Dynos, Gears and RWHP?

[GR8_ASP]

I will try also:

1. The dyno measures the surface speed of the roller (really angular speed but I won't cut hairs with that) and the rpm of your engine. 1:1 is used as it generally produces the best results and is easiest in the transmission.

2. It calculates the average ratio based on the difference between measured dyno roller speed and engine speed. That ratio is not the final drive but the combination of transmission, final drive and rolling tire radius. This is very important with an automatic transmission which does not have a constant ratio through a run due to varying degrees of torque converter slip.

3. What Joe said. I would guess 2-3% loss for a Viper

The dyno actually measures roller speed and has a formula to translate the speed change per time period into average hp for that step. The one I use uses a 0.005 mph step and records the time for that unit of change. Knowing the inertia (of the dyno), the roller speed and the delta speed and delta time, horseppower can be calculated directly. It is then translated into RWHP by the average engine speed to roller speed that it measured. It then calculates the torque from the horsepower and dyno speed, and again translates that into what people call RWTQ.

For any true engineering nuts here (hey Tom) we all know that the RWTQ numbers generated are not RWTQ. They are translated to represent engine speed based torque so that they can be graphed together in the traditional manner. Also, because the dyno does not know the rolling radius of the tire it cannot calculate the real RWTQ. It could calculate the rear wheel thrust though.

 

[ViperRichRT10]

I will try also:

1. The dyno measures the surface speed of the roller (really angular speed but I won't cut hairs with that) and the rpm of your engine. 1:1 is used as it generally produces the best results and is easiest in the transmission.

2. It calculates the average ratio based on the difference between measured dyno roller speed and engine speed. That ratio is not the final drive but the combination of transmission, final drive and rolling tire radius. This is very important with an automatic transmission which does not have a constant ratio through a run due to varying degrees of torque converter slip.

3. What Joe said. I would guess 2-3% loss for a Viper

The dyno actually measures roller speed and has a formula to translate the speed change per time period into average hp for that step. The one I use uses a 0.005 mph step and records the time for that unit of change. Knowing the inertia (of the dyno), the roller speed and the delta speed and delta time, horseppower can be calculated directly. It is then translated into RWHP by the average engine speed to roller speed that it measured. It then calculates the torque from the horsepower and dyno speed, and again translates that into what people call RWTQ.

For any true engineering nuts here (hey Tom) we all know that the RWTQ numbers generated are not RWTQ. They are translated to represent engine speed based torque so that they can be graphed together in the traditional manner. Also, because the dyno does not know the rolling radius of the tire it cannot calculate the real RWTQ. It could calculate the rear wheel thrust though.

Ron, I greatly appreciate your reply. You could have just typed sentence number three and I would have been happy! Thanks again!

 

[GR8_ASP]

I know but some of us techno-nerds need enjoyment too!

 

[ViperRichRT10]

I know but some of us techno-nerds need enjoyment too!

Well I must say I now know everything I ever wanted to know about dyno calculations...and things I didn't! Thanks!

 

[1TONY1]

I have the 3:45 gears and 600 dyno rwhp,and are fine for short tracks, but I had my ass handed to me by a stock Porsche twin turbo after 130 mph last year on our club road tour in Houston. Anything other than the stock 3:08s sure hurts on the top end side.

There must be something wrong with you car if a 911TT went by you with 600RWHP with those gears at that speed..I have 640 RWHP with 3.55s and I can tell you it aint even close past the launch..hard to beat em off the line for sure...Al

There is nothing wrong with my car and I hope you too get to compete and get your ass kicked with a much finer car, i.e. Porsche twin turbo. There was no threat until past 130 mph and then he was GONE. My point is the gears, and I am sure it would have been a better chase with my old stock 3:08 gears. With your 3:55s you would have been really TOASTED on the long end with this Porsche. Dont think the Viper is the best and fastest car out there.

Your gears have NOTHING to do with getting beat.....NOTHING. Stock Porsche and you should not have gotten beat. Maybe it was not stock, maybe it had nitrous. Maybe there is something wrong with your car. I am assuming you are a competent driver....shouldn't take much skill to win that race if everything else is ok etc. What speed did you start out at ? Anything 70 or below you should have been so far ahead at 130 you would be letting out anyway because the race was over.

I have 3.55's and have raced a duplicate of my car with a very good driver. The only differences: He had 3.07's and I had 3.55's and roller rockers...due to the age of my engine the rockers probably just brought us back to even. The race was dead even.

I may not know much, but I do know racing

 

[DEADEYE]

I have the 3:45 gears and 600 dyno rwhp,and are fine for short tracks, but I had my ass handed to me by a stock Porsche twin turbo after 130 mph last year on our club road tour in Houston. Anything other than the stock 3:08s sure hurts on the top end side.

There must be something wrong with you car if a 911TT went by you with 600RWHP with those gears at that speed..I have 640 RWHP with 3.55s and I can tell you it aint even close past the launch..hard to beat em off the line for sure...Al

There is nothing wrong with my car and I hope you too get to compete and get your ass kicked with a much finer car, i.e. Porsche twin turbo. There was no threat until past 130 mph and then he was GONE. My point is the gears, and I am sure it would have been a better chase with my old stock 3:08 gears. With your 3:55s you would have been really TOASTED on the long end with this Porsche. Dont think the Viper is the best and fastest car out there.

Your gears have NOTHING to do with getting beat.....NOTHING. Stock Porsche and you should not have gotten beat. Maybe it was not stock, maybe it had nitrous. Maybe there is something wrong with your car. I am assuming you are a competent driver....shouldn't take much skill to win that race if everything else is ok etc. What speed did you start out at ? Anything 70 or below you should have been so far ahead at 130 you would be letting out anyway because the race was over.

I have 3.55's and have raced a duplicate of my car with a very good driver. The only differences: He had 3.07's and I had 3.55's and roller rockers...due to the age of my engine the rockers probably just brought us back to even. The race was dead even.

I may not know much, but I do know racing

So the 3:55's brought no improvement over the 3:07's in that straight line test. Just as I suspected.

Also, the estimated loss percentage will obviously change with different ratio's.

 

[joe117]

After you get up to high speed, there will be little difference between the performance of one rear ratio and another.

One rear may have you in 4th while the other has you in 3rd.
The overall ratio may be very close to the same.

I believe that the right rear for any given road race will depend on what gear you want to be in at critical places on the track.

 

[00prowler]

I have the 3:45 gears and 600 dyno rwhp,and are fine for short tracks, but I had my ass handed to me by a stock Porsche twin turbo after 130 mph last year on our club road tour in Houston. Anything other than the stock 3:08s sure hurts on the top end side.

There must be something wrong with you car if a 911TT went by you with 600RWHP with those gears at that speed..I have 640 RWHP with 3.55s and I can tell you it aint even close past the launch..hard to beat em off the line for sure...Al

There is nothing wrong with my car and I hope you too get to compete and get your ass kicked with a much finer car, i.e. Porsche twin turbo. There was no threat until past 130 mph and then he was GONE. My point is the gears, and I am sure it would have been a better chase with my old stock 3:08 gears. With your 3:55s you would have been really TOASTED on the long end with this Porsche. Dont think the Viper is the best and fastest car out there.

Your gears have NOTHING to do with getting beat.....NOTHING. Stock Porsche and you should not have gotten beat. Maybe it was not stock, maybe it had nitrous. Maybe there is something wrong with your car. I am assuming you are a competent driver....shouldn't take much skill to win that race if everything else is ok etc. What speed did you start out at ? Anything 70 or below you should have been so far ahead at 130 you would be letting out anyway because the race was over.

I have 3.55's and have raced a duplicate of my car with a very good driver. The only differences: He had 3.07's and I had 3.55's and roller rockers...due to the age of my engine the rockers probably just brought us back to even. The race was dead even.

I may not know much, but I do know racing

AGREED Tony...AL...

 

[KepRght]

thanks for the answers!