View Full Version : tune car with 3.42's that is going to 4.10's

11-04-2006, 06:52 PM
well just wondering because a tune is set up basically for what the next rpm will do at a certain time. if i switch from 3.42's => 4.10's will the rpms go thru too fast and ultimately change the tune? causing it to run lean?

not worried about the gear change in the computer my buddy can do that in the computer in like 5 minutes im more worried about the tune.

99 Z-28 with zo6 intake, intake, headers, exhaust, no cats, and t-rex cam

11-05-2006, 08:25 PM
not one person?

11-05-2006, 09:16 PM
tune shouldnt be affected much.

11-05-2006, 09:23 PM
It won't matter at all what axle ratios are in the car when your tuning.

11-06-2006, 07:00 AM
The engine performance is independent of the differential.

11-06-2006, 07:41 AM
I wouldn't say it would be completely independent of the gear ratio selected...your tune depends on how the engine is loaded, and a higher (numerically) gear ratio will change the perceived load by the engine. If anything, it may just mean that you'll be hitting cells that you normally wouldn't, and those will need to be fine-tuned more. But I don't think it will be enough to amount to anything...FWIW, when my 3.42s go to the 4.11s in my garage, I don't plan on doing a retune (other than the speedo). I'll see how close it is.

11-06-2006, 09:33 AM
You should not have to retune, however, you may be able to run a little more timing advance due to the less actual load to your engine which may give you more power. It all has to do with changing the acceleration rate of your engine per a given MPH. Stated another way, if you were going the other way and you were at the ragged edge of detonation, you would have to retard the timing a little.

Gary Z
11-07-2006, 01:50 PM
a higher (numerically) gear ratio will change the perceived load by the engineThis seems like it makes sense, but it doesn't.

11-08-2006, 12:27 PM
This seems like it makes sense, but it doesn't.

Yes, me to, please enlighten us. I agree that the engine has less load at given MPHs, but to my knowledge, the PCM does not know or compensate for this, or does it?

Gary Z
11-08-2006, 04:51 PM
While your intuition may tell you that the engine load is reduced by a higher overall gear ratio, it is not. The engine load is largely independent of the overall gear ratio. You do not have (or need) a different tune in fourth gear than in first. In fact, the purpose of the gears, both rear and transmission, is match the speed of the car to a fixed tune. The "load" is more-or-less determined by throttle position. At WOT, the load in first gear is the same as the load in second, third, or fourth. You guys know all this but it is sometimes easy to get confused.

11-08-2006, 05:21 PM
The engine's dynamic load changes with the ability to accelerate the crankshaft and thus the piston. As the heat from the cylinder's combustion process expands and pushes down on the piston, its ability to move the piston changes with gearing. In first gear it is much easier for this process to push the piston down more rapidly and accelerate the RPM of the engine to a higher value then in sixth gear. The more resistance to movement by the piston increases combustion chamber pressures, usually changing the combustion characteristics such as how much ignition timing is optimum. A truck's engine is more prown to detonation pulling up a hill rather then down that hill at WOT, same principle. Also, this is why many engines tuned on an inertia dyno need to run less timing on the street or track then if they were tuned on a load dyno.

11-09-2006, 01:30 PM
don't try to make it too complicated? the AFR in the 1:1 gear will be the same. the conditions during tuning or the type of dyno you are using will make more of a difference than the gear ratio. All of which is going to be different at the track or on the street. If you tune it at the track for track passes, you might be able to get it the closest but for purposes of verifying a "safe" tune, tune it with whatever gear and specify the gear ratio in the tune for the future gear change to be and be done with it?