Chassis dyno question

[AdioSS]

How much of a difference does the gear ratio you dyno vehicle in? I know standard procedure is to do it in the gear ratio that is closest to 1:1. However, in my case it wasn't possible. My truck has the 4L60E, 3.23 gears, and 30.5" tires. I did my pulls from 2000-6000 RPM. In 3rd gear (1:1) that would put me at nearly 170mph. My govenor is set at 120mph, so clearly that wouldn't work. I had to use 2nd gear (1.625:1) and I'm wondering how much it affected my results.

Some people have told me that the extra gearing might've increased my numbers, but others say it probably lowered my results. Nobody seems to know for sure.

To make things stranger, it was done on a Mustang Dyno set to Dynojet settings.

[MileHighMan]

How much of a difference does the gear ratio you dyno vehicle in? I know standard procedure is to do it in the gear ratio that is closest to 1:1. However, in my case it wasn't possible. My truck has the 4L60E, 3.23 gears, and 30.5" tires. I did my pulls from 2000-6000 RPM. In 3rd gear (1:1) that would put me at nearly 170mph. My govenor is set at 120mph, so clearly that wouldn't work. I had to use 2nd gear (1.625:1) and I'm wondering how much it affected my results.

Some people have told me that the extra gearing might've increased my numbers, but others say it probably lowered my results. Nobody seems to know for sure.

To make things stranger, it was done on a Mustang Dyno set to Dynojet settings.

While I don't have an answer for you. I think I would've put it in third gear, and just took it to 120mph. Sounds like to me, your truck needs more gear considering the tire size. Just curious, what were your power numbers? Good Luck. Dan.

[rskrause]

You get slightly lower numbers in a lower gear. I am not sure why, but I see it pretty consistently. A chassis dyno is good for tuning, not necessarily getting an accurate number anyway.

Richard

[1989TransAm]

I had the same problem with my Tahoe. The tires did not like going above 120mph. So they had to run it in 2nd gear. They had a formula all worked out and on my vehicle they said to add something like 17% to the horsepower and torque numbers.

[AdioSS]

I know that more gear (or shorter tires) would help, but it's my daily driver, and I like the highway fuel economy it gives. If I keep it i my head that I want to get good mileage, I can get over 20mpg. My best has been 23.5 with the mirrors folded in and staying at 65mph one night.

it put down 255hp@5250 & 274tq@4050. Stock engine rating was 295hp@5200 & 335tq@4000. It only has a stock replacement K&N filter, cheap muffler exiting in front of the rear wheel, and a mail order 87 Octane tune from Charlie Wheatley. These are my "before" numbers.

[AdioSS]

I had the same problem with my Tahoe. The tires did not like going above 120mph. So they had to run it in 2nd gear. They had a formula all worked out and on my vehicle they said to add something like 17% to the horsepower and torque numbers.

that 17% should be to convert rear wheel numbers to engine numbers through the automatic.

[AdioSS]

and if anybody would be interested in a visual aid...

[1989TransAm]

That is correct. It was in addition to the normal percentage with a 1:1 drive. Also that was with my particular combination.

[don2]

The biggest worry, especially with the trucks is not necessarily the tire rating, but the critical speed of the stock driveshaft. A few people have thrown driveshafts when spinning them up past the 120mph mark. A lot of the difference in the hp readings between gears is the frictional losses that vary due to different acceleration rates and torques on the drivetrain components.
One thing you have to be careful of is the afr and spark requirements change between gears based on the loading. You may find that it is ok in 2nd, but will detonate or go leaner with the higher loading of the next gear (i.e. 3rd).

Not quite sure why they would want dyno-jet settings other than to inflate the numbers.

[AdioSS]

that's exactly why the operator set it to dynojet numbers. Of course he didn't even ask me what I wanted, and I didn't think to ask until after my truck was off and another car was strapped down.

With the 3.23 gears and 30.5" tires, I am able to hit 100mph in second gear. There was no reason for me to use third gear on the dyno since it is very unlikely that I'll ever be at WOT in 3rd gear. I'm not even sure if my truck has the long 1 pc or the 2pc driveshaft. I should crawl under there and look just to know for the future.

[rskrause]

I still think you are kind of missing the point of a chassis dyno. They do not produce the same numbers as an engine dyno. If you accept an engine dyno as the "gold standard", the chassis dyno is often quite inaccurate, regardless of any correction factor you apply. However, the numbers from the chassis dyno are reproducible (produces the same numbers under the same conditions) and fairly precise (detects small changes). This makes it an excellent tuning tool. Think of the numbers like trap speeds at the track. The advantage over the track is obvious though. It takes traction out of the equation, it is easier on the car, it is possible to get a number of passes in less time, it is easier to use instrumentation, you are working in a shop with access to a full range of tools and test equipment, etc. IOW, it is for testing and tuning, not for measuring hp per se. Also, with any luck the dyno operator is going to be very experienced and can often spot problems of make tuning suggestions.

Richard

[AdioSS]

Rick, I do get it. There was an engine dyno at SAM while I was going to school there. I saw many engines run on it.

In the future I will be going back to the same dyno to keep track of my progress.