"Tractive effort" output from a dyno?

jacksoni · Post by **jacksoni** » Wed Mar 28, 2007 9:19 pm

Guy tuned my car on Dyno Dynamics chassis dyno today. The for some reason could not get tach reading so no HP or Torque numbers available. he tuned it by holding load and setting advance and mixture with wideband tuning to peak torque (tractive effort he says in this setting). I was told because they didn't know the gear ratios couldn't back calculate any numbers either. Says it pulled well, tuned well and made 950 lbs or lbsft of "tractive effort" which is allegedly good (for a high revving 2 liter). Anybody have any idea how to get to an idea what the engine was putting out? Peak torque is predicted (EA Pro and Dynomation) at 8500-9000 range.(about 170lbsft) I'm going racing this weekend (Maxton Mile in North Carolina) and the track will tell the tale but was wondering if anyone has an idea.
Thanks
Jack

Pete · Post by **Pete** » Tue Apr 10, 2007 10:02 pm

Jack,
Have a look on this forum (limk below) which has a discussion on tractive effort curves;

http://fsae.com/eve/forums/a/tpc/f/3326 ... 8510378821

HTH , Cheers , Pete.

Shrinker · Post by **Shrinker** » Wed Apr 11, 2007 2:18 am

Hello; the tractive effort divided by 12" times the radius of the wheel in inches equals the axle torque. then divide that answer by the diff ratio and you have the drive shaft torque exiting the gearbox. Divide that answer by the gearbox ratio and you have the input shaft torque. If you do all the measurements this way you will get engine output torque in foot pounds. Not including losses for inertia and friction in the driveline etc. If its was steady state measurement you dont have to add acceleration inertia and the friction is not much. But you could add about 40 to 70 hp to the figure at the engine depending on the wheel speed used etc. if its a little car I would go 40 hp

The backwards calculating of power is not a percentage of the engine its always a fixed loss value on a local car basis. Some people keep saying "Oh its got 500 hp at the wheels and therefore it makes 800 at the flywheel, thats rubbish. The driveline only has the same inertia resistances irrespective of the power put through it. A very small factor of friction heat in the oil comes into it but by and large people expand their engines power by using rubbish physics. Regards Shrinker.

jacksoni · Post by **jacksoni** » Wed Apr 11, 2007 7:57 am

Thanks, Pete. I think I need more data and truly not sure understand all these guys are talking about in a quick scan. Will study it.

Shrinker: Thanks, this is the type of calculation I was looking for. Am interested in your comments about the differences wheel to flywheel. Someone else has suggested similar to me and these comments clearly go against the usual "If its stick, 15% loss, if its automatic 20% loss in the drive train so if it makes x at the wheels will be x+20% at the flywheel, etc". Just like one dyno to the other or one day to the other, there are just too many variables. So many people want a number so "mine is bigger than yours" and forget that the dyno really is a tuning tool to just get the engine to produce what it should, and learn about it so you can make it go down the track faster.

Thanks all.

Jack

Shrinker · Post by **Shrinker** » Wed Apr 11, 2007 10:51 am

hello; i dont know if you need a reply to your post but I am going to say this;

If i put a 100 horsepower at the flyweel engine in a ford pickup and dyno it at 50mph in top gear it will read a hp at the wheels of say 60 hp as an example. So we assume that the driveline losses total 40 hp. Then i change the engine to a 500 hp one and re dyno it at 50mph in top gear. The dyno will indicate 460hp at the wheels because the same driveline is still spinning at the same speed and therfore has the same losses. If we were to use the percentage loss theory the 500 hp engine would only be 300 hp at the wheels. So I always say to people where is the reason for all the extra energy loss. I know a little bit goes in extra heat in the oil etc from the increased loads at the gears but I'm not talking minor stuff. Any way if all the extra power loss was reasoned to be going into the oil the oil would get dam hot real quick.

jacksoni · Post by **jacksoni** » Wed Apr 11, 2007 3:06 pm

Thanks, I was not disagreeing with you, but to the contrary and indicating having heard this from another source. What you say make much sense and is contrary to "common opinion"..Thanks for the example.

Shrinker · Post by **Shrinker** » Wed Apr 11, 2007 7:09 pm

No Worries

RPM351 · Post by **RPM351** » Sat May 12, 2007 7:10 am

Shrinker,

I own a Dyno dynamics chassis dyno and let me say its almost impossible to determine exactly what each and every vehicle losses in power through the driveline.One thing for certain is that its definitly not an amount of power but more likely a percentage! I have dynoed many engines on engine dyno's and seen what they make once fitted to the car. When I get asked the BIG Question of engine power vrs rear wheel power I can only say that I have seen an 18% loss for manuals and a 24% loss for auto's which does seem to work out close when compared to said vehicles ET and mph on the black Dyno!

John Wallace · Post by **John Wallace** » Mon May 14, 2007 9:57 am

How can you tell if the 'power loss' is from the drivetrain loss or that the car isn't setup right ?
I've seen a lot of cars that make a lot of power but their ET's are bad in relation.

Shrinker · Post by **Shrinker** » Mon May 14, 2007 6:34 pm

Hello John; You have touched on my point, my point is that a drive line is a known rotational mass and it will consume the same amount of power to rotate it and angular accelerate it every time you test it. The drive line doesn't very its power requirement just because the engine is more powerful connected too it. If the more powerful engine is tested at a higher angular acceleration rate of the drive line then there will be more power consumed of course. There is no way the drive line's rotational mass can consume more energy from the engine in a static RPM measurement. The only additional losses from more power are the eventual heat into the oil from extra loads on gear mess and side loads on bearings etc. But its not much because the oil doesn't instantly boil. There is also some power loss in flex of the shafts but that is mainly only of importance if the engine power pulsing is rough, for a correctly operating engine it is no concern.

Some cars may make lots of power but achieve poor ET because the power is not arranged in an effective way for the available gears. If the power curve is all top end then the engine will need a few gears to keep the rpm in the power range. If the power is spread over a long torque curve then the gears don't matter as much but if its an automatic then the torque converter becomes very important. We did a test run once with our race car, it was running 8.38 seconds 1/4 mile at the time with a 2 speed glide. The car owner was considering going to a 3 speed box and I was advocating that we sort out the engine first as I thought the 3 speed box was not going to benefit us greatly. So we did one run by taking off in first then going to top before the car had gone 30'. The car slowed from an 8.38 to a 8.43. So basically a one gear run was half a tenth slower. We haven't bought the 3 speed box yet and we continued along engine style development. Now the engine is a high rpm screamer and it will get the 3 speed box as its nearly the right type of engine to do that with. But the car runs slow with this style of engine in a 2 speed setup.
So you see its a match up of drive line component's that allows your engine to produce an ET.

Don Terrill’s Speed-Talk

"Tractive effort" output from a dyno?

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