Inquisition

Read what I typed. Everyone else is half correct. I'm about 80-90% correct. Although I will agree with slo_crx mostly about the turbo spooling. at 0 inHG, I wouldn't be at 14's AFR and feel safe(would you feel safe if your NA engine ran that lean under that load??) but I do agree running a bit leaner there and retarding timing will increase spool time. 5-10ft/lbs in the midrange on a 300whp engine is a significant increase. Higher compression makes up for the shitty VE of a D-series engine.

Atticus

ya i most likely will go low comp maybe even try for faster spool with the afr's
but like i said i wanted to see what you guys thought and go from there

fe3tcourier

alone, that statement is untrue as a blanket statement.

this kinda gets to the point though

for a given two comp ratios and a fixed octane, if you run through and plot power vs boost, there will be a crossover point where the retard you need to avoid knock moves pcp too late in the stroke to be useful. at low boost high comp gives more power, and at high boost, low comp gives more power (if high comp can survive at all). somewhere there is a crossover point.

in the gsr example above, the 9:1 should have been running 15psi safely and easily whereas a tight tune would be required to make 15psi on 10:1. along those lines anyway. at 15psi, on the same pump gas, you would probably have found that the 9:1 engine would make more power than the 10:1 on pump gas and with a tune that wont ping no matter how hard it is flogged. ie, a safe good tune.

the spool difference is going to be fairly marginal, whereas the power increase from the higher boost allowed by the lower comp will be significant. that is what it comes down to.

evo_lucian

well said fred

Tough-guy

Wow, I think some of you have some serious misconceptions about how forced induction actually works.

When the charge-air density is increased (Via boost) The Effective Brake Mean Working Compression is increased. Example:

A 6:1 static comp in a 100cc cylinder in an atmo engine is still 6:1

A 6:1 static comp with a charge-air density of 1.25 (3.675psig) will have an brake effective C/R of 7.5:1 under boost

A 10:1 static comp in a 100cc cylinder in an atmo engine is still 10:1

A 10:1 static comp with a charge-air density of 1.25 (3.675psig) Will have a brake effective C/R of 12.5:1 under boost

A 12.5:1 static comp atmo 100cc cylinder is (once again) 12.5:1

A 12.5:1 static comp with a charge-air density of 1.25 (3.675psig) will have a BME C/r of 15.6:1

Cliff notes: The higher the static compression the higher the effective working C/R during the power stroke

Note: Temperature is a function of charge-air density, and obviously the adiabatic process comes into play here, however the system is squishing the air pre-induction charge then further compacting it in the cylinder. The harder that a charge is compressed, the high the average cylinder pressure will be, whether the charge is less-than-atmo , atmo, or positive atmo(boost). The result will be increasingly greater average cylinder pressure the higher the boost though.

The higher the static C/R is though, the less margin of safety the engine will be able to enjoy. E.G. High boost+high C/R+hot day = engine.

My bike's static compression is 11.8:1 and I've run up to 12 lbs on a 98o day in Tucson, AZ. so...I think you'll be alright with a slight bump and moderate boost levels.