Dyno transmission losses - appropriate estimate
10-01-2002, 03:37 AM
#1
Instructor
Thread Starter
Dyno transmission losses - appropriate estimate
Hi, I normally post stuff over on the 911 Board- I don't have a 993 (I wish 
). But I figure I'd better cast the knowledge net a little wider on this one.
As I am sure almost everyone reading this knows, chassis dynos measure rear wheel horsepower (RWHP) rather than flywheel horsepower (FHP). RWHP is lower due to the frictional losses getting from the flywheel to the wheels. FHP can be estimated from RWHP by assuming what this transmission loss is.
As FHP is a bigger number, it is the one everyone likes to quote. Unfortunately, from what I have read this causes certain tuners throughout the world to use "optimistic" assumptions of transmission losses.
For example, if a car makes 200 RWHP you could assume anywhere from 5% transmission loss to 30%. This would give a FHP estimate range of 211-286 - a 75hp difference!
So, I would like to initiate a bit of a discussion on what people have seen as an appropriate transmission loss (or correction factor) for 911s (or 930s, 964s, 993s).
My thoughts are:
- it might be different between dynos (my dyno tuning was done on a Dynapack which attaches directly to the hubs (wheels removed)) and this may cause a difference
- it might (should) be different between transmissions (ie a 901 transmission is lower than a 915 which might be lower than a G50 or a 930)
I would also expect that a transaxle transmission would have much lower losses than a front engine - prop shaft - rear diff drivetrain.
Finally (long post, huh), the dyno operator I used sticks to 1.1x (or 9.1% losses) for a 911. Bearing in mind this is on a hub dyno (which may measure lower losses than one which has wheels and tyres in the measuring path), this is still way down at the conservative end of the calculations I have seen. Gearbox is a 915 from an SC.
I have also seen 11-12% losses on the same car on the same shops other Dyno (a Bosch rolling road). It estimated the losses using coast-down. This was with the old 901 gearbox.
Useful (but not working today) links are at:
<a href="http://www.pumaracing.co.uk/power3.htm" target="_blank">http://www.pumaracing.co.uk/power3.htm</A>
<a href="http://www.pumaracing.co.uk/trans.htm" target="_blank">http://www.pumaracing.co.uk/trans.htm</A>
<a href="http://www.pumaracing.co.uk/coastdwn.htm" target="_blank">http://www.pumaracing.co.uk/coastdwn.htm</A>
). But I figure I'd better cast the knowledge net a little wider on this one.As I am sure almost everyone reading this knows, chassis dynos measure rear wheel horsepower (RWHP) rather than flywheel horsepower (FHP). RWHP is lower due to the frictional losses getting from the flywheel to the wheels. FHP can be estimated from RWHP by assuming what this transmission loss is.
As FHP is a bigger number, it is the one everyone likes to quote. Unfortunately, from what I have read this causes certain tuners throughout the world to use "optimistic" assumptions of transmission losses.
For example, if a car makes 200 RWHP you could assume anywhere from 5% transmission loss to 30%. This would give a FHP estimate range of 211-286 - a 75hp difference!
So, I would like to initiate a bit of a discussion on what people have seen as an appropriate transmission loss (or correction factor) for 911s (or 930s, 964s, 993s).
My thoughts are:
- it might be different between dynos (my dyno tuning was done on a Dynapack which attaches directly to the hubs (wheels removed)) and this may cause a difference
- it might (should) be different between transmissions (ie a 901 transmission is lower than a 915 which might be lower than a G50 or a 930)
I would also expect that a transaxle transmission would have much lower losses than a front engine - prop shaft - rear diff drivetrain.
Finally (long post, huh), the dyno operator I used sticks to 1.1x (or 9.1% losses) for a 911. Bearing in mind this is on a hub dyno (which may measure lower losses than one which has wheels and tyres in the measuring path), this is still way down at the conservative end of the calculations I have seen. Gearbox is a 915 from an SC.
I have also seen 11-12% losses on the same car on the same shops other Dyno (a Bosch rolling road). It estimated the losses using coast-down. This was with the old 901 gearbox.
Useful (but not working today) links are at:
<a href="http://www.pumaracing.co.uk/power3.htm" target="_blank">http://www.pumaracing.co.uk/power3.htm</A>
<a href="http://www.pumaracing.co.uk/trans.htm" target="_blank">http://www.pumaracing.co.uk/trans.htm</A>
<a href="http://www.pumaracing.co.uk/coastdwn.htm" target="_blank">http://www.pumaracing.co.uk/coastdwn.htm</A>
