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Educate me on Variable Valve timing

M

morrisond

Expert
Joined
Sep 10, 2003
Messages
472
Location
Muskoka/Toronto, Ontario
Theoretically if Yammie were to put VVT on the Nytro Motor and it made 165-170 at clutch friendly RPM, say 8700rpm, what kind of effect would it have on torque? More less? Broader torque band?

Assuming the Nytro is 138 HP and 94 lbs of Torque could VVT get this engine over 100 lbs ft?

I know Torque falls off on Nytro at higher RPM would VVT Keep the Torque High at High RPM's?

Thanks
 

I would like to make this very clear.....
Crankshaft torque is TOTALLY IRRELEVANT.

The only number that has ANY MEANING at the crankshaft is POWER.

If it makes 170 HP at, say, 8700 rpm, then its making 102.6 footpounds at the driveshaft. This number, of course, is irrelevant because there is additional gearing before getting to the track. Lets say that you're going 50 mph and you have 9 tooth drivers.... 2.52x9=22.68" circumference, 50 mph=1056 inches/min = 46.56rpm.

Now at driveshaft speed, given 170 hp, you have 19176 footpounds of torque. At 100 mph, 9588 footpounds. Your top speed is defined as the speed where driveshaft (not crankshaft) torque is equal to resistance.

(power x 5252) / rpm = torque
So you can see that when power is constant, torque varies inversely and linearly with speed.

Thats the magic of a CVT transmission -- that power becomes more or less a constant and driveshaft torque (where it matters) is always maximized.
 
VVT usually is used to help make small displacement engines more powerful (i.e. more torque) at lower rpms by having a low rpm cam profile and a high rpm cam profile. This feature isn't so important in our application because the CVT keeps the engine in it's proper rpm range better then a standard car transmision....but I guess it would allow some really aggressive cams and still keep the idle...

LB I wouldn't say that crankshaft torque is irrelevent..since horsepower is a function of torque and rpm. Low end torque may not be as important in a sled application as it is in a car or and outboard but it makes for good hole shots and easier clutching...
 
He's not asking about low end torque for holeshot, he's asking about peak torque. And it makes no difference for clutching since best performance is attained through following peak POWER. And its the width of the POWERband that makes clutching 4-strokes easy... because they'll still make very close to peak power within HUNDREDS of rpms of the point of absolute peak power (nice to be able to clutch for somewhere between 10000 and 11000 rpm, eh?), whereas a 2-smoke will drop off a cliff if even slightly off in either direction.
 
Broad powerbands are a result of a flat torque curve....Higher horespower levels 160-170 at lower revs 8700 (compared to the RX engine) is achieved with higher peak torque levels...ALL of which makes a BIG difference in clutching. Usually VVT is used to broaden the torque curve of a peaky high revving engine (which I would think 8700 would fall into) making more user friendly power curves.
 
You're missing the fact that in this application, there is NO DISTINCTION between power and torque. Its just a different way of looking at the EXACT SAME THING, therefore don't waste your time thinking about torque since that number is useless to us without taking SPEED into account. When you take speed into account, its called POWER, and *everything* is based on that number.

Case: A 1 hp engine can produce 1000000 footpounds of torque at 0.005252 rpm. Yes, it'll drag a bulldozer out of a mudhole (after waiting for a very long time), no, its not going to send you off at 150 mph. It'll probably reach its maximum speed of almost nothing almost instantly.
 
The question was about VVT and if it would help bump the torque (of 94lb/ft) of the 3 cylinder and still keep a reasonable clutch speed of 8700rpm. Which takes into account torque and speed. If we raise the torque curve in relation to the current engine's power (which is the question) using VVT horsepower will rise. He's not asking a general question...he's asking specific questions relating to a specific engine...not about a 1hp engine with 100000 lb/ft of torque (not foot pounds btw). I understood the question (maybe incorrectly) to relate to the torque curve rather then a specific torque number...

Crankshaft torque does matter...it's how the dyno calculates horsepower...
 
Look, very simple. The objective is to increase power. If the speed stays the same, then yes, that means that torque goes up, but since that number is totally irrelevant, WHO CARES??

And BTW: Dyno does not measure just torque, it measures torque AND SPEED = IT MEASURES POWER.
 
morrisond said:
Theoretically if Yammie were to put VVT on the Nytro Motor and it made 165-170 at clutch friendly RPM, say 8700rpm, what kind of effect would it have on torque? More less? Broader torque band?

Assuming the Nytro is 138 HP and 94 lbs of Torque could VVT get this engine over 100 lbs ft?

I know Torque falls off on Nytro at higher RPM would VVT Keep the Torque High at High RPM's?

Thanks
Click to expand...

As DoktorC has stated, VVT simply allows for a more agressive cam lift (that would be how it got to the 165-170hp as you want) and be able to maintain excellent low rpm characteristics. That being said, it means that the power band (TQ curve) will be much broader(more area under the curve) than that same motor with the bigger cam and NO VVT.

Obviously HP is a result of torque, so if the rpm range (peak) stays the same for your 165hp motor as it is with the stock motor, the TORQUE MUST BE HIGHER at peak also, so yes, over 100ftlbs is a reality.

Torque does fall with rpm....but if you did get 165hp at same rpm range as stock, then I'd say the torque increase would follow through nice and smoothly and not drop so bad.

Hope this explains it in an easier way for you....
 
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