Dyno driveline loss numbers ???

[Fastfred92]

How much of a difference in driveline loss % on a dyno is there when looking at FWD cars vs. RWD ??? I guess a better way of asking my question is which one is better ( less loss ) ???

[joeg]

Theoretically, there would be less loss with FWD because of no drive shaft and drive shaft U joints.

Very theoretical, however.

[mowog]

Based on many years spent on various dynos (Superflow engine and a couple different chassis dynos), I'll address some questions you didn't ask. Hopefully this will help you and others understand the benefits of using dynos.

I've been told the difference (loss) in dyno numbers between an engine dyno (flywheel numbers) and a chassis dyno (wheel numbers) is about 16-18%. The real answer depends on the individual dynos involved and the car. I doesn't really matter if it's FWD or RWD.

The term "best" depends on your goal. If it's presenting the best possibly numbers for resale, then you should go with an engine dyno. Even then, different brands and setups will yield different results. It will matter when it was last calibrated as well.

If your goal is to use the dyno as a tool to improve performance, then "best" doesn't matter as much as consistency and repeatability. This requires data acquisition on the dyno, and I believe the same dyno should be used each time, or you will have no chance of really knowing what change produced what result. If you are trying to find out optimum timing, or which exhaust has the least loss, or even what spark plugs to use, you need a dyno that can repeat within 1-2 horsepower. That means making a change, then putting the car back the way it was, and getting the same numbers you had previous to the change. Without that ability you will never really know what will work best in your car. Also don't just look at the top horsepower number, look at the power curve, and take into consideration the track(s) you run to determine what's best. Sometimes you will find a different setup is required.

Final advice, arrive with a plan and with parts to try. Then establish a baseline (engine and oil fully warmed up, numbers repeatable). Don't just show up, make one or two pulls, and see what you've got unless your goal is to have a printout for resale value.

[Fastfred92]

Mowog, thanks for the reply! I have a pretty good basic understanding of dyno theory, I guess I should have been more clear with my question.. I am looking to build a new IT car and I have several possible cars in mind ( each with a different driveline config ), what my real question is, I guess, is is there a trend for less dl loss in either a FWD or RWD platform ( considering chassis dyno only ). Should I look towards the high end 18-20% for RWD and lower for FWD ???? I know each car is different but wondered those with vast dyno experience could cite a trend one way or the other... Not really looking for a specific #, just trend.

Thanks!

[lateapex911]

I'll stick my neck out here, knowing tht I'll be 'corrected', LOL....
In general, FWD is more 'efficient' from a loss point of view.

General averages ae 18% for RWD, and 15% for FWD.

Of course, YMMV.

On top of that, when looking at FWD vs RWD for racing applications, factors like actual power vs tire width and suspension type come into play.

So, while a FWD car might have a lot of potential in a certain class powerwise, enough to overcome the RWD consideration, too much power for the allowable tire, and /or an ineffective suspension design could be mitigating factors. Thats more likely in ITS than ITC for example.

[MMiskoe]

Is it really a percentage of power?

Say you have a car w/ 100hp (wheel, crank, doesn't matter). You measure that you have 18 HP lost through the driveline. Same car w/ some improvements made to it now turns out 125hp (25% gain). Why doesn't the loss in the driveline remain the same 18hp? None of the parts got heavier or harder to turn.

I will admit that as you try to accellerate the driveline at a faster rate, it takes more energy to do so, but I don't see this change is a linear amount based on the original HP. I would expect it to be the original loss amount + some small increase.

Look at it on another scale - does a 5000hp top fuel car use up 900hp just in the driveline? Doubt it.

I've been reading about 18% losses on the BWW topic & have been turning it around in my head (nothing else up there), this seemed like a good place to ask.

Matt

[JimLill]

IIRC...

transverse engine FWD is the lowest loss, as there are no "right angle" gearsets.

longit. FWD may be the next lowest....

RWD

AWD

something like that order...

range can be anywhere from 12 to 22+% and it is not a fixed % across the speed/RPM range....

[mowog]

Matt, I really don't know, haven't done the back-to-back testing after gaining substantial increases in power. But in my mind numbers aren't all that important if your objective is to improve performance. So stick with one dyno, which should have some type of data acquisition/recording method, and see what your modifications show. Don't be at all concerned if someone else's dyno reads more, or less, than the one you use.

There can be rather bit differences in dynos, regardless of wheel or flywheel type. For example the same motor on a Superflow 901 read a consistent 122 bhp. The same motor on a Stuska, without computerization or data acquisition, reportedly had 131+ (1100cc Sprite motor). That was a rather optomistic dyno, to say the least. If you've ever seen a dyno like this you would appreciate how hard it is to tell the difference between 5400 and 5600 rpm, and a number like 15, or was that 14.5, or was that 15.5 - aw heck, show it as 5400 and 15.5.....sounds more impressive that way. In other words, you're pissin' in the wind if you are looking for 1-3 horsepower changes like you might have with different mufflers, but looking good of you are selling services based on high dyno numbers or looking to advertise a horsepower number for sales purposes. Even two different dynapacks can show different numbers. If you made a change significant to reflect 25 flywheel horsepower, you will certainly see a substantial gain in wheel horsepower too. If it was legal for IT, please tell me how - PM is fine. :D

[MMiskoe]

No, I didn't find 25hp on any motor, those #'s were just made up numbers that made the math easy to do in your head.

I had been thinking about it due to all the talk on the SIR page, using 18% for the driveline losses. Going on about a 210 WHP car then turning out say 175WHP all the while using the same 18%. Or the typical equation of 25% gain for IT mods, then 18% driveline loss - well is it 18% from the stock # or the improved #?

I was wondering if anyone had ever seen actual numbers posted. I don't disagree, they are all relative to the dyno being used. Although I was talking to a Dynojet guy who commented that any of the good ones will all be w/in about 3% of each other.


Matt