I assume this is a simple basic under carb plate kit we are talking about ??
I hope you have a good head seal & good set of head gaskets too...
A 75hp N20 shot should be relatively safe, as long as
if you have the needed added extra fuel supply
of 5-7psi flowing 70gph minimum for a smallish 50-75 HP plate kit...
you don't want to go lean ever !!
{flowing & not dead headed is important, you need to learn to tune it off the car flowing 1st}
with a stand alone dedicated appr. sized fuel pump & appr. sized fuel lines
{engine/carb & N20 fuel/gas supply should be 3/8" (line & pick-up) or #6AN min. preferably}
for the N20 system & specific hp that's being added...
& also a stand alone dedicated fuel pressure regulator {bypass style preferably, not mandatory thou}
for the N20 added "gas side needed" of the system & an accurate Fuel pressure gauge inline
& also the proper jetting/settings for the fuel & N20 side of the N20 system is all tuned/matched properly...
But
the 150hp N20 shot is pushing it slightly, especially with cast pistons, rings, bearing, stock rod bolts etc.
{especially if your 100% not sure of what you actually have},
I've seen it done thou, you can run it a little on the fat side, to be a little safer like
example 6psi of fuel pressure "flowing" instead of 5.5 psi etc.,
not kill allot of the extra power, but be a little safer/fater, no on the ragged edge...
A purge or solenoid valve, teed before the N20 solenoid,
to get full pure N20 {not just compressed warm air}
at the N20 solenoid, is a good idea too, purge before activation/run,
or the 1st hit each time, will be fat/rich & slow coming on, not work as well...
I'd suggest you back off the timing about 2*-4* for every about 50-75hp added,
as the N20 is added too activation/button or switch is pushed,
helps to save parts & study up on how & why you tune it etc...
I'd also highly suggest that you get a book
& study up on N20 tuning/applications & or read up on it "allot" before you buy & setup a system
it's not really that tough, not all that unsafe either, if done tuned properly {especially on small kits}
there are other safety stuff you can add like a full throttle switch or a hobb switch etc.
if you have the proper stuff,
spend a little more now, or you'll spend allot more later...
the N20 fuel/gasoline side will seem to fatten/richen up as N20 bottle pressure goes down
{especially as the bottle goes below 900psi}, more gas to N20 %'s
keep the bottle warm & it will stay up-to pressure longer
{N20 is 109* below zero with an oxygenator @ 1200psi, boost/blower in a bottle}
you'll get more consistent hit's out of a bottle that way too,
with a 75hp shot probably/possibly get 3- maybe 5 "full" 1/4 mile hits relatively safely IIRC,
then refill the N20 & carry extra "full" N20 bottle if you plan on using it allot...
good luck
