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Dyno 'Coast-down' tests for finding "drivetrain loss"
Was in a disscussion on another forum; a reputable tuner referenced the procedure as reliable. I have a huge problem with the tests, as I've done it on a couple cars with NO hint of accuracy... Basically, after you run a dyno pull, you let off the gas, keeping the car in-gear. The dyno keeps measuring and in the end, gives you a number of how many Ponies are lost thru the drivetrain.
Anyone have any experience in the accuracy of such tests? Here's why I'm not a fan, as posted on the other forum:
Originally Posted by dmp
1993 Probe GT - showed a 55hp loss thru the transaxle. The car dynoed at 158whp Engine was rated at 164hp, I had CAI and Catback. :bs:. Most Probe GTs w/ that engine dyno stock, in the mid 140s...mine did 148, so it was on the 'strong' side. Still, I only showed a 10hp gain w/ the two mods. Even at 148, that shows Mazda under-rated my car's engine by FORTY HP. Very Unlikely. 1993 Miata - showed 40HP loss. 101 rwhp with cone filter on MAF, timing advance and GReddy muffler. The engine was rated at 116hp stock. The Golden Rule for Miatas - accepted by 99% of the Miata Smart Guys is ~26hp loss thru the drive train. There are hours and hours of threads to support that figure. The Drag Down test showed the car 'really' had 140 hp at the crank...from a 116hp 1.6L Engine. Impossible. Drag Down tests are probably valid to show the which car among a group may 'consume' the most HP thru it's drivetrain...but the raw numbers shown just never make sense to me. |
totally agree- the numbers from those drag down tests are completely unreliable.
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What really bothers me is the loss of hp from the 8's drivetrain. Mazda rates this at 238, but most dynos are anywhere from the 180's to 190's with one lucky guy hitting 206. Thats a loss of almost 60hp for worst case.
Now the WRX which is rated at 220 hp dyno's about the same numbers as ours, and the cobalt with 205 is barely shy of our numbers along with the RSX. How is it that we are losing so much more hp to our drivetrain then others are? Is mazda that inefficient? |
the cobalt is under rated is front wheel drive and doesnt have the kind of limslipdif we do.
but lets leave that conversation in a different thread. this topic i believe is coast down on the dyno determining the loss thru the drive train it s ridiculous really - if you are coasting down you arent putting a load on the drive line- how can that determine anything? |
Wow Zoom, that was just 7 minutes...you must live on this forum...
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i have no idea what i'm talking about, so take this with a grain of salt...
i'd think that the only accurate way of determining drivetrain loss is to put the engine on an engine dyno, then put the engine back in the car and get the whp, then compare the two. the difference is the drivetrain loss. or am i'm completely off base here? |
Originally Posted by rodrigo67
Wow Zoom, that was just 7 minutes...you must live on this forum...
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Sounds like a crap attempt to measure something that doesn't take inertia of the engine and non-driveline components into account. This method would indicate an increase in driveline loss due to a lightened flywheel.. naturally most piston engines will benefit more from this sort of useless exercise than a rotary.
But it would be nice to have a way to measure driveline loss without pulling the engine for a baseline... |
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