left field tune-up?

[1320quick]

So I'm looking for thoughts and opinions on this one. I've been having problems getting my car to respond to my nitrous tuneup so as a last resort, I decided to run it on the wheel dyno yesterday. This is a gasoline 548ci bbc, 14.1 comp, Dart 345 pro1's, King demon 1190 and a fogger with .032n and .028f jets. At the start of the day, had main carb jets at 90 squared, power valves blocked and timing at 36 degrees. First pull on the motor, a disappointing 505hp. The wideband o2 sensor showed the a/f to be 15.10 to 1. I didn't think that could be possible so they calabrated the wideband, double checked it on another car and we made another pull. Same hp and same 15.10 a/f ratio. Plugs looked lean. We deciced to change the jets to 99 square. A/F improved to 13.2 and picked up 25hp. Ended up with 103 squared in the carb, the a/f at 12.85 and 555hp to the rear wheels. Plugs finally started showing some color. Then we made 8 nitrous passes and ended up going from almost 11psi of nos fuel pressure to 5.2psi. The a/f ratio on the bottle shows 11.2x's which still should be a bit fat. The plugs look a touch rich still. All in all, we picked up 50hp on the engine and 75hp on the bottle. The last dyno pass of the day on the bottle was 859hp at the rear wheels. Cold Fusion has been telling me that 5 to 5.5lbs of fuel pressure should be fine but the car would not go 20 feet without backfiring. I has to have the fuel pressure at 10+ psi to get the car to make a full pass. Could the motor tuneup been that far off and I was using the nos pressure to makeup for it? Anyone else use main jets sizes over 100? If the motor was that lean, how did it not hurt itself?

[John_Heard]

From what you've found on the dyno it certainly sounds like the motor was way lean, which will affect the nitrous tune up as well.

Cylinder heat is a bell curve, as you remove fuel temperature climbs to a point and then start dropping again after you move past peak temp so yes you can be very lean and still not hurt things. However putting fuel back into the engine at that point is going to heat things up before it cools down again. Without a chart this is kinda hard to explain, but if you do some searching on Aircraft Cyl Head temps and monitoring you'll find a lot of info on this. There's a company in Oklahoma that has done some amazing work in that field.

I have not had to run that much jet in my Holley dominators, they are usually around 88-94 or so depending on which one and the air.

[John_Heard]

This is a very good read fellas, I found that article I was talking about fuel ratios and cylinder temps. I have had some experience that verifies what this dude is saying he's on the money

http://www.gami.com/future.html

[ERV JR]

What kind of pump are you runing for the nitrous ? Is the pressure stable with the system on ?

[1320quick]

What kind of pump are you runing for the nitrous ? Is the pressure stable with the system on ?

I'm running a Product Engineering 460GPH pump for both the engine and the nitrous. It's -12 from the cell to the pump and then -10 to the front of the car. When the car is going down the track, I can't watch the fuel pressure guages but yesterday on the dyno, I was able to watch very very close and the pressure is stable on both the engine and nitrous sides. for the entire pull. Neither guage moves at all when the nitrous is activated, they both stay rock solid.

[ERV JR]

the pic of your car looks alot like a friends old car, have you had the car for long, it looks like Levi Hanna's old car , a psca wild street car, or maybe im crazy

[1320quick]

I've owned this car since 1992. The previous owner was a smoker and had tried to toss a butt out the window and it landed in the back seat and caused a pretty bad fire. That car was pretty much a mess when I drug it home.