Crank vs Real Numbers
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Category: Competition
Forum Name: Drag Racing
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URL: https://theamcforum.com/forum/forum_posts.asp?TID=93475
Printed Date: Apr/16/2024 at 12:31pm
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Topic: Crank vs Real Numbers
Posted By: rogue_66
Subject: Crank vs Real Numbers
Date Posted: Apr/06/2018 at 6:40pm
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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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Replies:
Posted By: PHAT69AMX
Date Posted: Apr/13/2018 at 12:34am
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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... -------------  Link to a http://www.youtube.com/watch?v=MySiKQsmWxU" rel="nofollow - Short YouTube Burnout Video |
Posted By: ccowx
Date Posted: Apr/13/2018 at 1:59am
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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 |
Posted By: rogue_66
Date Posted: Apr/13/2018 at 2:04am
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" rel="nofollow - 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  .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 -------------
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PHAT69AMX wrote:Posted By: WesternRed
Date Posted: Apr/13/2018 at 4:32am
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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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Posted By: Ken_Parkman
Date Posted: Apr/13/2018 at 9:37am
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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. |
Posted By: Ken_Parkman
Date 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. |
Posted By: PHAT69AMX
Date Posted: Apr/13/2018 at 11:07am
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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"...   ------------- Link to a http://www.youtube.com/watch?v=MySiKQsmWxU" rel="nofollow - Short YouTube Burnout Video |
Posted By: Ken_Parkman
Date Posted: Apr/13/2018 at 11:33am
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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. |
Posted By: ccowx
Date Posted: Apr/13/2018 at 11:49pm
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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 |
Posted By: 67 Marlin
Date Posted: Apr/15/2018 at 1:10am
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My 1967 TSM mentions "taxable horsepower." For the 343 with 280 hp, it says 57 taxable. When I saw that, I figured it meant 223 hp at the wheels. Sounds like an appropriate term if they're using it to go from crank to wheels, but I don't know if that's exactly what it meant, or if so, how precise the "tax rate" was. If this is an appropriate tax rate, can I use the 390 intake that was in the trunk when I bought it as a tax deduction
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Posted By: rogue_66
Date Posted: Apr/16/2018 at 12:51am
I've never run the car on the strip as we built it to drive across country to shows & such. My wife & I drove it from Denver to Kenosha & back in 2014 without incident or breakdown. 18 MPG with the AC running the whole way. Yes on altitude Ken...& thanks everyone for sharing info...the dyno run was at 5280 ft. The main purpose of the dyno run was for tuning purposes as I was wanting to know how close the car was jetted for this altitude. We'll be moving to sea level within the next 4 months & I thought it might be a good way to get an accurate AFR under load as a baseline. Per the video I shot, it took 3 seconds on the first run to accelerate from 2000 to 5000 RPM & 4 seconds on the second run to accelerate from 2000 to 5200 RPM. The drivetrain is as follows...T5Z World Class tranny with 2.95 first gear & 37% overdrive 5th gear mated to 354 AMC Twin Grip rear end gears. Torque links & stock 1972 Gremlin diameter tires...10.5" in width. The runs were done with the transmission in third gear. On the highway at 2200 RPM in 5th the car cruises @ 75MPH. IIRC fourth gear in that tranny is a 1 to 1 ratio. Could it be possible the operator entered the wrong parameters not knowing it was a five speed? BTY...I can't imagine the car weighs more than 2800 lbs full of gas with me in it but haven't verified that yet. Here's the printout for the second run mentioned above if it helps get to the bottom of this mystery.  Cheers & Thanks, Dean -------------
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Posted By: motorhead_1
Date Posted: Apr/16/2018 at 4:24am
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the type of dyno will make a difference. dynojet will read the highest IIRC. mustang ~12% lower then there's the heartbreaker dyno, the dyno dynamics reading lower yet. ------------- 69 SC/Rambler tribute 401/th400, 68 Rebel SST LSA, 66 F100 460/c6, 88 Merkur Xr4Ti, 71 Jaguar XJ6 LS1, 08 Supercharged Tundra |
Posted By: Ken_Parkman
Date Posted: Apr/16/2018 at 7:41pm
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Never been on a mustang, but have heard they give a lower number. Wonder what the reason is if true. Power is a basic engineering definition, so it should not be hard but it is. Also sorta humbling sometimes. Not knowing how the mustang calculates a dynojet would give a lower number in 3rd gear so that is some of your story. You should make the pull in 4th. The concern with OD is wheel speed, and in 5th at 5200 rpm would be a whole lot. Tires do come apart on chassis dynos (been there done that - not good) so you don't want to be too far outside the tire speed rating. Use it for tuning, then go to the track and you really know.
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Posted By: motorhead_1
Date Posted: Apr/17/2018 at 3:08am
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forgot about the dyna-pack hub mounted dyno, no need to worry about blowing up tires. takes the weight of the wheel and tire out of the equation, so will read slightly higher. all different dynos aside, they are just a tool to aid in tuning. for accurate before and after shots use the same dyno, same operator etc. unless you are bench racing and want to brag about 5 more hp than the next guy. tune the engine on the dyno and the chassis at the track...
------------- 69 SC/Rambler tribute 401/th400, 68 Rebel SST LSA, 66 F100 460/c6, 88 Merkur Xr4Ti, 71 Jaguar XJ6 LS1, 08 Supercharged Tundra |
Posted By: BassBoat
Date Posted: Apr/30/2018 at 10:09am
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Dyno results can be humbling. I was quite proud of my AMX but on a low barometer muggy day I could only get 283 HP through the mufflers and 294 with open exhaust to the wheels on the dyno at GAD. That car would typically run 110 to 112 in the quarter and on that day my 283 HP would only get me to 106. I had a very close to stock AMX with BW automatic and 3.15 gears that ran 14.50's at 94 mph. So that is what I think a 315 HP factory rated AMX is good for if you want to plug numbers into one of those calculators. BB |
Posted By: BassBoat
Date Posted: Apr/30/2018 at 10:11am
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Oh, and your dyno readings will be different/wrong if the operator doesn't put the right gearing in or if you maybe ran it in 5th gear instead of 4th... BB |
Posted By: amxdreamer
Date Posted: Apr/30/2018 at 11:47am
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Your numbers on a track will tell the truth. Take her over to Bandimere before you move! ------------- Tony Vancouver, BC 1970 AMX 1972 Badassador AMO#10333 |
Posted By: rogue_66
Date Posted: May/01/2018 at 12:24am
Makes sense! I failed to mention that my original post in pictures included concerns about jetting at my altitude... http://theamcforum.com/forum/gremlin-x-dyno-run_topic93460_post826726.html?KW=#826726" rel="nofollow - http://theamcforum.com/forum/gremlin-x-dyno-run_topic93460_post826726.html?KW=#826726 ...so the primary reason for the dyno run was for tuning purposes, as I wanted to know how close we were tuned for 5280 ft. The operator was baffled by the extremely low HP/Torque numbers...but he did say the reading indicated that we were jetted way rich which is cool...because we're moving to sea level in a few months  .Tony...To be quite honest, I've never made a pass down the track nor am I qualified to do so..but it's on my bucket list now that I'm officially a "geezer"  .  Dean -------------
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Posted By: tomslik
Date Posted: May/01/2018 at 6:46am
Geez Dean, how qualified do you need to be? I could be talked into driving if ya want..... ------------- 67 american 290/4speed |
Posted By: rms827
Date Posted: May/01/2018 at 1:53pm
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I'd bet your altitude was definitely part of the cause for the low readings. When I was in school, the instructor spent about an hour or so covering that issue with carbs. Now if I could only remember what he said, LOL. Gotta agree with Tomslik too... No real qualifications to hit the track other than the cost of the run. Used to be a joke in Atlanta because we'd see kids and 20 somethings with rental cars hitting the track. NOT rented sports/muscle cars either; HiPo cars like Chevy Cavaliers, LOL. ------------- 1964 AMC Rambler Classic 660 "You can think I'm an idiot, just don't talk to me like I'm one." - Batman |
Posted By: amcenthusiast
Date Posted: May/01/2018 at 11:02pm
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http://www.epi-eng.com/piston_engine_technology/thermal_efficiency.htm http://performancedevelopments.com/fuel-flow-calculations-for-horsepower/ https://wahiduddin.net/calc/cf.htm ------------- 443 XRV8 Gremlin YouTube video: https://www.youtube.com/watch?reload=9&v=2DmFOKRuzUc XRV8 Race Parts website: http://amcramblermarlin.1colony.com/ |