Power output V8 Petrol vs V8 Diesel





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  1. #1
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    Default Power output V8 Petrol vs V8 Diesel

    I'm perplexed...
    Please explain to me: If the power output of my LC 100 VX, 4.7l V8(petrol) is rated at 175kW, and the newer V8 Turbo Diesel on the LC 200 is rated 173 kW or there abouts, WHY is it that my 20 year old petrol V8 FEELS so much more responsive?
    It runs out of steam sooner, but hey! it is 20 years old!
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    Default Re: Power output V8 Petrol vs V8 Diesel

    Turbo lag?
    Last edited by douglash; 2020/04/09 at 11:58 AM.
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    Default Re: Power output V8 Petrol vs V8 Diesel

    Gosh this is a long and involved discussion, but simply power is the rate of doing work. In automotive terms, the torque gives you the ability to do the work, but the power determines how fast you can do it.

    Generally speaking, although modern Turbo Diesels are changing many of the rules, petrol engines like to rev and typically develop most of their torque high in the rev range. Remember revs = power (as long as you don't loose too much torque.) So the petrol engine is developing more torque as it revs harder, making is feel and be more responsive.

    Diesels are usually mechanically heavier engines, and often optimised more for torque. They often have a longer stroke which helps with the torque, but also limits revs and piston speeds need to be kept in check. So trucks often redline below 2000rpm (I am not sure of the newer ones) but are developing torque in excess of 1000nm from very low revs.

    Typically diesels are very strong at lower revs, but run out of puff as their whole intake system is designed to deliver that low rpm torque. My amaock delivers max torque at 1750-2250 rpm. After 2250 is falls off pretty fast, so unlike a petrol engine, there is no gain in power by revving it higher. Plus with the higher piston speeds, etc, it just does not like to rev. Remember that the power is the rate of doing the work, but as the torque starts to drop off, the higher revs do not equal more power in a diesel as the torque drops off unlike your petrol which is just getting into its stride as the revs pick up.

    This is where some of the newer diesels have changed the game. I would imagine the BMW 3ltr for example. It is still not 9000rpm honda s2000, but it can pull more cleanly to say 4000rpm still with the high torque.

    That being said in traffic I would regard a good turbo dielse as being more responsive as it has bags of torque and can just pull cleanly to move you along. With a similar petrol vehicle you have to drop a gear or 2 to really get onto the power peak which is a chore if you are not a boy racer.

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    Default Re: Power output V8 Petrol vs V8 Diesel

    Quote Originally Posted by stephenplumb View Post

    That being said in traffic I would regard a good turbo dielse as being more responsive as it has bags of torque and can just pull cleanly to move you along. With a similar petrol vehicle you have to drop a gear or 2 to really get onto the power peak which is a chore if you are not a boy racer.
    That sums it up nicely.
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    Default Re: Power output V8 Petrol vs V8 Diesel

    It's all about max RPM, torque and power curves, and gearing. And perception.
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    Default Re: Power output V8 Petrol vs V8 Diesel

    From a VERY quick google, it seems the 200 is almost a ton heavier than the 100?
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    Default Re: Power output V8 Petrol vs V8 Diesel

    SOme vehicles also give a false perception of speed due to poorer NVH. A CTI Golf spinning its wheels and revving its nuts of feels a lot faster than my awd, auto 200kw Forester.

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    Default Re: Power output V8 Petrol vs V8 Diesel

    First, I guess, would be to determine what you mean by "responsive"?

    As in the engine responds quicker to throttle inputs, or the car feels faster?

    Stephenplumb covered many possibilities,

    Another one might be flywheel weights, this has a big effect on how quickly the engine would respond to inputs.

    Also, naturally aspirated petrol engines tend to respond quicker than turbo diesel engines due to a few factors but turbo lag and more rotating mass of a diesel could also factor in.

    As an owner of a S2000 and a few roots super charger driven engines, I tend do get annoyed with turbo diesels, you get used to them, but compared to a good NA petrol, most diesels are lazy in their responsiveness.
    Last edited by Cyclone101; 2020/04/09 at 05:04 PM. Reason: Spelling

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    Default Re: Power output V8 Petrol vs V8 Diesel

    Quote Originally Posted by Graham View Post
    From a VERY quick google, it seems the 200 is almost a ton heavier than the 100?
    I'm not even going to Google this but I somehow doubt this?
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    Default Re: Power output V8 Petrol vs V8 Diesel

    Quote Originally Posted by Graham View Post
    From a VERY quick google, it seems the 200 is almost a ton heavier than the 100?

    Thats was a VERY VERY QUICK GOOGLE....so quick in fact that I dont think anyone will be able to obtain the same result....

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    Default Re: Power output V8 Petrol vs V8 Diesel

    100 VX 4.7P 2270kg with 0-100 in 11.2sec,
    200 VX 4.5D 2740kg with 0-100 in 8.6sec,
    200 VX 4.7P 2585kg with 0-100 in 8.6sec,
    200 VX 5.7P 2581kg with 0-100 in 7.4sec,
    Quote Originally Posted by dirtshark View Post
    I'm not even going to Google this but I somehow doubt this?
    Last edited by Tom13; 2020/04/10 at 07:22 AM.
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    Default Re: Power output V8 Petrol vs V8 Diesel

    With all this lockdown time on hand I googled too.
    Gross weight and Kerb/Curb weight differs by approximately a ton. And even within each different weights are list, difference by 200-300kg.
    Best is put each on a scale with full tanks LOL
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    Default Re: Power output V8 Petrol vs V8 Diesel

    Again lots of muddled thinking about torque/power etc.

    To accelerate a mass it requires the injection of energy (in the form of joules). This energy in the case of a car must be supplied by an engine. The power of the engine is quoted in Watts. (A Watt is one joule per second).

    If engine A and engine B have the same power quoted in kilo Watts then they will accelerate at the same rate. Forget about all the quasi-science regarding torque etc., just stay with basic physics - it works the same throughout the Universe.

    Since engine A (petrol) and B (turbodiesel) do not perform equally it is safe to assume they do not deliver the same power.

    And the answer is right there: The diesel relies on a turbo/intercooler system to deliver it's rated power, and a turbo/intercooler system rarely performs optimally. Hence the engine rarely, if ever, delivers it's rated power. There's other threads on this forum to explain why this is, so I won't repeat why this is.

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    Default Re: Power output V8 Petrol vs V8 Diesel

    Quote Originally Posted by Fyko View Post
    Again lots of muddled thinking about torque/power etc.

    To accelerate a mass it requires the injection of energy (in the form of joules). This energy in the case of a car must be supplied by an engine. The power of the engine is quoted in Watts. (A Watt is one joule per second).

    If engine A and engine B have the same power quoted in kilo Watts then they will accelerate at the same rate. Forget about all the quasi-science regarding torque etc., just stay with basic physics - it works the same throughout the Universe.

    Since engine A (petrol) and B (turbodiesel) do not perform equally it is safe to assume they do not deliver the same power.

    And the answer is right there: The diesel relies on a turbo/intercooler system to deliver it's rated power, and a turbo/intercooler system rarely performs optimally. Hence the engine rarely, if ever, delivers it's rated power. There's other threads on this forum to explain why this is, so I won't repeat why this is.
    Engines may produce/deliver the same power, though if have to carry grossly different weights it naturally won’t do it the same
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    Default Re: Power output V8 Petrol vs V8 Diesel

    Quote Originally Posted by AK77 View Post
    Engines may produce/deliver the same power, though if have to carry grossly different weights it naturally won’t do it the same
    Install the new UNICHIP X Chip or re- map your current ECU on the V8 diesel. The responce is enormous

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    Default Re: Power output V8 Petrol vs V8 Diesel

    Quote Originally Posted by Fyko View Post
    Again lots of muddled thinking about torque/power etc.

    To accelerate a mass it requires the injection of energy (in the form of joules). This energy in the case of a car must be supplied by an engine. The power of the engine is quoted in Watts. (A Watt is one joule per second).

    If engine A and engine B have the same power quoted in kilo Watts then they will accelerate at the same rate. Forget about all the quasi-science regarding torque etc., just stay with basic physics - it works the same throughout the Universe.

    Since engine A (petrol) and B (turbodiesel) do not perform equally it is safe to assume they do not deliver the same power.

    And the answer is right there: The diesel relies on a turbo/intercooler system to deliver it's rated power, and a turbo/intercooler system rarely performs optimally. Hence the engine rarely, if ever, delivers it's rated power. There's other threads on this forum to explain why this is, so I won't repeat why this is.
    Peak power figures are also misleading, power under the curve is more important, or average power if you will.

    If you Dyno a turbo car you will also notice that larger turbos barely make any power in 1st gear as the rpm rise quicker than the pressure can build.

    Gearing also plays a big role, larger tyres and higher gearing will have slower acceleration.

    Interestingly, a car can have a slower 0-100 time than another car and yet be in front of a drag race due to non-linear acceleration. This was true of the WRX and STI of a certain year (don't recall which now) where the WRX could reach 100kph in 2nd gear and the STI only in 3rd. The STI had a slower 0-100 but was in front the whole time.

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    Default Re: Power output V8 Petrol vs V8 Diesel

    Power under the curve is the important bit:
    Everything being equal, tyre size, weight etc. A specific engine (powerplant) has a power curve, which is calculated from the torque produced by the engine at a specific rpm.

    This power curve will look different for different powerplants/units.

    To accelerate a mass you need work done. The more work which can be done by an engine is the area below the power curve. . .

    You can thus have an engine with no power low down and very high peak power which does less work that an engine with lower peak power.

    Thus if you drive in a certain RPM range to accelerate you need to calculate the area underneath the power curve say in 3rd gear from 3000rpm to 4000rpm to get the work done in that time.

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    Default Re: Power output V8 Petrol vs V8 Diesel

    Quote Originally Posted by Graham View Post
    From a VERY quick google, it seems the 200 is almost a ton heavier than the 100?
    I think you have nailed it right there.

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    Default Re: Power output V8 Petrol vs V8 Diesel

    I see lots of relevant and good and also muddled comment, but none that shows insight into how power and torque are related. Two cars with same rated power in kW (presumably meaning same top of power curve but not necessarily at same rpm) will NOT accelerate identically if the underlying torque curves are different because (1) when torque at the wheels differ on the way up to higher revs, then the forward force on the car (which is the torque divided by wheel radius) also differ, and different force means different acceleration. Even more so when the wheels are different diameter, and car masses differ. Not to speak of different drive trains having different losses and so forth. And (2) because IC cars do not start accelerating at the top of the power curve, they start at the low end. The physics is a little bit more involved. And that is why all manner of races are held to see the final outcome.


    I can fill in the gap and remove some hazyness in this area. First a few basic notions.
    Power is expressed in W or kW.
    W (Watt) is same as Nm/s (Newton meter per second). And is also J/s (Joule/second),.

    The essential formula relating power to torque is :
    Power in W or Nm/second = Torque in Nm X rotational speed per second.


    The rotational speed to use is not revs/minute, not even rev/second, but radians per second.

    A radian is the angle in a cirlce that spans a stretch of the circumference equal to the length of the circle’s radius. Some will know this from high school but some may not have heard this, or have forgotten.
    Since the length of the circumference of a circle is 2 * PI * radius, there are 2 PI radians in the full 360 degree revolution. Where PI is the famous number 3.14159265…

    Hence one radian is 360 / 2 PI = 360/6.28318...= 57.2957… degrees (a bit under 60 degrees).

    We do not need this number for the power to torque relationship, but it helps to understand or recall the concept of radian. In short, 1 rev = 2 * PI radians = 6.28318… radians.


    We are nearly there.
    First go from rpm to radians/second. Example for 2100 rpm
    2100 rpm = 2100/60 = 35 rev/s = 35 * 2 PI radians/s = 35 * 2 * 3.14159 = 219.9 rad/s

    Say 220 rad/s. The factor 1/60 * 2 * PI = 2 X 3.14159 / 60 = 0.1047.


    Back to the formula for power and torque:
    Power in W or Nm/s = Torque in Nm X rotational speed in radians/sec


    Say a torque curve shows an engine at 2100 rpm has 260 Nm torque.
    Then at that engine speed it develops 260 Nm x 220 rad/s = 57200 Nm/s = 57200 W = 57.2 kW.

    Taking a torque curve (as published in motoring journals) we can do this for multiple points along the curve and calculated the power at any point. Simply multiply the torque value (in Nm) with rpm and with 2 PI/60 = 0.1047, then we have the power in Watt. Divide by 1000 to get kW. Doing this for a range of points along the torque curve, delivers the power curve. Anybody can do this calculation, and check the result with the published power curve which is usually shown next to the torque curve. So the primary curve is the torque curve, what the engine is capable of producing when pistons push on the crank. Therefore long-stroke engines have more torque because they they have longer cranks.


    Engines that have max torque at low revs, such as 2500 rpm, do not develop max power at that point because while the torque goes down gradually past that max point towards higher revs, for a fair stretch of revs beyond peak torque, the higher revs manage to keep the product of torque and (rpm x 0.1047) climbing, but eventually the drop in torque bends the power curve down. So a torque peak at say 2500 rpm can go together with a power peak at 4000 rpm. But before reaching those 4000 rpm the torque may be sofar down that there is insufficient force to overcome wind resistance or uphills, and one needs a lower gear to create higher torque. Alternatively one can get past the max power point and torque is sofar down that there is no acceleration left against the wind resistance or uphill. In other words max speed has been reached.

    This is actually only the beginning of the story. There is more to explore after this in terms of max up-slope driveable, in which gear, with what torque available, slipping on low friction ground with too much torque, and some more. Great fun.


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    Default Re: Power output V8 Petrol vs V8 Diesel

    I have once before likened a diesel engine to a Sumo Wrestler and a petrol engine to a Ninja Warrior.
    The one is slow, but strong and the other less strong, but fast. The number of bricks he can carry at once is his torque, but the number of bricks he can deliver over a specific time period is his power.
    For the example below I'm going to use the Prado models as an example, in other words the diesel version has more torque, but less power than its petrol cousin.
    If we were to put the Sumo and the Ninja in a construction site, with their sole purpose being to carry bricks, it will look something like this:
    • if, at their best effort, the Sumo can carry 10 bricks at a time and the Ninja only 6, but the Sumo can only walk while carrying his 10 bricks and the Ninja is able to do it on the run, then after an hour the Ninja will have delivered more bricks than the Sumo.
    • what the Toyota engineers have done in their infinite wisdom (as more than half of the car buying public believes), is to have given the diesel and petrol models the same gearing (diff and gearbox). In effect they have confined the Ninja to walk at the same pace as the Sumo, but carrying less bricks.
    • at cruising speed, in a flat landscape, this is fine. What this equates to is bricks being delivered to a building site that is really not trying to build fast.
    • if the building foreman cracks his whip and demands faster building, which equates to ascending a hill or accelerating to overtake, then the Sumo is still fine because he is delivering more than enough bricks at his walking pace. The Ninja however is now in trouble because he can not supply enough bricks at a walking pace. He now has to break into a run to keep up. For the driver this equates to a drop in gear. We know the Ninja is fit and happy to run, but something has now changed in the quiet harmony that existed before the foreman cracked his whip. This forum has numerous posts about V6 Prado owners criticizing the cruise control on their cars because of this phenomenon.
    • if Toyota had followed the practice of most of the other manufacturers and given their petrol models a lower gearing setup, then the Ninja would be jogging while his Sumo partner is walking, yet delivering enough bricks to accommodate an increase in brick demand at the build site.
    • the reality is that if things get really hectic at the build site and there is a big demand for bricks, then the Sumo is buggered because he simply cannot run, but the Ninja can run at a good speed and get more bricks on site than the Sumo
    Last edited by RoelfleRoux; 2020/04/14 at 05:29 PM.

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