost, and the more air/fuel is pushed in the cylinder, the more likely a misfire will occur also. It's like trying to light a match(fire a spark plug) while wind is blowing on it. The more air/fuel, the more wind. Try to combine really high boost with a really rich a/f ratio, and that makes the chances of misfire much higher then lower boost and leaner a/f ratios.

The limiting factor isn't octane, it's heat. We add fuel and retard timing to keep the in cylinder temps down to try and fight detonation. High egts' can cause other parts of the cylinder(like exhaust valves) to get hot which can also cause detonation. Higher octane lets us run the engine harder while keeping things like egts and in cylinder temps the same. This is why higher octane lets us run more boost and what not. It compensates for the extra heat created by more load put on by having a larger a/f mixture in the engine and the engine working harder. More advanced timing can lower egt's but raise in cylinder temps, and retarded timing can raise egt's while lowering in cylinder temps. As always, this is just what I've heard. I'm not sure how it relates to max boost even though it probably does, and I'm no expert.

peace

jblaine · 03-19-2005, 11:15 PM

So, yes, the limiting factor is octane... because of heat. A little nitpicky, eh?

hippy · 03-19-2005, 11:41 PM

It's heat, and I don't think I'm being nitpicky

. If there's a way to keep the temps down(like water injection or whatever), the octane of the fuel doesn't have to be as high to get results like someone would get by using higher octane fuel.

peace

Uncle Scotty · 03-19-2005, 11:55 PM

A dissertation could(and has been) easily be written on this subject....several, in fact.....

jblaine · 03-20-2005, 09:18 AM

Quote:

Originally Posted by hippy

It's heat, and I don't think I'm being nitpicky

. If there's a way to keep the temps down(like water injection or whatever), the octane of the fuel doesn't have to be as high to get results like someone would get by using higher octane fuel.

peace

Water injection, when added, is considered part of the fuel.

Fuel is not "gasoline". Fuel is whatever you mix with the air.

And WI, in effect, is doing a lot of the same things higher octane gasoline would do.

hippy · 03-20-2005, 10:10 AM

Water injection is doing a lot of the same things higher octane fuel would do along with a lot of the same things dumping extra normal octane fuel on the mix would do. It does other things too, but it doesn't actually raise the octane of the fuel. This is why I said heat is the limiting factor and not octane. When ya raise the boost you're usually gonna raise the heat created by combustion. Higher octane might be a way to deal with the heat better, but it's not the only way.

Like you and saab said, different engine dinamics(like better parts, lower compression ratio, different burn characteristics) can also change how much boost someone can run safely on a given octane. It's pretty easy to see this type of difference when ya compare an ej205 engine to an ej207. The ej207 has a higher compression ratio(from what I remember, could be wrong), but because of the way it's made, it might be able to run the same boost as an ej205 on the same fuel with the same a/f ratio, but have more advanced timing. It might also be able to run more boost with the same timing as the other engine on the same fuel. There's also something to be said about different fuels even at the same octane level, like what happens when ya have oxyginated gas?

Heat's the product of many things which are all limiting factors. This might include fuel, but fuel isn't the only thing that dictates the heat created druing/after combustion. The engine, turbo, exhaust, load created by weight(like weight of the car and it's moving parts, or gearing), and so many other things contribute to the heat created. When ya change any of those things, the max boost can also change even on the same fuel. I mean you can run more boost safely with 1 person in the car then 4 people. Driving alone compared to driving with 4 people in the car might do some of the same things as using higher octane fuel(like reduce in cylinder heat along with egts, and let us run more boost and/or more timing with leaner a/f ratios), but it's not the same thing.

peace

03-19-2005, 08:05 AM	#1
bugeyes Scooby Specialist Member#: 64494 Join Date: Jun 2004 Location: Australia Vehicle: 2001 V7 sti	OK, heres a question for you all you tuning techo's What ultimately limits the maximum boost? Lets say theoretically the engine holds together. If you keep winding up the boost you normally induce detonation, this can be reduced by adding fuel or retarding timing. Retarding timing tends to increase egt's, running too rich can also send the egt's high. Do you get to a point where you can no longer retard the timing any further or add more fuel due to high egt's. Therefore leaving no other way of getting rid of detonation (unless you go for water injection which slows the combustion of fuel and can reduce detonation). Is this then the ultimate maximum boost of the engine ? Any tuners explain this? * Registered users of the site do not see these ads.

03-19-2005, 11:41 PM	#6
hippy Scooby Specialist Member#: 36528 Join Date: May 2003 Location: florida Vehicle: 2002 Impreza Wrx Silver	It's heat, and I don't think I'm being nitpicky. If there's a way to keep the temps down(like water injection or whatever), the octane of the fuel doesn't have to be as high to get results like someone would get by using higher octane fuel. peace

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03-19-2005, 09:46 AM	#2
jblaine Scooby Guru Member#: 8512 Join Date: Jul 2001 Chapter/Region: South East Location: St. Pete, FL Vehicle: 2002 WRX chassis... stage-infinity.com	You have it right. Octane is your limiting factor in a theoretical/ideal world where your engine can handle the power and your turbo can make the boost. Your only other option is to reduce the compression ratio of the engine which will buy you some more room.

03-19-2005, 11:04 PM	#4
hippy Scooby Specialist Member#: 36528 Join Date: May 2003 Location: florida Vehicle: 2002 Impreza Wrx Silver	More fuel makes the egts and in cylinder temps cooler. The extra fuel(beyond 12.5 to 1 or whatever) never gets used because there isn't enough air to create the chemical reaction needed for it to burn. It just absorbs some heat from the combustion along with slowing down the burn, and gets pushed out the exhaust. The more fuel ya use(beyond 12.5 to 1 or whatever), the higher the chances of misfiring. This is cause more fuel makes it harder for the plugs to light the a/f mixture. Kinda like trying to poke a knife through a piece of plastic. The knife being the spark, and the fuel being the plastic. The thicker the plastic(the more fuel beyond 12.5 to 1 or whatever), the harder it is for the knife(the spark) to make it through. The higher the boost, and the more air/fuel is pushed in the cylinder, the more likely a misfire will occur also. It's like trying to light a match(fire a spark plug) while wind is blowing on it. The more air/fuel, the more wind. Try to combine really high boost with a really rich a/f ratio, and that makes the chances of misfire much higher then lower boost and leaner a/f ratios. The limiting factor isn't octane, it's heat. We add fuel and retard timing to keep the in cylinder temps down to try and fight detonation. High egts' can cause other parts of the cylinder(like exhaust valves) to get hot which can also cause detonation. Higher octane lets us run the engine harder while keeping things like egts and in cylinder temps the same. This is why higher octane lets us run more boost and what not. It compensates for the extra heat created by more load put on by having a larger a/f mixture in the engine and the engine working harder. More advanced timing can lower egt's but raise in cylinder temps, and retarded timing can raise egt's while lowering in cylinder temps. As always, this is just what I've heard. I'm not sure how it relates to max boost even though it probably does, and I'm no expert. peace

03-19-2005, 11:15 PM	#5
jblaine Scooby Guru Member#: 8512 Join Date: Jul 2001 Chapter/Region: South East Location: St. Pete, FL Vehicle: 2002 WRX chassis... stage-infinity.com	So, yes, the limiting factor is octane... because of heat. A little nitpicky, eh?

03-19-2005, 11:55 PM	#7
Uncle Scotty * Banned * Member#: 16200 Join Date: Mar 2002 Vehicle: OK buy Nates beans westcoastroasting.com	A dissertation could(and has been) easily be written on this subject....several, in fact.....

03-20-2005, 10:10 AM	#9
hippy Scooby Specialist Member#: 36528 Join Date: May 2003 Location: florida Vehicle: 2002 Impreza Wrx Silver	Water injection is doing a lot of the same things higher octane fuel would do along with a lot of the same things dumping extra normal octane fuel on the mix would do. It does other things too, but it doesn't actually raise the octane of the fuel. This is why I said heat is the limiting factor and not octane. When ya raise the boost you're usually gonna raise the heat created by combustion. Higher octane might be a way to deal with the heat better, but it's not the only way. Like you and saab said, different engine dinamics(like better parts, lower compression ratio, different burn characteristics) can also change how much boost someone can run safely on a given octane. It's pretty easy to see this type of difference when ya compare an ej205 engine to an ej207. The ej207 has a higher compression ratio(from what I remember, could be wrong), but because of the way it's made, it might be able to run the same boost as an ej205 on the same fuel with the same a/f ratio, but have more advanced timing. It might also be able to run more boost with the same timing as the other engine on the same fuel. There's also something to be said about different fuels even at the same octane level, like what happens when ya have oxyginated gas? Heat's the product of many things which are all limiting factors. This might include fuel, but fuel isn't the only thing that dictates the heat created druing/after combustion. The engine, turbo, exhaust, load created by weight(like weight of the car and it's moving parts, or gearing), and so many other things contribute to the heat created. When ya change any of those things, the max boost can also change even on the same fuel. I mean you can run more boost safely with 1 person in the car then 4 people. Driving alone compared to driving with 4 people in the car might do some of the same things as using higher octane fuel(like reduce in cylinder heat along with egts, and let us run more boost and/or more timing with leaner a/f ratios), but it's not the same thing. peace