93ChevExt5.7

So how much truth would there be in the mph of a dyno? If dynos can be off by about 20hp (which is somewhere around what I've read here before), then how accurate can the mph be?

gametech

iTrader: (1)

On a properly setup dyno, the MPH will be extremely accurate, as long as the tires aren't spinning on it. The dyno MPH calculations are based on the rpm of the steel roller. As long as you have no tire spin and the steel roller size is properly input to the dyno software, the MPH will be dead-on.

KrautBurner

iTrader: (5)

I would have to think that something was F*CKED up to get an incorect speed.

mine was nuts on with my spedo in the car.

GOaT Cheese

I have a sugestion... Let us just assume ALL chasis dyno numbers are bullshit, and only track times speak of power output.

93ChevExt5.7

Hi GOaT Cheese - I would, the problem with my vehicle is this.... Speedo only goes to 85 (its a truck right - guess no one went past 85 in 1993

On top of that, tracks only calculate quarter mile. I could go to a race track I suppose, but I don't know who is going to let me run around that thing in my truck to see what its top speed was.

This is what thought was interesting and why I asked the question in the first place. The dyno reported back the truck had gone 142mph....only a few mph off from the "all powerful" Dodge RAM SRT-10.... thought it intersting that I could "hang" with a 500ci engine.

dynocar

Wow, you sure have it in for chassis dynos. I'm not to sure that I can change that but I am pretty sure about this, if you put your car on my chassis dyno and I find a way for you to average more HP, if you do your part, you will go faster. As many pro racing teams have often stated, "we can accomplish more on a chassis dyno in an hour then several weekends at the track, one of the best tools ever invented." Most have them.

sprayjunkie

I see your point, and you are right, but for the majority of internet dyno racers, this is not how chassis dynos are used. Very few people could tell you how much power their car made on a chassis dyno at any point other than the peaks. Most of these guys think you can compare a dyno run done in Florida in July to one done in Maine in December, and as we know, that is just not the case. The point about using the track to determine power output is a good one as everyone is running 1/4 mile and the MPH will indicate true power potential.

dynocar

I agree with peak Vs average dyno numbers.

If the dyno is properly operated with SAE atmospheric correction, which many are not, and similar types of dynos, I would trust your dyno example much, much more, in theory, no difference. A 1/4 mile track near sea level shows a much higher MPH then most other parts of the country such as here in Omaha or Denver.

2xLS1

iTrader: (25)

Just because your truck can spin the dyno drums to 142 mph doesn't mean it has the HP to push the vehicle that speed on the road.

93ChevExt5.7

^^ which is the reason for me starting the string and this specific question.... I know the answer to some point...you've got wind resistance and blah, blah, blah and you have SAE corrections which are supposed to take into account temperatures and blah, blah, blah.....that's why I thought it was a good question to ask.

So the question is....how accurate can the MPH really be?

waSStock

+1 for wind resistance. I hate the internet sometimes. A dyno is a tool to help you find power and problems. It is not magic. It is not some kind of godly being that predicts the future. It is just a fancy bathroom scale. Use it to tune and diagnose, and it will become your best friend. Use it for bar stool bragging and bench racing, it will drive everyone crazy. Like I have always said, I dont care if the dyno tells me I am making 3 warm fuzzies, or 4 warm fuzzies. Just be consistent, and show me the warm fuzzy difference after I make a change. If you want to find out how fast your truck goes, buy/rent a radar gun. A loading dyno can simulate wind resistance, but it is not an atmosphere simulator. Besides, you would end up spending more in proper fans and ventilation than the building itself. (wind tunnels are expensive, however, the ideal chassis dyno cell) I apologize if I come off like a dick, I am really not trying to be one. I just can't stand the myths and confusions of dynamometers. Peace.

gametech

iTrader: (1)

I completely misunderstood the point of the original post. A load bearing dyno could possibly be setup to account for high speed aero drag, but in practice the numbers would need wind tunnel data from your vehicle to be accurate. I thought you were just asking something to do with speedo calibration.

93ChevExt5.7

Nuf said - let the thread die. Yes...agreed the best way is to take it out to a track....what the dyno was telling me was the reason for the question. Even out at the track, your gonna have heat, driver, wind resistance, tires..other stuff (besides the darn thing not breaking down on you while your out there - right?)...its not accurate.....good enough for me...

GOaT Cheese

Oh, yes, I agree completely, chassis dynos are invaluable for measuring changes in average h.p. output. If all is equal you can easily tell if a given mod helped or hurt you. My problem is the actual number produced. For example if my stock '98 Z-28 put out 1000 RWHP on a chassis dyno, and I change out the camshaft, now I have 1200 RWHP, the engine is making more power, but do I believe the number? No, but at least I know the camshaft did help.