dizwiz24

TItle says all.

I keep hearing people say gears are a torque multiplier. So if I have a given rwTQ, will I now have more rwTQ with gears? :confused:

Nathan Plemons

Your HP and Torque numbers will be lower on the dyno.

True the gears multiply your torque but since the dyno calculates this back out of the equation it won't show up. Now why higher ratio gears = lower numbers is another argument that you can look up in the archives but it's a dyno proven fact.

In my mind I just like to think that the higher ratio gears are simply less efficient. Granted the acceleration benefit outweighs the inefficiency.

C4DC

Your rwhp will go down a little. I believe the dyno takes into effect the rear end ratio and trans gear you are running to extrapolate the hp at rpm. I wanted to see 300 hp when I dynoed mine and the operator said to do a run in 3rd vs 4th and the #'s would put me over 300, but the parameters for the hp calculation would be off. Others will likley be able to better say why this is...

Nathan Plemons

Actually he was wrong. If you ran in third it would be the same as raising your rear end ratio and effectively lower your output. If the theory works both ways you would want to run in 5th to see higher dyno numbers. I've got a graph that proves that 3rd produces lower results than 4th, let me dig it up.



The only difference between these dyno runs was that #3 was done in 3rd gear.


[Modified by Nathan Plemons, 11:47 AM 8/29/2003]

Strick

Hey Nathan,
I've never been on these new chassy dynos. Do you go through the gears as you would on the road, or do you start out in the gear you want to run?

Nathan Plemons

Well the drums on the dyno don't turn themselves so you have to start somewhere. They weight quite a bit so it's best to just start in 1st gear as if you were driving. Once you get moving the drums will have their own inertia and will keep them moving. After you're moving you just need to get at whatever gear you want to be in at whatever RPM. I like to use the dyno to gather as much data as possible so I don't go very high in 4th gear. Essentially you just "drive" up to speed and put it in the gear you want, press the button to start recording and nail the gas. When you want to stop you let off and hit the stop button, which will turn on the brakes on the dyno and slow it down. For a 6-spd car you can just press the clutch as soon as you're done and throw it in neutral. That way your engine can return to idle without having to wait on the drums to stop.

As for how you get the car up to speed that depends on preference. I always start in first gear and slowly accelerate upwards. Sometimes I'll go straight for 3rd, sometimes I'll go through second. It really doesn't matter. Once I'm in 3rd I'll accelerate up to a speed that I know will be above idle in 4th gear and then I'll go ahead and put it in fourth. Say I shift into fourth and it puts me at 1500 rpm's. I gradually give it a little more throttle until it brings the RPM's up to 2000 or so. Give either punch the button or give the signal to the operator that you are ready to begin and he'll punch the button. When the button has been pressed they dyno is now recording, FLOOR IT.

I don't like running my engine at 6500 RPM's for extended periods of time so I like to have my foot hovering over the clutch. As soon as I hit the rev limiter I hit the clutch and kick the car in neutral.

Of course in my case my dyno operator is a good friend of mine so we've got this little system thing down. We don't even need to talk to each other. :cheers:

LT4BUD

NATHAN, As you know I am perplexed by this set of dyno runs,as I still think 3rd gear would give higher dyno readings than 4th gear. I was trying to follow the green torque curve for run#3. It looks like it finally moves in with the rest of the curves around 5000 rpm. Could the higher torque of 3rd gear resulted in some wheel slippage at lower speeds on this run?? Anyhow I was wondering do have another set of curves to support your view??

I am still trying to understand dyno runs. Right or wrong I keep thinking the reason you would get a higher reading in 3rd is because the driveshaft, wheels etc would be moving slower in relation to motor thus not using as much hp as they would in 4th where they would be moving faster, thus you would have more power left over to reach the dyno??

Geez, wish I were smarter!!!

bogus

these shananagans are the exact reason why horse power numbers are so much horse hockey....

the only numbers that matter are how fast you can go down the strip, or around the course.

Other than that, dyno numbers are entirely to manipulatable to be real. They are a nice reference point, but they are not something to swear to...

Nathan Plemons

I'm not real sure why it takes that spike at the end but I don't think it's a wheelspin related issue.

Essentially every time you enter a gear multiplication in there you are introducing some inefficiency. That's why you always do your dyno run in 4th gear, it's a 1:1 ratio so it gives the most accurate reading.

Right off hand I don't have any other graphs to prove it but you'll just have to trust me on this one. The higher gear ratio always dyno slightly lower. The only other example I can mention that might mean something to you would be looking at my friend's car in the dyno graphs above and my car.

His car is a 93 Z-28. For all intents and purposes our cars are the same. He's got the same cam, etc. The only practical difference was that he had shorty headers when I had long tubes. In theory he should make less power than I did with the same setup. I don't have the actual dyno graph but with this setup he made 342 horsepower. My car with long tubes made 336.

Is that a stunning difference? No, but lets look at it. I've got long tubes instead of shorties, a true dual exhaust instead of a single, and a more direct air intake path. So why does he dyno more than me? I beat my head over this one for a long time until I finally figured it out. His car is a 93, which came with 3.23 gears. Compare that to my 3.45's and you can easily account for the small difference, knowing what we do about the relationship between 3.45's and 4.10's.

So the tranny works in the same way. The difference in dyno data is dependent on the total overall gear ratio, this includes the rear end and the tranny. It doesn't matter if you actually change the tranny gear or if you change the final drive gear, it's the same effect. The overal gear ratio is still changed.

I know, it makes your head hurt.

Bogus,

That's a little bit harsh. True dyno numbers leave something to be desired but they certainly have their place too. Granted you can't always compare apples to apples but you can get pretty darn close. If Honda A dyno's 150 horsepower and Mustang B dyno's 220 then you still know that Mustang B is gonna be faster. Sure it might not be exactly what you would expect for 70 horsepower. Flywheel it might only be 50 horsepower difference or it could really be 100. You still know that it makes more. The dyno's best use is for comparing cars of the same drivetrain or similar and for tracking the benefits of certain modifications.

:cheers:

I'll agree it can be a little misleading, for example with the power I'm making I should run better track times than I do, BUT not everything is at the track. Granted my clutch might not hold up to the stresses of racing but it will still out accelerate car X on the street, etc. I like to use both track and dyno numbers. The dyno gives me some idea what I SHOULD be able to run at the track, regardless of what ever factors might be keeping me from getting there (driver skill being one of these)

Dave C. '04 Z06

Actually, 4th gear is 3.45:1 to the drum in a 6-speed car due to the rear ratio.

6th is as close to a 1:1 ratio as you can get to the drum, at 1.72:1. (3.45 (rear) * 0.5 (6th))

But the faster you spin everything in the drivetrain, the more lossy it becomes due to vibration and heat and such. (especially the driveshaft, which in a car having 4.1:1 gears is spinning almost 16% faster than a 3.45:1 car)

Like the one guy said, dyno's are tricky and can be manipulated. Air temp, Engine coolant temp, engine oil temp, gear oil, trans oil, how straight the car is tied down, etc... all those play a part to a degree on how lossy your engine and drivetrain are. Also there are calibration factors you can enter into the calculations for altitude and such.

So dyno numbers are good for tuning, but run the car on the track to see what it's really got. And don't look at the time...that's you. Look at the MPH..that's the car.

-Dave





[Modified by Dave C. '97 Z28, 9:15 PM 8/29/2003]

Nathan Plemons

That's a good way to look at it. There is also the problem that most dyno's would like you to spin the wheels at 200MPH+ while you tach it out in 6th.

dizwiz24

Well, since I already feel like my Vette doesnt have enough high RPM HP, gears is the wrong way to go for me.

A race for me isnt 1/4 mile stop. A race is from a roll to 140 or so....

mistaben

"roll from a 140 or so..."

:hurray: :hurray: :hurray: :hurray:

Dave C. '04 Z06

Oh yea, speaking of running a car up throught the gears on a dyno, here is a Very true event I witnessed..it was hilarious:

A company brought a portable chassis dyno to a local cruise we were having. This kid with this mustang got up there and insisted his car was going to put400 to the ground, but it was a stock 5.0 (early 90-ish) with no nitrous or anything. So everyone was like, "yea right!"

So the kid gets in and makes a dyno pull and he puts down 190 to the drum. Well, everyone was laughing because of his original outrageous prediction. Well, he gets pissed off so on his second run he revs the engine and pops the clutch in 1st gear! He starts doing a burnout on the dyno drum!! Well, it eventually gets to turning and he runs up through the gears like he's making a quarter mile pass, speed-shifting the gears! Well, all this was great and dandy until he tried to go from 3rd to 4th, and hit second instead. Keep in mind the car is strapped down to the dyno. Well, when he hit second the dyno was already spinning pretty fast and wasn't going to slow down that quick, so his rear tires started bouncing really violently as they skidded on the faster moving drum. But the problem was he didn't know he hit second so he kept it floored. Well, then engine was singing along there pretty well, probably around 8000RPM or so because the dyno was pulling it up far past the rev limiter(which you could hear working it's heart out), until the rattling and brown smoke out the tail pipe started to occur. At that point the poor pony car coasted to a stop and sat there in a cloud of smoke until they could to push it off the dyno.

I still remember vividly the people screaming and running from the dyno while those wheels were bouncing and slamming up and down, with the smoke coming out. It was like a B-rated horror movie. The funniest thing about the whole incident, the dyno guy printed up charts for everyone and the poor mustang made 190-200HP through 1st, second, and 3rd, but in 4th (2nd the second time) it made (-)310HP! Yes, I didn't believe a dyno could measure negative power, but they can. The car made more negative power than positive power.

Just thought I'd share...
-Dave C.





[Modified by Dave C. '97 Z28, 5:34 PM 9/4/2003]