Cylinder pressures juicing at low rpm. What is and what isn’t ‘safe’ and why...?

The patient – 355, 11.5:1 comp, solid bottom end, ported LT1 heads, LT4 HOT cam……

The juice – compucar wet plate, 175 shot, Jabobs NMM.

Previously we’d launched at 1,000rpm on our 2,400 stall and 3.42 gears and got a 1.59 60ft with the NMM set to come on at 2,000rpm and reach 100% at 2,500 on a 150shot and 6-deg of retard. I personally felt this was safe. Some have stated this is spraying too low due to the high cylinder pressures generated. I know the potential dangers of spraying at low rpm, but how low is too low and what factors influence this….? I have seen countless folks spraying 150 shots on heads/cam, stock bottom ends from the line (off idle) and having zero known problems. I would just like some help determining the comfort-zone as far as cylinder pressures are concerned when spraying at a particular RPM. I don’t want the pins, bearings, crank, etc reduced to matter that filters past a kitchen strainer.

So, what can I do here...? We have good rods and heavy SRP pistons. I shift at only 6,100rpm so we’re not running the thing to death. Can I start the spray at a lower rpm or should I leave it alone on the 175shot…? I do not like hitting the whole deal at once because of the shock to the drivetrain and chassis. We’ve made several upgrades here, but I do not want to come out trying to set the world on fire. I do, however, want to get this turd moving and run a good number.

Would launching off-idle (700-800rpm) and begin introducing the spray at 1,500rpm be okay or should we try launching at a higher rpm. Say we brake it to 1,500rpm and leave the nitrous coming on at 2,000 and reaching 100% flow at 2,500. I do not want to blow the tires away (11.5 ET Streets) with too much too soon. I prefer to keep the car easy to control, but am experienced enough to ‘drive’ it if I have to though I am fairly confident (no comment…there’s always an element of ‘what if’) the car will stick and go straight.

many thanks....

 

You might want to see what your convertor flashes to under a heavy hit of juice.... it's probably going to be a lot higher than the advertised "stall". If it flashes high enough, you might be out of the woods. I use exactly that approach, activating the juice at less than 2,000rpm. With a 275-shot, the convertor flashes instantly to 5,000rpm. Yet running NA, it won't flash over 3,000rpm.

In any case, you will get more responses to this on the "N2O Tech" forum than on Advanced Tech.

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Fred
94 Formula A3+1: 381/TH400+OD/N2O

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11.513@115.59 on motor; 11.162@127.67, 1.643 60' on a 125-shot. Going with a 275-shot this year