Crank horsepower conversion from chassis dyno

[CDMBill]

We had a long (12 hours) dyno day thrash last Friday with the super cool crew at Westech and despite my pathetic engine assembly skills we manged to get 708 HP at the tires from the pump gas BBF. ;D

Huge thanks to Scott (Diesel Geek), Chad, Shaun, Brian Rock and Ray, Mike and Jim at Dougan's and Westech particularly Steve and Ernie.

So the question is what does that translate to in crank HP? The dyno is a SuperFlow in ground unit which I'm told is calibrated weekly.

In case it matters the car has an AOD with an approximately 4200 stall converter, a 3.89 geared 9", 315/60/15 MT Drag Radials and it weighs 4460 with me and Scott sitting in it.

[TC]

Re: Crank horsepower conversion from chassis dyno

Just figure in a 20% drivetrain loss. So your roughly making 840hp at the crank.

[Scott Liggett]

Re: Crank horsepower conversion from chassis dyno

I saw two different engines dynoed on the engine dyno, then both were stuck in cars and both lost right around 24%. Both articles were in PHR and both were did all the dyno work at Westech.

[BlackoutSteve]

Re: Crank horsepower conversion from chassis dyno

How much power, and at what rpm will make the percentages vary also. -Let alone the endless drivetrain variables..

It's my belief that smaller power outputs loose more power in terms of percentage than high power outputs.
I also expect low rpm to loose less power than compared to high rpm, which is the otherway around in terms of losses.

Scott, do you recall the issues of PHR? or is the stroy still online do you think? :-*

[JeffMcKC]

Re: Crank horsepower conversion from chassis dyno

Bill it will be hard to figure, if you take your car, with a high rear gear, and taller tire on the back that will give a higher number than the same set up with a 295 and a 4.10 in the back.

I have been looking into finding the tipping point, the flywheel does not get the car down the track, the rear wheel power does. Converting it over will be a geuss at best.

If you can find from a mustang to a super flow, Docs Dynoed 960 to the flywheel and 630 at the rear wheels on a mustang I beleive. Thats with a way to lose 7800 convertor and a 4.71 gear with 33 inch tall tires

[DanStokes]

Re: Crank horsepower conversion from chassis dyno

Chassis dyno to engine dyno values are always difficult to compare. You're fortunate that Westech has done some back-to-back comparos (most shops don't bother) so I'd go with the 24% number. This is likely as good a bogey as you'll come up with. The only way to REALLY get a handle on this is to pull your engine, change NOTHING (keep your exhaust system, etc), run it on their engine dyno and see what the numbers turn into. I'm guessing it isn't worth it.

SAE has attempted to develop standard models for this to no avail. There are just too many variables. As you may know, SAE is THE authority on HP standards.

Dan

[dieselgeek]

Re: Crank horsepower conversion from chassis dyno

Bill's last combo made 850 at the crank and lost 37% to the wheels through his AOD, but a less efficient converter. That combo was dyno'd on a superflow engine dyno, then chassis dyno'd on a Mustang brand (notorious for low numbers).

20% is more like what front driver loses, or what the Dynojet guy tries to tell you when you spin his drum of concrete to some magical number. No combo like bill's has ever lost less than 30% to the wheels, of the ones I've seen dyno'd on both types. Auto trannies and high stall converters kill the numbers that reach the wheels, in trade for keeping a higher average power during the a 1/4 mile pass... which is what makes them faster.


From my observation, the biggest factor is converter. I do not believe tire size matters (if not spinning)... we once swapped a T56 car making similar power to Bill's (supercharged LT1 built to the max) with a TH400 and lost 150hp to the wheels (car went a half second faster too). I think that most guys who see/claim losses in the 20's are talking about OEM powertrains and power levels. BlackoutSteve, didn't you do both engine and chassis dyno? but got confusing numbers or something like that?


The biggest thing I am going by, however, is NOT the dyno numbers at all. I know his last combo made 850ish peak at the flywheel, and this time around I had to add around 12% more fuel to maintain the same AFR. That's at the exact same fuel pressure as before. So that tells me that he's making 950+.

Shaun Callaway has both a Superflow enine dyno, and the same Superflow chassis dyno at his shop at Edelbrock. He estimated 930-940hp based on a similar engine that has been on both dynos. Again, the converter / trans are going to be huge factors and not consistent between cars. But it's an interesting point, he came up with that number before I looked at the fuel quantities in the datalogs.

-Scott

[dieselgeek]

Re: Crank horsepower conversion from chassis dyno

I saw two different engines dynoed on the engine dyno, then both were stuck in cars and both lost right around 24%. Both articles were in PHR and both were did all the dyno work at Westech.

What transmisions were in the cars? I think you and BlackouSteve have good info here, he's right - the power loss is closer to a fixed amount than "percentage" - but likely somewhere in between.

*all* of my comparisons have been to Mustang dynos. People say Mustang brands are "low numbers" but I know they are 100% dead nuts consistent with other mustang dynos which I like! Anyways, the comparisons I have done were all with a Superflow engine dyno (considered very accurate!) and Mustang chassis dyno. I've seen as little as 24% loss on a front drive car with manual trans, and as high as 40% loss on a drag car with auto trans and high stall converter.

What's hilarious is, track times never correlate to any of this.

[TC]

Re: Crank horsepower conversion from chassis dyno

What's hilarious is, track times never correlate to any of this.

So basically dyno numbers are bullshit. Because that is what I'm getting from what your saying.

To me all a dyno is good for is getting a baseline tune on your car. Because the real power output will be determined in the MPH and ET that you run.

[Barry Donovan]

Re: Crank horsepower conversion from chassis dyno

I saw two different engines dynoed on the engine dyno, then both were stuck in cars and both lost right around 24%. Both articles were in PHR and both were did all the dyno work at Westech.

And that is good. the factors are everything that needs to be moved by engine power. The guessing game useless.
counterbalance-less engines hardly have an accurate guess, but it is much lower than the 24% (ie: a flat benz twe cyl, or even more famously, the 3 main boxer). I pondered this a long time ago, and just ride the facts and real world without a number of "crank HP". thenm, some engines gain even more as torque is demanded. Lower compression is like this (a 400 something cubic inch gasser 9:1 in a dump truck). therte is many things to take note of... why guess it. the number written is what matters, and then one could challenge how good the dyno functions...

[Scott Liggett]

Re: Crank horsepower conversion from chassis dyno

Both cars had autos. I think one was their street/ strip 68 Chevelle with, I think, the 496, th400. I can't remember the other car right now.

[dieselgeek]

Re: Crank horsepower conversion from chassis dyno

Both cars had autos. I think one was their street/ strip 68 Chevelle with, I think, the 496, th400. I can't remember the other car right now.

Any idea what kind of stall speed on the converter?


it's intersting, some people feel the Flywheel HP number is worthless but that's how the OEM's rate their engines, and if you're searching for every last big of power from a combo, you don't put it on the chassis dyno - you take it toWestech and put it on the engine dyno.

For me, the chassis dyno is strictly the tuning tool. But, it can be used to get a rough idea of flywheel horsepower, as we're doing here for Bill.

I think the most accurate indicator for Bill, is the fuel consumption increase. If it needs 12% more fuel to maintain the same AFR, then it's burning 12% more available oxygen, and it's making about 12% more power over the previous, engine-dyno'd combo. It's just more supporting info when the guys at Westech, and Edelbrock, both some up with numbers very similar to what we derived based on fuel consumption.

[BlackoutSteve]

Re: Crank horsepower conversion from chassis dyno

BlackoutSteve, didn't you do both engine and chassis dyno? but got confusing numbers or something like that?

Yep.. My flywheel number was lower than expected and because of my cam change, I'm not really comparing apples. So, I'll be back to the chassis dyno when the car is ready.
Otherwise, I'd be boasting that my 'Glide and 9" is losing less than 10%.

I really felt the chassis number was correct because it was within 5hp of what the performance formulas I used to predict before I got even to the track! Then, there's the huge dabate about formulas being flywheel or rearwheel.
I don't what to go there, but the example of a ZZ502/502 in 3500lbs NOT being able to run near a 10.8 @ 123 is a good one.

I thing the amount of power lost in any given drivetrain is a constant variable. Not a fixed amount, not a fixed percentage. It varies with rpm, just like wind resistance.. The faster, the more.

[SpiderGearsMan]

Re: Crank horsepower conversion from chassis dyno

manual trans and a 12 bolt will put the most power down
the race dynos ! don't they ?
how much did all of that dyno time cost - in the dealership 8 hours of my time is now 1000.00 +7% NJ sales tax , your charge

[SpiderGearsMan]

Re: Crank horsepower conversion from chassis dyno

I only get a fraction of that , and I get yelled at if I only turn 8 hours