20 replies to this topic

[Spider]

One thing, i have heard that some operators will "massage" the figures slightly either through lack of knowledge, or because they know the owner of the car just wants to see big numbers.

 

'Funny' you mention this. There's a goose around these parts who always seem to get between 10 to 20 HPs more on his Dyno than anyone else.

 

As a guy from Mini Spares said to me not long ago, "Yes, his legend is well known over here, he gets 50% more on his Dyno that anyone else can"

 

I think it's important that the corrections are applied for the day, as they will vary with weather and other factors. It certainly gets hard to get repeatable results on a rolling road, but far from impossible, just a few more checks to be done.

 

A few ways of looking at this and where best to measure. The Flywheel while in the chain, isn't what drives the car, the tyres do, so anything that's measured before these only shows part of the story, though measuring at the Flywheel is easier (the lazy way) to get repeatable results - of the engine.

[Turbo Phil]

I've always seen flywheel figures as fairly pointless, as said above it's what's hitting the Tarmac that counts. Consider the whole engine and drivetrain as a complete package where everything needs to be optimised to give best power.

Phil.

[Cooperman]

For any car, but especially an old classic car, the actual bhp & torque figures are really quite meaningless. You can have a really powerful engine which is combined with an badly-matched gear train and it will not go as well as a less powerful engine on a  better-matched transmission. It is the driveability which is important.

A dynamometer is the real way to go rather than a 'roller-brake' rolling road, as it measures the true direct output from an engine. That way the engine can be developed by changing the critical parts and the result measured at each stage. You can test it and measure everything, then change the head or cam or induction system and test it again until it is giving the best you can get from it. You will also have the necessary torque & power graphs to enable the best transmission to be determined.

For an engine in a vehicle the R-R is really just for optimisation of the settings rather than evolving more power/torque.

A few bhp will make virtually no difference on the road. For example, suppose I have an engine with 75 bhp corrected flywheel figure at 5700 rpm and you have 80 bhp at 6000 rpm, but assume that I'm running a 3.44:1 FDR and you have a 3.1:1 FDR. My car is going to drive better and be quicker through the gears, although you may have a couple of mph more top speed (where would you use this?).

Classic cars tend to be judged on 'how nicely they drive' rather than the power at the wheels.

[grizzler73]

The idea of a RR session is not to give you big numbers but dial in the various different components so they work to there optimum, I remember going to the RR with my father 

and his racing Fairthorpe years ago, the first year we went it had 95bhp at the wheels but was giving very little at the bottom end with the shallow skirt racing pistons we had, we changed the pistons (powermax) and rods to modified A series by machining the crank  the following year and it peaked at 85bhp at the wheels, but it revved to 8000 all day long and the tourque kicked in at 2000 rpm and stayed, suddenly we started winning...

[AlexMozza]

Once I've built my 1330, I'm going to have a RR session, but I can't give a crap about the power. Driviabllity is far more important.

[Northernpower]

Once I've built my 1330, I'm going to have a RR session, but I can't give a crap about the power. Driviabllity is far more important.

That's the biggest difference on mine. I went yesterday and while the figures are great to see, and also some confirmation you've done the job right, its the complete change in drivability that's the best reward. It also helped to reduce the fuel consumption significantly as well.