IAN

iTrader: (3)

I was just wondering since I'm not all that knowledgeable on timing:

My car was tuned for 15psi but we did hit a few 18 to 19psi on the dyno. My question since I've hit this a few times on the street is that the timing map stays at 14° all the way from 14psi to 30psi from 3000rpm till 10,000rpm. I was wondering if this is ok?

This is on a haltech E6K t04b 60-1 etc etc.

Thanks,
Ian

pistonsuk

If you can maintain the same timing at 19psi as you do at 15psi and your not detonating anywhere then your timing at 15psi is too conservative.

Justin

IAN

iTrader: (3)

Actually looking at it again it is at 15psi these values:

5000rpm= 14°
6000rpm= 14°
7000rpm= 15°
8000rpm= 16°


I guess then I'm safe I will leave it at that.

pistonsuk

This is showing an increase in timing vs RPM, not boost. This is due to the drop in VE of your engine as you give to less time to fill.

Justin

IAN

iTrader: (3)

Sorry. i'm pretty vague.

At all boost levels above 14psi these are the timing values.

thanks,


I seriously need to pick up a book on rotary timing.........

GUITARJUNKIE28

if you haven't tuned for the higher psi, ramp your timing way down and put it back on the dyno. you might be able to get away with hitting 18-20 psi a couple times for a couple seconds each time, but have you held 19si to redline and watched the egt's?

pistonsuk

The goal is for your combustion pressures to peak about 17ATDC, this is where the pressure can to the most work on the e-shaft. The onset of detonation is your design variable. Assuming a constant octane, both heat and pressure are the factors that initiate detonation.

Pressure comes from:

1) compression ratio: This is the mechanical act of compressing one volume to another. The greater this difference the greater the pressure.
2) manifold pressure: When this is raised more air enters the chamber to be compressed, and pressures rise.
3) heat: Heat expands what is already there, this increases pressure.
5) VE: This includes inertia effects among others. Basicly, wherever your engine makes peak torque is your engines highest VE. This means the chamber is receiving the most air possible given the manifold pressure. This will also yield higher pressures.

Flame Propagation:

The entire reason for ignition advance is the burning of the fuel takes time. So you must start the burn before 17ATDC to get the peak pressure here (at higher rpm you have less time to get it there, hence more timing advance). Flame speed is dependant on:

1) turbulences of the mixture: Turbulence helps speed it up.
2) AFR: AFR of max speed is around12-13AFR, due to highest probability of every air molecule being next to a fuel molecule
3) shape of the combustion chamber: symmetric, small chamber centered around the spark plug is good. Minimal flame-traveling distance is good.

Another key aspect associated with rotaries is the intense squish flow that results from the fact that just after TDC the trailing area combustion chamber is decreasing in size and the leading is expanding. This mass flow actually keeps the flames from propagating fast (or at all temporally) in the trailing direction.

In short rotaries have #1 going for them (but only in the leading direction), our AFR’s are usually a little rich for max speed (leaning, then cooling with water would help), and our combustion chamber shapes suck due to it length and a large quench area (reduces flame speed, due to cooling) in the trailing end.

Timing:

The reason timing is pulled as boost increases is that the sum of advanced timing (which adds pressure from the heat of an early burning mixture) AND added pressure from the manifold exceeds the detonation threshold. For the same reason the most retarded timing is usually centered about the engine’s peak VE (and can sometimes be bumped after peak VE, sometimes not due to the added heat which also initiates detonation). Again higher compression ratios generally have less advanced timing.

Heat:

Heat is generated in every combustion cycle. The removal of heat takes time, therefore at higher rpms the surfaces of the combustion chamber will increase due to more often combustion events. This temperature rise can approached the detonation threshold. Compression (of any type, from either the turbo or engine, i.e. compression ratio) also introduced heat (that’s why we use IC’s). Heat can also be more intense with leaner mixtures (up to 16-17AFR, then is deceases).

As you can see there are a lot of competing factors. Good luck.

Justin