Gross vs Nett Horsepower Ratings

[Vetman]

My 1975 C3 left the factory with only 165 nett bhp (now addressed) due to several smog engineering features and camshaft timing changes. This output looks very poor against say a 1971 LT1 with a rating of 330 gross bhp. This same engine in 1972 was rated at 255 nett bhp. The only change being the SAE testing method where all engine ancillaries were now included. These ratings are at the flywheel, and not to be confused with rear wheel dyno figures.
An interesting YouTube video that addresses this is: Horsepower in the 1970’s: No longer Gross.
Corvettes are included later in the video.

 

[Chuffer]

My 1975 C3 left the factory with only 165 nett bhp (now addressed) due to several smog engineering features and camshaft timing changes. This output looks very poor against say a 1971 LT1 with a rating of 330 gross bhp. This same engine in 1972 was rated at 255 nett bhp. The only change being the SAE testing method where all engine ancillaries were now included. These ratings are at the flywheel, and not to be confused with rear wheel dyno figures.
An interesting YouTube video that addresses this is: Horsepower in the 1970’s: No longer Gross.
Corvettes are included later in the video.

It makes for interesting viewing and shows what a minefield power ratings can be to the uninitiated . It sure makes bragging rights very speculative . My 1980 left the factory with a base 350 rated at 195bhp nett but now has a 1985 Camaro IROC - Z 305 cu inch with the TPI junked in favour of Edlebrock Torker 2 inlet manifold and 600 cfm Edelbrock Performer carb . All the smog gubbins is disconnected and has no Cat fitted . Anybodies guess was the bhp is ??

 

[Vetman]

It makes for interesting viewing and shows what a minefield power ratings can be to the uninitiated . It sure makes bragging rights very speculative . My 1980 left the factory with a base 350 rated at 195bhp nett but now has a 1985 Camaro IROC - Z 305 cu inch with the TPI junked in favour of Edlebrock Torker 2 inlet manifold and 600 cfm Edelbrock Performer carb . All the smog gubbins is disconnected and has no Cat fitted . Anybodies guess was the bhp is ??

I hoped to do a rolling road dyno test before I changed the cylinder heads on mine, and then a similar test after the upgrades. The before test would have benefited from existing mods to the intake, carb, exhaust, and camshaft, so more of an intermediate stage. Alas on the day of the dyno test I was distracted at the petrol pump, by a C3 admirer, and put diesel in the car. Not serious but expensive to have the new fuel extracted and thrown away. I cancelled the dyno session. I made another appointment and drove to the facility but by the time I got there the car was pouring black smoke and would hardly run. This turned out to be dirt in the fuel and carburettor and made it impossible to proceed with the dyno test. I have since fitted the new heads and I am none the wiser on current torque and bhp. I will try to measure the 0 to 60 mph acceleration on a phone app and then back figure the likely bhp. The car certainly goes well. It seems that you need a big increase in bhp for a small improvement in acceleration times.

 

[Chuffer]

I hoped to do a rolling road dyno test before I changed the cylinder heads on mine, and then a similar test after the upgrades. The before test would have benefited from existing mods to the intake, carb, exhaust, and camshaft, so more of an intermediate stage. Alas on the day of the dyno test I was distracted at the petrol pump, by a C3 admirer, and put diesel in the car. Not serious but expensive to have the new fuel extracted and thrown away. I cancelled the dyno session. I made another appointment and drove to the facility but by the time I got there the car was pouring black smoke and would hardly run. This turned out to be dirt in the fuel and carburettor and made it impossible to proceed with the dyno test. I have since fitted the new heads and I am none the wiser on current torque and bhp. I will try to measure the 0 to 60 mph acceleration on a phone app and then back figure the likely bhp. The car certainly goes well. It seems that you need a big increase in bhp for a small improvement in acceleration times.

You do indeed need big increases for small gains due to the laws of physics and the movement of a mass through the air and all other associated friction losses . Also bare in mind that it is the peak torque figure that really counts in acceleration times . TBO I am not fussed about cranking up peak performance figures as I don`t race the C3 or do track days in it so no point really , I just like it to run well and have enough poke to put a smile on my face . Oh , and make a good noise doing it !

 

[Vetman]

You do indeed need big increases for small gains due to the laws of physics and the movement of a mass through the air and all other associated friction losses . Also bare in mind that it is the peak torque figure that really counts in acceleration times . TBO I am not fussed about cranking up peak performance figures as I don`t race the C3 or do track days in it so no point really , I just like it to run well and have enough poke to put a smile on my face . Oh , and make a good noise doing it !

Yes, torque is important for acceleration to 60mph. No replacement for displacement.

 

[Roscobbc]

It makes for interesting viewing and shows what a minefield power ratings can be to the uninitiated . It sure makes bragging rights very speculative . My 1980 left the factory with a base 350 rated at 195bhp nett but now has a 1985 Camaro IROC - Z 305 cu inch with the TPI junked in favour of Edlebrock Torker 2 inlet manifold and 600 cfm Edelbrock Performer carb . All the smog gubbins is disconnected and has no Cat fitted . Anybodies guess was the bhp is ??

220 hp? - you really need to change that Torker inlet for a dual plane Edelbrock Performer (if you have enough height under the hood) and keep the carb. The change of manifold will improve low-end grunt - especially if you have low numerical rear end gearing.

 

[Roscobbc]

My 1975 C3 left the factory with only 165 nett bhp (now addressed) due to several smog engineering features and camshaft timing changes. This output looks very poor against say a 1971 LT1 with a rating of 330 gross bhp. This same engine in 1972 was rated at 255 nett bhp. The only change being the SAE testing method where all engine ancillaries were now included. These ratings are at the flywheel, and not to be confused with rear wheel dyno figures.
An interesting YouTube video that addresses this is: Horsepower in the 1970’s: No longer Gross.
Corvettes are included later in the video.

Ignore all the cobblers 'downing' smog era hp figures. Ok so they are low compression etc and don't compare well against pre smog era hp figures......
But they still have low end 'grunt' and are fun to drive. Simple changes to timing curve can perk things up nicely.
I recall the RHD 1977 Mustang Ghia my missus had back in the day. 4.9 ltd, auto box all in a body based on a Pinto (USA equivalent of a Cortina)......rated at 138 bhp or similar.....less than a 3 litre Granada...... supposedly. People would laugh at them.....hairdresser or poofter car it was known as.......far, far quicker than hp figures lead one to believe....much like mid/late 79's Vette (except for the pee take factor)

 

[Vetman]

Ignore all the cobblers 'downing' smog era hp figures. Ok so they are low compression etc and don't compare well against pre smog era hp figures......
But they still have low end 'grunt' and are fun to drive. Simple changes to timing curve can perk things up nicely.
I recall the RHD 1977 Mustang Ghia my missus had back in the day. 4.9 ltd, auto box all in a body based on a Pinto (USA equivalent of a Cortina)......rated at 138 bhp or similar.....less than a 3 litre Granada...... supposedly. People would laugh at them.....hairdresser or poofter car it was known as.......far, far quicker than hp figures lead one to believe....much like mid/late 79's Vette (except for the pee take factor)

This is my point also. A lot of the lost power is due to the reporting standard, and it doesn’t take much too regain power. The intake and exhaust changes alone wake up the performance.

 

[Chuffer]

220 hp? - you really need to change that Torker inlet for a dual plane Edelbrock Performer (if you have enough height under the hood) and keep the carb. The change of manifold will improve low-end grunt - especially if you have low numerical rear end gearing.

Not enough clearance under the hood Ross as it`s tight already , even with a drop base air filter . I know the trade off with a Torker 2 is the loss of some low end power until you get to a higher rev band but I like the fact that the peak power and torque curve has a longer flatter peak instead of dropping off a cliff as the revs rise . The IROC - Z 305 lump revs OK due to the shorter stroke than the base 350 and also is higher compression . I run a standard 3.08 : 1 diff and as I am not bothered about lighting up the rear tyres the acceleration off the mark is quite satisfactory .

 

[CaptainK]

Same thing with my 327 / 300hp in 1968, which of course is GROSS rating. I believe than in NET is about 270hp looking at the Vettes of the time that were rated 300hp, but the next year the exact same engine was 270hp. I know later on they went lower compression and more smog and so forth, hence losing more power etc.

I took my Vette to a rolling road many years back and on the day it ran about 245hp / 300 lbs/ft. I can't remember if that was "at the fly" or "at the wheels". Remember also a car will change its power output depending on the day and conditions for that day etc. So some days it'll produce more, some days less. But to me it doesn't matter as long as the engine appears to be running ok, it makes a good noise, and is quick enough for my simple needs. And to be fair, my little 327 is plenty quick enough down the country roads for me. Like Chuffer above, I also run a 3.08 rear gear with LSD. So its more setup for cruising than acceleration, but still accelerates fine for me.

 

[Roscobbc]

I used to be fascinated by the sheer rpm flexibility of my original oem 390 hp 427 engine. With its 3.08 rear and M21 once I'd installed the MSD system and good ht leads it would comfortably pull from 700rpm in any gear, even from standstill with no difficulty in 3rd gear. Current 489 isn't happy pulling away below 1100 rpm - which on a modern car would be acceptable.

 

[Vetman]

Same thing with my 327 / 300hp in 1968, which of course is GROSS rating. I believe than in NET is about 270hp looking at the Vettes of the time that were rated 300hp, but the next year the exact same engine was 270hp. I know later on they went lower compression and more smog and so forth, hence losing more power etc.

I took my Vette to a rolling road many years back and on the day it ran about 245hp / 300 lbs/ft. I can't remember if that was "at the fly" or "at the wheels". Remember also a car will change its power output depending on the day and conditions for that day etc. So some days it'll produce more, some days less. But to me it doesn't matter as long as the engine appears to be running ok, it makes a good noise, and is quick enough for my simple needs. And to be fair, my little 327 is plenty quick enough down the country roads for me. Like Chuffer above, I also run a 3.08 rear gear with LSD. So its more setup for cruising than acceleration, but still accelerates fine for me.

I am surprised that you quote only -30bhp gross to nett, as the video I recommended in the initial post indicated larger differences in general for a range of engines. Also I was surprised by how large the drops were reported in the video, and other sources. If the only difference in measurement are for some ancillaries like water pump, alternator, power steering pump, air pump, etc , then I would be inclined to believe your figure. Perhaps something else is going on. Up to say 1971 the manufactures were probably looking to maximise reported power to boost sales, then for reasons of politics, or environment, or oil crisis, they started to downplay power outputs. Any thoughts?

 

[teamzr1]

Many factors are involved with dyno numbers
Due to that, nameplates could play games to get the marking numbers they want

1, Because of the SAE came up with a standard that nameplates have to adhere to and also someone from SAE
must be present at time the nameplate dyno tests are done
These numbers come as used is an engine dyno, so reason like GM reports flywheel and not rear wheel numbers
since the later C6s GM has done and reported the SAE engine dyno results

Now must deduct the loss over the drivetrain to the rear wheels
Common loss is deducted, about 10% for a manual tranny and 15% for an auto tranny

Types of chassis dynos

2 Non load bearing
Results from these are about useless, but vehicle owners love them as results are bloated high numbers
One reason is to get the most customers wanting dyno numbers the drum is around 2,700 pounds, so lighter ricer mobiles could be customer base

This leaves the bogus part of results as let's say a C3 and say with driver in car weight is 3,700 pounds that is about 25 % difference of weight mass between drum and car.
Thus, the end numbers would report higher but in fact the car has to only overcome the drum's 2,700 pounds of mass
Meaning if the drum and car were the same weight it would take more HP/Tq to turn that mass.

This type of chassis dyno simply measures, time, that is the time when driver takes the engine close to redline (tranny in 4th gear as 1:1)
and jumps off the gas pedal and the machine than times it took to get the mass to come to a stop

These funny numbers do not factor in weather or elevation, so non-corrected HP/Tq

3. Load bearing chassis dyno

The results would be closer to fact but also leaves for lots of games on cheating the system

The drum is controlled by either water or electrical in adding the load mass of drum to equal the weight of vehicle/driver
and has a weather station to factor in the weather and elevation
Hooked up to this dyno is a P/C with software and this type of dyno (which cannot be done with non-loading type) is the vehicle can be driven like on a road or track thus allowing truer number in the real world
Cannot be done on non load bearing as the simple math explained about, simple is go to 4th gear take to redline and jump off pedal

With load bearing, the shop can make any numbers they want to lie about, such as override what the weight is and what the correct D/A is

An Example I caught as a shop when wanting to market mods was they put the weather station on top of the hot engine during the pulls
Thus, the weather factors are reported much higher and is computed for a higher corrected D/A results

Here are 2 examples using my 1999 C5 on a load bearing chassis dyno and making sure the weight, corrected D/A were correct and
making sure the dyno was calibrated each time

One test is N/A LS1, that I did engine mods to and other run was with 200 HP shot of NOS
These numbers are at the rear wheels.

 

[Chuffer]

I am surprised that you quote only -30bhp gross to nett, as the video I recommended in the initial post indicated larger differences in general for a range of engines. Also I was surprised by how large the drops were reported in the video, and other sources. If the only difference in measurement are for some ancillaries like water pump, alternator, power steering pump, air pump, etc , then I would be inclined to believe your figure. Perhaps something else is going on. Up to say 1971 the manufactures were probably looking to maximise reported power to boost sales, then for reasons of politics, or environment, or oil crisis, they started to downplay power outputs. Any thoughts?

There was a lot of fibbing and misrepresentation of facts going on in those days . Even well into the 1970`s some manufacturers in the UK were very liberal with their performance figures to boost sales.

 

[Roscobbc]

There was a lot of fibbing and misrepresentation of facts going on in those days . Even well into the 1970`s some manufacturers in the UK were very liberal with their performance figures to boost sales.

I wonder how many people were badly disppointed back in the early/mid 60's when they read the road tests for E Type Jags indicating 150+mph performance, chose to buy one based on that understanding (and never ever to realise those sort of speeds)........wonder when they later found-out the cars were factory tweaked what their thoughts were?

 

[Chuffer]

I wonder how many people were badly disppointed back in the early/mid 60's when they read the road tests for E Type Jags indicating 150+mph performance, chose to buy one based on that understanding (and never ever to realise those sort of speeds)........wonder when they later found-out the cars were factory tweaked what their thoughts were?

In today`s world it would end in litigation for misrepresentation . Back then , they just sucked it up . Marcos were a passed master " upping" their performance figures to keep up with or outdo TVR .

 

[Mr. Cricket]

From the Black Book:

Horsepower rating decreases in 1972 were due mainly to a change in the measuring criteria. Previously, horsepower ratings were measured as the "gross" output of the engine alone. Starting in 1972, ratings were a more realistic, real-world "net" which included losses from air cleaners, exhaust systems and accessories.

My 1971 LT-1 is rated at 330hp gross whereas the following year '72 was rated 255hp net and I doubt there's much difference in performance.

When I bought the LT-1 it was far too quite and I'd always wanted to put Cherry Bombs on something so I did. Christ it was loud and it got a lot of attention but one day, several years later I found myself all grown up and decided to go back to stock. It was doing that, that made me realise just how much the Cherry Bombs choked the engine and now with a stock set I had all that power back at my disposal and what a difference. Nice to be able to hear that 'sewing machine sound' from the solid lifters again too!

Being all original it still has the SMOG set up too

 

[Nassau65]

Always thought cherry bombs were a bit too loud. Don’t know if they did/do different versions of loudness?

 

[teamzr1]

On my 1999 C5 since 2009
When taking tail pipes just past rear fascia, using sound meter at right angle, 86 Dba


Trust me do not do this

I tested this at 104 Dba as sound bounced off the ground :-(

 

[Roscobbc]

Always thought cherry bombs were a bit too loud. Don’t know if they did/do different versions of loudness?

There is only one sound with Cherry Bombs - unpleasantly raucus........and as (unlike other manufacturers they seem to only use glass fibre packing around the inner absorsion tube) they get louder with time as the 'glass wadding gets covered with exhaust waste. Other brands use a combination of ceramic, stainless and 'glass wadding which seems less affected by coke and oil etc.