Why Don’t We Like To Quote Flywheel Power?
The most common question we get asked is: “What’s the power at the flywheel?” And the only answer an honest chassis dyno operator can give you is: “We don’t know” There, we have said it.
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But Other Places Tell Us!
Indeed they do – and we will give you a flywheel power graph too. However, they are mere guesstimates and the problem is, some guess high to inflate your ego, some guess a little low and some aren’t too bad at all. But NONE of them are 100% guaranteed accurate because a chassis dyno can ONLY measure the power at the tyres... nowhere else.
If you want to measure the flywheel power accurately, you need to take the engine out and measure its power on an engine dyno – it’s as simple as that.
.jpg "Power figures on rolling roads")
Why Is that?
The way a chassis dyno works is that it measures the tractive effort at the wheels against a known resistance and then when plotted against RPM and a set of correction factors (See other signs), it outputs a pretty damn accurate torque and power graph. However, this graph does of course only tell us the power at the point of measurement... The tyres.
What then happens is an extra “percentage” is normally added to the graph to represent the amount of power the dyno maker assumes a transmission system will absorb. Now I have seen these as low as 10%, but as high as 35%. This means a known good 200bhp at the wheels vehicle will have an estimated flywheel power of anywhere between 220bhp and 270bhp if it has a go on each of these dynos. It certainly won’t accelerate any faster as it still has exactly 200bhp at the wheels, but the owner would probably smile a lot more and greatly impress his mates at the pub if he uses the one that gives him 270bhp and sends his mate and his identical car to the one that will tell him he has only 220!
Why Is The Power At The Wheels The Most Important Power?
The power at the wheels themselves is what propels your vehicle forward. When tuning on our dyno, we take a baseline wheel figure over three runs and then tune for more power at the wheels. As long as we get more power at the wheels we know for a fact the car will accelerate better. It’s as simple as that. All we need is repeatability so we can be certain that changes we have made have had the desired effect.
.jpg "Power figures on rolling roads")
Is There A Ballpark Accurate Figure To Get A Flywheel Estimate?
No – There really isn’t – however, we tend to find that a reasonably good set of figures to use is as follows:
- Front Wheel Drive = Add 15%
- Rear Wheel Drive = Add 18%
- Four Wheel Drive = Add 22%
Please understand that these figures are just guidelines – To suggest that a 1000bhp at the wheels 4wd vehicle has just lost 220bhp through the transmission is ludicrous – that is enough heat to melt a gearbox casing in seconds! The figures do however normally work out pretty accurate for most types of sub 250 bhp road car.