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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Re: Chassis dyno question
 

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.

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

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

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

Ed- i am a common sense practical kind of guy ..so what you just said makes very good sense to me..

Brian

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

billhendren thank you for that. That was just what I was looking for now my mind is clear. Wesley

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

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

The chassis dyno will get you very close,we tune top running cars all the time and usually pick up some power which shows up on the strip in the time slips.the key is the weight of the drum never changes so there are no variables. if you don't change anything the runs will overlay within one or two H.P. if your tuning on the drag strip its hard to see 2 H.P. gain but on the dyno if you do several things that pick up two H.P. each you will see the results at the strip.about the only thing you cant do on the dyno is tune for traction on the starting line or converter flash.most runs need to be started at an rpm that is close to converter stall.find a chassis dyno near you that has a wide band A.F meter and you can get your jetting dead on,play with ign timing,valve lash different headers or collector length,engine trans and rear end oil weights and types and many other things,all without using up tires and having to wait for other cars to clear the starting line so you can get another pass.Bill

Do not get me wrong. I love chassis dynos.

I like to be able to get my customers to get on them, becuase it isd a wonderful tool to help with a lot of tweeking. You will learn things on a chassis dayno that could take you many months of track testing to learn when it is used as a tool.

I hate chassis dynos when it comes to quickee 3 pulls to get rear wheel horsepower for the sake of bragging rights. I had/hve a customer that has a Mopar RB BB and we got him a little over 670 hp using his Mopar heads when measured on an engiine dyno. He put his car on a chassis dyno and the rear wheels showed a little over 360 hp. He was furious with me at that time. What happened to my 670 hp? He asked screaming at me. The engine hp was still there but not on a chassis dyno it wasn''t showing, becuase he has an old school 5,500 rpm TCI torque convertor in his car.

If you have some extra money and want a little fun, seek out all the chassi dyno in your city, {in mine there are many] and take the same car to all of them. You will think that you were testing with different cars when you see all the results side by side.

If you use a chassis dyno as a tool to check and get a baseline of what the car is now, and then use it to monitor any changes in performance and go to the same chassis dyno ecah time, I love them.

Fact of the matter any dyno, chassis or engine, is still to be used as a worthwhile and valuable tool. Even with engine dynos you will not get repetablity from one dyno to the other.

By all means, I do highly recommend using any dyno you have an opportunity to use and to pay for some real dyno testing. It is money well spent and can save you lots of future grief.

Ed

On your mopar, converter or trans slip would affect H.P. as well as any brake drag,excessive rear end friction etc. could also be fuel supply,200 deg air entering the carb,dist got bumped when installing the engine, or a ton of other stuff. a good dyno operator should have been able to find the problem.that engine if it was an automatic trans should put down around 530/550 to the wheels so there was a problem somewhere.
Since dyno Jet drums are all weighed and the electronics calibrated at the factory to all read the same and have absolutely no user changeable adjustments they should show the same numbers going from one to the other assuming same car and same air temp, pressure etc. other chassis dynos that use a load control instead of a fixed weight will vary quite a bit.Bill

Bill an Ed. Point well taken and I will use this info to my advantage for sure. I do understand that there is no time like track time but I am new in this game of Procharger engines and also this amount of power. I feel that I have a very large learning curve looking at me with this whole set up of car being run off of a lap top and so on. But Conley's in Houston has been do this for a long time and don't seam to be the kind that tells you what you want to hear but the truth. So will give it a go and learn something in the meantime. Thank you guy's for your point of views because there are so many ways to look at the same picture. Wesley

ok so let me get in on this.... how much power does my motor make??? i've never had it on a dyno but i do know what the car weight is and i have a best et to go off of....

2750 ready to race and best et of 8.962

i run a glide with 4.33 rear gears.....i know that it won't be exact, but just a ball park figure would be nice 8)

buffjhsn1 there two places that I like to play around on one is Tim McAmis race cars under cool stuff and Wallace Racing automotive calculators go to these web sites and you can learn something and have some good fun playing around. Wesley

Woodsman Wesley :),
Conleys' has a Mustang 1750 with the Eddy Current attachments if I am correct. The results you see there will be very helpful for you. Jeff and I think it is Chris know what they are doing.

Let me tell you an unbelievable but true story just recently. A car from Louisiana late model Purple or Blue (?) Mustand with a Pro Charger was on a Chassis Dyno for several days with the making adjustments and tuning corrections. The following week they loaded it on the trailer and went to Indianapolis with the car never yet going down a race track. It qualified as # 1 in the Radial Tire Class the first day. By Eliminations it had been bumpoed back to 2nd qualifier. When the race was over it had won the whole shebang. Fisrt time down the race track and it got everyone's attention. First race and it won him many thousnd dollars.

So,
Does that answer your question about how much a chassis dyno can help if you are using it as a tool, instead of just a horsepower detector? lol.

Are you planning on coming to the big East vs West vs Gulfcoast Shootout at Evadale on July 21st? I will be there. If you are there just ask one of the two promoters who I am. They both are my customers.
We are having big heavy hitters form both coasts coming in for certain. Two magazines will be covering it, and it should be a fun and good show. Look forward to meeting with you. Now If I could get one of those guys to pay attention to my Chassis Article maybe his car would quit playing leap frog. lol

Ed

Ed you are right about Chris's car and that was a very good news story.Vicki and I are going to Paris, France to look around for a week and will do our best to go see the boy's run on the 21st July never been to that track but its not more then 2 1/2 hours away and will look you up to put a face with a name. Looking forward to meeting you in person. Wesley

Guys, Thanks for all the replys. I asked the original question because I have 4.10's in my car now, but am going to change to 4.88's, and was wondering if nothing else was changed but the gears between dyno pulls, would the runs look different? Reading all this it looks like maybe yes, maybe not, but to just use it as a tool to get higher readings at the same dyno. If I have the extra coin to run it with the 4.10's first, then change, I will post what I find. Unfortuantly, I think I am only going to be able to do it after the 4.88's are installed, but now I am curious! Thanks again, Sean
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On the same dyno with exactly the same atmospheric conditions changing from a 4.10 to a 4.88 will show a few less H.P. at the wheels because your using some of the H.P. to accelerate the entire rotating assy at a quicker rate.its just like on an engine dyno if you go to full throttle and hold the engine steady at say 7000 rpm and take a reading then run a sweep test at say 600 rpm per second increase as it passes 7000 and takes a reading it will show 10/15 H.P. less than the steady state test because of the power being used to accelerate the internal engine parts.on the strip the lower gear may help you because the engine will get into the power band quicker after leaving the line.Bill

buffjhsn I was wondering if you got some ball park #'s I came up with 620 hp at the rear wheel an 689 hp at the flywheel. I think these calculators are close but I dont know about being spot on. Wesley

well i have a formula i use to get rear wheel hp.... it is as follows

hp=weight*200/et*et*et
hp=2750*200/8.962*8.962*8.962
hp=550000/719.8
hp=764

i went to tim mccamis like you said and used the calculator. it says that i have
755.1 rwhp and 981.63 at the flywheel.

i just have a hard time believing this pooch of mine make almost 1000hp... i also have a hard time believing i lose 230 horse to the tires....


i have used my formula on a buddies dragster that makes 900 hp on the dyno and he had 900 horse at the rear wheels on his best pass....so thats why i wanted some input. making 1.512 horse per cube is respectable outta a 505 with box stock heads and intake.

i don't believe that you lose 25% to the tires. i understand it takes hp to initially turn the drive train but everything you put into the rear gears, converter, and trans is multiplied to the tires. granted you lose some through converter slippage, but 25%???

once you get the mass moving in stays moving (newtons law)....

anyone have any thoughts on this???

what numbers and or calculator/s did you use?

buffjhsn1 I used Wallace Racing Calculators (hp from 1/4 mph,et and weight calculator) I think they are the closest. I just put your et and weight in to get the #'s if you put in the MPH than you come up with 755hp as you did and that looks much better. Sorry if I stole some of your pony's. I am a little new at this and did not input all the info. Well I have learned another good lesson on RJ. Wesley

wesley,

no big deal, i dunno what is right or wrong. all i know is what it runs in the 1/4 and what it weighs.

buffjhsn1 I think I am trying to help to soon. So will just sit back and let the boy's that know what they are talking about give the advice. And thanks for your understanding. I love this stuff and will get great enjoyment out of it. Thanks again. Wesley

The ET calculator says about 750 RWHP