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Discussion Starter #1
Excuse my ignorance, but when someone is talking horsepower ratings, what is the difference between Gross HP and Net HP?
 

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Don't know the answer, but also have a HP question.

Do all mfgs state HP at the same place. Flywheel, rear wheels, drive shaft, etc....
 

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As I understand it, gross is at flywheel with no accesories, net is with accesories--alternator, air breather,water pump, etc, and maybe(?) driveline too
 

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Here is another idiot question that I should probably know. Has the horsepower rating changed since the 60's and 70's? The reason I ask is that all the old muscle cars have rating of like 450 to 500 hp. Yet, now days only the elite cars reach that. I have driven my dads 65' stingray and it is supposed to have 405 hp and yet, I think my mustang has better exceleration (sp?) than dad's vette. So, am I missing something or what is the deal here. Any help would be welcomed.
 

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Yes,
In the early 70's they were able to run external water and electric fuel and probably anything else they could think of and rate the engines HP that way. I believe it was 72 they stopped that and that is why you see a huge drop in HP claims. Plus this was all rated at the flywheel.
 

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in 1971, they also dropped compression ratios in order to handle the newer (lower octane, unleaded which had it's own changes in ratings) fuel requirements as well as coming up with the new method for calculating horsepower. So, the '71, '72, '73 engines were rated significantly lower in horsepower. Most hot rodders felt at that time that it was the beginning of the end for high performance automobiles. However, here we are in 2001 and we have the Bullitt, which puts out more raw HP than many of those supercars, lower emissions and much better mileage. The only bad news is the MUCH higher purchase prices ( the average musclecar back then went for $3000.00-$4,000.00 out the door. If you apply about a 5% per year inflation factor compounded, those same cars should sell today for roughly $15,000.00, but would more than likely sell for $30,000.00 instead).
Sorry, I got off topic a bit, but hope I shed some light on the changes in horsepower ratings over the years.
 

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Discussion Starter #7
EXACTLY...I noticed that Pre-'72 is referred to as "Gross HP" and Post-'72 is referred to as "Net HP".

So how do we really compare...how can we create relative ratings so that we can accurately compare? Is there maybe an approximate percentage we can use?
 

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So the bullitts and so forth are rated by net hp? Then Gross HP is a lot higher... hehehehehe... I can confuse some people then...

hahahhaha
 

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Test brakes (the B in BHP) all measure at the flywheel. Gross didn't have any front end accessories - no alternator, no water pump nothing. All those functions were provided by equipment connected to but not driven by the engine. Net has the engine front eqpt. but not the tranny, driveline, differential and rear axle. Why you ask? Trannies and diffs are technically torque(force) multipliers, a class of machine which includes things as simple as pulleys. That's why one man with a rope can lift a piano. It's also why the engine in a semi can move 80000+ lbs. without being overly large.
 

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The difference between pre 1971 and post 1972 HP is gross vs SAE. You are correct in stating that the post 1972 hp rating is with all accessories operating. Road and track did some empirical testing comparing a 70 boss to a 1987 lx 5.0. In the same issue, they compared a 1969 TA to a 1987 305 formula. The formula they came up with to compare gross to sae net was: SAE net hp = 0.797 x gross BHP. This puts the bullitt at 332.5 gross hp. (September 1987 issue page 49). The motor heads of the past don't realize that many of the cars of today really can kick some old butt. The new 405HP Z06 is actually an old 500+hp car.
 

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Another aspect of horsepower rating to consider is that rating an engine is statistical. As a result of manufacturing variations the power output of nominally identical engines will not be the same. The manufacturers test a group of engines and then rate the horsepower somewhere in the range the of the measurements. Years ago the manufacturer would rate an engine at or near the power output of the best engine in the sample. In todays world because of lawsuites and other problems manufacturers usually rate the engines at the sample mean or sometimes below the mean. The 5.0 liter V8 in the Mustang was derated in 1992 from 225 hp to 215 hp. This was actually just an adjustment of where in the sample distribution the engine was rated.
 

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I used to have a 1995 Mustang GTS. This car came standard with the 5.0. Its funny how many people would say that I should've gotten an older GT as these cars had more HP. I tried telling them that the engines were no different and that both 5.0's would dyno at approx the same HP, but no one believed me. They would always respond that they either read an article or saw it on a TV show and that I was wrong. I would always respond by telling them to either read the whole article or to pay attention while they watched TV.

Regards,
Dfresh DHG 294
 

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I would like to thank everyone for that info on the HP ratings..that had been confusing me for about a year now. It just didn't make sense. At least now I can tell my dad that his vette isn't so hot anymore. Of course he special ordered his for $4500 back in 65 so..I guess he did kick my butt in the cost..plus his car is appraised at over 40K now..whereas mine is still dropping in value. I guess daddy knows best :smile:
 

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I just found this out the other day. There are different standards for rating Hp. SAE and DIN. Basically English and European ratings. The Euro ratings tend to be higher than the American ratings due to how they calculate power.
 

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Ok, fellow Bullitheads. A quiz for you.

How is engine output in HP really measured?

Stay tuned for the answer. My fellow engineers are exempt from answering.
 

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Two more things about gross hp ratings. I read in article about it in Car and Driver, I believe, that stated that gross hp was engine out of car, non production exhaust (basically open) and non-production carb tuning. The carb itself was a production piece. How much could an EECV engine gain if someone were tickling a laptop the whole time? I love the Nov 1996 issue of Mustang Monthly. The one where a 1996 Cobra was faster in the quarter than a 1970 Boss 302, a '71 Boss 351 and a '69 Boss 429. The good old days are NOW.

_________________
True Blue Bullitt #4398

<font size=-1>[ This Message was edited by: gbranton on 2001-12-01 22:38 ]</font>
 

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OK Bullitheads, the answer is ...

Horsepower is not measured. Test brakes, dynos, etc. all measure torque. Horsepower is CALCULATED from torque and RPMs. That's why max HP occurs at a higher RPM than max torque.

The best engine would have output along a nearly flat torque curve at maximum torque across the RPM range from zero to the redline.
 

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Hey TXBullitt, can you explain why horsepower and torque curves always intersect at 5,230 RPM (I think that's the number)??

Someone told me once, but...

Thanks!
 

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The intersection depends on the specific engine. The RPMs become the controlling factor in the calculation.

For example, the Ford/International Powerstroke diesel produces max torque of 500+ ft-lbs at 1700 rpm and max HP of 210-230 at 3200 RPM. The redline is about 3600 rpm. Not a lot of HP, but try towing 12000 lbs in a truck with a Bullitt engine and you'd have a hard time. Most Jap engines produce their max torque way up in the rpm range, usually 4000+, and their max Hp near the redline.
 

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Yes,todays muscle is quicker than that of 30 years ago....Stock! But remember those big and small blocks,varied a lot in their factory tune,not as sharp as today's computer run engines. The old musclecars would respond big time to simple carb/ignition/header mods....much more so than todays Mustangs. Headers on some of the wilder 60s,1970 musclecars would show as much as 40-50 HP gains! I have one test,where a 70 Hemi Roadrunner was turning mid 12s at 112-114 with headers and a good tune on slicks of course. The old Boss 302s,Z/28s needed a lot of gear to run their best,much more than our highly tuned 4.6s
 
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