Dyno HP #'s, 3.55's vs 3.07's

[joe117]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

"What is the trade-off? You have to shift sooner. Now the redline in first gear comes at 48 mph instead of 58 mph. Second gear maxes out at 63 mph instead of 76 mph."

See? the car with the 3.73 has to shift out of 1st at 48mph.
When he does that, the 3.08 now has a better ratio until he hits 58mph.

When the 3.73 shifts out of second at 63mph. the 3.08 has a better ratio until 73mph.

While it's true that the car with the 3.73 will pull harder in any gear, the real story is how the cars pull from any given speed.

How about you race the two from a roll 60 mph to 100 mph?

The 3.73 would be starting the race in second but shifting almost immediately to third at 63.

The 3.08 would start in second and run all the way up to 73 in second, shift one time and get to 100 in third.

The 3.73 must shift again at 94 to reach 100.

I believe that under this 60 to 100 stomp. The 3.73 would be at a gearing disadvantage for much of the run.

If you want to race from a roll and have both cars stay in the same gear, then the 3.73 will always pull harder. Do you ever challenge anyone to a race where you have to stay in the same gear?

 

[Greg 99ACR Red and Silver]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

I keep reading that top speed is sacrificed with a rear gear change. It is not so - you just get into sixth and keep increasing speed. Although it is probably not recommended to use that small gear for extended top speed runs (heat, etc.), sixth gear becomes useful. I have 3:45s and can easily go much faster than the redline indicates in fifth gear.

Top Speed seems to be a wash between the 3:07 and 3:45 ratios. There is a certain 90 mile stretch of highway located in Nevada at 5000 feet elevation where more than a few of us have proved this.

And....when you average 150 mph for 90 miles in sixth, you actually get better gas mileage.

 

[AG98RT10]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

Joe, you forgot to quote the "good stuff."
For a stock Viper GTS, peak rear-wheel torque in first gear is:

450 ft-lb x 2.66 First gear ratio x 3.07 rear end gear = 3670 ft-lb

So, if you swap in a 3.73 rear differential gear, peak torque at the rear wheels in first gear is now:

450 ft-lb x 2.66 x 3.73 = 4460 ft-lb

Yep, a 21% increase in rear wheel torque without touching the motor! That's the same as keeping the stock gear and modifying the engine to increase its torque output to 630 ft-lb (Take away 13% for driveline losses and you get 546 ft-lb).

546 ft-lb x 2.66 x 3.07 also = 4460 ft-lb

You will gain 21% more "pull" in every gear with a 3.73 gear swap. This modification is indeed one of the best "bangs for the buck" you can do to your Viper. The difference in acceleration is phenomenal.

Now, see, the key word is "acceleration." But anyway, not everyone seems to be getting this big result in measured times. I think that's because of traction limitations. (Most people aren't running sticky enough tires for gears - I've yet to break loose my Kumho V700s in second or third while Michelins were all over the road).

Greg, top speed is limited in sixth due to the RPMs being in a lower torque range - it's all explained in the article posted a couple back.

 

[joe117]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

Greg,
Gear ratio wise, you should be able to go a little over 180mph at redline with 3.45 in 5th gear.

Doing that at 5000 feet is quite a trick.

5th is .75 overdrive, 6th is .5 to one.

If you can shift from 5th to 6th at 180mph and keep accelerating, you must have giant power.

I would think that the shift to 6th would drop you to a little over 4k rpm.

You have power enough at 4k rpm to go faster than 180mph, wow!

And at 5000 feet, wow!

 

[joe117]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

AG98RT/10,
Like I said, I've got the Kumho V700s and the 3.55 too.

Look, when you do the math that shows 2.66 x 3.73 x torque, of course you will find a big increase over 2.66 x 3.08 x torque.

What you are not figuring is the ratio for the next gear in the box.

The guy with the 3.73 is going to be in second gear 10mph sooner than the 3.08.

During that 10mph the 3.73 car will not have as much torque as the 3.08 car. See?

What have you got to say about the 60 to 100 roll?

Do you understand what I'm pointing out?

 

[Greg 99ACR Red and Silver]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

I read the article. I didn't slow down as it says - I red line in fifth and speed up after the shift to sixth. It is fact. I did 184 mph and it still had more in it. I have a pic off my GPS I can post if will help prove any point. I had dynoed at 410rwhp when this occured. It is possible that I was on a slight down hill slope during the run, although it looked pretty level. This in fairness could be the case.

Joe: When you run a 60-100 race (with 3:73s) would you start out near the redline and shift almost immediately? Or would you just start in 3rd? I don't buy that you shift more often - just at different points. You can easily go through the 1/4 mile traps with 3:07s in third gear, but 4th produces better ETs. A change to 3:45s/3:55s puts you in a sweeter part of 4th gear at the end of the quarter with the same number of shifts.

Interesting dialog by the way - this makes for great info exchange.

 

[AG98RT10]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

Joe, I think I'm following you. But what I'm not clear about is how the 3.73 guy being in second while the 3.07 guy is still in first is a disadvantage? Seems to me he has a fresh gear, which he'll also run through quicker btw, while the other guy is near the top (and falling off) HP/Torque range of first? The visual I'm using to help understand is like having a bicycle with fifteen versus ten speeds. Up to a certain MPH, the lower geared Viper rear will mean more gears can be used to get there, right? (for example the stock rear will only need four gears to get to 140 or so, while the 3.55/3.73 rear will need (part of) five, right? Well, to me, it's intuitive that given the same powerplant (in the case of the bike, my legs!) I'm going to be able to get up to any given speed quicker if I am using more gears! (Or even a one more!) Same thing with the Viper (assuming you don't lose too much time shifting). Each gear (First, redline, bam!~ Second, redline, bam! Third, etc.) is using all the available power of the engine over a shorter period of time, just like running a bicycle with more gears would do. The 3.07 never catches up acceleration-wise because it's like that bike with fewer gears in that it is using 15-20 per cent less "gear" for any given MPH. Now, I realize the difference in real life may not be great, but 15-20 per cent more available torque is certainly significant.
I also realize the taller gears will give better top end in most cases.

As for your analogy of the 60-100 roll-on, I certainly would not bother shifting to second (@ 5700 RPM) only to have to switch to third almost immediately, but would roll on from third. My guess is this would be pretty much of a wash between 3.07 and 3.55.

 

[AG98RT10]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

Greg, I gotta agree with Joe that you must be making some real HP to accelerate to 186 in sixth. According to the chart posted, you should have less than 900 ft-lb of torque in sixth above 180, which is not enough to overcome resistance. Maybe your Viper is pushing more HP than you think! (Or speedo could be off a bit, maybe downhill, tailwind, I dunno?)
Anyways, the charts seem to be useful, but are still only a guide and, as always, your mileage may vary! Interesting discussion.

Can you tell I don't feel like working today.

 

[joe117]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

And if Greg's spedo was wrong, showing higher mph than reality, his odometer would be wrong too, showing great gas mileage.
Something is wrong. Perhaps you have 3.73 gears without the spedo cal box?
Got a GPS? put it on your dash and see if it agrees with your spedo.

AG98TR10,
The whole point here is, again, the 3.73 will pull better in any given gear.
It will not pull better at any given mph.

There are some speeds where the 3.08 is going to let that car be in a lower gear.

And again, I have the 3.55 and the kouhmo V700 stickys. I like the gears, I had them changed. They give real sotp kick.
But, I just don't think they change the car very much after you get out of first gear. The reason I wanted them was so I could use all six gears. I live in the country and I don't often get on the interstate where I could go 80+ with the normal traffic.
Fast normal traffic where I live is about 70mph tops, and now I can do it in 6th.

Not that I don't blow it out from time to time on a back road. I'm just talking about normal driving with the fast traffic here in the country. Now I have a true six speed.

 

[AG98RT10]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

The whole point here is, again, the 3.73 will pull better in any given gear.
It will not pull better at any given mph.

There are some speeds where the 3.08 is going to let that car be in a lower gear.

Agree in part, but can't agree with "not pull better at any given mph." The chart (back on page one now) clearly shows a torque/tractive effort advantage for lower gears all the way to each car's top speed. (It is most obvious in first gear, though, as you also point out.) When the 3.07 "lets that car be in a lower gear," the 3.55/3.73 has already used that gear and is working its way through the meaty portion of the next. Yes the taller gear car has more fp of torque at that moment, but had less torque in all the moments right up until the lower geared car shifted, and will have less torque in all those moments that the lower geared car has already pulled through that next gear at a more advantageous final ratio.

Best analogy is still the bike: to make analogies more visible, it's useful to exaggerate them: pit a three-speed Schwinn versus a ten-speed. The ten-speed has to go through more time-consuming gear changes to reach top speed but is there any doubt whatsoever which will get up to speed the quickest? Not to me, and I own both bikes. The powerplant can only do so much with taller (fewer) gears.

And again, I have the 3.55 and the kouhmo V700 stickys. I like the gears, I had them changed. They give real sotp kick.
But, I just don't think they change the car very much after you get out of first gear. The reason I wanted them was so I could use all six gears. I live in the country and I don't often get on the interstate where I could go 80+ with the normal traffic.

I absolutely agree. And first is the most noticeable change. And yes, it's great to be able to use sixth gear!

 

[joe117]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

The 3.73 car is out of 1st gear at 48mph, right?
Now he's in second.

The 3.08 car is still in 1st until 58mph, right?

so from 48 to 58 the 3.08 car has more torque to the ground because he's still in first when the 3.73 car is in second.

That's why I said that the 3.73 will pull harder in any given gear but not at any given speed.

 

[AG98RT10]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

Yep, I see that. So, a snapshot of each car's power to the ground at any given MPH may show the taller gear car "winning" for that given moment. OK. But the rest of the story, I believe at least, is that the lower geared car will "win out" for the largest number of data points taken along any given 0-*** mph run, since it has a 15-20 per cent advantage overall. Agreed?

 

[joe117]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

Well, yes.
If you look at the area under the curve, you will see that the 3.73 has more power to the ground than the stock rear at probably more points.

It doesn't seem to equate to faster quarter mile times, probably because the Viper has trouble using all the power to the ground that it gets in the lower gears, with the stock rear.

So I would have to say that the claim of 21% more power to the ground is misleading.
If you had the same gears and 21% more power to the ground, say from a supercharger, you would see a real difference in quarter mile times.

 

[joe117]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

And remember, the 0 to *** run is going to take full advantage of the difference in first gear.

When both cars are in first there will be a difference.

Remember, I said that after you get out of first there isn't much difference.

 

[AG98RT10]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

Put it like this, would you swap 'em back to 3.07s? Me neither. I drove an SRT-10 and really missed the 3.55 SOTP feel, too. All things considered, they are just a great mod and I can't think of any real disadvantages, even on the track. Maybe if you ran 600-plus HP they'd be too low, but the Kumhos do a good job planting the power...

 

[Greg 99ACR Red and Silver]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

Joe:
My speedo is dead on with the recal box (very accurate). At the finish line of the Silverstate I was clocked (Radar) at 165mph, my GPS said 165 and my speedo said 165.

A not too noticable down hill stretch probably is more likely the reason. This I can't verify.

 

[joe117]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

No, I'm keeping my gears.
There might be a disadvantage sometimes.
I believe on a road course, some of the guys say that the 3.55 or the 3.73 or the 3.08 put them in different gears at different parts of the course.

There are some places on a road course where you really don't want to shift.

If there is a certain turn where you could do the turn at, say 75mph and one of those ratios would put you in the top of a gear or cause a shift where they didn't want one, they might change the rear.

They don't always go for the highest numerical ratio because they spend so little time in first gear.

 

[joe117]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

Greg,
It must be something, look at the numbers, rpm, torque, gear ratios and on top of all that, the power deficit that you must have at 5,000 feet.

Driving your 3.45 geared car up to 180+ in fifth and shifting to sixth and accelerating to even higher speed seems strange.

 

[XS TORQ]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

Been lookin' for this, finally found it with search: web page

Has good straight info, albeit concerning 3.73 swap. For 3.55, etc., just scale down expectations slightly.

Great info from Dave Atkins from WAY back in the day (probably 9 years ago or so, when I first bought my Viper)

 

[Martin D]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

http://www.cartestsoftware.com/ This is a place I picked up some great software to run simulations of speed, gearing, weight, horsepower, etc...

I have determined that with my power lever, the car will be faster from a roll with the 3.55's from any speed up to about 135. Then the 3.07 has an advantage to about 150.

It is really useful in illustrating Joe's point that racing another car does not take place in the same gear. Anyway, the software is cool.

Regards,

 

[Steve 00RT/10]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

I have the same program. I had a DOS version of it many years ago and found it again earlier this winter. It helped in providing the information I wanted to change rear ends. As there were no 3:33's available from Unitrax this spring, I went with a 3:45. Joe's argument about the power trading back and forth through the gears is valid. However, a snapshot of time/speed in any gear for both rear ends will have the lower rear end beating the stock rear end. Top speed also comes much quicker with the lower gear. The program shows my top end higher with the 3:45 gear due to being able to achieve a higher RPM in 5th. The 3:07 hits the wall around 5300 RPM at 180. I plugged my dyno numbers into the program. The program may not be perfect, but it's all relative. It treats every vehicle with the same set of criteria.

Steve

 

[GR8_ASP]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

Not sure if this helps but this is a graph for the SRT with several gears. It uses stock horsepower and torque data. It demonstrates what several are mentioning and that is that the major advantage for lower gears is the first gear range. From then through 5th the advantage shifts back and forth, depending on which needed to shift. Top end is also affected as top speed with the 3.07 is in 5th whereas 3.73 tops out in 6th. The 3.55 tops out about the same in 5th or 6th. Each of the lower gears loses top speed compared to the 3.07.

 

[Paolo Castellano]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

The 3:07 hits the wall around 5300 RPM at 180.
Steve

Steve, I hit 190+ with stock power and gearing with my 1996 GTS.

Changing the rear end gears to a 3.33 or 3.55 makes the car slightly quicker on the acceleration through 2nd and 3rd if you can shift quickly. I remember being able to pull TOOOFST pretty easily with both of us having 3.07's. Once he changed to a 3.33 and then a 3.55, he was slightly quicker form a roll but the launch was more of a touchy situation. Just a little too much throttle input and I was gone. He went back to the 3.07.

 

[VIPERnXr4ti]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

its a good point that the longer geard car is pulling to 58mph, while the shorter has just shifted while he is at 48, but then the shift takes place and he is backin the sweet spot of the power curve, and as the longer geard car is not making his shift out of 1st the shorter geared car is rushing through the RPMS in 2nd up to redline, so that really negates the advantage of being able to hold the gear our longer....the real benefit is the ability to stay in the sweet spot...the longer geard car's rpms will fall off further down the range than the shorter car's not too mention it then takes longer to get back to redline...of course when boost is added to the equation all bets are off because in many cases with boost longer gears are better because you can hold the car in boost longer...

Anthony

 

[VIPERnXr4ti]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

and btw, you guys are lucky to have even 3.08's stock...Ford gave me and my Mustang brethren 2.73's!!! 3.08's were optional, and standard on Cobras and Automatic GT's got 3.27's but must of us lowly 5spd GT drivers got the bid daddy 2.73's...woohoo!

Anthony

 

[joe117]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

VIPERnXr4ti,
From 48 to 58 the 3.08 car has the gearing advantage, the shorter geared car isn't "rushing" through second gear, he is in second and the other guy is still in first. The guy in second is at a disadvantage over that 10mph band.
That's part of the trade off he get's with the gear change.
After the shift he will regain the gearing advantage and then lose it at his next shift.
Period....


Just take a look at the graph a few posts above. You will see the back and forth. it is real.
Take a look and you will see that after the cars get out of first, the advantage trades back and forth.
In third and above, there is really no advantage to any of them.

 

[Steve 00RT/10]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

The 3.55 tops out about the same in 5th or 6th. Each of the lower gears loses top speed compared to the 3.07.

Hi Ron,
The 3:45 gear in the Cartest program has a few more MPH and higher RPM in 5th gear than the stock 3:07.

Steve, I hit 190+ with stock power and gearing with my 1996 GTS.

Paolo,
My car is an RT and probably about 10 slower to start with due to the drag in the roadster.

Just take a look at the graph a few posts above. You will see the back and forth. it is real.
Take a look and you will see that after the cars get out of first, the advantage trades back and forth.
In third and above, there is really no advantage to any of them.

Joe,
The lower geared cars will obtain top speed much quicker than the stock rear end. Per the software, the time difference to top speed kicks in after 150 when the 3:07 goes into overdrive. 0-150 is about a wash for all the ratios I plugged into this program. (3:07,3:45,3:55,3:73 were within a second of each other to 150. The 3:07 won by less than tenth over the 3:45)

Steve

 

[joe117]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

I'm not sure what the point is about higher ratio rears reaching top speed sooner since their top speed isn't as high.

The big jump in ratio from 5th to 6th is the problem. The change is from .75 to .5 and that is enough to slow down the engine to the point where there isn't enough torque to maintain the speed that the car reached in 5th.

Also,
one second is not just a little. A second at 150 mph is 220 feet.

 

[Steve 00RT/10]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

Joe,

The program shows top speed for my car higher than the stock rear end by over 5MPH. There is a considerable gap in ET to top speed from 150 on. There was a tenth of a second between the 3:45 and 3:07 to 150 MPH--not much difference there. I'm not hanging my hat on this software program being perfect, but it does provide some good info for comparison. In reality, I will probably never be at top speed so therefore your position on speeds to 150 is true. I wanted the punch for autocrossing and more grunt coming out of corners. The downside on a shorter road track is that I did run out of 3rd gear at Gingerman this year. I'm done just over a 100 MPH with the 3:45. That would be true for Blackhawk Farms as well. Larger tracks would easily get into 4th or higher. Seeing as these track experiences only happen a time or two a year, I am happy with the swap for all the reasons mentioned above.

Steve

 

[VIPERnXr4ti]

Re: Dyno HP #\'s, 3.55\'s vs 3.07\'s

there may be an advantage in the length he can hold out first gear, but once he shifts the RPMS will fall off more and more with every gear, while the shorter geared car's rpms will not fall off as much therefore keeping him more in the sweet spot giving him the advantage....


Anthony