dyno conundrum

[jlevie]

There are a lot of factors that affect the absolute numbers produced by an inertial dyno from session to session. The variance, in my experience, can be about 15%. The only way that I know to have repeatable results from session to session is with an engine stand using a water break or hysteresis dyno. Well there exist hub attach hysteresis dynos that should be repeatable from session to session.

Inertial dynos are convienent and quite good at relative results when the changes are introduced during the dyno session. They aren't so good if you dyno now than again a while later.

[2002maniac]

Are you sure the new throttle body is opening up all the way when the pedal is down?

[hoveringuy]

Are you sure the new throttle body is opening up all the way when the pedal is down?

Yep, I checked that. It uses the same bellcrank and TPS as the M50 unit, just with a larger bore.

I'm going to put the OBD2 cats and muffler on and re-test. Hate to say it, but I don't think I can improve on the factory OBD2 mids even with a 2 1/2" straight-through.

The merge is probably killing me.

[etxxz]

how is the obd2 cats and muffler going to make you more power than a straight-through?
sounds like you need to shift those curves up a couple octaves... or conversely shift the firstone down..

[matt]

Would this make a difference (different gears!)?

Only one way to find out....

[nrubenstein]

FWIW, the 4wd dynojet does seem to read lower in my experience.

[nando]

different gears does make a difference, but not a 25hp difference. on my car it was only 2-3hp from 3rd to 4th gear.

as far as the AWD dynojets - that could be a factor. You always hear how dynojets read high, blah blah blah. But I've seen many cars go through the AWD dynojet at carbconnection in kirkland and all of them seemed pretty dead on. There was even an E92 M3 owner who was a little pissed because he wasn't getting the power that it should have been reading.

where's this dyno at BTW? I wish there was one closer to me than Kirkland.. we have a mustang dyno but it's 2WD. :(

[hoveringuy]

how is the obd2 cats and muffler going to make you more power than a straight-through?
sounds like you need to shift those curves up a couple octaves... or conversely shift the firstone down..

Logically the 2 1/2" should be fine, but it also has a merge and less cross-setional area than the dual OBD2 pipes, which are widely regarded as very good. I recognize that exhaust is as much an art as it is a science and need to give credence to the data which shows my power is down.

Maybe it will test the same.. we'll see.

MaxRPM is in Bremerton near the ferry terminal. The owner swears that the dyno tested accurately against his baseline car after the upgrade.

[etxxz]

yeah but you can't regard the cross sectional area simply as m=pVA, but rather Bernoulli's equation ~ restriction, major and minor losses. The more efficiently and less restrictive you can evacuate gases from your cylinder the more power you will make. Guaranteed that a 2.5" straight will flow better than the OE OBD2 dual pipe.. besides it weighs a ton and junkyards will give you ~ $125 per cat.

when are you planning to re-dyno?

[hoveringuy]

when are you planning to re-dyno?

It's out of the way for me and a PITA to get there these days. OBD2 stuff is back on, so maybe next week or two?

[etxxz]

kewl, keep us posted then!

[balooshinakus]

Steve, let me know if you go on a weekend, I'll head out too.

[hoveringuy]

I e-mailed Dynojet.

Dynojet:

"I believe they have the LINX coupling system. This is a belt system that mechanically links the front and rear drums. There is a different configuration for each drum to be used when the LINX is engaged. This includes mass and bearing drag differences. If they were incorrectly using the linked configuration file for your unlinked run, the software would show the car down on power."

Me:
"The front drum was driving my front wheels"

Dynojet:
"If the front drum was linked, then you car was also turning the front wheels of your car, which certainly has parasitic loss due to the interia of the wheels, friction of the tires to the drum, brake drag, etc. For repeatability compared to your old runs, have them unlink the LINX system and run the UNLINKED option in the software."

Me:
"It sounds like they should run it unlinked for any RWD car, and there was no chance of it being accurate! "

Dynojet:
"Correct. The LINX option is really only for running AWD cars."


I am going to send him the run files where he can see exactly how the dyno was configured. If they blamed my car when they actually screwed-up the dyno configuration I will bitch to no end!

[nando]

hey cool. So you aren't down on power. they should give you some free pulls since they had it set up wrong - you definitely don't want the front drum connected. I've been to carbconn in kirkland a few times, they never run the front drums on anything but an AWD car.

I was surprised at how much drivetrain loss is in an AWD car. I'd be impressed if a bone stock 325ix broke 125whp. :p

there's supposed to be an AWD mustang dyno somewhere in the seattle area too, but the kirkland one is a lot closer to me.

[matt]

It's a huge PITA to tie a car down to use all the rollers... I really wonder why they would do that instead of just disengaging the clutch for the front rollers.