Engine/Transmission (1994 - 1998) higher timing.....why not?

[NBowlin]

how come the majority of people seem to stop at 16 or 17*?

whats to stop a person from running more?



i have been calling around and its seems that 22 and 23* seem to be the answer i am getting. now i was told as well that on some of the straight pulling trucks they run upwards of 50*.



just wondering what the down side is. thanks

 

[RustyJC]

To oversimplify, the diesel fuel injected into the cylinder takes a set amount of time to burn, regardless of engine RPM. For maximum efficiency, engine designers generally try to achieve a peak pressure angle (aka PPA, the angle of crankshaft rotation when peak pressure is reached in the cylinder) of about 16-18 degrees ATC. If you back up from this by the injection and combustion time required, then the timing for the start of injection can be established.



Since the injection and combustion time is constant, but the RPM is not, it's obvious that the higher the RPM, the more injection timing advance will be required. Those pulling trucks that are running 50 degrees BTC advance are also spinning the engine 4500-5000 RPM or more - they have to start injecting the fuel that early to reach the 16-18 degrees ATC PPA.



Now, try 50 degrees BTC injection timing at 2000 RPM and the PPA will be reached far too early - maybe even before the crank and piston reach TDC. This will (1. ) produce negative work and (2. ) excessive cylinder firing pressures. That's assuming there's even enough temperature rise due to compression to light off the mixture that early in the compression stroke!



Rusty

 

[Green Machine]

I disagree rusty. 17 degress is the same no matter how fast its turning.

 

[Jim Fulmer]

15-18 is fine for a street truck, 20 if you have an O-Ring to control the cyl pressure that is raised with the increased timing, now when you get past 20 with say 370 injectors you start loosing bottom end... ... . later spooling. With my current EDM's I ran them at 20 then raised it up from there and notice very little if and bottom end loss, why, different spray angle that there set at, bottom line was I asked my injector guy what timing I need to run as maximum and went with it.



Jim

 

[Vaughn MacKenzie]

There's certainly a number of factors affecting timing. What RPM range you operate at certainly plays a role.



If you aggressively advance timing it becomes more difficult to start in the cold. It may give you better mileage up to a point. Another thing to think about is if you start injecting too soon the piston is further from TDC and there is less pressure and not enough heat to light off as soon, so when it does ignite it causes more rattle . . . and more of a wasted pressure spike.



Green Machine I'm not sure what you mean. Yes 17 degrees is always 17 degrees at any RPM, but injected fuel takes a certain number of microseconds to ignite. So at higher RPM more engine rotation occurs from the time injection starts until fuel ignites. Thus the benefit of more advanced timing for the pulling guys. Make sense?



Alas the P7100 is not a perfect pump, due to its fixed timing. That's one of the few advantages of a VP44. . . timing adjustment based on RPM.



Vaughn

 

[Bob V]

You want to inject the fuel in the combustion bowl. Some of the folks running more advance have had work done to lower the tips in the head. Also, the 215 pumps do have some advance built into them.

Bob V

 

[RustyJC]

I disagree rusty. 17 degress is the same no matter how fast its turning.

Please see Vaughn's comment - he's right on the money. It takes a certain amount of time as measured in milliseconds to inject and burn the fuel. The number of crankshaft degrees of rotation that take place in those milliseconds will vary as a function of engine RPM - the higher the RPM, the more crankshaft rotation, and thus the need for more timing advance. That's why gassers had centrifugal advance weights in their distributors back in the "breaker points" days - as RPMs went up, timing advance went up.



Rusty

 

[Green Machine]

Thanks for the info I am a wiser man now.