Educating Sarnie

[mugwump]

Its really quite funky when you play with the numbers in the graph, if you are *really* that interested I will post the xls spreadsheet and you can play with it yourself. Or are you just winding me up?

 

but if you think about the equation for a moment (if I have understood your question correctly) , torque = power * 5252 / rpm. Lets compare two engines running at 1000 rpm and 5252 rpm (the diesel and the petrol).

 

at 1000 rpm the equation looks like this

torque = power * 5.2

 

(as 5.2 = 5252/1000rpm)

 

whilst at 5252 rpm the equation looks like this

torque = power

 

(as 5252/5252rpm =1)

 

So at 1000 rpm you need 5.2 times the torque to generate 1 more BHP, or just 1 more BHP at 1000rpm is 5.2 times the torque that same BHP would give you at 5252rpm. So a very small increase in engine power at low rpm equates to a huge return in torque, this tails off as the rpm increases. This is why the two graphs appear so different, the two engines produce similar power but the diesel does so at a much lower rpm.

 

not sure I have explained that very well?

 

So the question is how does the diesel achieve this? My understanding is that its mostly due to the fact that diesels are built like tanks to run much higher compression ratios because it is actually a spontaneous explosion of the fuel brought about by the compression raising its temperature. In turn the power stroke provides much higher force to the piston hence more torque. I think! Sure someone can explain that better than me....

 

What was sarnies question again? Something about whats BHP, we seem to have wandered somewhat?

[Sarnie]

 

Ok. So how does the size of the engine (3.5l or 2.0l etc) affect the ability to produce bhp and torque?

[mugwump]

Sarnie, just noticed your location!

[Sarnie]

Sarnie, just noticed your location!

 

Do you know it?

[ir_fuel]

 

Ok. So how does the size of the engine (3.5l or 2.0l etc) affect the ability to produce bhp and torque?

 

 

Thats easy,

 

bigger engine size + more cylinders -> more torque.

 

But do not compare N/A engines and FI ones here, because adding a turbo is "cheating"

 

Lets try simplifying it:

 

More engine size means bigger cylinders means more fuel in the cylinders in total, means bigger explosion = more force generated by the fuel exploding. Now there are some other issues influencing the final result:

 

If, for the same engine size, lets say 4 liters, you have very few cylinders it would mean each cylinder is very big, so the pistons and all the connected parts are very big/heavy too. This means that lot of the force of the fuel explosion will be used to get all this mass moving = loss of efficiency. Now if you have 4 liters and lots of cylinders (8 for instance) each cylinder will be smaller so there is less energy wasted getting everything to move. Now you cannot continue doing that infinitly, coz when you would lets say make a 4 liter 20 cylinder car you are also loosing efficiency because of all the added components, the weight of having lots of them and the fact that there is getting a lot more friction and so on. So engine manufacturers have to find a balance between all this. lots of small cylinders = quicker revving coz less mass to move, too many small cylinders == you get too much "stuff" in the engine to move and the engine gets too heavy with too many moving components. Few big cylinders == very hard to make lots of revs because of the mass of the pistons, hard to accellerate the engine because of that mass etc etc.

 

Is that clear?

[ir_fuel]

One thing i didnt touch is big bore vs small bore engines, meaning the typical american muscle car vs european race-cars. The way you construct your cylinders (big diameter, small height, or vice versa) also influences all this

[Sarnie]

So how do EVO's have 2.0l engines and still produce 340+ BHP

[H5]

So how do EVO's have 2.0l engines and still produce 340+ BHP

 

As I understand it the net result of the turbo is a greater 'explosion' in the cylinder so more force produced (and torque and power therefore increased).

 

I'll take cover now....!!

[ir_fuel]

So how do EVO's have 2.0l engines and still produce 340+ BHP

 

As i said (you should read everything ) Turbos are "cheating".

 

Basically they pump more air in the cylinder, hence you can get more fuel inthere too to get a good mixture, so the generated explosion is bigger.

[H5]

So how do EVO's have 2.0l engines and still produce 340+ BHP

 

As i said (you should read everything ) Turbos are "cheating".

 

Basically they pump more air in the cylinder, hence you can get more fuel inthere too to get a good mixture, so the generated explosion is bigger.

 

[mugwump]

The EVO engines with 340BHP+ are also very "peaky"

 

The turbo, intake, exhaust, cam profile and ECU are all designed to give that performance over a small rpm range (its optimised for ragging the @r$e off it). Catch one un-awares and you will leave it for dead because the turbo is MASSIVE as well and takes a while to spin up if off boost.

 

It makes for a great track/rally car (which is what it is after all) but its a pig to live with everyday.

 

Dont know if you ever saw the topgear episode when they had the EVO FQ racing the Lambo? The Lambo (being driven by a pro driver) could not drop Clarkson in the FQ round the track.

 

However, they then raced a hired Renault Scenic and the FQ from a rolling start at 30mph in 5th gear throttle on the floor, the Scenic streaked away (if its possible for a scenic to streak) and it took the whole mile and a half runway for the turbo to spin up in 5th and start gaining on the Scenic.

 

OK, in the real world you would drop a few cogs and floor it but what it shows is how intractable the engine is. Its not that flexible and you will spend your life switching cogs to keep it in the powerband.

 

Great fun, but not everyday

[Sarnie]

The EVO engines with 340BHP+ are also very "peaky"

 

The turbo, intake, exhaust, cam profile and ECU are all designed to give that performance over a small rpm range (its optimised for ragging the @r$e off it). Catch one un-awares and you will leave it for dead because the turbo is MASSIVE as well and takes a while to spin up if off boost.

 

It makes for a great track/rally car (which is what it is after all) but its a pig to live with everyday.

 

Dont know if you ever saw the topgear episode when they had the EVO FQ racing the Lambo? The Lambo (being driven by a pro driver) could not drop Clarkson in the FQ round the track.

 

However, they then raced a hired Renault Scenic and the FQ from a rolling start at 30mph in 5th gear throttle on the floor, the Scenic streaked away (if its possible for a scenic to streak) and it took the whole mile and a half runway for the turbo to spin up in 5th and start gaining on the Scenic.

 

OK, in the real world you would drop a few cogs and floor it but what it shows is how intractable the engine is. Its not that flexible and you will spend your life switching cogs to keep it in the powerband.

 

Great fun, but not everyday

 

And they look like dog turd

[mugwump]

And they look like dog turd

 

 

As ever Sarnie I am dumbfounded by your repartee......

[Dan]

In answer to your other question, and quoting Mike Costin from Cosworth Engineering : "BHP figures sell engines, Torque wins races "

[Liam's Z]

In answer to your other question, and quoting Mike Costin from Cosworth Engineering : "BHP figures sell engines, Torque wins races "