Tuning and afr's affect on detonation

[Bdpprocessors]

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.

[45T-XR]

Im even greener than you, but just out of curiosity what were the afr's on your mates honda during the run?

[BigDaddyCool]

12.5 is for aspirated engines. 11.7-11.9 is a better mark for a turbo or supercharged anything

[4evervette]

sometimes in the dyno trying different target ( AFR's) will get wide range of results ..u shoot for 12.8 on ls7 ..u get 480..then u try 12.2 u get 490.. assuming same timing ..

so u think richer is better try that on ls2 ..it does the opposite .. some times different combination will like different targets..i know that is not what u asking but to my limited knowledge i would say cylinder peak pressure plays a big rule on determining peak HP with certain fuel mixture and fuel grade as well some combination can tolerate 93 octane some dont

[Atomic]

The further away you move from stoich (either direction), the colder the combustion temperature...excess heat in the combustion chamber and/or heating of the air coming into it is what will make it detonate the fastest. Running it slightly richer will reduce the temperature slightly, but not a huge amount and at some point its too rich and misfires or starts washing the cylinder walls.

[Bdpprocessors]

Afr did not go richer than 12.3. Which is why im left thinking there is more in the setup on pump gas Atleast safe power

My trans am is 12.5 from 0-3 psi
12.0 from 3-6 psi
And 11.6 from 6-12 psi

[eficalibrator]

Yes, adding fuel to a lambda below ~0.90 will progressively cool and slow the flame front which may help in knock resistance with all else being equal. I.e.: going from a target (and delivered!) lambda of 0.82 down to 0.79 should allow you to add more timing before you encounter knock, perhaps getting closer to MBT timing without losing much due to carbon balance (AFR control). I tend to err on the rich side, knowing that fuel quality will change from tank to tank and I like having more safety margin to the actual knock limit.

[LSxpwrdZ]

Cylinder pressure and ignition timing have larger affects on knock/detonation than just fueling. Too much ignition timing and it will generate more heat in the cylinder as well and will develope hot spots which will auto ignite the pump fuel much easier than a higher octane fuel.

This is one of the reason's E85 is so famous because it keeps those hot spots to a minimum due to it's natural cooling properties. It's not that high of an octane however compared to C16 or the like.

Airflow is key, and the only way a smaller cubic inch engine can make the power is RPM and forcing more air into the limited cubic inch of the motor. Theoretically a 1.8L honda running 200kPA would be equivalent to an extent to a 3.6L NA engine running 100kPA. Just remember every 100kPA is an atmousphere so you basically double your cubic inch every atmousphere you force into it. Being 4 less cylinders and the capability of about 2000rpm more the Honda's are generally much more efficient at producing power than a V8 pushrod engine.

[BigDaddyCool]

sometimes in the dyno trying different target ( AFR's) will get wide range of results ..u shoot for 12.8 on ls7 ..u get 480..then u try 12.2 u get 490.. assuming same timing ..

so u think richer is better try that on ls2 ..it does the opposite .. some times different combination will like different targets..i know that is not what u asking but to my limited knowledge i would say cylinder peak pressure plays a big rule on determining peak HP with certain fuel mixture and fuel grade as well some combination can tolerate 93 octane some dont

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agree'd leaner = more power....... but also more risk of detonation due to temps.

ATR is a very hard thing to nail down. There are pros and cons to leaner vs richer....

Richer is generally safer and compensates more for weather changes. Plus you can generally squeeze more timing in at the top end. That said, I have run a very lean ratio ........ and it made a tonne more power with lower timing.......... so tough one