I am still trying to get more sampling and would love input from our tuners and others who have compared dyno pulls before and after gear change but this is what I have gotten off of our dyno so far:

3:42 to 3:73 ==cost you 9rwhp
3:42 to 3:90 ==cost you 14rwhp
3:42 to 4.10 ==cost you 20rwhp

the 3:73 based on 3 c5s all m6 tranny's 2 pulls were slightly over 9rwhp and one was just under 11rwhp

the 3:90 based on 1 c5 and 2 pulls on my c5 1 m12 and my m6 tranny the pulls ranged from slighly over 14rwhp to just under 15rwhp

the 4:10 based on 2 c5's and 1 fbody all 6 speeds and all 3 were just under 20rwhp--

all these runs on diff day with diff DA

we are using the most conservative 9, 14 and 20--now for an interesting fact: this may not apply to all rearends but I have always thought that the diff in dyno pulls was the fact that the dyno was setup to read hp based on a final drive ratio of 1 to 1 which is distorted with the rear end change this lowers the rearend ratio and thus hp readings--if you don't believe it dyno in a lower gear like third and you will have considerably less hp and then dyno in a higher than 1 to 1 gear like 5th and you will have tons of hp--

I know its been argue'd but on these 3 rearends if you take the % diff between say the 3:73 and the 3:42 there is a 9% diff in the gear ratio's and the pulls seem to indicate 9hp diff--the same applies to the other 2 gear sets--a diff of 14% going to 390s and just under 20% going to 410s and this happens to be the hp drop we are seeing---this is for what its worth and at least gives you something to go by---I would love to get any input on these changes from others to have a larger sampling---all the cars we tested has at least some mods and were for the most part 400rwhp and higher cars I don't have any data on basically stock cars--


[Modified by NEPTUNEBILL, 3:00 PM 3/15/2004]

 

Bill, going from 3.42's and SY3500 to 3.90's and PT4000, there was a 7HP loss for my car.

 

Thanks Joe I have very little data on a4's and no virtually nothing about TC's and there affect--

 

Good info. Thanks. I guess torque off the line is more important sometimes................along with a HUGE stroker...you head will spin for days :D :crazy: :cheers:

God I love these motors................and will love it even more real soon :D

 

Joe I was just looking at some notes and another guy with a TC went from 315s to 373s and dropped 9 like the m6 going from 342 to 373--I wonder since you had already altered your final drive ratio going from the 315s to 342s if your total drop from 315s was about the same as an m6 going from 342 to 390s--

 

I don't know, since I don't have a direct comparison with 3.15's.


[Modified by gojo, 10:02 PM 3/15/2004]

 

I am about to get 4.10s.

Depending on what stage my car is in, I might be able to do a before/after dyno.

 

that would be some interesting info for the database--

 

Tommy's car went from 470 to 463 with a swap of 3.42's to 4.10s. So we only saw 7 HP or so.

 

I know this subject has been beat to death but I will try again to explain this phenomena... It's all in the physics and it is a difficult concept to grasp.
1. Changing the gear ratio does not alter the power of the engine, only the reading on an enertial drum dyno at the rear wheels.
2. The loss you are seeing is real under the (accelerated) conditions that exist on a chassis dyno.
3. The loss is NOT due to friction.
4. If all the engines in your sample were tested at constant rpm points no loss would be evident. This is done by stabilizing the engine every 500 rpm and taking a reading. This is not possible with an enertial drum dyno.
5. The reason the deeper gears cost HP is because they allow a slightly higher engine accel rate on the inertial dyno...
6. If all the cars in your sample were taken to top speed in any gear (where steady state engine speed exists) all the power of the engine would be delivered to the wheels (minus about 4% for drivetrain frictional losses)...
Inertial losses are proportional to the accel rate of the rotating masses and the higher the accel rate, the higher the losses (lower hp reading). This is the reason that dynoing in a lower gear shows low HP and in a higher gear more HP. A dyno pull in 3rd gear causes the engine accel rate to be much higher and hence more energy is expended accelerating all the rotating mass and less is delivered to the rear wheels. With an electric or water brake load absorber dyno all data is taken at constant rpm points and no rotating components are being accelerated. This results in no inertial losses and the most accurate representation of the engine output.
Hope this helps but I know some will never get it...
Shirl
SDRE

 

I realize there is no actual loss of power I am just trying to have a sampling of data for comparing apples to apples on dyno pulls on our particular setups. For instance I have spent some time mapping the DA and comparing readings on the dynojet I realize that the power will not be different its just an apples to apples thing--for instance Rogers dyno was off by at least 7% in the SAE corrections and that amounted to 37rwhp and 40lbs of tq diff--if I didn't check and just went by what the other tuners are pulling on their dyno's I would have to think I have done something terribly wrong--I will not be able to build the power that our top tuners do I'm sure but I could never buy a 100hp diff when the components are pretty close to the same---thanks for the input again Shirl I know you have to be tired of posting this--I agree 100% on you explanation I am just posting what I can find on real world bottom line on our cars to have some rule of thumb for testing purposes and again for apples to apples dyno runs--

My point is that regardless of why there is a dyno diff with the rear gear change here is what diff you can expect to see--if you add a cam and a set of 410 gears and the cam gain 20rwhp and the gears cost you 20rwhp (on the dyno) you would not want to think gee I wasted my money on that cam I didn't gain a thing--no I gained 20rwhp because the 410s are costing me 20 on the chart---I'm not trying to argue the laws of physics just some real world data for comparison for the hot rodders with our particular setup--


[Modified by NEPTUNEBILL, 8:11 AM 3/16/2004]

 

how do you calibrate a chassis dyno? i know how to do this on a engine dyno but i a wondering how you do this on a chassis dyno. :chevy

 

Thanks J--I think Dave (cartek) told me once he thought they lost about 10rw so maybe once again I need comparison of diff dnyo's so this other input is great--I don't want the headache or expense but I would love to slap the 342s back on for a new comparison on more than one dyno--

 

It'd be nice if folks could use load dynos rather than slung weight dynos, then you could apply a load and get truer figures.

 

How do load dyno's compensate for DA or is there any need, I hate to keep harping on the subject but I just don't think the SAE corrections on the dynojets are correct--Mike Norris told me last week in Orlando they had great air for Orlando and the dyno was doing 95% adjustments--from what I can find on DA last week in Orlando it was better than usual but no negative numbers by any means--a minus 5% would have to be in the -1450' range on the DA scale and that would be one of the better days they have had at EnglishTown for god sake--I don't believe the DA in FL has ever been below sealevel--I realize that the dyno is just a tool and what matters is what the car does, but when I see a car elsewhere with a smaller cam, less CR, more restrictive exhaust system with similar heads dyno 40 more hp than mine it makes me wonder what I did wrong--for the tuner standpoint how do they look to the unknowing particullarly when someone looks at 2 427 packages and one dyno's a 100 more hp for the same money which would you buy---I have had the pleasure of meeting some of our better tuners and I have no doubt they can squeeze 20 or even 30 more hp out of the same motor but not 100---I ran the most conservative DA numbers for last Friday and the DA was 1429' or a postive 5% and the dyno took 2 points that is a diff of 7points and in my case 37rwhp and 40rwtq and that is significant in anyones book--I am not trying to set the world on fire or be a dyno queen I would just like to know if I am overlooking something or did something wrong--if its a tool I would like it to be accurate--a 9/16" wrench fits all 9/16" bolts--I choose to use the uncorrected numbers and calc my own DA until someone shows me a machine that has the accurate DA count--hell the guys at Cartek and at Lous have great numbers and for all I know the dynojet is screwing them as well--