Mobile RR conundrum

[DangerousDerek]

I know for a fact my car is faster than it was when I first had it mapped at Bailey Performance. That day it made 330 bhp at the Rev limit of 6300 rpm. The next day it ran 108 mph terminal speed at York. Since then the engine has fully bed in, freed up and now runs nearly 116 mph terminal. That's quite a difference.
So I was at Total Retro this weekend and there was a mobile RR so I thought a great opportunity to see if my theory was correct and I had a fair bit more hp.

The man told me the RR was good for 1200 bhp and had been used up and down the country at all sorts of events etc.
The car was strapped on, I told him it was good for 6200 rpm. He had some problems with the electric supply and claimed he was getting static shocks off my roll cage so we used my jump leads to earth the car to the rollers.

Then the power run gave this



I was happy to see the power get to 292 at under 5300 rpm but can not figure out why the sudden drop from there.

Any theories?

[DangerousDerek]

Here is the run from Dales for comparison. Climbing all the way to 6300

[Pupp]

If it wasn't for the fact the torque had already fallen off a cliff, I'd say it might be wheelspin... but, then again, the two traces are not crossing where they are supposed to (5,200 rpm), are they?

Why do these bloody graphs never have sensibly graduated axis scales (or even any axis scales)?

Only other thing that occurs to me is air temperature/density differences if it was especially hot when the mobile run was done (your ECU not pulling advance by any chance?)

[gacksen]

Anytime use the same RR for comparison. Do another run on a third one and you will a different result..... dont worry that much....

[DangerousDerek]

I am not in the slightest worried and know the car will perform on the drag strip next time as it has all this year. Low 12's are easy now and I can push it into the 11's at 115 when needed.
It's just this graph that confused me. The sudden drop off in torque and power at such a low rpm.

I want to just get through to September with enough reliability and to give me the biggest chance of winning the NSCC after competing in the series for a good few years now.
Then I will go see Dale and get a fresh mapping and check over and head to Surrey Rolling Road for a shootout with the usual suspects. Then we will see

[ClassiChimi]

Wheel spin, but then why does the torque drop off so early!

The Mazda's my mate races uses a hub Dyno, controlled Ecu etc and the power graphs are all very similar, different days etc but the Bhp figures are often right on the upper level allowed which suggests it's very accurate?

at one race weekend many cars were checked for cheating,, they all had a similar curve which suggests to me the hub connected Dyno seems accurate car to car.

I watched the Dyno sessions at the wedge fest, a number of times cars needed strapping down more because of wheel spin.

As you say Derek, the times don't lie but that would be bugging the crap out of me now, compression test etc just to be safe maybe!

[DangerousDerek]

Had a chat with Dale last night and I need to check the breathing on my petrol tank. We have a theory that the tank is suffering vac and possibly affecting fuel pressure. I have rich ended the map top end as I was seeing over 13:1 on the strip flat out. But I know the cap was loose on the rolling road so maybe it was very rich top end. I will do a compression check also as Alun suggests. Be intereresting to see at this stage anyway.

Watch this space.

[QBee]

There has to be something wrong with that session. Ignore it completely, but I suggest you get the car on another set of rollers asap.
Jools is at Chesterfield if you are passing that way. J28, M1. If you call him ahead I am sure he will stick it on the rollers for you. Number's on Www.kitsandclassics.co.uk

[ChimpOnGas]

Surely a hub dyno will reveal the answer you seek?

[Hoofa]

What cam have you got in it ?? Think I would be inclined to get another run done on another dyno, doubt it's wheel spin ,

Edited by Hoofa on Tuesday 26th July 23:33

[ClassiChimi]

Hub Dyno should be as accurate as any, another time we got pulled in for a power test I looked at the car before's graph, both curves from both cars Were increadably similar, they rose in exactly the same manner,, as is proven on track they are very similar in pace.

I concluded both the controlled mapping and the hub Dyno was accurate to a pretty fine degree on many of the cars

What I don't know is if the Dyno used at the race circuits is the same one used to do the controlled mapping for the race series in the first place,, if it is then I can see why the bhp figures are so close to the upper limit allowed, if it's a different Dyno but of the same type ( hub) then that would suggest the Dyno's record the information accurately,,, I'll find out.

I'm starting to feel a days Hub Dyno session coming on for Tvr Boys,, it's the only way we can work it all out! I wonder if Sprint Will sponsor it !

Hope this is nothing but a red herring Derek, fingers crossed, you have a lot at stake,, keep pushing fella

[phazed]

I will be looking for a dyno run in a couple of months time.

We must get together for a session and a looksee!

[Hoofa]

Yeah mines back on the dyno for some further mapping soon, yeah a rr day sounds good

[QBee]

There's one quite near me that's central to all of us:



.....though I can think of better places to eat.

[sheel]

Get them on lloyds hub dyno,that was accurate enough to pick up the change in oil temp and viscosity through the drive line with mine,consistently and repetitively accurate
Rich

[DangerousDerek]

Hoofa said:

What cam have you got in it ?? Think I would be inclined to get another run done on another dyno, doubt it's wheel spin ,

Edited by Hoofa on Tuesday 26th July 23:33

It's a mech Crower 50304 and went to 6300 easy first time round. Something strange happened on this rr on Saturday. I have some ideas after chatting with Dale.
I will also be doing a compression test on Friday

[ClassiChimi]

sheel said:

Get them on lloyds hub dyno,that was accurate enough to pick up the change in oil temp and viscosity through the drive line with mine,consistently and repetitively accurate
Rich

You see I didn't realise that Rich,

So when you go on the roller type Dyno does someone accurately work out the outside tyre circumference of your car, I'm on 17 inch rims with a 45 profile I'm not sure if that is the same size as a standard 16 inch wheel and tyre, wouldn't that give different readings etc if the gearing was lower because of a smaller circumference for instance? Where's a mathematician when you need one

[QBee]

Standard 225 50 16 has a diameter of 628mm.
Your ? 235 45 17 has a diameter of 643mm. If it was 225 45 17 it would be 634mm, 215 45 17 would be 625 mm.

So the answer depends on the tyre width.

And on how worn the tyres are - brand new tyres are 12mm more in diameter than virtually worn out tyres.

[Pupp]

Why would tyre diameter materially affect RR results; the RR is measuring the ability of the car to spin up a known mass against a known braked resistance so as to determine its output, not 'road' speed or acceleration per se - the software takes into account wheel and engine revs when doing the calcs and fourth gear is commonly used as that is often the most direct drive. Whether or not you have Fandangos fitted is going to matter a jot provided the rollers are not overriding grip...

[ClassiChimi]

QBee said:

Standard 225 50 16 has a diameter of 628mm.
Your ? 235 45 17 has a diameter of 643mm. If it was 225 45 17 it would be 634mm, 215 45 17 would be 625 mm.

So the answer depends on the tyre width.

And on how worn the tyres are - brand new tyres are 12mm more in diameter than virtually worn out tyres.

Thanks for the question mark Anthony,,

I dunno where that size came from !!! My tyres are of course 245/40/17.
Can you tell me the size of this combination please mate.

Pupp, yes that's also rather obvious when you think about it, cheers.