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Crank vs Real Numbers

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rogue_66 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rogue_66 Quote  Post ReplyReply Direct Link To This Post Topic: Crank vs Real Numbers
    Posted: Apr/06/2018 at 6:40pm
Can any of you race guys give me an idea of how much HP/Torque is lost between the factory 1968 390 numbers & real world numbers from a Dyno run? A rough percentage please.
Thanks,
Dean

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PHAT69AMX View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PHAT69AMX Quote  Post ReplyReply Direct Link To This Post Posted: Apr/13/2018 at 12:34am
Are you asking about REAR WHEEL Horsepower versus FLYWHEEL Horsepower ?
All stock accessories?  Water Pump, Alternator, Flex Fan, etc both on Dyno Stand and in Car?
Manual or Automatic Trans?  Which model Trans, like 727 or 904?  What Stall Converter?
What Rear End?  What Size Rim and Tire, Aluminum or Steel Rim (weight)?
imho, about 20% HP Loss for a "regular hot rod street car"...
all accessories, 727, "cheap" 3k stall (not a $1k 9"), AMC 20, 26x8.5 MT ETs, light 15x8 Weld Lites
maybe 17% or so if a 904, maybe 15% or so if a T-10 4-spd, all just opinions...
HP is "lost" by having to Spin Weight, total weight of spinning parts between Flexplate and the ground.
And Tire diameter or radius actually is a "factor", like a "backwards torque wrench"...
Max Torque is a "moment" at the centerline point of the rear axle,
then the farther the tire contact patch is out from the axle centerline,
the fewer effective "Pound-Feet" there is at the contact patch...
Remember a Torque Converters "momentarily" Multiply the actual torque at the crank
and depending on the converter and/or "design" there's a range of how much multiplication... 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ccowx Quote  Post ReplyReply Direct Link To This Post Posted: Apr/13/2018 at 1:59am
There was a thread a while back on this subject that would probably answer most of your questions. I believe it worked out to about 25% on a relatively mild 390 with a T10, maybe 30% with an automatic. All of the things mentioned above would be a factor of course, but without a dyno run, you would probably be mainly guessing.

Chris
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rogue_66 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rogue_66 Quote  Post ReplyReply Direct Link To This Post Posted: Apr/13/2018 at 2:04am
Originally posted by PHAT69AMX PHAT69AMX wrote:

Are you asking about REAR WHEEL Horsepower versus FLYWHEEL Horsepower ?
All stock accessories?  Water Pump, Alternator, Flex Fan, etc both on Dyno Stand and in Car?
Manual or Automatic Trans?  Which model Trans, like 727 or 904?  What Stall Converter?
What Rear End?  What Size Rim and Tire, Aluminum or Steel Rim (weight)?
imho, about 20% HP Loss for a "regular hot rod street car"...
all accessories, 727, "cheap" 3k stall (not a $1k 9"), AMC 20, 26x8.5 MT ETs, light 15x8 Weld Lites
maybe 17% or so if a 904, maybe 15% or so if a T-10 4-spd, all just opinions...
HP is "lost" by having to Spin Weight, total weight of spinning parts between Flexplate and the ground.
And Tire diameter or radius actually is a "factor", like a "backwards torque wrench"...
Max Torque is a "moment" at the centerline point of the rear axle,
then the farther the tire contact patch is out from the axle centerline,
the fewer effective "Pound-Feet" there is at the contact patch...
Remember a Torque Converters "momentarily" Multiply the actual torque at the crank
and depending on the converter and/or "design" there's a range of how much multiplication... 


Thanks for the reply's & yes...crank vs ground. I paid for a couple Dyno runs with more than disappointing results. Here's a link...https://vimeo.com/263100757...My buddy's VW puts more to the ground than these numbers so please tell me I've either got a big head or the numbers are so far off I can rest peaceful knowing that I got ripped off...cmon man...ain't no way this thing is only putting 119 HP & 185 ft lbs of torque to the ground Ouch.

This Gremlin X is an original & titled 1972 V-8 factory equipped car that came with & is still equipped with torque links. We welded in a six point roll cage for support & safety instead of sub frame connectors so it's solid. Everything on the build is AMC from different years except the T5Z racing tranny that I mated using a steel Lakewood bellhousing to a 1968 390 from an AMX bored 30 over with custom built forged pistons. We dropped the piston compression to 9.5 to be able to run premium gas. The heads are 1970 291c doglegs I had milled & installed SS valves with cam matching springs. Mid range cam, roller rockers, oil line mod, Performer intake, Eddlebrock 650 Thunder series naturally aspirated carb running at an altitude of 5,280 ft. Fresh AMC 20 twin grip with 10.5 inch street rubber on the rear. PS, AC, Alternator...it's a street car...we drove it from Denver to Kenosha & back in 2014.

I'm not a racer & I thought a Dyno run would answer my questions but all it did was create more of the same. I posted the Dyno Run sheet over in the picture section.

Cheers,
Dean




Edited by rogue_66 - Apr/13/2018 at 2:15am

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Post Options Post Options   Thanks (0) Thanks(0)   Quote WesternRed Quote  Post ReplyReply Direct Link To This Post Posted: Apr/13/2018 at 4:32am
Something seriously wrong with either the dyno numbers or the engine there.

Factory 325 HP was at the crank with no accessories, even if you said 25% for accessories and 25% for drive line losses, you would still be at 180 HP. My otherwise stock 343 made 143 HP on a chassis dyno the first time I ran it, but it had been rebuilt with the wrong pistons and would have had less then 8:1 compression.

You could try timing it over 0-60 mph or a set distance like 1/8th mile on the road and use those numbers to estimate horsepower as a cross check.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ken_Parkman Quote  Post ReplyReply Direct Link To This Post Posted: Apr/13/2018 at 9:37am
We chassis dynoed a 68 or 69 390 AMX, as stock as can be as far as I know. It was stock manifolds (had to swap to the stock big port intake cause someone had installed a small port) stock carb, full exhaust, factory air cleaner, stock fan, and a T10. A little effort was made to ensure the tuning was decent. It was the car used for a Dream Car Garage factory stock shootout. Then we found out the other cars in the shootout were not stock. Oh well.

IIRC it was about 225 RWHP. Can't remember for sure but I think on the TV show it ran in the mid 14's. Decent, and pretty close to testing back in the day.

There is so much BS out there on hp numbers it is unbelievable. There is gross, net, but the net could be 2 different SAE specs - J607 or J1341. Then there are imaginary hp numbers. You can make a dyno read anything you want by screwing with the weather inputs, and there is some seriously BS numbers out there. Really bad. You can get really nice advertising or bragging numbers if you are not totally ethical.

The real answer is 1/4 mile mph vs weight. That is the most trustworthy judge of true power. A dyno can be a very accurate machine, the problem is which measurement standard, are the atmospheric conditions properly corrected, and what engine was tested (air cleaner, exhaust, coolant temperature, water pump, fan, oil pan, etc.). This stuff makes a giant difference. Of course the lying numbers are the biggest difference.

Having done a bunch of this, both engine and chassis, I have a few opinions and some facts to back me up. One is never even look at a torque number from a chassis dyno. It measures HP, and back calculates torque from an engine rpm signal via a formula. If you have a race converter this methodology is simply useless. With a stock converter maybe, and a stick is better still. But in general simply ignore a chassis dyno torque number.

Using a percentage power drop calculation is also pretty useless. The problem is even if both dynos used the same standard, the engine tested is never the same. Dyno headers, exhaust, fan, dyno coolant tower temperature, etc. Again this stuff makes a giant difference.

A couple examples on something I have direct data on - my street Rambler with the mild 401, then the little more serious 426. The testing was as controlled as I could with the engine dyno work directly back to back to back, pulled one engine off and installed the other, same everything. This dyno correction factor was J607, it used 1 3/4" headers CJ fenderwell headers without mufflers, no air cleaner, an external electric water pump drive and a 170 F external coolant tower thermostat. The chassis dyno work could not be properly back to back, but was still same car. The dyno uses J1341 correction (about 4% lower reading) and the car has 1 3/4" hooker headers 2 1/2" exhaust to the bumper with "relatively" quiet dynomax mufflers, a 14" x 3" K&N filter, mechanical water pump drive, with a slight underdrive, electric fan, alternator, and a TKO tranny. Unfortunately there was quite a bit of time between chassis dyno runs, so there may have some tuning drift.

The 401 made 444 hp at the flywheel and 354 rwhp. You would think 20% loss, but that is not true. Take 16 hp for J1341, 6 hp for the water pump, and 25 (estimate) for the exhaust and the percent drop is approximately 8.5. With that combo the car ran 11.99 @ 118, works out to about 380 rwhp, but that was an awesome mineshaft air day. It all works out decent.

The 426 made 567 flywheel, and 472 rwhp. Take 23 hp for J1341, 6 for the water pump, and 30 for exhaust and the drop is actually a slightly smaller % at 6.5. That combo ran 11.05 @ 125.5, calculates to 476 rwhp. Very close.

To the original post, yep there is something wrong. Look to tuning first, and check things like is the throttle fully opening.

Edited by Ken_Parkman - Apr/13/2018 at 9:40am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ken_Parkman Quote  Post ReplyReply Direct Link To This Post Posted: Apr/13/2018 at 9:56am
BTW another difference on a chassis dyno is rate of acceleration. The faster the acceleration, the less accurate as the cars change in rotational inertia absorbs power. A pull in a lower gear, or a higher number rear axle or heavier tire makes a lower power reading.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PHAT69AMX Quote  Post ReplyReply Direct Link To This Post Posted: Apr/13/2018 at 11:07am
Here are the formulas mentioned that use a car and drivers actual weight and the
actual 1/4 mile trap speed, regardless of E.T., to calculate NET HP to the ground.
Then use calculated NET HP and Weight to calculate a "target optimum" 1/4 Mile E.T.
that I would assume represents "peak efficiency" use of the Net Horsepower.
The results can help in selecting a Rear Gear Ratio and Rear Tire Size / Diameter
that achieves the desired "optimum" MPH at the appropriate engine RPM,
- IF - optimum 1/4 Mile performance is the goal, which is not always the case.
I made up a MS Excel Spreadsheet using these formulas and the values
agreed pretty well with Timeslips and numbers from a Chassis Dyno Pull.
Some details can get tricky, like top end converter slip percentage
or "effective" rear Tire Radius, ? "squished", ? "true", or "grown", lol
But at least it lets one know if they're "in the ballpark"...



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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ken_Parkman Quote  Post ReplyReply Direct Link To This Post Posted: Apr/13/2018 at 11:33am
I didn't catch that you are at altitude, assume you dynoed there? At altitude true hp is way down, but that's why you have correction factors to the same atmospheric conditions. As long as the weather conditions and math is right it should not matter for the hp number. As long as it is right.

But the 1/4 mile based hp number will be less for sure, as the actual uncorrected power number would be way down.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ccowx Quote  Post ReplyReply Direct Link To This Post Posted: Apr/13/2018 at 11:49pm
I ran my car through one of the online calculators with my trap speed, weight, etc. I suspect that it is around 375 or a few more horses old school at the crank. It came out to about 295 hp at the rear wheels. That was a Javelin, 4.10's, 3585 pounds with driver and a trap speed of 102mph.

FWIW
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