Hey guys!

Love the forum. Been reading for a while and finally decided to sign up and join the chat!

I have a question regarding dyno testing that I think some smart people in this forum can help me out with. I recently took my car (2012 G37) to a car shop that had a chassis dyno. I am a little confused with the results and with what the shop owner said, and I wanted to bounce some ideas off you guys.

He told me that cars need to be tested at 1:1 in order to get a "correct" dyno reading. He proved this by doing a dyno test in 1st gear and the HP curve output was about 50% less than in 5th gear (1:1). I found this very surprising.

I asked him why and he didn't know why, just that it was what he has learned in order to produce more accurate numbers. Thus, I have been looking across forums and called a few other car shops, but no one has given me an answer that has satisfied me. And actually, some people (on forums) have mentioned that this 1:1 rule is bogus and others not. Lots of conflicting answers.

It makes sense to me that if I ran it in a gear ratio other than 1:1, then there will be some more friction involved due to the gears being utilized, thus giving me a slightly less reading (maybe a few percent?), but 50% percent seems outrageous. Why do you guys think that his dyno was giving this low HP output in 1st gear? Is there something wrong with his dyno, or have you experienced this as well?

Thoughts?

Thanks!
Panboy

 

Time to hit up the Interwebs to look for that info.

 

From a Hot Rod magazine web page:

"Will different final drive ratios affect the dyno readings?
This one’s tricky. First, there are potential discrepancies because different gears have different inertia values, generate more friction, and change the amount of tire slip. Higher numerical gears tend to be more inefficient, so as gear ratios increase numerically, power levels tend to slightly drop, particularly on an inertia dyno. When torque is multiplied by steeper gears, tire slippage also tends to increase.

However, there’s another, often overlooked, factor in the brew: rpm and torque are inversely related to calculating horsepower, so changing the rear axle ratio or testing in other than a 1:1 transmission gear seemingly shouldn’t change the horsepower numbers. But this doesn’t take into consideration the fact that changing gear ratios changes the engine’s rate of acceleration. For example: We know that on an engine dyno, if you change a sweep test’s acceleration rate from, say, 300 rpm/second to 600 rpm/second, the flywheel power number (bhp) drops due to the faster rate of acceleration. As an engine accelerates at a higher rate, the power required to accelerate the engine increases, and a greater portion is consumed before it gets to the flywheel. Going to numerically higher gear ratios - whether in the trans (testing in a lower gear) or in the rearend - is like increasing the rate of acceleration in a sweep test. Whether this actually changes a given chassis dyno’s reported results depends on how the specific dyno manufacturer does its math. For the most consistent results, always test in the same trans gear (generally 1:1) and rebaseline the vehicle after a rear-axle ratio change."

I'm not going to link to the page because the web site redraws the screen every thirty seconds for ads making it impossible to read. I copied the page and pasted in Word to actually read the thing and snip the part above.

Google is your friend. Really.

 

Nice find slartibartfast. I definitely did some google searches before this post but didn’t run into that one. You must be a better googler than I am.

So sounds like the they are saying that the power loss for lower transmission gears (below my 5th gear) is due to additional friction and windage within the engine itself. The article also mentions that the losses may be noticeable or not depending on how the Dyno computer calculates the HP. This means to me that even if any differences/losses are seen, that it should be fairly small. Pretty much what I was saying before.

Sounds like I need to go to another Dyno shop?

 

Well, the dyno operator did know to test using the 1:1 ratio, so unless you're tuning using step or sweeps (mid-range/part-throttle), I think that shop is okay.

 

So, the reason for these questions is because I am attempting to measure my HP from an accelerometer and camera. In order to do so, in a safe and controlled environment, I am running tests in 1st gear in a parking lot. Higher gears require too much distance. I wanted to have dyno data in 1st gear also, to have data to compare to, hence my questions. If I can't get accurate 1st gear data at this shop, I might have to go somewhere else. However, if dynos at any shop won't get me accurate data in 1st gear, then I'll have to figure something else out.

Based on our convo, seems like dynos should be able to give me accurate 1st gear data. Any power losses seen on the dyno, will be seen during my tests so that isn't a worry to me.

Any ideas on how to get accurate 1st gear data? Or why this shops dyno is off at 1st gear?

 

Most dyno shops will be off, it's the nature of the beast. Contact Dynojet or Mustang for first-hand knowledge.

Or, go to the drag strip. You can find the empirical formula to calculate hp from drag strip data on the net.