Stock 944 S2 dyno expectations

[drive418]

Does anyone know what I should expect to get at the rear wheels for a stock 944 S2? Taking it for a dyno next week.

[v2rocket_aka944]

If a Dynojet and SAE corrected I'd expect in the low 170s rwhp.
"944 convention" puts the drivetrain loss as 15%, which would put a factory-fresh car at about 176, allow some loss for age etc...

[drive418]

Thanks! I will see what I get and post for informational purposes.

[azbanks]

Here is the dyno charts from my 1990 S2 cab. I got right at 185 HP to the wheels, stock. 172 TQ.
As you can see it was done on a Dyno Dynamics dynamometer.

The first is HP and TQ, the second is HP and AFR.

You can see some strange behavior in my AFR and the doodling on the charts was done by my dyno guy to show what I should be seeing if the AFR was correct.

[v2rocket_aka944]

azbanks, was there any correction involved?
by the 15% convention that would put your car at 217hp at the crank, stock 951 numbers

[sausagehacker]

I've always wondered how conservative those Porsche-released figures really are. Wouldn't be shocked at 217bhp honestly.

[DannoXYZ]

Which line is AFR on the 2nd-graph?
The one that's 14.5:1 to 4500rpm and drops to 10.25:1 by redline?
Or the that drops to 12.5:1 @ 3250rpms?

There's variations between samples. I've seen S2 dynos from 150-190rwhp depending upon wear & tear. Over the years, I think S2s hold up better than 951s due to lack of easy upgrades. It's just too easy to turn up the boost on a 951 and cause damage.

[flash968]

comparing dynos is like comparing children. there are so many variables that it is impossible. even tire pressure can change the readings. dynos are great tools for tuning, but worthless for comparing numbers, unless you are going to do it to cars, on the same dyno on the same day, under the same conditions.

another thing that is a common misconception is the issue of drivetrain loss. it is NOT a fixed percentage. it is a measurable number, assuming you have a dyno that can measure the load. yes, the frictional losses increase with power output, but not in a direct percentage. these cars generally have between 30 and 40hp loss via the drivetrain. this will be affected by things like tire and wheel weight, flywheel weight, tire pressure, camber angle, yada yada.

frankly, calculating to the flywheel is pointless anyway, and only good for bragging rights. it's what you get at the wheels that matters.

[azbanks]

In my charts, the green lines are the actual numbers. The black lines(pen) are what my dyno guy wrote.

The actual AFR line is the one that is 14.5 to 4500 and then drops to 10.5 at redline.

I believe these are corrected.

[DannoXYZ]

another thing that is a common misconception is the issue of drivetrain loss. it is NOT a fixed percentage. it is a measurable number, assuming you have a dyno that can measure the load. yes, the frictional losses increase with power output, but not in a direct percentage. these cars generally have between 30 and 40hp loss via the drivetrain. this will be affected by things like tire and wheel weight, flywheel weight, tire pressure, camber angle, yada yada.

frankly, calculating to the flywheel is pointless anyway, and only good for bragging rights. it's what you get at the wheels that matters.

yeah, thrust at the contact patches is what pushes you down the F=ma.

Drivetrain frictional loss is dependent upon load f=Nu. Imagine dragging a brick across the floor, it takes a certain amount of force to overcome the friction between the brick and floor. If you double the load by stacking a 2nd brick on top of the 1st one, it will require more force to overcome the friction and slide the stack at the same speed as before.

A 500rwhp 951 will lose A LOT more power (70-80hp) through the drivetrain at full throttle compared to a stock 150bhp 944. Imagine you've grabbed the rear wheels and force it to stop. There will be a certain amount of grinding and friction between the gears, bearings, etc. The higher the power, the harder the load and the more grinding and friction you'll have between the parts.

With practically zero load with the wheels in the air, I can turn the crankshaft by hand in 4th gear and spin the rear wheels. I'm putting out nowhere near 30-40hp with my arm to overcome the drivetrain losses just to spin the crank once.

[flash968]

as an engineer, i understand what you are saying. it just doesn't work out that way in reality though. it's not nearly as much as you would think. i've actually measured the loss. i do that pretty much every time i tune a car. that's why i don't use a dynojet. but, i can tell you for certain that a straight percentage loss calculation does not work. the loss percentage decreases as you add power. it's not huge, but it does start to taper off.

as an example, the drivetrain loss between a 236hp engine and a 340hp engine in a 968, same engine, same car (pre and post supercharger) was a whopping 2hp (that crunched to 15% at 240 but only 13.6% at 340). i'm not even sure it was additional loss, as it was so small that it could have been just about anything.

as i said, there are a number of factors that can affect the numbers too. besides the atmospheric conditions, tire and wheel weight, flywheel weight, tire pressure, strap tension, and even camber angle can skew the numbers.

good for tuning, but horrible for comparisons between cars unless you do them all at the same time and place. most dyno shops won't tell you that though because they want your money. i've spent thousands of dollars tuning on dynos. i've been fortunate enough to work with some great guys who don't cream me with wasted runs.

[v2rocket_aka944]

flash, was your motor put on an engine dyno with all accessories hooked up as in the car?

[DannoXYZ]

the loss percentage decreases as you add power. it's not huge, but it does start to taper off.

True, there are multiple components adding to total drivetrain losses. Some may be a fixed amount, others may scale linearly, and others may be exponential. The exact mix and ratios if those factors are unknown. Also the condition of the drivetrain between differen cars will be different. Some may have more wear & tear than others. The type of oil makes a difference. Ultimately, it's whatever gets to the ground that matters.

[flash968]

v2 - nope - didn't want to pull the motor. the loss comparisons were done before the build.

danno - exactly. that's why a fixed percentage doesn't work. it's "close" but not accurate. that's also why i don't worry about it. i only do the loss readings to see if there is anything anomalous, and it's already hooked up, so not a big deal.

it's always good for a giggle though when i see some shops posting inflated numbers based on some factor they pulled out of the air, rather than just post the wheel numbers.

and then there is always the standard "well, our dyno reads low, and you're really making more power" kind of thing.

[drive418]

Recent dyno results. 175 hp, and 184 tq.

[flash968]

is the exhaust stock on that?

[drive418]

Nope. It's a race car with a straight pipe.

[flash968]

thought so - that explains the torque dip at 3250. it's pretty typical of systems that flow too openly. not as much of a problem on a race car. major bummer on a street car.

[DannoXYZ]

Reversion. Not as bad with EFI, but caused triple carburetion in the old days.

[azbanks]

Originally Posted by flash968

thought so - that explains the torque dip at 3250. it's pretty typical of systems that flow too openly. not as much of a problem on a race car. major bummer on a street car.

I got the same dip(see posted dyno charts above) and my car was completely stock at the time.