Timing and knock with nitrous

[importracer150]

I just finished installing a 80hp NOS direct port setup that I got from Nathan at DSMotorsport. Its actually the same kit he posted pics of in the forums a few weeks ago. We're heading to the track on thursday and am looking for some help with tuning.

We have been tuning the car to run about 22 degrees of timing with race gas. So far with 720cc injectors my knock counts are almost nonexistent. Should I be tuning to the same thing with the nitrous? Thanx for the help guys.

 

[NosLaser]

...We have been tuning the car to run about 22 degrees of timing with race gas. So far with 720cc injectors my knock counts are almost nonexistent. Should I be tuning to the same thing with the nitrous? Thanx for the help guys....

Nitrous is quite a bit different than what you are used to. While the chemical composition N2O will actually act as a compression buffer (as opposed to pure oxygen) it will still cause a much quicker cylinder pressure rise than a turbocharged engine. Combine the two, and pressure can get huge. With nitrous, you can actually make more power at lower timing than with just a turbocharged engine. You want to start off small, and work your way up, but don't work too far. Set your base timing back to 5 degrees BTDC. Ideally, in a nitrous application, you want peak cylinder pressure to occur with the piston roughly 10-15 degrees ATDC. Picture hitting a punching bag. Your fist is spark/explosion/combustion, and the bag is the piston. Punch the bag, and start it moving. If you hit the bag while it's coming at you, it will likely stop because the force of it swaying is meeting the force of your fist punching. Apply this to what's going on in the engine. The piston is coming up, and then combustion is forcing the piston back down the hole. The only thing driving the piston upwards is the inertia from the crankshaft. This can have adverse if not detrimental affects on your rod, and rod barings, not to mention even the main bearings when you are dealing with high rpm and cranskshaft deflection. Same scenario, you have the punching bad moving, now hit it at the peak of it's travel. The bag will sway away pretty well. You are timing combustion to occur at TDC, which is okay, but there is more power to be had. Now, back to the bag; this time, hit it just as it starts to swing away from you. The bag goes flying! Imagine combustion occuring at 10-15 degrees of crank rotation ATDC. The crank is already pulling the piston back down, and combustion is going to force the piston down even faster. You have the pulling force of the crankshaft, AND the force of combustion both working for you in the same direction. You are basically taking advantage of the leverage offered by the crankshaft as cylinder pressure pushes down on the piston. This is why over-advanced ignition timing even in a normally aspirated engine makes less power because the combustion process is so early that the engin is working against cylinder pressure as the piston approaches TDC. Nitrous is going to greatly increase the burn rate in the combustion chamber so retarding the timing can actually yield more power. Start off slow, and work your way up. Make a pass, look at the plugs. Any hot spots? Any flecks of detonation? Your timing may be too advanced if so, and you may want to retard it a bit. never underestimate the power of reading your spark plugs, as they are the only true window into the combustion chamber. I can get into spark plug projection, and all sorts of things, but this should get you going for now. I hope this helped.

Regards,

 

[Turbo Guru]

Nice explanation, well said.

Joe

 

[importracer150]

WOW. I'll try setting the base timing to 5deg BTDC and go from there. I really appreciate all of the advice. I'll let you all know how things go at the track on Friday.

 

[NosLaser]

Good luck! Let me know how it turns out.

Regards,

 

[importracer150]

Aslan, I really appreciate the advice. We went to set base timing tonite and it was so far advanced you couldn't even see the mark with the timing light. We retarded it back to 5deg BTDC, so we'll see how it goes at the track

 

[NosLaser]

Wow..I'm glad you checked that. Well? It's Saturday! I know it's early, but how did you do??

Regards,

 

[nano]

I have a 2g so I can retard my timming. But normally I get about 21 degrees on race gas and 20psi. But on the 75 shot and race gas still at 20psi I can only get about 17degrees of advanced. It seems to work fine. Hopefully it's not hurting anything.

 

[NosLaser]

We want updates Geshdurnit!

Regards,