Calculate ET change from final ratio change?

[6667fan]

Can this be done? I have looked at many on line calculators but cannot deduce from what I have seen how to learn the impact of a gear ratio change. RPM & MPH changes no problem.

Existing numbers are 125 mph at 7000 rpm. Horsepower could be anywhere from 575 to 625. Probably 575 at this point as engine is tired. Current ratio is 4.86. Considering 4.71s to take a couple hundred rpm off of trap rpm.

Is ET too subjective to formulate?

Thanks

[mike7570]

Might be simpler to change tire diameter for such a small gear difference. (If you have room)

[Rory428]

Kinda hard to tell, if the engine is stilling making power, and pulling strong, likely hurt it some, but if it`s tongue is hanging out at 1200 feet, may pick up some. Even still, I have to think the MPH change would be more affected than the ET.

[Jb427]

Not got anything to do with gear ratio but I thought this was cool when i watch a video about it. could be some good et gains.

https://bangshiftbilly.com/products/the-bangshift-billy?variant=42410512810179

[Posi67]

That's not nearly enough ratio change to make any difference. I have 3 different sets of rear tires with different diameters which also don't seem to make any difference in ET and "maybe" a hundred or two RPM. If you were to change gear, I'd go down to a 4.57 which may still be a bit much but at least it won't hurt your ET and will lower the top end RPM a little. Might even pick up a MPH as a bonus.   As Rory mentioned, once you are out of RPM then the rest is just noise. 

[6667fan]

I feel like its tongue is hanging out. That is why I was hoping to roll back the rpms. Can’t fit a taller tire so that is out. The hydro roller stick did peak at 6500-6600 when fresh but the valve springs might not be what they were four years ago.

Ran 4.57s for a few seasons and the ET and mph were not as good as with the 4.86s

Calculators show the drop to 4.71s would shave 200 rpm, perhaps enough to stave off valve float?

Thanks for all the replies.

[mike7570]

In that case, new valve springs and a compression check and run it 4 more years 

[shady]

ET is subjective to traction so there is no way to calculate it. most calculators use hp and weight and perfect traction (no wheel spin) for results which has no bearing on gear ratios.

[Jim Comet]

Last spring I switched from 4.10 gears I had been running for 2 years to 4.71's. In my first test and tune and a week later ET was the same at 10.75ish. One month later I made some timing and suspension changes and was rewarded with a 10.63 pass. The next pass to improve or back up the time the motor dropped a valve. Season done. I am running an automatic and was trapping at 126 at 7400rpm. I am going back to 4.10 gears in an attempt to be easier on the motor. While it is always fun to go faster I would consider 4.33 or 4.56 gears and not worry about motor carnage from spinning 7000 plus.

[6667fan]

Thanks for sharing that Jim. 7400 seems high, is that with a 28” tire?

If you are through with the 4.71s I would be interested in buying them. If so please contact me:

jwysocki@charter.net

Thanks

[Rory428]

Agreed, that seems like a lot of RPM for that speed with 4.71s, my Fairmont is just below 7000 RPM at the finish line at 133 MPH with 5.14 gears, although my slicks are 31" tall, plus I have a 4 speed so no convertor slippage. Jim how tall are your rear tires, and do you know how much converter slippage you have? I have always found the old Moroso Power Speed calculator slide rule to be very accurate at both HP based on weight, ET vs MPH, and RPM/gear ratio and tire size for a zero slippage application, like a stickshift, but some converter manufacturers seem to have a better control of slippage than others. If I ever got so physically damaged that I had to run an automatic, I would be looking at what converter companies the Stock and Super Stock racers are using in your area.