At a local drag racing event recently, we helped one of the racers chop 0.3 seconds off his ET slip with a correct addition of a low percentage of nitro. The racecar was an early model, low compression Hemi drag car with a small 6 GPM fuel pump. It ran solid 7.6 seconds in the quarter mile. The owner / driver said it was close to the limit on methanol. He had a few gallons of nitro to pump up the power. Rather than guess how much to run, we did an analysis using our resources.
Engine Analysis: His combination was entered into our ProCalc jetting calculator. His air to fuel ratio on methanol in this low compression, 6-71 blown Hemi was at 4.3 to 1. That would be lean for a higher compression blown engine, according to our new nitro book. The main bypass jet was removed from his jetting profile in the calculator. That was done to determine how much more fuel could be put into the engine. With more fuel into the engine, enrichment from some amount of nitro could be done. ProCalc was used to determined the air to fuel ratio (AFR) for various percentages of nitro. That took only a few minutes.
Nitro Addition Analysis: The AFR requirements for different percentages of nitro were determined from our new nitro book. I asked if he preferred measuring the nitro by weight or by volume. He preferred measuring it by volume. At 10% by volume, info from our nitro book indicated a 3.8 to 1 air to fuel ratio ‘on the pump’ for his combination. That took a few more minutes.
Good Combination: With 10% by volume nitro entered into ProCalc with all the fuel to the motor, his air to fuel ratio was 3.8 to 1 from his fuel system. That was the cross over point. If he ran more nitro, his engine would be out of fuel pump and too lean. If he ran less nitro, his engine would be too rich. That could cause a power reduction over straight methanol. All total, we spent about a half hour sorting that out.
Fuel Pressure Analysis: ProCalc was also used to check the fuel pressure increase with the removal of the bypass. This check determined that the pressure increase would be about 10 psi more than the straight methanol setup with the main bypass. That was still within the fuel pump rating.
Successful Result: He removed the bypass jet, added the the correct amount of fuel mixture, and ran effortless 7.3 sec ETs with no reported damage. Cost data in our nitro book revealed an increase of $45 in added fuel costs per round. Not bad for a 0.3 second drop in ET!
Even More Power: Using ProCalc, we examined how to turn the power up if necessary. To do this, we looked at dropping the blower overdrive. That would allow an increase in nitro percentage. With 24% less blower overdrive ratio, 15% more nitro could be run on a safe air to fuel ratio at a different cross over point, within the limits of the fuel pump that was installed. The power would be down from a slower blower. However, it would be up from more nitro. Using our BTU’s table in our nitro book and adjusting for a slower blower, the power would still be increased further by about 7% over the previous lower percentage of nitro that was successfully run. Although the owner / driver did not have to run this combination, that would drop the ET another tenth or two if needed. That took less than a half hour to determine.
It demonstrates that blower overdrive and nitro percentage can be juggled in virtual reality with ProCalc to get the most power within the limits of the fuel pump and the fuel injection jetting setup. This combination represents almost a half second drop in ET. According to a cost table in our nitro book, that percentage would be about $62 more in added fuel costs. Where can you go anywhere on earth for $62 a run and get a half second drop in ET? Try “Blown Nitro on a Budget”.