Discussion
I will be swapping heads over soon, and may take the opportunity to change the cam timing, but wanted to understand the consequences of doing so...
In simplistic terms, as I understand it, by retarding cam timing 2-3 degrees I will gain BHP but lose max torque . I presume the torque curve will be transposed further up the rpm range, although I don't know if the turque curve will change shape?
So here is my simplified question - which would be quicker overall in a mix of situations, either a car with 300BHP 330Lb/ft or the same car with 330BHP 300Lb/ft..?
Assuming chassis, suspension, etc are exactly the same - which setup would be faster: more BHP/less torque, or less BHP/more torque..?
Thanks,
Dom
In simplistic terms, as I understand it, by retarding cam timing 2-3 degrees I will gain BHP but lose max torque . I presume the torque curve will be transposed further up the rpm range, although I don't know if the turque curve will change shape?
So here is my simplified question - which would be quicker overall in a mix of situations, either a car with 300BHP 330Lb/ft or the same car with 330BHP 300Lb/ft..?
Assuming chassis, suspension, etc are exactly the same - which setup would be faster: more BHP/less torque, or less BHP/more torque..?
Thanks,
Dom
I really don’t know what I am on about here but it is an interesting question.
Plainly if you had a constantly variable transmission which allowed you to always run the engine at peak power 330bhp would accelerate the car more quickly than 300bhp regardless of the torque it is producing. Following on from that logic if your cam timing change produces exactly the same torque curve but higher up the rev range the car has to be quicker provided you use the appropriate gears to transfer the available power to the road.
Thing is do you what the car to be quicker or do you want it to feel quicker ? Lots of shove low down the rev range feels very satisfactory and makes the car feel quick but in reality making torque higher up will always make more power and therefore be quicker.
There are some useful software tools that let you enter your torque curve and gear ratios to calculate when changing gear will produce more torque at the rear wheels, which is after all what accelerates the car. Provided your ratios allow you to use the available power band fully more power has to be quicker. If you move too high up the rev range you may find that you need to exceed the rev limit before the next gear makes more torque.
I suppose a relevant point may be to ask is will your induction system and heads work as effectively at the higher flow rates, you will need quite a lot more air to make the same torque at higher rpm.
Plainly if you had a constantly variable transmission which allowed you to always run the engine at peak power 330bhp would accelerate the car more quickly than 300bhp regardless of the torque it is producing. Following on from that logic if your cam timing change produces exactly the same torque curve but higher up the rev range the car has to be quicker provided you use the appropriate gears to transfer the available power to the road.
Thing is do you what the car to be quicker or do you want it to feel quicker ? Lots of shove low down the rev range feels very satisfactory and makes the car feel quick but in reality making torque higher up will always make more power and therefore be quicker.
There are some useful software tools that let you enter your torque curve and gear ratios to calculate when changing gear will produce more torque at the rear wheels, which is after all what accelerates the car. Provided your ratios allow you to use the available power band fully more power has to be quicker. If you move too high up the rev range you may find that you need to exceed the rev limit before the next gear makes more torque.
I suppose a relevant point may be to ask is will your induction system and heads work as effectively at the higher flow rates, you will need quite a lot more air to make the same torque at higher rpm.
Interesting.
My pal's 5.2 grief with all the bells and whistles, (including triple act plenem) is tuned for torque.
On the track it fires out of bends with little effort, in fact rarely using 2nd gear and the acceleration in those situations is fantastic and that advantage clearly puts it ahead!
On the flip side, side by side down a long straight, it was no faster then my at the time, 275 bhp 4.6!
My pal's 5.2 grief with all the bells and whistles, (including triple act plenem) is tuned for torque.
On the track it fires out of bends with little effort, in fact rarely using 2nd gear and the acceleration in those situations is fantastic and that advantage clearly puts it ahead!
On the flip side, side by side down a long straight, it was no faster then my at the time, 275 bhp 4.6!
900T-R said:
Torque at the wheels = acceleration. Whether this is achieved at a given road speed by an engine producing 100 lbs.ft @ 9,000 rpm or 200 lbs.ft @ 4,500 rpm is immaterial.
Now would that be torque or BHP @ wheels..? (your calc looks like a simplified version of BHP)I know the conventional wisdom as stated above by many posters - but I guess I was looking for something more qualitative...
Interestingly, it was something on a thread a couple of months ago about chassis stiffness that 900T-R made a comment about tuning the RV8 for BHP rather than outright toque, and making the car more drivable in the process - that got me thinking about this...
And as Alex said above, do I really need >300lb/ft at 2000rpm? - I only ever find myself *touching* the pedals at this point in the rev-range, otherwise the wheels spin pointlessly with me going nowhere...
I guess I was looking for peoples take on the different environments that the 2 set-ups will perform well & badly in - for instance..:
"A car with more torque will perform better in the twisties, as it will have more torque available immediately on gearchange, thus enabling quicker acceleration out from each corner to the next - whereas a high BHP car will perform better on the long straights where the car can reach the powerband, blah blah blah."
...or something similar...
Does anybody know - will changing the cam timing affect the *shape* of the torque curve (and therefore the area under it), or simply move it up the rev-range..?
Dom
domV8 said:
Now would that be torque or BHP @ wheels..? (your calc looks like a simplified version of BHP)
If you look at the engine side of things, yes; but the clue is that at the given road speed, the wheels of the car are turning equally fast in both instances - independently of how fast the engine at the other end of the gearbox is spinning.Both engines have the same power, one has twice as much torque as the other
But because with the high rpm, low torque engine the torque at the wheels is multiplied by a factor 2 compared to the low rpm, high torque engine, torque at the wheels and hence acceleration will be the same.
900T-R said:
Torque at the wheels = acceleration. Whether this is achieved at a given road speed by an engine producing 100 lbs.ft @ 9,000 rpm or 200 lbs.ft @ 4,500 rpm is immaterial.
Almost true. Tractive force (or effort) at the tyre/road interface is the parameter influencing maximum acceleration and top speed. Fit smaller diameter tyres and acceleration will improve (same gear without wheelspin). Similar effects occur under braking.Tractive force accelerates vehicle mass but driveline torque accelerates rotating components. Effective inertia is proportional to square of overall ratio: (diff ratio x gear ratio)^2. That's why "heavy" flywheels and to a lesser extent, "heavy" wheels/tyres are bad news (translational and rotational penalties).
domV8 said:
Does anybody know - will changing the cam timing affect the *shape* of the torque curve (and therefore the area under it), or simply move it up the rev-range..?
Dom
Basically the latter, although some cam profiles are a bit 'cleverer' than others that might needlessly give a bit of torque away at some point due to them not being tailored to the engine (+ intake system)'s airflow characteristics closely enough...Dom
If you want across-the-board torque improvements, generally head development/preparation and induction are where they are to be found.
bigdog3 said:
Almost true. Tractive force (or effort) at the tyre/road interface is the parameter influencing maximum acceleration and top speed. Fit smaller diameter tyres and acceleration will improve (same gear without wheelspin). Similar effects occur under braking.
Tractive force accelerates vehicle mass but driveline torque accelerates rotating components. Effective inertia is proportional to square of overall ratio: (diff ratio x gear ratio)^2. That's why "heavy" flywheels and to a lesser extent, "heavy" wheels/tyres are bad news (translational and rotational penalties).
Er yes, but I was restricting myself to the very basics. Tractive force accelerates vehicle mass but driveline torque accelerates rotating components. Effective inertia is proportional to square of overall ratio: (diff ratio x gear ratio)^2. That's why "heavy" flywheels and to a lesser extent, "heavy" wheels/tyres are bad news (translational and rotational penalties).
The effects of bigger/heavier wheels bolted to a modestly powered vehicle (i.e. cheapish aftermarket 18" wheels and wide tyres on a boggo BMW 316i) on acceleration (and braking) are quite noticeable indeed.
domV8 said:
Does anybody know - will changing the cam timing affect the *shape* of the torque curve (and therefore the area under it), or simply move it up the rev-range..?
Directionally retarding cam timing will move peak torque up the rev-range with proportional increase in power, and without a significant change in the integrated area under it.But what are you expecting from 2 to 3 degrees cam timing change? I've played with retarding cams on racing A-series and picked up top end slightly, but the effects are small. Suspect you could make 2 to 3 deg change on a V8 TVR and not notice any difference in engine characteristics.
900T-R said:
bigdog3 said:
Almost true. Tractive force (or effort) at the tyre/road interface is the parameter influencing maximum acceleration and top speed. Fit smaller diameter tyres and acceleration will improve (same gear without wheelspin). Similar effects occur under braking.
Tractive force accelerates vehicle mass but driveline torque accelerates rotating components. Effective inertia is proportional to square of overall ratio: (diff ratio x gear ratio)^2. That's why "heavy" flywheels and to a lesser extent, "heavy" wheels/tyres are bad news (translational and rotational penalties).
Er yes, but I was restricting myself to the very basics. Tractive force accelerates vehicle mass but driveline torque accelerates rotating components. Effective inertia is proportional to square of overall ratio: (diff ratio x gear ratio)^2. That's why "heavy" flywheels and to a lesser extent, "heavy" wheels/tyres are bad news (translational and rotational penalties).
The effects of bigger/heavier wheels bolted to a modestly powered vehicle (i.e. cheapish aftermarket 18" wheels and wide tyres on a boggo BMW 316i) on acceleration (and braking) are quite noticeable indeed.
Unfortunately when coupled, its extra rotational inertia becomes a penalty 
Perhaps we should get back to cam timing?
domV8 said:
I will be swapping heads over soon, and may take the opportunity to change the cam timing, but wanted to understand the consequences of doing so...
In simplistic terms, as I understand it, by retarding cam timing 2-3 degrees I will gain BHP but lose max torque . I presume the torque curve will be transposed further up the rpm range, although I don't know if the turque curve will change shape?
So here is my simplified question - which would be quicker overall in a mix of situations, either a car with 300BHP 330Lb/ft or the same car with 330BHP 300Lb/ft..?
Assuming chassis, suspension, etc are exactly the same - which setup would be faster: more BHP/less torque, or less BHP/more torque..?
Thanks,
Dom
Changing the cam or timing's strongest effect is in changing the driving style. In simplistic terms, as I understand it, by retarding cam timing 2-3 degrees I will gain BHP but lose max torque . I presume the torque curve will be transposed further up the rpm range, although I don't know if the turque curve will change shape?
So here is my simplified question - which would be quicker overall in a mix of situations, either a car with 300BHP 330Lb/ft or the same car with 330BHP 300Lb/ft..?
Assuming chassis, suspension, etc are exactly the same - which setup would be faster: more BHP/less torque, or less BHP/more torque..?
Thanks,
Dom
Personally the only thing that is relevant is whether you want your max power at low rpm or high rpm.
I wanted it higher up and a more revvy engine so it was more fun for me to drive. It means you need to change down when cruising to get max acceleration but that is a small and almost irrelevant loss given the sheer joy of hooninh down a good set of twisties pulling high rpm and really driving the car.
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