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

[Viper X]

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

Does anyone have a good handle on how changing gears changes you horsepower and torque numbers? I have been told that typically, as you lower your gears from 3.07 (in this case) to 3.55, 3.73 or 4.11, etc. you will loose "dyno" horsepower. Is there a rough percentage value or ?

 

[Sean Roe]

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

We typically see a decrease of 10-15 RWHP when switching to 3.55 from 3.07 on a stock car (410 RWHP range).

 

[Snakester]

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

But it definitely feels like you are gaining power!

 

[Greg 99ACR Red and Silver]

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

Sean:
That's some good input about the 10-15 loss. Everyone tosses this around, but for some reason it isn't well known.

Only someone with many Dyno pulls under their belt really knows.

Thanks.

 

[Snakester]

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

The hard part in documenting something like this is that there can easily be a 10-15HP variance between different dyno pulls on a Viper anyway, depending on heat soak, temp, humidity, etc. And it's not a simple swap like an air filter or pulling the belt off of a blower to swap out the gears. Therefore the before and after runs with a gear changes are usually done on different days.

 

[Martin D]

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

Figure between 2-3 percent loss at the wheels.

 

[AG98RT10]

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

Does anyone have a good handle on how changing gears changes you horsepower and torque numbers? I have been told that typically, as you lower your gears from 3.07 (in this case) to 3.55, 3.73 or 4.11, etc. you will loose "dyno" horsepower. Is there a rough percentage value or ?

You may lose a few HP but you will gain mucho torque!

 

[Hans Christian]

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

You may lose a few HP but you will gain mucho torque!

[/QUOTE]

my thought exactly !!

 

[VIPERnXr4ti]

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

enlighten me on the loss of HP with gears...this should in theory have no effect on the power the motor is making...the gears just act as torque multipliers, and allow for the car to get into its powerband quicker...they should have no effect on the motor's actual power....in essense all they do is make the driveshaft spin faster...for an example....jack up the rear end of the car...put a mark on the driveshaft...spin the rear wheels ONCE around...if the driveshaft spins exactly 3 times...thats gonna be 3.00:1 gears...if it spins 4 times its 4.00:1 gears....

....the cars feel faster because the gears are multiplying the torue more...they shouldnt be effecting hp....i can only imagine the loss of power to be attributed to variences in dyno runs not the gears themselves...

 

[VIPERnXr4ti]

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

for an interesting quick little read check out this....


http://forums.stangnet.com/showthread.php?t=381060&highlight=dyno+gears

 

[joe117]

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

I can see why there would be a difference between the friction. The 3.55 would have more teeth on the ring or fewer on the pinion gear.

I don't know if the 3.55 or the 3.08 has more gear to gear contact.
I guess the one with the more contact would have more loss.

The part that I can't see is why there would be 2% or 3% difference in hp at the wheels just because of the rear gear change.

I think most guys would say there is 15% loss in the whole drivetrain. If that's true, then a 3% change because of a rear gear ratio change would be a 20% change in the total drive train loss. 3% is 20% of 15%

This seems like a lot. I wonder if it's really that much.

And about the drivetrain loss itself,
If you do the conversion from horsepower to BTUs, you get an idea of the amount of heat that 15% of 450 horsepower represents.

I guess 15% must be a good number but I always wondered where all that heat was going.

 

[Snakester]

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

From what I've read there is no actual power gain/loss, but it's how the dyno reads the differently gears cars that shows up a difference.

 

[Mamba man steve]

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

as far as i know the dyno (dyno jet) knows the size of the drum 4' and if it knows your engine speed RPM it can figure out your gear and tire ratio and combine the to give you your actual speed. knowing that it the measures torque and plugs in the formilar for Hp. they cross at 5250 rpm. it does this no mater what ratio you are running so i can't figure why there would be a difference, but i'm no engineer.

 

[Viper X]

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

Thanks Sean.

Thanks for the thread VipernXr4ti. That was interesting reading.

I ask the question because as Greg says, everyone is aware of some sort of change on paper, but no one seems to have given me / us a good answer on this until now. Any idea why horsepower shows lower when switching gears?

Is torque (as measured by the dyno) also measured at a lower level when switching gears? I understand that it is muliplied, but on paper, would you need to add 2 - 3% when comparing 3.07's with 3.55's for torque as well as horsepower?

 

[Cris]

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

It is mostly about the inertia. Most chassis dynos used are inertial dynos. That is they measure the rate of drum speed increase to calculate the horsepower, then use the measured engine speed to calculate the torque and apply both to engine speed. Because lower gears allows the engine and driveline to speed up faster, albeit at a lower dyno drum speed, the effect of the engine and driveline inertia is higher.

The only way to truly measure the actual frictional losses is on a normal chassis dyno where the car can be run steady state at several engine speeds, or a low speed sweep made (slow enough to minimize inertia effects). Not sure I have seen dyno results of the gear change alone but I would be surprised if it resulted in any significant driveline loss increase. But, of course, on an inertial dyno like Mustang or Dynojet, a measured loss due to inertia would be expected. And with the Viper having pretty high inertia the effect would be largest with the Viper.

 

[Tom and Vipers]

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

It would be nice Dyno Jet had a way to vary its effective inertia for true back to back correlations run at the same time rate of change of rpm.

 

[SpinMonster]

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

Going from 3.42's to 4.10's on my corvette was a net 11hp loss due to friction. I haven't dyno'ed the viper yet but I'll get to it and let you know after aircraft inspections are done with.

 

[VIPERnXr4ti]

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

i just dont buy the loss...there should be no real loss of power...all the gears do is just multiply the torque more...and make the driveshaft spin more....


Anthony

 

[joe117]

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

He thinks there is 11 hp worth of difference in the friction of the two ratios.

I'm not sure there is any difference between the two as far as gear contact area.

I don't buy the loss either. I believe it is likely an artifact of the way the dyno measures the torque.

 

[Cris]

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

Read above.It is not a loss due to friction. It is a change in the dyno's measured horsepower due to the engine, transmission, differential and wheels/tires inertia. Due to the lower gear ratio all of those things are speeding up quicker than they do with a higher ratio. That means the total driveline plus tire/wheel inertia of the Viper (not the dyno) is a player in the measured power.

As long as you use an inertial dyno you will have that effect. Not a big deal. Rebaseline your car and go from there.

 

[ViperRichRT10]

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

Regardless of what the Dyno says, I could care less. I had 3.55 gears installed this week and they are GREAT!!! They sure are allot more fun!!!

Rich
2001 RT10
Muffler Delete, X-Pipe, RT High Flow cats, BTR Intake, 3.55 Gears

 

[Paolo Castellano]

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

enlighten me on the loss of HP with gears...this should in theory have no effect on the power the motor is making...the gears just act as torque multipliers, and allow for the car to get into its powerband quicker...they should have no effect on the motor's actual power....in essense all they do is make the driveshaft spin faster...for an example....jack up the rear end of the car...put a mark on the driveshaft...spin the rear wheels ONCE around...if the driveshaft spins exactly 3 times...thats gonna be 3.00:1 gears...if it spins 4 times its 4.00:1 gears....

....the cars feel faster because the gears are multiplying the torue more...they shouldnt be effecting hp....i can only imagine the loss of power to be attributed to variences in dyno runs not the gears themselves...

From my understanding, there is a software upgrade that is necessary for the dyno th properly read the 3.55 gear.

 

[joe117]

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

ViperRichRT10,
I have the 3.55 gears. I like them compared to the stock that I had.
The only place you will feel anything with the 3.55 that you wouldn't feel with the 3.08 is in first gear.

In any other gear you will not have a ratio that was unavailable with the stock rear.

First is going to be a higher ratio with the 3.55 but second with the 3.55 is going to feel like part of first gear with the 3.08 rear.

In other words, with a 3.55 all the gears are going to give you more thrust than they did with the 3.08 but you will be shifting out of any given gear before you would with the stock rear. I don't think it gives you much SOTP after you get out of first gear.

I like the 3.55 because it let's me use 6th without having to go 80 mph.

 

[Snakester]

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

I put 4.10s in my previous Corvette, and 3.55s in the Viper and I definitely feel the stronger pull in every gear relative to the stock 3.07s. In fact 1st gear is the least difference to me because it always pulled strongly, but with all of the gears it feels like you are always on the meat of the power curve. And now it even pulls strongly in 5th, and isn't asleep in 6th when you are cruising on the freeway and want to move.

If I really want the power (NOW), I'll certainly drop down to 4th. But 6th pulls enough so that you don't feel like a turbocharged car off boost when you need to accelerate on the freeway in top gear.

My take on it is that both 5th and 6th gears (combined with the 3.07 rear) were meant to raise the overall mileage of the Viper to improve Dodge's corporate average (even though the Viper still is classified as a gas guzzler).

And the 3.55 gears simply make a correction that the factory should have, to make all 6 gears more useful.

-Dean.

 

[joe117]

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

Dean,
Don't think of this in terms of how hard it pulls. You need to think about the overall gear ratio.

Take the trans ratio in first times the rear ratio. This will give you the overall ratio for the drivetrain in first gear.
Now do it for all the gears with 3.08 and 3.55.

Then figure how fast you can go in each gear.
You will see that with the 3.55 you will be shifting at a lower mph.
After you make that shift, the guy with the stock 3.08 will not have shifted and he will have a more favorable overall ratio.

This back and forth will even out the difference between the rear ratios, except in first gear.

You can't simply say, "it pulls better in second". You have to take into consideration the fact that you can't use second, or any of the gears in the same mph range as before.

 

[FE 065]

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

The guy with the 3.08 will be well past his torque peak while the 3.55 geared car will be in the fat part of it in 2nd gear and walking away. If your supposition were that simply true, a car with ridiculously high axle ratio, say in the 2's would accelerate just as fast as one with a ratio in the 3's, due to less shifting and more MPH in each gear. Which doesn't fly in the real world does it? My '94 Viper with 3.73 gears was a rocket after I added those gears. Things happened much faster when passing cars at around 45mph. Quicker than it could have in any gear with the 3.07.Moundlr made a post once that his geared near-stock car (pre DLM) easily stomped modded 3.07 geared Vipers.

 

[AG98RT10]

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

I believe Drag racers have understood the benefit of gears for a long time. It's a trade-off to put the performance band of the drivetrain within the range of speeds desired. Top end drops, you get there quicker. If 3.55's didn't have an advantage over 3.07's, they wouldn't add "points" to your car in the technical inspection for Viperdays for having them.
And those who say 3.55s aren't as good for the track - well I'd like to be able to run on their tracks - 5th still redlines at around 170. Must be some long straight-away!
The analogy given about gears only affecting first is too simplistic and doesn't explain the torque multiplication in every gear. One thing for sure: if you change gears you'll either have to lose the Michelins or travel with a very light foot!

 

[Viper X]

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

Hey Joe,

The idea is to put your engine into the fat part of the power band as often as you can. AG98RT10 has is right, mostly. He may be running at lower RPM's. My stock SRT-10 pulls hard all the way to the rev limiter - 6250 RPM. With an SRT-10 in 5th gear at 6000 RPM with 3.55's and 27.4 inch tall (stock) Michelins in the rear, your speed tops out at just over 186 mph. This is plenty fast. If you increase your RPM with a 650 package (comp coupe cam, heads, roller rockers, headers, tuning and increase the rev limiter to 6750 as I plan to do) and run to 6500 RPM, you can now theoretically go just over 201 mph at the top of 5th gear. I don't think I'll ever try this, but this is where the numbers put you.

Joe, if you lived closer, a demonstration of geared vs. non-geared could be arranged. The difference between the same car with 3.55's and 3.07's is substantial. Perhaps you could hook up with someone in your area for some spirited driving / comparisons.

 

[joe117]

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

I know what the 3.55 does. I have one, I changed from 3.08 and I like it.
I'm not saying there's anything wrong with it.

Here's the thing. When you change the rear gear ratio, you are just moving the speed range available in each gear.

For example,
A guy with a 3.55 is going to use third gear over a certain mph range.
The guy with a 3.08 is going to use a different mph range.

To say that the car with the 3.55 will pull harder in third is of course correct, but people don't race that way.

What you do is shift gears. The guy with the 3.55 will have to shift out of any given gear sooner than the 3.08.
When he does, the 3.08 car will have better gearing until he has to shift.

In first gear there is an advantage. After that first shift, there is little difference as the two cars will trade advantage back and forth as they go through the gears.

 

[AG98RT10]

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.