tuning and what octane fuel to use / its limits
08-09-2012, 12:30 AM
#1
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tuning and what octane fuel to use / its limits ive been street tuning my turbo'd ls1 trans am with HP Tuners for about 6 months now upping the boost and playing with timing and haven’t had any knock issues running 15 psi from a 67/66 t4 precision turbo on 93 octane.
last week i went with my buddy to get his Honda dyno tuned it is running a built gsr block and head with a gt35r tubro from reading about similar builds online we were expecting to see 550 or so to the wheels on 93 octane pump gas
after a few pulls the tuner came and said it’s done we look at the numbers and were disappointed with 415 hp to the wheels.
i began to pick his brain asking why he stopped there and what the limitation was. (I’m no tuning expert by any means but I’ve done a bit of research as well as looked at similar builds and seen what they made on pump gas)
he said the limiting factor was 93 octane injectors were only running at 52% duty cycle (id1000's) and the turbo only running at 16 psi.
he wouldn’t really explain it any further he seemed to be secretive about his whole tuning process
my question is what makes 93 octane the limiting factor at that point i know octane numbers represent the resistance to detonation.
but can’t you just run a much richer mixture to keep detonation from occurring?
for instance if you experience knock at 12.5:1 in a certain cell wouldn’t running richer say 11.8:1 in the same cell under the same conditions reduce or eliminate the knock? sure its not near as efficient but should still make more power allowing the tune to be pushed further on the same fuel?
i know pulling timing is another option but i'm more curious as to why a richer mixture wouldnt fix the problem instead
any info would be greatly appreciated as im still trying to learn as much about tuning as possible.
last week i went with my buddy to get his Honda dyno tuned it is running a built gsr block and head with a gt35r tubro from reading about similar builds online we were expecting to see 550 or so to the wheels on 93 octane pump gas
after a few pulls the tuner came and said it’s done we look at the numbers and were disappointed with 415 hp to the wheels.
i began to pick his brain asking why he stopped there and what the limitation was. (I’m no tuning expert by any means but I’ve done a bit of research as well as looked at similar builds and seen what they made on pump gas)
he said the limiting factor was 93 octane injectors were only running at 52% duty cycle (id1000's) and the turbo only running at 16 psi.
he wouldn’t really explain it any further he seemed to be secretive about his whole tuning process
my question is what makes 93 octane the limiting factor at that point i know octane numbers represent the resistance to detonation.
but can’t you just run a much richer mixture to keep detonation from occurring?
for instance if you experience knock at 12.5:1 in a certain cell wouldn’t running richer say 11.8:1 in the same cell under the same conditions reduce or eliminate the knock? sure its not near as efficient but should still make more power allowing the tune to be pushed further on the same fuel?
i know pulling timing is another option but i'm more curious as to why a richer mixture wouldnt fix the problem instead
any info would be greatly appreciated as im still trying to learn as much about tuning as possible.
08-09-2012, 06:44 AM
#2
I'm no expert either but it seems like running extra fuel to fight off detonation is a band-aid and it probably wont make any more power. I'm guessing the reason the extra fuel will fight detonation is because the charge will burn cooler (fuel burns the hottest and makes the most power at a certain afr) and the extra unburned fuel might also provide some cooling affect to the cylinder walls. This is just my best guess; I will leave the facts to the experts...
Adding more timing does not always equal more power. Think of it this way, the more timing you have the more time the piston is fighting the pressure of combustion on it's way up. Some fuels burn much slower (high octane) and require more timing for combustion to occur at the optimum time (when the piston is on it's way down - approx 8* atdc iirc). Every engine / fuel combination has a certain requirement for timing advance which yields the best power (at WOT). The more efficient the chamber design, etc the faster the fuel burns and the less timing is required. More is not always better
Adding more timing does not always equal more power. Think of it this way, the more timing you have the more time the piston is fighting the pressure of combustion on it's way up. Some fuels burn much slower (high octane) and require more timing for combustion to occur at the optimum time (when the piston is on it's way down - approx 8* atdc iirc). Every engine / fuel combination has a certain requirement for timing advance which yields the best power (at WOT). The more efficient the chamber design, etc the faster the fuel burns and the less timing is required. More is not always better
08-09-2012, 09:59 AM
#4
I think that a meth kit would help both of you guys especially your honda friend. I run 100LL avgas in mine and its tuned to 23* timing with no knock, beyond that it didnt gain anything. Another option if you can get it is e85, it is like 116 race gas you can buy at the pump for less $ than 87 octane. Only downside is you will need to upgrade to like say 160# injectors.
