94GreenMachine

iTrader: (16)

I see a lot of people in the engine tech sections talking about different head and cam and intake and exhaust set ups making lots of different hp#'s and lots of guys with nearly the same set up's making drastically different dyno numbers and blaming their tunes and a whole bunch of other stuff but I occurred to me that in an m6 car for example with 3.42 rear gears, 4th gear is the 1:1 gear, wich I have been told is the gear that you would do your dyno pull in to get accurate #'s. Now, the question is: going up to 4:10's in the rear as i have for example, won't that change my final gear ratio to where 4th is no longer 1:1 and as a result, won't I get higher and possibly false dyno numbers if I pull in 4th? By that logic, would I then need to dyno in 5th and can somebody throw in the calculation for what your ratios would change to with different gears, etc.?

thanks in advance for making sense of the ramble and adding your $0.02

Carter01

iTrader: (11)

Can't answer all your questions but I will try to hit a few high points. Changing you rear gears is not going to change your 1:1 ratio in 4th. All previous cars I've owned making from 280 rwhp to 480rwhp and torque in the 500s have been dynoed in 4th. I have never tested this but I've heard people actually loosing hp with the lower rear gears. I think it has something to do with having less load on the engine than you would with a taller gear. If you look at most of the turbo cars they all run taller gears that way the car stays under a load in order to keep their boost up. As far as the all the different HP #s you see from similar setups is very possible due to tunes and mismatched combos. I've seen stock vechiles pick up as much as 35hp in just a tune and some only 10hp depending how close the stock tune was for the specific combo. If the engine is n/a or f/i you will also see drastic number differences in the tunes as well. Where a decent tuned n/a car may pull into a shop, get strapped down and a good tuner may find 20-30 rwhp. That would be considered a respectable amount over whaat you had. Now car #2 FI pulls into the same shop with a decent tune might pick up 50-80 rwhp depending on the tune and boost. There's so many differnt variables to add into the equation when tuning so you will get so many different results. Just my .02!