exactly how much top end does 3:55's kill?

[JGK95]

Re: exactly how much top end does 3:55\'s kill?

OneRareV..

I appreciatively disagree with you. Your page albeit helpful isn't dead on the money but rather an estimation. The question specifically asked for a final figure for which I thought I provided...273 MPH HAHAHHAHA.




Seriously, I did the math with the variables above. Go to the website I've listed and calculate the figures yourself. Come back to us w/what you've seen rather than take the easy way out!

Respectfully,

Jay K.

 

[WCKDVPR]

Re: exactly how much top end does 3:55\'s kill?

Onerareviper,

That is great information but the real benefit only really applies to first gear. What is not clear in this article is that when you shift to the next gear (earlier with lower gears e.g. 3.55) you now have LESS torque until the car with higher gears would have to shift. Example at 50 mph you are in first with 3.07s:
3.07 x 2.66 (first gear) x 490 lb-ft = 4000 lb-ft
with 3.55 you are in second gear:
3.55 x 1.78 (second gear) x 490 lb-ft = 3100 lb-ft
Basically, the torque advantage at the wheels goes up and back between higher and lower gears once the car with the lowest gear ratio has to shift to second.

There is a more detailed explanation here:
http://socal.viperclub.org/
click on "tech tips",
click on "Benefits of changing rear end gears" which is where you find the referenced article,
then click on "Click here for David's updated version"
which goes through the math for all the gears. You will see that if acceleration from zero is important to you (and you can hook your tires from zero), lower gears might be nice. If you are road racing, lower gears may actually slow you down! More shifting without the torque benefit, as I don't know of any road courses on the west coast where we are in first gear.

This also just assumes an even torque distribution throughout the useable rpm range. The viper engine makes a pretty wide power band so you can use this type of analysis for generalizations. Shifting at 6000 drops you back to 4000 rpm where plenty of power is available. If your motor is very peaky, the lower gears may benefit you more because you will not have as wide of rpm range in each gear (e.g. F1 cars).

The best way to plot this is plug in your dyno numbers for torque at incremental rpm's in each transmission gear for each rear end gear ratio.

Hope this helps,

 

[viper spray]

Re: exactly how much top end does 3:55\'s kill?

Joe's right about the 70mm tb's.

I will buy your 70mm tb set or swap my stock units and some cash , if you want to sell them.

 

[Steve 00RT/10]

Re: exactly how much top end does 3:55\'s kill?

The site listed below offers a program which allows you to input any parameters you want for any car and run acceleration tests from 0-top speed,1/8 miles, 1/4 miles, and just about anything else you can think of. You can pick speeds-- from/to. ie. 10-50,20-60,40-120---and so on. There is a data base of several hundred cars in the program, including vipers. The interesting thing I discoverd was that the main difference was the 'snapshot' time in the lower gears--where there is a gain with a lower gear ratio. 0-150 produced only a second or two difference between 3:33,3:45,3:55, and 3:73's. This concurs with the SoCal website referral above. The biggest difference was the time to top speed where the lower gears made a substantial difference. In other words...from 150 to top speed.

On my car, inputting my dyno numbers, the difference in top speed from a stock 3:07 to a 3:45 ratio was only about 2 MPH(183/181). It has to do with the .74 tranny gear ratio to rear end ratio. You can get about 500 more RPM with the 3:45. The 3:55 gear shows 180 to be the top speed at 5938 RPM. (The 3:07 rear end tops out just over 5100 RPM in this program with my car) While there may be some deficiencies in this program--it's all relative and based on the input. It worked quite well for my purposes.


Car Test Software

Steve

 

[Martin D]

Re: exactly how much top end does 3:55\'s kill?

I have the same program and it shows that the 3.55 is the gear to have for a roll on race that does not exceed 135 - assuming you are in the right gear. And that is NOT horsepower dependant. After 135 the edge goes to the 3.07.
Unless you change the 5th and 6th gear Tranny ratio.....

 

[Steve 00RT/10]

Re: exactly how much top end does 3:55\'s kill?

After 135 the edge goes to the 3.07.

You're right Martin, but only till 4th gear runs out at 150 or so. In fact, from a torque standpoint, most Vipers are past their torque curve by 6000 RPM. As soon as you shift to the .74 gear, the lower ratios will take over again. With my car inputted ..top speed occurs in the following time frames for the gear sets I plugged.

3:07------115.36 sec. 183MPH 5206RPM (150-170 - 13.32 sec.)
3:33------100.81 sec 183MPH 5642RPM (150-170 - 11.46)
3:45-------93.59 sec. 181MPH 5808RPM (150-170 - 11.06)
3:55-------90.61 sec 180MPH 5938RPM (150-170 - 10.93)

Surprisingly, when plugging in 150-180, the 3:55 gear lags all. I would guess it has something to do with running out of torque approaching top end

3:07----32.22
3:33----27.67
3:45----29.48
3:55----37.68


Steve