Boost Level vs. Octane

[DrifterTom]

i am looking for some info on what point do we need to start adding race fuel. i have never seen anyones dyno sheet on nico that was anything other than 93 octane. my tuner never goes above 19psi with out race fuel or alcohol. mostly supras and drag cars though.

so my question is at what point are you guys with some serious hp switching to a higher octane
Modified by DrifterTom at 12:02 PM 8/18/2006

[krayton]

isnt that gonna be dependent on the size turbo and tuning?

i was running 1.2 bar on 91 with a small t3/t4

[DrifterTom]

i don't think that the turbo size is going to change this. yes this will all be very clear towards the end of the tuning day. ( this weekend hopefully) i am just trying to prepare for draining the fuel tank and how much fuel to have on hand. i am not lloking for a end all answer, just prior experiences. anybody in the 20psi range?

[Joe]

turbo size plays a big role.

a t3 flows a HELL of alot less air at "10psi" than something like a T61 does at "10psi"

[DrifterTom]

ok maybe i am confused. how can 10 pounds of manifold pressure be different than another 10 pounds. i am not saying your wrong, i just want to know the physics behind it

[Joe]

pressure and flow are different things.

why do you think you will make WAY more horsepower running a bigger turbo at the same boost as a smaller one?

example: stock RB25 turbo, 14psi, maybe 275whp

T3/t4, 14psi, 325-340whp.

now at certain PSI's its basically a requirment to run race gas because you have so much air in the cylinder you NEED that much octane, but there is no set number and it varies on your injectors, tune and tuners ability.

[DrifterTom]

i just remembered that my gt30r did feel a hell of a lot faster than my t3/to4e did and they ere on the same external wastegate. i thought it was just how fast it cam on line and responsiveness.

you have me conviinced, but i wanna see the math behind it

[Joe]

i wish i could give you the math but i suck at thermal dynamics and physics.

[DrifterTom]

i guess that i am no engineer either

[gawdzilla]

ok maybe i am confused. how can 10 pounds of manifold pressure be different than another 10 pounds. i am not saying your wrong, i just want to know the physics behind it

there is no difference, 10 psi is 10 psi. however there are many factors which affect pressure, like volume and temperature. safe to assume your volume doesnt change with a bigger turbo, but the efficiency of a bigger turbo will be better at certain flow rates. thus less temperature to make the same pressure, giving you more air. thats why you get the "10 psi on a big turbo" makes more power than "10 on your stock turbo".

[DrifterTom]

now that makes sense. more efficient=less heat generated due to friction and resistance

[IowaRB240SX]

I would run 7 gal of 93 with 3 gal of 116 octane. That will put you at 100 octane. It will never hurt to have too much octane. When you are tuning you can add more timing this way and thus make more power.

[Joe]

I would run 7 gal of 93 with 3 gal of 116 octane. That will put you at 100 octane. It will never hurt to have too much octane. When you are tuning you can add more timing this way and thus make more power.

ehh i disagree with that.

too much octane and you loose power. you want to run as low as possible octane without detonation to make the most power. octane is a measure of how hard it is for fuel to combust. the higher the number, the HARDER it is to burn.

and if he isnt gonna run 100 octane all the time, he shouldnt tune with 100 octane. tune with what you are gonna use each time you drive the car.

[krayton]

we tuned for 91, put race gas in for safety, ended up misfiring.

went back to 91 and it was fine.

[DrifterTom]

i believe that kamin is right. you don't want to tune with too much octane.

and to what he said, i definately won't flex the race fuel around town. i am tuning for a low boost setting and for one that is for drag/dyno or when ever i want to go balls out and get crazy

crazy menaing risk of engine and bodily damage

[gawdzilla]

race gas is a good idea to let you know how close to the limit you're running with your timing. for example, with my knock counts, i havent really gotten to the fine line between real knock and false knock. race gas is a good way to edge closer to that line. with 93 pump, 19 psi for me is about 9 degrees retarded compared to stock timing up top...

[Kansei240sx]

Tom another way to look at different turbo and air flow rates is this..

A turbo the size of a walnut at 7psi flows as much air as gt30 at 7psi right?

INCORRECT. Just thing of the logic on that one. The pressure generated is the same, but how much of that air going through at that rate of pressure is completely different.

[wantsansr]

if you want to see the math there is a good book that has it all its called maximum bosst its by corky bell. talks all about design and the requirments of a turbo charger system. i picked it up and read it and then realized that that much math was outta my league.

[StricNyne]

alky is your friend, u can over come the need for high oct with the use of larger inj to simply supply more gas, or you can use alky to cool combustion which effectly raises oct rating, alky kits cost around 400 for a good one, and alky is 10 dollers a gallon which inturn will last u like 200 miles i think so in theory u could run cheapo 89 oct gas and tune for alky which will add around 7 or 8 points and be running like 96 or 97

[DrifterTom]

yeah thats what i want to do, and my map-ecu will control the injection. i may use it to control some juice for that 3500 rpm of spool time. only problem is i am not sure that i could drive a 5 speed juiced through the lag. i tried to drive a 6 speed supra with 800 whp and livin on the bottle. i was nowhere near fast enough to beat my friends widebody twin turbo sc430. he has somewhere in the 700's and a gm automatic. it wasn't much of a race, he stomped me out. and then he almost beat me in his 06 m5. i guess i suck at shifting when the gear last about 1.5 seconds

[Kansei240sx]

Yeah.... i would also call that not fun to drive...

Gotta love that 300zx and supra comparison quote

"What do 800 rwhp Supras and 400 rwhp 300zxtt's have in common? 12 second time slips."

[Shocker]

Yeah.... i would also call that not fun to drive...

Gotta love that 300zx and supra comparison quote

"What do 800 rwhp Supras and 400 rwhp 300zxtt's have in common? 12 second time slips."

and only a 20mph trap difference.... depends if the driver can drive or not. BPU's have gone 10's gotta respect the supra.

and the whole octane thing, i ran pump 110 with a 93 mix at the track before, averaged to around 101-102 octane. hurt my mph big time, 4 mph lose. only thing it helped with my 1/8 mile gave me 4 mph there and .2 seconds!.... end result was the same tho. confusing.. i dont think the ECU liked it to much. The following time i started my car up and went WOT it missed like hell around 6 k, big time bog. Took about 4-5 pulls to clear it up, my guess was lead on the o2 messing with it. 93 for me only.

[GTR Shop]

and the whole octane thing, i ran pump 110 with a 93 mix at the track before, averaged to around 101-102 octane. hurt my mph big time, 4 mph lose. only thing it helped with my 1/8 mile gave me 4 mph there and .2 seconds!.... end result was the same tho. confusing.. i dont think the ECU liked it to much. The following time i started my car up and went WOT it missed like hell around 6 k, big time bog. Took about 4-5 pulls to clear it up, my guess was lead on the o2 messing with it. 93 for me only.

Reason being is that your tune wasn't here for the higher octane.The more octane you run the slower the fuel burns (also the more resistance to knock it has) therefore to create the maximum cylinder pressure, you have to advance the timing.We did alot of testing a few years ago with sprint Karts.. we were deliberatly letting the fuel go off and doing thing like adding terps or weed killer to fuel to knock the octane back.. what we did find is certain types of weedkiller actually oxygen the the fuel.. I can tell you, it didn't take long for they to ban it and implement fuel testing.

[rbspitfire]

i have to agree that psi and volume are differnet, just look at fuel pumps, i know its hard to swallow but youre stock pump at 60psi isnt going to flow half as much as 255 pump.

[rbspitfire]

heres some numbers http://gnttype.org/techarea/im...k.gifsometimes physics is like magic, you just go with it

[BoostFab]

Tom another way to look at different turbo and air flow rates is this..

A turbo the size of a walnut at 7psi flows as much air as gt30 at 7psi right?

INCORRECT. Just thing of the logic on that one. The pressure generated is the same, but how much of that air going through at that rate of pressure is completely different.

just think of it this way, think of blowing hard through a drinking straw to 1psi, then through a 2" pipe to 1psi. thus the 2" pipe will have more air per 1psi than the straw. of cousre temperature play an important role.

[Carl H]

simply put:psi = pressure pressure != airflowboost is a measure of pressure, how much air a turbo can flow is difrrent from turbo to turbo.

[Darius]

My question is: How does compression ratio factor into the amount of boost and octane level of the fuel you are running? What are the upper and lower limits (reasonably speaking) of compression ratio that should be run in a turbocharged application with a standard pump gas of 93 octane for example?

[eh?]

CR raises cylinder pressure and increases the chance of preignition. The upside is higher HP. There was someone who had a RB25DE+T, had something like ~430whp but he eventually popped the HG. RB25DE's are 10:1 CR motors...

[Carl H]

also have to think that on a FI motor that the motor has a 'static' and a 'dynamic' CR, while its static cr might be 8.5:1 off boost, while on boost the cr increaces with boost pressure...