Bill

I would have to agree to the incorrect dyno reading theory.

Two months ago I made several major modifications. SFR Stage II bb turbo, maf, test pipe, tial wastgate, Arc II. I then put the 951 onto a dyno. With 13psi of boost and 75k miles on the odometer, print out showed a SAE corrected reading of 275 rwhp.

Last week end here in Nor Cal, I attended a dyno day at a different shop. We were getting readings that I felt were high. A stock 944 S2 with 110 mi. got 220 rwhp. So being curious as I am, I put up the $100us and did 2 pulls. This time the print out showed 245 rwhp!

In the two months that had passed, I have done absolutly no changes on the car with the exception of adding 500 miles to the odometer. I keep my tire pressures religously at 36 psi and neither shop bothered to ask the weight of my vehicle.

I have the print outs to back this story up.
The funny thing about it, my air/fuel curve is exactly the same on both sheets.

The part that burns me....how could a stock 944 S2 put down 220 rwhp first off. And then 2 hours later my modified 951 only put down 245 which is a drop of 30 rwhp from my prior dyno reading?

I left that session feeling embarrased by my readings and ripped off for five minutes of dyno time that cost $100. At least at the previous dyno shop, I got an hour on the rack with many pulls/adjustments...and checked my speedometer accuracy.

andyrew

Bill,
I might be wrong, but I think the S2 had 188hp. Granted your's was low. I didn't know you had a 275 reading from before.

As far as the turbo cars go, we didn't get a good boost reading on several of the cars. I know I set my AVC-r to 1.3 bar and only peaked at 1.2 on the dyno.

Here's a link to the readings I got from the videos.

<a href="http://members.rennlist.org/dan10101/dyno_day_in_nor_cal.htm" target="_blank">Nor-cal dyno day</a>

If we do this again, we'll need to clarify that we can climb in the car and make adjustments on the fly. Could make a real difference, rather than just moving cars in and out. The operator seemed ok with us making changes. (even liked it)

DanD

woops, got the kids account again.
this is really me...

PorscheG96

[quote]Originally posted by Bill:
<strong>I would have to agree to the incorrect dyno reading theory.

The part that burns me....how could a stock 944 S2 put down 220 rwhp first off. And then 2 hours later my modified 951 only put down 245 which is a drop of 30 rwhp from my prior dyno reading.</strong><hr></blockquote>
That's inaccurate. Karl's S2 put down 187 RWHP which is perfect for that car, and Jeff's 968 put down 199 RWHP, again perfect for that car. When I dynoed my 968 last March down at the Dyno Shop in Santee the car pulled 198 RWHP before tuning, 205 RWHP after, so the dyno is pretty darn accurate. A stock S2 puts out 208 horsepower at the flywheel and a stock 968 is rated at 236.

Danno

"And then 2 hours later my modified 951 only put down 245 which is a drop of 30 rwhp from my prior dyno reading?"

Bill, sorry you had a bad experience. We'll talk to the dyno shop into giving you a little extra attention the next time. BTW, which chips where you using this last time around? And which ones did you have the last time?

QUICK'44

There's really only one way to get relevence from a dyno session described above.

Always test on the same dyno and pay attention to atmospheric conditions,tire pressures,etc.
i believe that any particular dyno's accuracy is fairly irrelevent as long as the dyno's repeatability is there...

I have always used the same dyno religiously for this reason...dont know if it's accurate and dont really care,but i "think" it's repeatable.
This allows me to accurately gauge various improvements (or detractions!) i've made to the car and THAT'S what i care about.
"Did this camshaft hurt me or help me?"
"Which one of this fist full of chips has the best power band?"

Bottom line..."dyno days" are just an excuse,and a good one, for car guys to congregate,bench race and have a good time...that's ALL.

guardsred951S

"Always test on the same dyno and pay attention to atmospheric conditions,tire pressures,etc"

Well said QUICK'44. I couldn't agree more! There are so many factors that come into play when dynoing a car that it's not surprising some results aren't what you'd expect.

Thanks for all the input guys!

Paul Bloomberg

It would be nice if all the magazine's would test ALL of the cars on the same dynojet. But different altitude's/temp's/dyno's would probably give different results. Shoot, if I put Viper sized tires on my 944, I am sure the rolling resistance would detract from the numbers.
So I can't see the concern for flywheel HP figures? for comparing before/after mods on a dynojet, YEAH!
IMHO, whether I use 10% or 30% to figure my flywheel HP, the HP to the road is the same. I will go conservative when I dyno my 944. (10%)
Jon

wjk_glynn

[quote]Originally posted by PorscheG96:
<strong>
...Karl's S2 put down 187 RWHP which is perfect for that car, and Jeff's 968 put down 199 RWHP...</strong><hr></blockquote>

Trevor is correct. I pulled 187 RWHP which implies 220 BHP at the crank (assuming 15% loss).

Here's a parrallel thread on the subject:

<a href="http://forums.rennlist.com/forums/ultimatebb.php?ubb=get_topic&f=15&t=011134" target="_blank">http://forums.rennlist.com/forums/ultimatebb.php?ubb=get_topic&f=15&t=011134</a>

Karl.