Chassis dyno question

Old 06-06-2007, 06:44 PM
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68bbnova
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Default Chassis dyno question

When running on a chassis dyno, I know they take the run in high gear in the trans (1:1), but will it make any difference on the charts with different rear end gears? I guess what I am asking is will the horsepower look the same with different gears, just the MPH would be different? Sean
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Old 06-06-2007, 07:17 PM
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obsessedtruck
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Default Re: Chassis dyno question

Originally Posted by 68bbnova
When running on a chassis dyno, I know they take the run in high gear in the trans (1:1), but will it make any difference on the charts with different rear end gears? I guess what I am asking is will the horsepower look the same with different gears, just the MPH would be different? Sean
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We have a Dyno Jet Chassis dyno at the shop. Yes the hp and tq readings WILL be differant. Heres a good example.

My pull truck has a 1003hp motor 6.20 gears makes 700hp at the wheel. thats a 30% loss.

Friend of mine has a 970hp motor with 5.38 gears makes 740 at the wheel. Same drivelines just differant gear ratios.

As the numericaly lower gears it gets closer to the engine hp. Cause it is getting closer to the 1 to 1 final ratio through the drivline.

Hope this explains things some.
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Old 06-06-2007, 07:39 PM
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woodsman
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obsessedtruck at the risk of sounding dumb which of the two trucks would have the fastest ET not MPH. I am just trying to get my head around this. Wesley
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Old 06-07-2007, 05:06 AM
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billhendren
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The transmission is a big power hog,far more than the gear ratio.our 420 cu.in SB2.2 super late model engine makes 844 H.P. and 714 at the rollers on our Dyno jet. trans is a Bert which is straight through and rear end is quick change.this combination averages 18.2 loss as compared to the same engine on a engine dyno.drag cars with powrglides and a 9" average 25-28% loss on the same dynos. changing gear ratio on the car has the same affect on power as changing acceleration rates on a engine dyno.if you test an engine at 600 rpm per second increase then test the same engine at 300 rpm per second increase the 300 will always show more power because the engine is using less power to accelerate itself and the drive train at the faster rate.Bill
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Old 06-07-2007, 07:15 AM
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edvancedengines
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I am not a Dyno Jet operator but I have had several customers putting their cars on them. My experience is that on chassis dynos at least of the rolling inertia wheel variety, that there can be no reasonable comparison to real world going down the track rear wheel horsepower is.

I think that a lot of this is becuase cars with any power and racing torque convertors are just blowing through the convertor. I see no other way to explain losing several hundred hp away from what was on engine dyno or for ending up with some outrageous horsepower curves that are far different.
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I see much better hp numbers in essentially the same engine combinations when the car is sporting a clutch and manual gear box.

Another huge probloem in my mind is the way they weight the wheel. The only way it an be close is if your vehicle is for what the wheel is weighted for.

So in summation, for me rear wheelo horsepower on a chassis dyno is used as a standard only and when used again can show results of changes only. The actual numbers are not real as compared to going down the track.

Most chassis dyno operators will tell you that the gearing of the car does not make much difference. I know better. Like was earlier mentioned, it does play a major part in how quick you get the rollers to moving quicker.

There is no set percentage of loos when using an automatic tranny because each convertor all by it's self will show diferent results. The old school high stall torque convertors that use the reverse pitch technology really suck.

Ed
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Old 06-07-2007, 08:23 AM
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Ed- i am a common sense practical kind of guy ..so what you just said makes very good sense to me..

Brian
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Old 06-07-2007, 02:31 PM
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The fixed weight dyno is by far the most accurate because there are no variables. if I try and overlay three runs from my Superflow engine dyno there will be variations from 10-15 H.P. because of the load control valve trying to maintain 300 rpm per second.with the Dyno Jet all the runs will be within 1-2 H.p. unless something changes on the engine or drive train. the power is calculated by Newtons 2nd law mass X acceleration so if you accelerate that 5000 lbs or so it will show you exactly what you have at the wheels.as I said before power glides average 25/28% with a 9.0 ford,same engine with Lenco in direct is 15% same with a super T10
what gets you down the drag strip is the net rear wheel H.P. not what the engine dyno says.
drag cars with N20 converters will show more rear wheel H.P than engines with very loose converters because there is less H.P. wasted generating heat in the converter. two cars one lenco and one powerglide that weigh the same,hook the same and are aerodynamically the same and both put out 700 H.P. at the wheels would run the same number even though they had several hundred H.P. difference on the same engine dyno.Bill
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Old 06-07-2007, 02:42 PM
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billhendren thank you for that. That was just what I was looking for now my mind is clear. Wesley
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Old 06-07-2007, 10:19 PM
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edvancedengines
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I do disagree but no need in me arguing. I just know what has happened with my own customer's cars and the chassis dyno is off by a mile compared to drag strip performances. In those examples though the convertors are old school and were high stall.

I akso know of a mild warmed over 350 in a Camaro with a Vette Convertor that did the same exact number to the rear wheels as his friend's raunchy 421 SB with a 4,500 convertor. The 350 was much slower on the track but on the chassis dyno they were both the same. those guys worked at the dyno shop too.

Ed
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Old 06-07-2007, 11:35 PM
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Ed let me ask you one thing about the chassis dyno would it help to get you close before you get to the track. I just need to know if it will help because when I get my car together I want to get it close before I go. But if its a waste of time than I will save my money. What can I do with a new car and a new set up give me some kind of heads up. Wesley
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