Much loss on high rpm

[Uboat]

Gents,
I made a Rolling road dyno test and got + 139 bhp at 6050 rpm, very happy with that (big bore manifold + exhaust, electronic ignition, a little work on ports, original sprint camshafts). However loss was 45 kW at 6050 rpm. Is that normal for a twin cam, and what could be the cause, riff, gearbox bearings?
Ulf

[rgh0]

Hi Ulf

A normal big valve twin cam will put out about 70 kw / 95 hp +/- 5 hp at the rear wheels on a rolling road. This corresponds to the nominal Lotus quoted 126hp at the flywheel. What you get can vary with many factors apart from the engine itself. Based on your description of your engine you may be getting up to around 130 hp but I would not expect much more. Thus I would have expected to see around 70+ kw at the rear wheels. From your graph it looks like around 60kw which is a little low and whether this is due to dyno calibration or losses in drive train or on the dyno at the wheels is not easy to tell versus an engine issue itself. 10hp plus 10% to 15% is a good rough rule of thumb conversion from a rolling road to engine output hp with the 10% to 15% range depending on exactly how the dyno test was setup and done and details of the drive train that affect internal losses

Type of tyres, tyre pressures, slip on the rollers, the gear the test was done in, was it steady state at set rpm steps or an acceleration run, temperature and atmospheric pressure all affect the results recorded in addition to internal drive train losses and the actual engine output

How people estimate dyno losses varies greatly but most people on most dynos overestimate it so they can quote a high engine hp to the customers which keeps the customers happy after they have spent lots of money getting their engines built and tested.

Rolling road dynos are best used to ensure the cam timing, carb and ignition settings are optimal for the engine build and the fuel being used. The results in terms of actual hp are best used for conversations at the bar over a few beers.

cheers
Rohan

[Chancer]

Was the dyno test done in top gear?

If not then the losses dont surprise me.

The only other one that can make a significant difference is sticky slick tyres on a hot day.

Anything like diff or gearbox bearings you would hear and if of such a magnitude the tester would refuse.

[Uboat]

Thanks for your advice,
Indeed the Rolling road was used to calibrate and set up carbs...
Output on wheel is very low, 55 kW, but output on engine is measured to 139 BHP. Do not ask me how this is calculated, but it is done by a very experianced tuning expert. All tests are done in forth gear. I do have sticky tyres. (Yoko 21R, 185x13)

[denicholls2]

Methinks the title of your message is backwards, should be "much gain through driveline math".

A rolling road dyno measures only power delivered to the dyno roller, never power at the flywheel. You need the engine out of the car and on an appropriate dyno to get that number reliably. Power at the flywheel is always an estimate involving calculating backwards from the roller result. This calculation can be informed by variables within the test run(s) by a knowledgeable dyno operator. I am not one of these.

As Rohan notes, the real purpose of a roller dyno is to tune on it, attaining the best results for the car in question. Repeatability on the same device is a good indicator that your rear-wheel power and torque is reliable. The actual numbers generated aren't good for all that much outside the barroom.

In a thread contemporary with my last dyno session some years ago, several posters noted horsepower differences of up to 50% on the same car at different shops. It is the shop's setup plus environmental variables that controls your raw numbers. Most shops strive to tell you your car is more potent than it is, but some do not.

On my particular dyno day, sunny and sultry in Massachusetts, a Maserati Biturbo owner came in expecting well in excess of 500 flywheel horsepower. He got a sheet indicating something under 300. The Lotus club customers present recorded numbers generally lower than expected, but not by this gap. I was quite happy that my lowly S2 Hermes Europa generated power numbers in the same class with the stock Twincams there (measured 89 at the rollers) and a much fatter torque curve.

This particular shop would not do test runs in top gear for safety reasons -- bad things that can happen happen faster at higher wheel speeds. My test runs were in third gear, some others were in second. As I recall, each run began at 20 mph and measured up to the redline specified by the owner.

In this realm, it is "all in how you look at things." Be happy you are not the Biturbo owner.