Understanding dyno results after a final drive ratio change

[Rob-bb]

Hi,

I am looking to be schooled about dynos and final drive ratios. I have been using a dyno near me, I don't know the brand, but its a chassis dyno with dual rollers, eddy current type.

I only look at power curve at the wheels, I don't try to back calculate engine power

Since the last time the car was on the dyno I have changed the final drive ratio. Does this mean when I go back soon I wont be able to compare my new charts with the older charts, or am I still OK look to look at the new and old charts and see what gains have been made?

[F-BIRD'88]

Everything effects everything.

[Rob-bb]

Everything effects everything.

Dude, thanks for entering the competition for the most pointless answer, you're a contender, I think you have a good shot at 1st place

[MadBill]

Hi,

I am looking to be schooled about dynos and final drive ratios. I have been using a dyno near me, I don't know the brand, but its a chassis dyno with dual rollers, eddy current type.

I only look at power curve at the wheels, I don't try to back calculate engine power

Since the last time the car was on the dyno I have changed the final drive ratio. Does this mean when I go back soon I wont be able to compare my new charts with the older charts, or am I still OK look to look at the new and old charts and see what gains have been made?

The gear ratio does not materially affect chassis dyno numbers. If you were going from say a 3.08-1 to a 4.88-1 you might see a slight drop due to the higher friction of a higher ratio and the inertia effect of the faster RPM climb rate.

For the learning if you have a manual, you could do pulls in say 2nd, 3rd and then (direct-drive) 4th, where the former two would also show some slight loss due to the extra gear pair in play.

[Rob-bb]

The gear ratio does not materially affect chassis dyno numbers. If you were going from say a 3.08-1 to a 4.88-1 you might see a slight drop due to the higher friction of a higher ratio and the inertia effect of the faster RPM climb rate.

For the learning if you have a manual, you could do pulls in say 2nd, 3rd and then (direct-drive) 4th, where the former two would also show some slight loss due to the extra gear pair in play.

Previous runs were done in 3rd gear (auto), basically for roller speed as far as I know, with the previous diff 4th gear was 260kph at high rpms, and the ratio has gone from 3.133 to 4.363.

So do you think it is fine to basically look at my last run, and use it to compare the effect of new headers and exhaust vs the previous ones without things being radically different on the dyno chart?

[MadBill]

I think a little math will show that the new gear will give about the same RPM at 'X' MPH in 4th as did the old gear in 3rd, making for an obvious solution. I don't know if internal transmission factors, lock-up converter, etc. also come into the '4th vs 3rd' equation, but in general the power numbers are only comparative anyway for automatics with high stall converters running on an inertia-type chassis dyno.

"So do you think it is fine to basically look at my last run, and use it to compare the effect of new headers and exhaust vs the previous ones without things being radically different on the dyno chart?"

So to summarize: yes.

[FC-Pilot]

I have an engine dyno and have no experience with a chassis dyno, but I would still compare previous data as you should see trends and change in power curve if nothing else. I think there is still value in your old data. Yes, everything effects everything, but while understanding that, you can still read between the lines.

Strap it down and make some pulls. Then report back what you find as far as if there are noticeable differences because if the gear change.

Paul

[Ken_Parkman]

One thing to note is a change in ratio will change the acceleration rate. Accelerating the drivetrain and rollers absorbs some of your power, and I do not believe the chassis dyno can properly asses this loss. The lowest acceleration rate (highest gear) will show a more accurate power reading.

[Rob-bb]

I think a little math will show that the new gear will give about the same RPM at 'X' MPH in 4th as did the old gear in 3rd, making for an obvious solution. I don't know if internal transmission factors, lock-up converter, etc. also come into the '4th vs 3rd' equation, but in general the power numbers are only comparative anyway for automatics with high stall converters running on an inertia-type chassis dyno.

So to summarize: yes.

It's an eddy current dyno not a inertia roller dyno, so the dyno controls the ramp speed, and the auto has a clutch locking converter, and there is about 25kph speed difference at 7000rpm between 3rd with the old diff and 4th with the new diff, but that wasn't my concern anyway, I watched a video this morning about different types of dynos and the presenter said different gears give different results and used 3rd and 4th as examples, so naturally from that you would extrapolate that different final drives will give different results because that is changing the total gearing.

It seems to me all the people on the internet saying you must use 1:1 gears are ignoring the fact the final drive gears mean no car is running a 1:1 total gearing so therefore transmission gear selected shouldn't make any difference either, and neither should final drive ratio.

My thinking is the engine makes the same torque at the crankshaft regardless of gear selected, but the trans and final drive gear multiplies the torque at the roller and also at the same time it changes the roller RPM, and since the formula for HP is (( Torque at the Roller x Roller RPM ) divided by 5252 ) if we pick an engine rpm, say 4000rpm, the Torque at the rollers is increased in a lower gear but the RPM is lower so the HP power at the roller should be the same. To me it seems the maths is saying the transmission or final drive gear selected makes no difference

For example (3x4)/5252 is the same as (4x3)/5252

Does that make sense or am I way off track?

[Rob-bb]

I have an engine dyno and have no experience with a chassis dyno, but I would still compare previous data as you should see trends and change in power curve if nothing else. I think there is still value in your old data. Yes, everything effects everything, but while understanding that, you can still read between the lines.

yea, from what I have read today I think it will be fine to look at previous run and the new run to see where things are going

[F-BIRD'88]

You are off track because everything effects everything.

[Stan Weiss]

I believe that it should be pretty close. Unlike a chassis dyno, this should not be measuring torque or HP but roller acceleration rate again some form of time and calculates torque and then HP using RPM. What is your torque convertor and will the gear change have any effect on slippage?

Stan

PS I see a couple of posts were made while I did my slow typing.

[Rob-bb]

I believe that it should be pretty close. Unlike a chassis dyno, this should not be measuring torque or HP but roller acceleration rate again some form of time and calculates torque and then HP using RPM. What is your torque convertor and will the gear change have any effect on slippage?

Stan

I don't understand the details, but from what I have herd the eddy current dynos are sort of backwards, and it measures how much power the dyno needs to prevent the wheel speed increasing, so you can hold an engine at a set rpm for steady state tuning etc, and for a power run the dyno controls the ramp rate not the car,

The torque converter has a lockup clutch so when powering hard the EUC triggers it to lock up like a manual and when cruising in allows slip so you can drive around in a car park. We did have some slippage on the rollers the last time, but the operator sorted it with some spray on sticky stuff

[In-Tech]

Everything effects everything.

Dude, thanks for entering the competition for the most pointless answer, you're a contender, I think you have a good shot at 1st place

Seriously DUDE, he's right and you are calling him the idiot yet you are the one
I am on an engine dyno or chassis dyno 300 days a year.
A person is looking for trends or shape changes, fock the numbers. I've changed tires on the same chassis dyno in minutes and picked up over 150 whp, what a hero

[Rob-bb]

Seriously DUDE, he's right and you are calling him the idiot yet you are the one
I am on an engine dyno or chassis dyno 300 days a year.
A person is looking for trends or shape changes, fock the numbers. I've changed tires on the same chassis dyno in minutes and picked up over 150 whp, what a hero

I never said he was an idiot, that is your word, I don't know if you know the guy or not, maybe you know something about him that I don't? What I did say was his reply was pointless, as in unhelpful. And I did it in a slightly funny way rather than a nasty way such as what you have attempted. He didn't answer the question or even try to be helpful with understanding what is going on. Neither have you, all you have done is brag that you can change tires and go to work 300 days a year, totally pointless and unhelpful but if those things define you as a person then well done