Savington

iTrader: (31)

You treasonous *****

Ben

iTrader: (33)

I'm saying it's possible that 2 scenarios can make the same power

• Scenario 1 is slightly leaner and has a little less advance
• Scenario 2 is slightly richer and runs a little more advance

I have read arguments from respected tuners and engineers over which approach is best, but unless the car is running enduros my preferred approach is slightly rich and slightly less advance of peak torque. Most people I speak to gladly trade off a few hp for added peace of mind.

JasonC SBB

Agreed.

You would have MORE torque if the burn rate at 12:1 was faster than at 11.6:1. However your description sounds like 11.6:1 was faster.

Faeflora

iTrader: (2)

Assuming you're not detonation limited. Just wanted to mention the caveat

JKav

iTrader: (1)

All other discussion aside, leaner + less advance = more higher EGT in your cornhole

y8s

iTrader: (8)

OK Mister turbo engineer type... does that higher EGT spool your cornhole any Harder Better Faster Stronger?

Or maybe the question is: is peak cylinder pressure worth more power than high turbine inlet enthalpy?

JKav

iTrader: (1)

yes, higher inlet temp for turbine will result in somewhat lower exp ratio (lower exh man press). but not enough better to be worth the much higher head temp.

example - 1.8 liter at 4000 rpm, 18 psi boost, smallish turbo, lots of assumptions (VE, i/c parameters etc etc)... assume 1600 deg F EGT and 28% flow wastegated. If EGT is increased to 1700 deg F, backpressure drops by less than 1 psi.

-1 psi not worth +100 deg EGT

JasonC SBB

Turbos aside, I do believe that fastest burn happens at some theoretical AFR (12.5:1?) which then yields lowest MBT and most torque.

Savington

iTrader: (31)

I think burn rate is dependent on way more than just AFR (specifically, chamber turbulence has a HUGE effect).

y8s

iTrader: (8)

Most cars can't do much about combustion chamber shape while they're running. Some can change tumble and swirl characteristics (2001 miata!) and some actually have variable chambers (Some crazy Saab motor) but I think we're comparing similar operating parameters... for example: 6000 rpm on a 99 miata motor without vvt or VTCS.

changing spark and fuel a few percent wont really affect chamber turbulence, will it?

JasonC SBB

As I said earlier
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The reason is that later spark (at MBT) means less work goes into compressing the charge up to TDC as the burn begins to raise cylinder pressure.

Fast burn combustion chamber design is THE main reason more modern engines make more BMEP (more torque, see my other thread ) than older (e.g. 1960s) designs. The old-school "hemi" chamber designs were bad (with domes pistons and thus an orange-peel shaped combustion chamber), later design pentroofs with narrower angles between the valves and flat-top pistons for the same compression ratio, were much better. Swirl also improves burn rate. Pentroof 4-valve combustion chambers gained as much from the fast burn chamber shape as they did from the improved hi-RPM VE. Modern LS motors show the same compact combustion chamber (short distances from the plug to the farthest corners of the chamber), as well as strong swirl.

Again for the particular motor Sav was tuning, fastest burn may have been at said indicated 11.6:1, due to perhaps an off-cal AFR gauge, lopsided injector imbalance, or huge injectors injecting short squirts at a non-optimal point in the cycle.