Chassis vs Engine Dyno

[firebob]

I had heard in the past that the reduction was 15-20% but recently I have been hearing more like 30%.
Does anybody have both back to back numbers? I know rear gear, trans and tire size all have some effect on chasis figures.

Robert

[oestek]

We did a story on Hot Rod TV several years ago with Dave McClelland's 1970 Buick GS455 4-speed ragtop... the engine was tuned on the engine dyno, then installed and run on the chassis dyno. It was a stick, but it lost about 14% once it was put in the car with and with the accessories.

[silver69camaro]

What dyno? Mustang or Dynojet? Those numbers are completely different from each other. I've seen a 427 make 530+HP on a engine dyno then lay down 350HP on a Mustang chassis dyno. Obviously it's putting more than that at the wheels, but the numbers are only good for back-to-back comparisons anyway.

[firebob]

I'll have to go back and check on what type.
I remember when I picked up my last motor from the machine shop and the guy said they had a dyno there that guys bring their cars in to get them dialed in and then go down the street to the other place to get the slips from the other shop to show their friends. He said his dyno only had one sensor and the other one had like 27. I guess you get what you pay for.

Robert

[ss dave]

Steve Rupp is a moderator on this site and owns Bad Penny. His LS2 had 575hp on engine then a Mustang dyno had Bad Penny at 441hp. 23% loss with a manual tranny.

[HarleyR]

Unless the dynos runs were on the same day in the same conditions your factors are bound to be off... Most people read way to into dyno numbers....Calibration of equipment...correction factors.....And 99% of shops out there that cater to a "street" crowd are bound to inflate numers or move corrections around to make the customer feel better..I have been on both ends.... Sometimes people's feelings get hurt....

[silver69camaro]

Steve Rupp is a moderator on this site and owns Bad Penny. His LS2 had 575hp on engine then a Mustang dyno had Bad Penny at 441hp. 23% loss with a manual tranny.

A Dynojet would have measured about 480HP, a 17% loss at the wheels. So which one is correct? Better yet, who cares?

[ss dave]

That's what I've heard, that Mustang and DD dynos measure less. Matt you state the difference between the Mustang and the Dynojet is 8%, and I've been told that the diff between a DD and a Dynojet can even be as high as 15%. More trivia.

[Fuelie Fan]

Like somebody already said, there's so many oppportunities for dyno operators to screw with settings that you can't even assume such percentages between dyno manufacturers.

Bottom line: if you want to know how your engine performs but aren't intending on making changes, don't waste your time on a dyno. Go to a track. It's cheaper, more fun, and ultimately the only "true" gauge. If you want to begin making changes to your engine in an iterative approach and want to see before/after, then a dyno is very repeatable and hence quite useful for such work. Also, if you want to play with gearing, a dyno curve will, regardless of scale, give you a good picture of the engine's best operating range, so that can have a lot of value as well.

[68sixspeed]

the 1/4 mile dyno don't lie... but shows up a lot of lies from tweaked engine dynos.

[streetk14]

I think 15-20% is the norm depending on manual or automatic transmission. 30% is a big loss, and the only way I see that happening is if you had a very inefficient drivetrain with an auto and very high stall speed torque converter. It could also seem like a larger power loss if the engine dyno session was done with no accessories, electric water pump, better exhaust, etc.

But yes, all dynos read different and some shops do inflate numbers to make themselves look better. There are also differnt brands that will provide different results. As stated, Mustang dynos always read lower power numbers than other dynos.

A good, modern chassis dyno is a great tuning tool and is almost required for modern, fuel injected motors, especially if there is forced induction involved. I has my car dyno tuned at A&A Corvette, who have a brand new Dynojet and do a lot of high-end cars. Their dyno also has a reputation of being very consistant and actually reading a little on the low side as far as Dynojets go.

My car put down 505 hp to the rear wheels on the dyno, which should equal around 600 hp at the crank. That was with all the belt driven accessories and a 2.5" exhaust system, so these are certainly real-world power numbers. In my opinion, the chassis dyno is a much better tool than an engine dyno for that very reason. It actually tells you how the engine is performing in the car. It's not a substitute for the dragstrip, but is the next best thing.

Andy

[Rhino]

Like somebody already said, there's so many oppportunities for dyno operators to screw with settings that you can't even assume such percentages between dyno manufacturers.

Bottom line: if you want to know how your engine performs but aren't intending on making changes, don't waste your time on a dyno. Go to a track. It's cheaper, more fun, and ultimately the only "true" gauge. If you want to begin making changes to your engine in an iterative approach and want to see before/after, then a dyno is very repeatable and hence quite useful for such work. Also, if you want to play with gearing, a dyno curve will, regardless of scale, give you a good picture of the engine's best operating range, so that can have a lot of value as well.

The situation could easily be reversed as well. Track conditions, reaction time, tires, gearing, and suspension tuning could change from car to car, causing different 1/4 times. Unless the same driver drove a similar car down the same track, on the same day, under the same conditions, that comparison is meaningless as well.

It's also pretty hard to get A/F ratio from a track

[HarleyR]

No it's not.... http://www.motec.com/products/plm/plm.htm

[Fuelie Fan]

points well taken. Note that i said that if you are specifically trying to track before/after changes accurately and in a controlled fashion, that is where a dyno shines. So, I don't think we're really in disagreement there. However, I prefer trap speed for an "absolute" value to compare, as opposed to indicated rwhp. But yes, you can't compare your chevelle to a GSXR-1000 and expect anything meaningful.

[70camaro406]

the 1/4 mile dyno don't lie... but shows up a lot of lies from tweaked engine dynos.

That's right Dan. I haven't had my engine dynoed alone, but from the sum of all parts and calculations, I figured it to be 525-550hp. I do however have dragstip times from my local track (Mason Dixon in Hagerstown, MD) and chassis dyno numbers from the GM Show at Carlisle last year. Since I had a 4000 stall converter in the car, it never got a really good dyno run, since evey graph had gaps and the lines jumped around. But on the first run, it showed 398 rwhp, which I though was a little low, but I also have an 11.05 time slip to show too. So is it more impressive to have a lower rwhp number and a faster ET, or a higher hp number? I think I'll take the ET.