I stand to be corrected by tuners much more competent than me to answer you - but in my experience, comparing dyno numbers is a bit like comparing hp over a few beers. I've seen different and same dynos from all over compared and the chances of getting identical readings are almost nil.
I'd be more concerned if your performance was down, or if you went back to same dyno and got corrected figures down on before.
Bench racing is a lot of fun, but if you want accuracy you gotta go back to the same place for subsequent readings - and then there is still a margin of error.
So often guys will get their cars dyno'd and get a real good number, go out and run some times, then decide to go quicker, add a few mods, but go to a different tuner - maybe because now they are getting more serious so they go to a shop with a good reputation, and they come out with less power than before! But they go to the track and run better times and run more mph!
Mph between two points will give you the only real world reliable guide as to whether you are improving or going backwards - and that will change with the weather as you're aware.
Another point - on top of the fact that different dyno's give different readings - one dyno operator - wanting to impress - may spike the engine to a very impressive number. The other operator has a holding policy over a certain time so power must be maintained for a number of seconds before taking the reading.... that will always be a lower figure. Not saying that happened to you - but it does happen. All guys want to see is a big number, and a dyno operator can give it to them if they want to
My advice is to go find a bit of road you know real well and make a few passes with a passenger running a stop watch. Choose a gear and a speed range and nail it. Record the end speed over the elapsed time. Do it a few times and average it to get a baseline. Note the aprox weather. Now you have a benchmark.
I used to use 60-75mph as it was easy to do without getting caught, and I could run the stopwatch myself.... maybe not the safest way - but I had no real problems. I could then compare mods done and the affect of weather. My best time was 1.5 seconds on a cold night (turbo's love the cold). You'll beat that in your Viper I'd think. My range used to be from 2.2 to 2.5 seconds pre-mods to 1.5 to 1.9 seconds post mods - depending upon day (and how good a grip I got on the stopwatch button while driving
A wider speed range will be more accurate, but try to use one gear to avoid other variables like gear change speed differences etc. You'll get a real good feel for the car over a time period in different conditions, then you will be able to measure changes you make. Its a lot more fun than using up your motor on rollers too
I like dynos for tuning - for taking a base number and improving it that same day. Thats their true place in my opinion. The real power is what it does on the road or track, and there's a lot of "lower" powered cars beating "higher" powered ones because the power figures are not relative to eachother. Hope that helps.
