Rolling road power figure variation

[Clem]

Just wondering what the usual variation is between trips to rolling roads. I know that rolling roads vary but I am talking about the same dyno different day.

The story is as follows:

My car was modified by BBR and came out with 207bhp ... excellent.

7 months down the line I take it in for the update and it comes up as 200bhp before 206bhp after ... I am happy with the improvements on the same day but a bit confused as to where the 7bhp went

Now, its only about a 3-4% variation across a large chunk of time, but do people reckon the majority is actually lost power ... or mostly change of conditions between the two days. Both were with Shell V-power petrol.

More importantly ... what figure do I use for bragging?! 207 (but thats before ...) or do I go for 212 ... because thats the plus 5ish i got from the update ... or 210ish and be vague and classy ... or go full secretive and just go "lots". This is clearly an important issue

[Horney]

Atmospheric conditions and tyre pressures make a difference.

I'd go with 210.

[Clem]

Good point ... I've had some pressure issues lately ...

It's probably about 230

[Woofwoof]

I've used a couple of RR's but not with my MX5. I've never really concerned myself with the BHP figure simply because if it drove well and the AFR's were good that's all that counted for me.

If you're happy with the performance and all the engine parameters are good, call it 250BHP!

[BikeTuna]

It sounds like these things clearly matter, so with a decent set of stickers and a good polish its got to be 270BHP...

[hayling5]

Bhp is unimportant, all MX5's should be measured in lateral G's for their cornering ability

[BikeTuna]

Nov 24, 2016 8:06:22 GMT hayling5 said:

Bhp is unimportant, all MX5's should be measured in lateral G's for their cornering ability

Well if you're measuring in lateral G then it's gotta be good for 290BHP...

[Clem]

Ok that settles the bragging ... its 300bhp of pops and bangs bro. Pullin all the gees all the time...

But anyone had any experience of running the same car on a dyno on different days? Does make me wonder what the error is on these things.

[Horney]

My supercharged one has made between 176 and 189 at the wheels on Skuzzle's Dyno although I have had more variables like boost leaks etc to contend with. It's only a small percentage difference so I wouldn't worry about it.

[Clem]

I'm not worried, this is basically academic interest.

Also if its that variable, what happens when you are tuning a car and the weather changes during the day, surely then even back-to-backs prove tricky ...

[Horney]

The headline figures will change a bit but any tuner is not looking at the headline figures. Firstly they're looking to adjust the fuelling to to get optimum AFRs across the range, then they'll tweak the timing, and then the fuelling again until it's doing the best it can (In the case of the MK3 they can also adjust the cam timing via the VVT controller). Once they've done that they check the curves as what they really want to achieve is the car running as close as it can to the limits safely and with a good spread of torque and power.

THe one thing they will always want to be able to see clearly is AFRs and to hear when it suffers Pre-Det.

[Clem]

But surely the AFR will vary by similar amounts? Given that the density of the air input is one of the factors that change with the weather/day/etc. I suppose there is an element of a closed loop on that as there is the MAF sensor reading which can be used to adjust the fueling for a given day, but then if that was effective then tuners wouldnt need to worry about the AFR.

[myothercarsa2cv]

I used the same RR on two different days with the 2CV to measure the impact of a few mods. The second run was about 10% higher than the first, which was what I expected for the work done. The RR operator had a conditions adjustment built in to the machine, so who knows whether the conditions make any difference? I wouldn't worry too much about the number. If someone like Nick at Skuzzle says it's good, it's good

[quinvy]

I always take RR figures with a very large pinch of salt. Too many variables to be consistent. All that's important is that your car is running as efficiently as possible, and within safe parameters.

Rolling road days are to be avoided at all costs, I've seen the consequences of an unexpectedly low figure.Massive fall outs on forums, and tuners dragged over the coals and threatened with litigation.

How it drives on the road is all that matters.

Is that you Robin?