Big engine, low HP output - why?
Discussion
From time to time, a vehicle gets posted with what appears to be a large engine displacement and a comparatively small bhp output. Usually resulting in what appears to be a poor bhp/litre ratio, or specific output.
This happened most recently in last weeks shed of the week, which was a Jeep Grand Cherokee, with a 4.0 litre S6 engine.
Which sparked comments of:
I thought I'd have an attempt at explaining why this is.
During the SOTW thread, someone made a very good comment, that Rover/Land Rover at the same time sold one vehicle with a 4.0 litre and 182bhp and another with a 2.5 litre and 177bhp. Very similar outputs, so it begs the question of why did they bother with the 4.0 litre engine, if a modern 2.5 could make pretty much the same power..
The answer is simple --- torque. And more importantly low and mid range torque.
It is important to remember, for automotive use, HP or power is a representation of:
HP = torque x rpm /5252
Which is essentially torque at speed. This means for a given engine rpm, the more torque you are making, you will also be making more HP.
What I've done to give a visual aid for this is, to compile and compare four different engines power curves. This highlights that while the PEAK outputs are often similar, the power mid way through the rev range is very different for these engines.
Ideally I wanted to use data for a Rover 3.9/4.0 V8 as it produces such a similar output to the KV6, however I couldn't find any reliable data, so have substituted it with the output of the 4.6 Rover V8 instead, which were rated at 225hp. A big of a hike, but the point of showing low and mid rpm power gains is still valid.
For additional comparison I've found data for a late model Jeep 4.0 litre engine and the high revving FA20 2.0 litre unit currently found in the Toyota GT-86.
As you can see, the FA20 and the Jeep 4.0 make quite similar PEAK power, although at very different rpm. This means to get this power in the GT-86 you need to make full use of the revs.
But more importantly you need to look lower down the rev range. At 2000rpm the story is quite illuminating.
The FA20 is making circa 50hp @ 2000rpm while the Jeep 4.0 is making around 85bhp.
That's a pretty substantial difference. And for towing, off road or even just normal non spirited driving, the Jeep's engine makes a lot more power at these rev ranges.
Of course there is more to the story, and that is gearing. So you could adjust the gearing to have the same road speed and the same engine output, but you would require much higher revs to attain this.
In the case of the FA20, you'd need just over 3000rpm to match the Jeep's 4.0 litre @ 2000rpm, in terms of Horse Power. The downside is, it would feel like you are revving the engine all the time. And will likely require far more gear ratios on hand, to give you the spread of road speeds required for a car. And you will have to perform a lot more gear changes because of this. Hence why some types of cars, usually those that are lighter, smaller, won't be towing and have less need for low end grunt, such as off roading. Tend to favour higher revving engines with higher specific output. And larger cars that are heavier and may want a more relaxed power delivery, or need more low end grunt will often use a larger displacement engine.
Looking further at the graph, even the Rover 2.5KV6 in it's lowest state of tune (177PS variant), shows significant power advantages over the likes of the FA20 from 1500rpm, right through to 5500rpm. It may make less PEAK HP than the FA20, but for most of the time it will be making more power. Which is probably why something like an MG ZS180 will feel far more grunty than a GT-86 does on the road.
For road use this is a very important trait, as it is part of what dictates a cars character and performance delivery.
The Rover 4.6 vs the FA20 shows this clearly. Despite not a massive difference in PEAK power. The Rover makes almost the same Horse Power @ 2500rpm as the FA20 does @ 5000rpm.
Which means a Range Rover can waft along with the V8 engine, but if someone fitted the FA20 to the Range Rover you'd have to have it screaming at 5000rpm just to make normal progress.
I hope this helps illustrate that a big engine and low output are not always a bad thing. And that an appropriate engine for it's intended use, is so much more than just it's PEAK power output.
Thank you.
This happened most recently in last weeks shed of the week, which was a Jeep Grand Cherokee, with a 4.0 litre S6 engine.
Which sparked comments of:
unnamed person said:
Only an American engine could take 4 litres to churn out 176 bhp
A common theme and often the norm when many N. American vehicles are talked about, or more traditional off road biased 4x4's.I thought I'd have an attempt at explaining why this is.
During the SOTW thread, someone made a very good comment, that Rover/Land Rover at the same time sold one vehicle with a 4.0 litre and 182bhp and another with a 2.5 litre and 177bhp. Very similar outputs, so it begs the question of why did they bother with the 4.0 litre engine, if a modern 2.5 could make pretty much the same power..
The answer is simple --- torque. And more importantly low and mid range torque.
It is important to remember, for automotive use, HP or power is a representation of:
HP = torque x rpm /5252
Which is essentially torque at speed. This means for a given engine rpm, the more torque you are making, you will also be making more HP.
What I've done to give a visual aid for this is, to compile and compare four different engines power curves. This highlights that while the PEAK outputs are often similar, the power mid way through the rev range is very different for these engines.
Ideally I wanted to use data for a Rover 3.9/4.0 V8 as it produces such a similar output to the KV6, however I couldn't find any reliable data, so have substituted it with the output of the 4.6 Rover V8 instead, which were rated at 225hp. A big of a hike, but the point of showing low and mid rpm power gains is still valid.
For additional comparison I've found data for a late model Jeep 4.0 litre engine and the high revving FA20 2.0 litre unit currently found in the Toyota GT-86.
As you can see, the FA20 and the Jeep 4.0 make quite similar PEAK power, although at very different rpm. This means to get this power in the GT-86 you need to make full use of the revs.
But more importantly you need to look lower down the rev range. At 2000rpm the story is quite illuminating.
The FA20 is making circa 50hp @ 2000rpm while the Jeep 4.0 is making around 85bhp.
That's a pretty substantial difference. And for towing, off road or even just normal non spirited driving, the Jeep's engine makes a lot more power at these rev ranges.
Of course there is more to the story, and that is gearing. So you could adjust the gearing to have the same road speed and the same engine output, but you would require much higher revs to attain this.
In the case of the FA20, you'd need just over 3000rpm to match the Jeep's 4.0 litre @ 2000rpm, in terms of Horse Power. The downside is, it would feel like you are revving the engine all the time. And will likely require far more gear ratios on hand, to give you the spread of road speeds required for a car. And you will have to perform a lot more gear changes because of this. Hence why some types of cars, usually those that are lighter, smaller, won't be towing and have less need for low end grunt, such as off roading. Tend to favour higher revving engines with higher specific output. And larger cars that are heavier and may want a more relaxed power delivery, or need more low end grunt will often use a larger displacement engine.
Looking further at the graph, even the Rover 2.5KV6 in it's lowest state of tune (177PS variant), shows significant power advantages over the likes of the FA20 from 1500rpm, right through to 5500rpm. It may make less PEAK HP than the FA20, but for most of the time it will be making more power. Which is probably why something like an MG ZS180 will feel far more grunty than a GT-86 does on the road.
For road use this is a very important trait, as it is part of what dictates a cars character and performance delivery.
The Rover 4.6 vs the FA20 shows this clearly. Despite not a massive difference in PEAK power. The Rover makes almost the same Horse Power @ 2500rpm as the FA20 does @ 5000rpm.
Which means a Range Rover can waft along with the V8 engine, but if someone fitted the FA20 to the Range Rover you'd have to have it screaming at 5000rpm just to make normal progress.
I hope this helps illustrate that a big engine and low output are not always a bad thing. And that an appropriate engine for it's intended use, is so much more than just it's PEAK power output.
Thank you.
Some good points, my version is drive it before deciding how fast it is/feels or how pleasant to drive it it.
I learn it early on, mk1 Capri, I was offered an engine swap, my standard one, plus cash for a 1700 cc version with bigger valves, bigger carb and a "Race" cam, apparently 120 plus bhp, drove it whilst still in the Escort and it was a pain in the arse, shuddered, lurched and bogged down unless hammering it, it was fast but the trade off wasn't worth it.
Two Golfs, 100 bhp 1.6 Petrol vs 100 bhp diesel Auto, diesel was way better in normal driving.
So, its not always the numbers,
I learn it early on, mk1 Capri, I was offered an engine swap, my standard one, plus cash for a 1700 cc version with bigger valves, bigger carb and a "Race" cam, apparently 120 plus bhp, drove it whilst still in the Escort and it was a pain in the arse, shuddered, lurched and bogged down unless hammering it, it was fast but the trade off wasn't worth it.
Two Golfs, 100 bhp 1.6 Petrol vs 100 bhp diesel Auto, diesel was way better in normal driving.
So, its not always the numbers,
ZOLLAR said:
Emeye said:
I always thought that large American engines with low power were also related to making them unstressed and therefore more reliable.
This was my understanding.Watch any of the 'Roadkill' epsiodes on YouTube where they seem to be able to get utter derelicts and just put some fresh fuel and some clean plugs in them and off they go. Unkillable.
Emeye said:
I always thought that large American engines with low power were also related to making them unstressed and therefore more reliable.
I think that's a fair assessment, American engines have never been the most technologically advanced things in the world so if they want to create more power you get a bigger engine. Also their fuel octane is crap and anything too highly compressed is likely to go bang.Additionally most diesels create far more torque but they not only have an image problem in the US and availability issues and it's more expensive, they also have to meet stricter US emissions which require more cost associated with it.
So if an American wants a torquey motor it's much cheaper to buy a petrol motor than a diesel
ZOLLAR said:
Emeye said:
I always thought that large American engines with low power were also related to making them unstressed and therefore more reliable.
This was my understanding.If you don't need to work the engine as hard, it'll produce less heat, be less stressed and can usually be made with cheaper parts and bigger tolerances.
Larger displacement will also generally be better than smaller displacement for high altitude work.
If you spend 2-3 hours winding yourself through the mountains of the mid west USA. A car able to run at 2000-3000rpm rather than 5000-6000rpm for the same power will be far more preferable for normal non spirited driving.
Isn't it more the case of production costs. Lower specific output means lower tolerances, quality of materials (for NA engine power), complexity of the engine design (2 valve heads, no variable valve timing, etc) consequently you can produce them cheaply and rely on the displacement to give you sufficient power for a low cost (for a NA engine).
Emeye said:
I always thought that large American engines with low power were also related to making them unstressed and therefore more reliable.
A variation thereof is probably more accurate, why make a small engine a (less reliable) screamer when you can just put in a larger engine for the same performance?The Americans, with cheap fuel, etc, have never had hangups about large engines.
If they also want large HP figures to go with those large engines, they can do that too, but there is no expectation, no "performance anxiety" related to having plenty of cubic inches.
The rest of what has been discussed is true.
I also find that a diesel engined car is much easier to live with on a daily basis than a far superior in peak HP petrol car of the same capacity. Even an NA diesel like VW's 1.9L SDI.
Truckosaurus said:
ZOLLAR said:
Emeye said:
I always thought that large American engines with low power were also related to making them unstressed and therefore more reliable.
This was my understanding.Watch any of the 'Roadkill' epsiodes on YouTube where they seem to be able to get utter derelicts and just put some fresh fuel and some clean plugs in them and off they go. Unkillable.
Edited by Otispunkmeyer on Thursday 12th May 13:08
deuchars said:
Not worth covering the effects gearing also, might change your argument...
These have an even bigger impact, but the OP's point is a valid one. Lots of smaller engines PEAK figures are the same as lots of larger engines PEAK figures, yet the range at which the smaller engine can produce higher outputs is much narrower than the larger engines.Valgar said:
I think that's a fair assessment, American engines have never been the most technologically advanced things in the world
What makes you think that?They had mass market turbocharged engines in the 1970's.
Have been using common rail diesel engines and variable vane turbos for longer than most Jap or European car makers.
Blocks made out of alloy, modular design, multivavle heads, cylinder deactivation. And many other things.
Of course not all of their engines use this technology, but it doesn't mean they don't sell it.
And technically General Motors and Ford are American motor companies and have been building "European engines" for almost a 100 years for our market.
Valgar said:
so if they want to create more power you get a bigger engine.
Nope, for many many years Ford's approach was multivalve technology and forced induction.Base model car might have a 2v SOHV V8, while a top of the range model had an all alloy DOHC 4v V8 or one with a supercharger.
GM did similar. The Northstar V8's used in Cadillacs for instance, as modern and technically advanced (maybe more so) than anything BMW or Toyota.
Valgar said:
Also their fuel octane is crap and anything too highly compressed is likely to go bang.
This is a complete misnomer. There are essentially 3 octane rating scales, RON, MON and PON (AKI).The US simply use a different scale to us. Their 93 octane fuel is about equal or better than most superunleaded fuel in the UK. And higher octane fuel is generally available more easily in the US than in the UK.
Certainly in aviation its all about reliability, the little Rolls Royce O-200-A engine in the Cessna 150 that I fly has a capacity of 3.3L and produces a modest 100hp; or 30hp/L it also revs out at 2600rpm (normal cruise 2350rpm), but that is as much for the benefit of the propeller as it is for the benefit of engine longevity. However, despite (in PH terms) its paltry power output it is wonderfully under-stressed and though in the UK runs on 100LL (100 Octane Low Lead) Avgas, it is rated to run on fuel as low as 80 octane; to put that into perspective 95RON is about 91 Octane iirc.
The Mustang that I drive in its stock form produces 300hp and 320lb/ft of torque from its 4.6L V8, though it will run on 87 octane fuel if you want it to. I have heard it referred to as a woeful power output for such an engine on these forums. I always found it strange that nobody mentions the AMG mercedes V8 from the same period, 5.0L producing 340ish HP; hardly pushing it.
The Mustang that I drive in its stock form produces 300hp and 320lb/ft of torque from its 4.6L V8, though it will run on 87 octane fuel if you want it to. I have heard it referred to as a woeful power output for such an engine on these forums. I always found it strange that nobody mentions the AMG mercedes V8 from the same period, 5.0L producing 340ish HP; hardly pushing it.
ZOLLAR said:
Emeye said:
I always thought that large American engines with low power were also related to making them unstressed and therefore more reliable.
This was my understanding.Americans on average drive double the amount of miles annually compared to us. Fuel is cheaper too.
When I worked in a garage during my student exchange in the US, years ago!, the work was 90% routine servicing; oil and filters plus brakes. I don't remember ever seeing colleagues doing serious engine work. As cars got older there'd be issues with ancillary stuff like alternators, cooling systems and aircon but rarely the engine. So there is logic too it.
ZOLLAR said:
Emeye said:
I always thought that large American engines with low power were also related to making them unstressed and therefore more reliable.
This was my understanding.Gassing Station | General Gassing | Top of Page | What's New | My Stuff