Rebel_Heart

Just had my engines run on the dyno. I was wondering if all dynos would come back with the same results? Is it a function of the dyno or the person operating the dyno? How many hp or ft lbs of torque might it be off? I am new at comprehending the dyno and trying to understand if these are comparable numbers to what Mercury publishes for their engines? Thanks.

Wobble

In a word, NO.

All dyno's are supposed to be referenced to an SAE standard in theory. In actuality a dyno test is only repeatable with the same dyno and operator if you want back to back comparisons.

Two of the better dyno shops here in Houston dynoed a small block motor on the same day in the morning for just this reason and came up with a 20hp difference at 400hp. Same brand of dyno also. They eventually were able to get them to read fairly close with some input from the manufacturer.

Edit: this comparison was done with uncorrected data

Whiteknuckle

In theory they should be, but they are kind of bathroom scales. Each one will be calibrated different than each other, and some are just higher quality than others. Also add the variable of the dyno operator and how he makes his correction factors. When I have a engine run on a dyno after a rebuild I do it for several reasons. First, for cam break in, oil pressure checks, and to see if everything is tight, i.e. no leaks. It is better to get all major issues taken care of before it gets in the boat. For a tuning aid to at least get it close for initial boat trials. And as a comparison to the last rebuild. You can get a good idea of the success of the rebuild, but the real test is when it is in the boat.

Rebel_Heart

Thanks for the replies. I had no idea there was that kind of disparity. I was speaking with someone about my numbers and he said that the dyno/and or operator may not be accurate. Now I know what he was talking about. Just how much can a dyno be off?

articfriends

iTrader: (1)

A good dyno with the right sae correction factor for actual conditions (altitude,air density,air temperature etc) should be accurate within a few percent to another dyno that is also corrected PROPERLY and using the same exact brand and style of exhaust . Lately though all I hear about is OUTRAGEOUS dyno claims with BS numbers. Lets say you build a good 502 with excellent heads,cammed perfect and it dynos at 565 hp using marine water cooled exhaust and all your stock accessories and total timing that can actually be ran for a sustained period of time,jets in your carbs that will give you a MARINE safe air fuel ratio (lets say 12.5-1). Another guy pays a shop BIG BUCKS to build him a motor and he is expecting BIG numbers for the bucks he spent. Lets say they build him a motor IDENTICAL to yours down to the final details. Now they dyno it using dry,stepped tube dyno headers and no accessories,WOW, this other guys motor makes 60 more hp (625hp)than yours and its identical. Now lets take it a step further,they PROMISED him he'd make 675hp,so they bump the timing to 38 dgrees total (yours was at 34) and they dump a 50/50 mix of race gas in,they jet it down to the point the air/fuel ratio is no longer safe for a sustained run and they do a QUICK pull,holy smokes,now this thing is making 650Hp,still not the 675 they promised though. Motor builder who is also the dyno operator takes a turn on the OL' correction facto knob a extra 5%,another quick pull,bang we're making682.5hp. This guy has 117 more hp than your motor with IDENTICAL cam,heads,induction etc. They print him off a pretty piece of paper for bragging rights,settle up on the bill,jet it back safe,turn the timing safe and send him on his way. You think you got decent power until you run into this guy and you lay your dyno sheets next to each other and can't believe the kinda power his identical build is making! The only problem is lets say his old motor made 430hp before the big bucks were spent and his boat went 65mph and now it goes 72 mph (1 mph gain for every 20 ACTUAL hp gain),he was thinking he'd see 80 mph. Its because he actually only made 565 hp also,the rest was smoke and mirrors and bragging rights.
I here and see this ALL the time,Smitty

onesickpantera

iTrader: (1)

Well said!!!

Happens in the snowmobiling industry ALL THE TIME!

cstraub

On a 500 inch boat engine with supposed 750Hp I have seen a correction factor of 120%. On 358 CID circle track engines I have seen a dyno'd 70HP increase equate to a .5 second slower lap time on a 3/8" mile track. I have seen a 330HP 4 cylinder sprint engine lap a 450HP 4 cylinder sprint engine. Depends on what you are doing with the dyno, R&D or M&S...

Wobble

Marketing and Sales... LOL

KAAMA

iTrader: (1)

Yeah, a friend of mine sent his naturally aspirated 540cid engines to some smooth talkin' guys we entrusted our engines to for a rebuild a few years ago. They gave him a shady looking dyno sheet that said his engines made somthing like 704hp. He put them in the boat and it didn't go any faster on the GPS than when they originally made 585hp with the same props. They didn't even act like they wanted to REV any more either.

The same people who rebuilt his engines proceeded to tell me my 565cid naturally aspirated engines made 787hp on the dyno after they rebuilt them and they always had some excuse as to why they didn't provide the dyno sheet. No wonder, ONE (just one) of the reasons was because the "custom cams" I pulled out were only 220*/220* Heck, we ended up finding so much stuff wrong including wrong sized head gaskets that we never did get them in the boat. Ended up having to strip them completely down and start all over again. Live and learn.

Unfortunately, my current engine builder says he has seen this kind of stuff happen all the time as well----especially when guys bring their engines they had built from other shops. Not only do some have inflated/exagerated HP numbers, they also seem to have lots of other problems like reliability issues as well.

Now, you guys be careful----there's some SHADY FELLERS out there lurkin' for ya'll

articfriends

iTrader: (1)

I went off in left field about all the dyno numbers and "variables" surrounding them in my first post,to help get your question back on track I have heard that merc uses a MORE conservative correction factor (something about 77 degrees vs 65 degrees corrected temp) and their motors normally exceed their ratings when dynoed at a std correction factor by a few percent. There are others on this board who can explain it better than me as I don't have any personal experience dynoing a stock merc motor,hopefully they will chime in,Smitty