Re: chassis dyno
 

Only have used Dynojet on it you get car in hi gear or 1-1 get rpm steady as can for start RPM hand held controller gives operator green light then driver goes WOT get to upper RPM point let off throttle dynojet has airbrakes on drum that stops drum after pull.
You can start whatever RPM you want but has to be around converter stall,if you try to start below that it's just going to go straight to flash point of converter anyway.
Dynojet has knurled rollers the one I used has 48'' rollers the bigger the rollers the better the traction you get I've heard small rollers are hard to get repeatable results on,I still like engine dyno over chassis anyday,It's alot easier to get them to repeat,one reason is you eliminate a bunch of varibles getting rid of driveline and you can control your oil/water/air temps on engine dyno.
As far as numbers go the Dynojet numbers were real close to what they should've been compared to what engine did on our Superflow 901,the HP was about 25% less at the wheels than engine had on Superflow 901,the dynojet is about 275 feet from the Superflow and cell.I'm just talking about one car I know well my own.
When you start getting big #'s it starts slipping tires,had Procharged SBC was in 990's HP range @ wheels,had 10.5 X 28.5 MT's it was spitting a little rubber mist out the back,had dodge w/cummins was flat lineing TQ from 55-85 mph on graph TQ shows up to 1200lbs graph went up steeply 55 mph would flat line till about 85mph don't know how high it peaked took about 1.7 seconds to go through pull,may had traction issues,but could'nt notice them.
The main thing is don't get caught up racing dyno's,don't worry about what someone makes on another dyno,learn how fast X amountHP&TQ on dyno you are using will make your car go.
Mike Taylor 3601

Re: chassis dyno
 

My car, running a normally-aspirated VERY MILD 360 Magnum Mopar engine made 260 RWHP on a Dyno Jet dyno, with a 2.76 rear axle ratio.

If I had swapped a 4.56 gear into my car on the spot and had immediately, re-run the test, I am SURE I could have accelerated that inertia roller a lot more quickly.

Would that have altered the "reported" HP output, since it did the same amount of work more quickly?

It should have, or maybe I don't understand physics.... a definite posibility.

If NOT, then how is the Dyno Jet type of computer able to deal with this variance, and give us a legitimate HP number with both rear axle ratios?


Just askin'....

Re: chassis dyno
 

UTI had a invite and I was able to have 2 free pulls. Like stated you need to be in high and close to stall. First pull n/a with 406sbc was 525 hp. Car was pretty steady tied down. Next pull with N2O car made 770 hp and with loose front being tied by operator you would have thought the car would not move. The higher the rpm's went the higher the front end would rise. It was getting pretty scary and the kids started moving away.Anyway with those numbers it showed around 20 percent loss using et of car and BG calculator to calculate flywheel hp. 10.2 et n/a and 9.1 et on n2o.

Re: chassis dyno
 

Uh, my understanding and Ive never run a car on an inertia dyno.

But Gear ratios are taken into account on a Dynapac.....

I wondered the same thing until we had to plug that data in.

Re: chassis dyno
 

A chassis dyno is only a tool for measuring changes on that car, on that day. You can move it off the rollers, then put it back on and tie it down again and often get a couple hp difference, due to tire's alignment with the rollers. Lower gears on a Dynojet (or any inertia dyno) will often show the added loss through the gear set. It calculates the actual gear ratio, so there is more to it than simply how fast the rollers spin up. Lighter tires, wheels, drive shafts, flywheels, etc, show more power because the rollers will spin up faster. Comparing rwhp between cars with different drive train components is a joke. They are not as accurate as an engine dyno, day to day, for many reasons. It is easier than pulling the engine to use an engine dyno. It is simply a tool, with it's limitations. All tools have their limitations. My car, with it's 6000 RPM converter, won't repeat within a couple hp because tranny fluid temps vary so much, effecting the converter's slippage. Two pulls within 5 rwhp is close. It's quicker and easier to check your air/fuel ratio and seat the rings. My car has not been on my own dyno for over a year now.

Re: chassis dyno
 

Very good point, I guess you just take certain things as a "given" but others may not know this.

Years ago on small engines my brother and law, we used a well pump with a captive air tank, hook the engine up and see how long it could it took to make X pressure, if the next run was better we knew we had an improvment, worked good for baseline from a given. No way to rate horsepower but it gave a refrence point, until the tank went farther south that it already was and we scrapped the whole contraption, I have always wanted to rebuild the design with computer refrence to somewhat accuratley measure HP.

It was actually a wonderfull tool, it really helped up peak out the engine we did it for, better than "seat of the pants" which at 75 across a plowed field was a bit dicey on the cart (an Oddesy like buggy)

A "hub" setup for a car , a "home" dyno as it were would be slick.

We had lots of problems getting an accutate measurment with our converter as wel.

Re: chassis dyno
 

I have personally done over 1000 runs on a Dynojet ( up to 800 HP at the wheels) and learned about them from a man who did some of the development with Dynojet and has about a half million runs under his belt. Mike gave some good info. The single roller type, big diameter roller are best especially for slicks. the double roller type are very hard on any tire. Dynojets with the 48" diameter drum are available with different units (slugs) of intertia. Perhaps the most common simulates a 4500 lb. vehicle. I have found NO difference from tire diameter or rear gearing. HP is defined as the rate of doing work and is therefore measured directly by an inertia dyno. You change (multiply) torque with gears, etc. but not HP. Once being sure to keep engine temp, fuel temp, etc. under control, I have found the Dynojet reproducible within about 2 HP run after run.

Re: chassis dyno
 

I was on a Dynojet the Wednesday before Commerce, And it was set up for a 3500 pound car. They told me the number would be wrong but you could still see a help or hurt situation. I didn't understand what they meant I thought it was just reseting some of the hardware. But now I see why they didn't recalibrate the dyno.
I made 217 whp, on there with it set up for 3500 pounds and I was 2500 Any idea of what I was really making?

Re: chassis dyno
 

I have used mine almost daily for over ten years now, and have had several repeat customers put lower gears in their cars and always show less hp than before the gear change. Every shop I work with has observed the same thing. I don't know about other brands, but it is common knowlege with Dynojet owners. 9 Ford rears always show a loss, as do Dana 60s, compared to 12 bolt GM rears, in the same car with no other changes.

Re: chassis dyno
 

Art,
I'm thinking 217 at the wheels. I think the weight is used to figure a quarter mile time.
Pete