14 replies to this topic

[Shooter63]

The above is the calculation you can use when you take your car to a rolling road, this is not my calculation but one from one of the great Dave's. While this calculation is correct to within a bhp or 2 it does rely on the RR figure being correct which obviously is where it can all fall apart. This works up to around 250 bhp after that it starts to over compensate. I'll give you a example, my Mrs mini made 72/74 on the rollers so

72 + 10 = 82 ÷ .9 = 91 at the flywheel. Which is about what you would expect from and engine of that spec, although if I'm honest I think the rollers were running high ( as most do ). Keith Calver states that a power unit normally looses around 17 bhp through the drive train, so it's pretty close.

Have fun

Shooter

[Tones61]

A mini gearbox will lose 15-18 bhp ,
So add this to the wheel power for crank power

[Steve220]

There is no defining calculation to it as there are way too many factors and variables. Crank bhp is a rough guess if you're using a wheel/hub dyno. Either WBHP or engine onto a dyno are the accurate methods.

[Spider]

There's many factors that need to be applied to Dyno readings, not just for losses between the flywheel and the tyres gripping the road, but also atmospheric temps, humidity, air pressure, fuel temp and many others.There's a few international standards that cover these. This is so that measurements taken on one dyno should be directly comparable to another on the other side of the world, however where it comes to engine tuners, it seems pretty consistent that certain operators always seem to get 10 - 20% more horsepower than the guy down the road

 

One other factor is that nearly all measurements I have seen on the internet with bragging graphs is that they are just a very basic power run and are a VERY long way from reality. Measurements are only true when localised temperatures in the combustion chamber have stabalised. Most 100 HP engines would in fact be closer to 85 HP.

 

There are many Rolling Road Dynos about that are able to determine the 'drive train losses' with a very high degree of accuracy and repeatability. Most of these will actually drive back from the rollers (with an electric motor) to directly measure the losses, wheel slip etc

[sonscar]

I dimly remember Vizard warning that power should be held for several seconds to obtain a true reading.This was to avoid potentially damaging tunes which gave false instantaneous readings?Steve..

[DeadSquare]

In the late 1950s, the was a chap from Doncaster that worked at the Malvern Royal Radar Establishment, who did a bit of Club Racing / Hill Climbing.  He had an 1172 Ford with a fiberglass "Mistral" body by Microplas of Mitcham, (Gateway to the South). and was the envy of a bunch of impoverished young motor enthusiasts.

 

After too many pints, resulting in wild claims of power out-puts, I made a tongue-in-cheek suggestion of how to establish an engine's BHP, and thought no more about it, until, one evening he drove up and proudly showed us the new gauge on his dash.  He had made my mad idea out of a Tachometer that gave a direct reading of BHP while driving.

 

He had done away with the engine mountings and made the engine pivot by replacing them with a bearing round the tail of the gearbox and another on the starting handle / crankshaft bolt.

 

He has stabilised the engine with an ex-WD (Draper) "Bendy-bar" torque wrench, so that as the engine twisted, the moving finger of the torque wrench pulled a bicycle brake cable that rotated the dial behind the needle of the tacho, which he'd calibrated it at work.

 

Of course, we all clamoured to see it work.  As it was my idea, I had the first go, and I was given careful instructions before the test run, up the Malvern hills, about gentle start and gentle gear changes.

 

I think it worked, but because the engine vibration was trying to remove my dental fillings, it was hard to see the gauges ! , and the next time he came, the engine mountings had been replaced.

[coopertaz]

In the late 1950s, the was a chap from Doncaster that worked at the Malvern Royal Radar Establishment, who did a bit of Club Racing / Hill Climbing.  He had an 1172 Ford with a fiberglass "Mistral" body by Microplas of Mitcham, (Gateway to the South). and was the envy of a bunch of impoverished young motor enthusiasts.

 

After too many pints, resulting in wild claims of power out-puts, I made a tongue-in-cheek suggestion of how to establish an engine's BHP, and thought no more about it, until, one evening he drove up and proudly showed us the new gauge on his dash.  He had made my mad idea out of a Tachometer that gave a direct reading of BHP while driving.

 

He had done away with the engine mountings and made the engine pivot by replacing them with a bearing round the tail of the gearbox and another on the starting handle / crankshaft bolt.

 

He has stabilised the engine with an ex-WD (Draper) "Bendy-bar" torque wrench, so that as the engine twisted, the moving finger of the torque wrench pulled a bicycle brake cable that rotated the dial behind the needle of the tacho, which he'd calibrated it at work.

 

Of course, we all clamoured to see it work.  As it was my idea, I had the first go, and I was given careful instructions before the test run, up the Malvern hills, about gentle start and gentle gear changes.

 

I think it worked, but because the engine vibration was trying to remove my dental fillings, it was hard to see the gauges ! , and the next time he came, the engine mountings had been replaced.

miss characters like this we used to have one in club we nicknamed the mad professor for some of his ideas, aslo miss good rolling roads with people who knew their way around carbs. this knowledge is being lost. even if some of their claims were 'optomistic'

[Spider]

After too many pints, resulting in wild claims of power out-puts, I made a tongue-in-cheek suggestion of how to establish an engine's BHP, and thought no more about it, until, one evening he drove up and proudly showed us the new gauge on his dash.  He had made my mad idea out of a Tachometer that gave a direct reading of BHP while driving.

 

He had done away with the engine mountings and made the engine pivot by replacing them with a bearing round the tail of the gearbox and another on the starting handle / crankshaft bolt.

 

He has stabilised the engine with an ex-WD (Draper) "Bendy-bar" torque wrench, so that as the engine twisted, the moving finger of the torque wrench pulled a bicycle brake cable that rotated the dial behind the needle of the tacho, which he'd calibrated it at work.

 

That's thinking outside the box !

[Gaz66]

Are those bendy torque wrenches that accurate? 😂

[coopertaz]

Are those bendy torque wrenches that accurate?

is that the only issue with this

[DeadSquare]

 

Are those bendy torque wrenches that accurate?

is that the only issue with this

 

No, it wasn't  the only issue. 

 

1)   Energy stored in the flywheel shows up as additional torque if you drop the clutch, (hence the gentle starting, not to brake the torque wrench handle), and why BHP readings are taken after 5 seconds of steady RPM, so it wouldn't be accurate until the vehicle stopped accelerating.

2)   The dyno at his work was only a static 3Ph / 7.5Kw, about 10 Hp, so he had to extrapolate the division on his dial, times at least 5 to cope with possibly 50 BHP

3)   My idea couldn't take account of the fact that the engine doesn't balance through the centre line of the crank, but putting a strain gauge in a Mini stabiliser and feeding it through a computer, might be possible, if you knew how to write the program.

[Gaz66]

To be fair, anyone who can invent a BHP/torque dash gauge would make a fortune with the go faster/bolt on goodies brigade 👍

[stuart bowes]

true but not if you hear a big snapping noise and the engine falls out on a hard launch 

[Spider]

To be fair, anyone who can invent a BHP/torque dash gauge would make a fortune with the go faster/bolt on goodies brigade

 

If your pockets are deep enough, they have been around a very long time, but frightfully expensive. The issue with doing 'in car on the road' measurements is rarely does one drive steady state long enough with wide open throttle to get meaningful measurements.

[Shooter63]

To be fair, anyone who can invent a BHP/torque dash gauge would make a fortune with the go faster/bolt on goodies brigade

 
The issue with doing 'in car on the road' measurements is rarely does one drive steady state long enough with wide open throttle to get meaningful measurements.

Speak for yourself:)

Shooter