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Old 07-08-2008, 12:12 PM   #1
oharajo
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Post Math for 3rd vs 4th gear dyno differences?

Hey guys, I just dyno'd my car on a Mustang Dyno putting down 290 HP / 285 TQ. I noticed the operator (Chris Tapp, knows his dsms) pulled in 4th. No problem, I'm just doing baselining and I'll be back on his dyno in a while with a few more mods, so I'm happy to be able to compare.

My theoretical question is what kind of numbers (say HP) would my car have put down in 3rd? Per the VFAQ (http://www.vfaq.com/mods/Trannies.html) 4th in my 2G AWD MT (which I converted to FWD since Tapp's dyno is only 2WD) has a gear ratio of 0.833. 3rd has a gear ratio of 1.115.

So I'm thinking the appropriate formula is:

3rd gear ratio / 4th gear ratio = 3rd gear hp / 4th gear hp
1.115 / 0.833 = 3rd gear hp / 290 hp
therefore:
3rd gear hp = 388.17 hp

Is that right? If I'd pulled in 3rd I'd have put down close to 400 whp?

Thanks for the wisdom guys!
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Old 07-08-2008, 01:59 PM   #2
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Re: Math for 3rd vs 4th gear dyno differences?

Quote:
Originally Posted by oharajo
So I'm thinking the appropriate formula is:

3rd gear ratio / 4th gear ratio = 3rd gear hp / 4th gear hp
1.115 / 0.833 = 3rd gear hp / 290 hp
therefore:
3rd gear hp = 388.17 hp

Is that right? If I'd pulled in 3rd I'd have put down close to 400 whp?

Thanks for the wisdom guys!
Nice Try And then you will say that if the dyno operator used 1st gear, then you would have "put down" 500WHP. Nope

The gear ratio multiplier does not work for adding power production on the dyno. If you have 290 WHP in 4th gear, you will have very close to 290 WHP in 3rd gear. In fact, TQ figures will go down in 3rd gear since the turbo will spool up later in the RPM curve.

If you have a datalogger, the best dyno is the 70-90 MPH times that your car puts out. 290 WHP is around 2.7-2.8 seconds 70-90.
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Old 07-08-2008, 02:50 PM   #3
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Re: Math for 3rd vs 4th gear dyno differences?

Quote:
Originally Posted by jeffgst
Nice Try And then you will say that if the dyno operator used 1st gear, then you would have "put down" 500WHP. Nope

The gear ratio multiplier does not work for adding power production on the dyno. If you have 290 WHP in 4th gear, you will have very close to 290 WHP in 3rd gear. In fact, TQ figures will go down in 3rd gear since the turbo will spool up later in the RPM curve.

If you have a datalogger, the best dyno is the 70-90 MPH times that your car puts out. 290 WHP is around 2.7-2.8 seconds 70-90.
Actually in 1st I'd have thought the dyno would have spit out more like 1077 whp given the 3.083 gear ratio.

So I can totally accept being wrong, but why? Isn't that what gearing is all about? Increasing torque you can apply? All that "give me a long enough lever and I can move the world"?

In my current FWD state I brake tires loose easily in 1st and 2nd, but not so much in 5th I'd attribute that to there being more torque at the wheels in 1st and 2nd because of gearing. I don't think the dyno knows anything about my gearing, so it just tells me what's at the wheels and that should decrease the higher gear based on gear ratio, no?

Like I say I appreciate the wisdom!
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Old 07-08-2008, 03:05 PM   #4
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Re: Math for 3rd vs 4th gear dyno differences?

Your goal on a Dyno is to get the most accurate reading. I'm not sure why your Manual tranny's 4th gear is .833, I always assumed it was 1:1, and 5th was "overdrive", but I could be wrong.
The point of a Dyno is to get the true reading of what your motor makes, not to "sugar coat" the numbers. You want the car to be in the gear closest to 1:1 so you get the true reading of what your motor makes at the wheels.
Why would you want to dyno your car, other than tuning? To get your HP and TQ numbers. Why skew the numbers by adding or taking away gear ratio?
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Old 07-08-2008, 03:33 PM   #5
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Re: Math for 3rd vs 4th gear dyno differences?

Quote:
Originally Posted by 93_GSX
Your goal on a Dyno is to get the most accurate reading. I'm not sure why your Manual tranny's 4th gear is .833, I always assumed it was 1:1, and 5th was "overdrive", but I could be wrong.
The point of a Dyno is to get the true reading of what your motor makes, not to "sugar coat" the numbers. You want the car to be in the gear closest to 1:1 so you get the true reading of what your motor makes at the wheels.
Why would you want to dyno your car, other than tuning? To get your HP and TQ numbers. Why skew the numbers by adding or taking away gear ratio?
From the reading I've done, the 1:1 in 4th is true for lots of cars like 'stangs and f-bodys, but not us! For us, 3rd is a bit closer to 1:1 than 4th, but lots of people say 4th is the right gear for tuning. I definitely wouldn't want to skew or inflate my numbers, but would like to do a little bit of comparison for ballparking my progress (hardly a science given SAE corrections, differences between dynos etc, and I believe the most meaningful numbers come from the track).

I wanted to know if 3rd vs 4th pulls give different numbers (I'm pretty sure they do) and by how much, which lead me to thinking the different gear ratios would tell me exactly how much of a difference. And then since neither 3rd or 4th are 1:1, what it would theoretically be for 1:1 to then guesstimate would the HP would be at crank. But I think I might be off track here, especially because there's a final gear ratio that I don't know how to factor in.

So, any engineers or science guys have some formulas for me?
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Old 07-08-2008, 03:36 PM   #6
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Re: Math for 3rd vs 4th gear dyno differences?

I would say if your that concerned, take 2 pulls. Make one in third gear, then a second pull in 4th. Average the two, and you'll have something close.
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Old 07-08-2008, 07:41 PM   #7
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Re: Math for 3rd vs 4th gear dyno differences?

I wish there was some type of formula but there is none. Real world experience proves that 3rd and 4th gear pulls put down almost the same numbers. This was actually proved somewhere on the clubprotege forums with a turbo BP. I'll see if I can dig up the thread and numbers.

4th gear pulls are usually recommended because it's usually the closest to 1:1/just plain a longer gear, and therefore will give better logging resolution. Front wheel drives and autos, will be closer to a 1:1 with 3rd gear.

P.S. Just so I don't get flamed...I'm sure it also depends on the dyno manufacturer, car, mods, power,etc,etc
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Old 07-08-2008, 08:35 PM   #8
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Re: Math for 3rd vs 4th gear dyno differences?

For a mustang, the difference between a 1.3X gearing in 3rd vs 1-1 in 4th is around 10RWHP. Its not a big difference...but with a turbo car it may be a bit more since your loading up the turbo a bit more.
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Old 07-08-2008, 09:38 PM   #9
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Re: Math for 3rd vs 4th gear dyno differences?

The gearing in the transmission and differential is for torque multiplication. To get engine horsepower and torque on a dyno you want to make a pull in the closes to 1:1 gear ratio as possible. However you are right, in 1st gear you are making a lot more torque than 4th gear, why because of torque multiplication. However because most people won't take there motor to an engine dyno its really hard to try mathmatically doing what you want to do. Also just remember, you use torque to figure out horsepower. ( HP=TQ x RPM / 5252 )
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Old 07-09-2008, 11:42 AM   #10
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Re: Math for 3rd vs 4th gear dyno differences?

Quote:
Originally Posted by punker462606
The gearing in the transmission and differential is for torque multiplication. To get engine horsepower and torque on a dyno you want to make a pull in the closes to 1:1 gear ratio as possible. However you are right, in 1st gear you are making a lot more torque than 4th gear, why because of torque multiplication. However because most people won't take there motor to an engine dyno its really hard to try mathmatically doing what you want to do. Also just remember, you use torque to figure out horsepower. ( HP=TQ x RPM / 5252 )
I'm glad you see where I'm going! What makes it mathematically hard to accomplish? If I know what is at the wheels, and I know the gear ratios for everything between the wheels and the engine, and I accept the 15% drivetrain loss for FWD, that's all the variables I need to do some math isn't it? Am I neglecting some other factors?
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Old 07-09-2008, 05:38 PM   #11
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Re: Math for 3rd vs 4th gear dyno differences?

Well with new motors the manufaturers put them on engine dynos to get the actual horsepower and torque numbers. What you then to is multiply the transmission gear ratio by the final drve ratio (Ex: 1st 3.083:1 x Diff 4.929 = 15.196:1 Final). You then take your peak torque and multiply that by the final gearing (Ex: 15.196 x 300ft/lbs = 4558.83 ft/lbs). Now before someone says i'm bull shitting I got this info from my teacher who for 25 years was an mechanical engineer at Ford, he's got a PhD in this stuff and he's a freakin genius.
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Old 07-09-2008, 08:04 PM   #12
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Re: Math for 3rd vs 4th gear dyno differences?

Quote:
Originally Posted by punker462606
Well with new motors the manufaturers put them on engine dynos to get the actual horsepower and torque numbers. What you then to is multiply the transmission gear ratio by the final drve ratio (Ex: 1st 3.083:1 x Diff 4.929 = 15.196:1 Final). You then take your peak torque and multiply that by the final gearing (Ex: 15.196 x 300ft/lbs = 4558.83 ft/lbs). Now before someone says i'm bull shitting I got this info from my teacher who for 25 years was an mechanical engineer at Ford, he's got a PhD in this stuff and he's a freakin genius.
And that absolutely make sense to me, gearing is all about torque multiplication. But that opens the question, how do dynos only spit out numbers that are same order of magnitude as our crank HP? My final drive ratio is 4.929, so once I factor that in, and then factor tire size on top of that (I'm thinking wheel torque = torque from differential / radius of tire in ft since torque = force * distance), I'm lost as to how a dyno puts out an accurate number, unless a whole lot is programmed into the dyno, which I don't think is the case, but I'm open to correction.
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Old 07-09-2008, 10:23 PM   #13
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Re: Math for 3rd vs 4th gear dyno differences?

Well again thats why when you dyno tune you want to do the pull in the gear closest to 1:1. However i'm like you I don't understand how the dyno works, I plan on learning that if I ever get into tuning more. I've only been to the dyno 3 times and that was with my friends car. I'm a backyard tuner, give me a boost controller and afc and i'll make power, maybe not as well as on a dyno. Just know that yes you were right and if you did make a pull in an lower gear you would be making more power. Hope this helps. I have found some of the notes I made from his info, if I find the rest i'll let you know to help out.
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