Changing Combos- From 871 Std. Helix TBS to 871 Hi-Helix Retro Littlefield-Need Help

Hey Fellas, new to the sight and I must say this is the best thing since sliced bread! Running T/D and changing combos now. Going from a 509 BBC (12:1 cr), 871 TBS (STD HELIX) Bird, Big Brodie's, BDS Intake with Hat and port nozzles, 110 enderle. Had Brendan Murry set up fuel system, worked great! #47 in hat, #36 ports @ 30% over. 155 main, no hi speed..Ran high 7.30's @ 5800' in 2200lb Altered. New combo going in 1800 lb Dragster: 548 BBC, BME 6.8" Rods, J.E. 12:1 Pistons, Callies 4.250 crank, same heads, cam, intake. Went to an 871 Littlefield Hi-Helix Retro...Going to start at 30% as well. Should I fatten the nozzles up or just fatten the main to get a baseline? Don't want to lean it out... Any help would be great. With the old combe..(knock on wood) I ran for 3 seasons and never hurt a thing... Thanks in advance.

 

be careful starting it

We set up a new combo last year. Ran 5% through 30%. However, we dropped the compression down to a little over 8 to 1. We used our Pro-Calc to determine nozzles & jetting for different air to fuel ratios for the low speed, air to fuel ratios for high speed, holding hat to port fuel split, fuel distribution around the port nozzles, fuel pressure, & high speed opening point; making nozzle & jetting selections from our inventory. I would not want to try to do all that trial and error. The numerical control made it easy to do without hurting anything.

At 5%, the nitro showed it presence. Idle speed went up 1,000 RPM. We readjusted for that. Our first run did some minor plug damage even though the overall mixture was fat. We redistributed the fuel to those holes. And kept the air to fuel ratio in control. Every percentage made more power. We went up in steps, and did not hurt anything else. Even close. I am convinces a low percentage in a blown alcohol engine is the cheapest way to more power.

In your setup, starting it up with 12 to 1 compression & 30% may be a big problem. You run the risk of detonation on startup. There may be lean cylinders as fuel pressure builds. Then compression heating may cause the fuel to go over the self ignition temperature. That is a possibility on start up before fuel volume builds up for cooling. Then off it goes. I believe top fuelers start with nitro in the lines. That would be since the mechanical compression is real low.

High compression AFDs must start on methanol with no nitro in the fuel lines. Then cross over to nitro while the engine is running. If it stalls, it goes back to the pits. To empty the fuel lines and get it ready to start on methanol.

Bob Szabo
Pro-Calcs
www.airdensityonline.com
www.racecarbook.com