The accuracy of dyno numbers

[bordseye]

Bit of a debate going on in the Jaguar area about dyno numbers, Jaguars claimed BHP and the discrepancy between the two. Not easy to solve without an engine dyno. But I did wonder - there must be lots of PHers who have dynoed their car whilst it has still been standard so there must be some useful info about how accurate rolling road dynos are.

If you have dynoed your standard car ( has to be standard since only then do we know the manufacturers claimed number) what was the result and what did the manufacturer claim?

[lostkiwi]

Dyno figures on a rolling road are at best an estimate when comparing to manufacturers figures.
Even on an engine dyno runs on different days with no other changes will give up to a 10% difference.
Runs on the same day on a rolling road can give 2-5% variations with no changes being made between runs.
An engine dyno can be more consistent as its possible to control the environment better hence the number of variables reduce and consistency improves.

Individual dynos are also variable so its really pretty pointless doing any comparisons by rolling road vs engine dyno or from dyno to dyno.

[Ozzie Osmond]

Dyno BHP = Bullst Horse Power

The people who sell dyno testing love to give optimistic results for the simple reason it makes their customers feel happy. No-one goes away pleased if their "chip 'n' zorst" have cost £1,500 and added nothing on paper.

[Ian_sUK]

I've gained 10% by the air temp sensor being put in the wrong place so have no faith in the figures.

The conversion from wheel to flywheel power is also mostly a guess, even if coast down is measured.

[iloveboost]

lostkiwi said:

Dyno figures on a rolling road are at best an estimate when comparing to manufacturers figures.
Even on an engine dyno runs on different days with no other changes will give up to a 10% difference.
Runs on the same day on a rolling road can give 2-5% variations with no changes being made between runs.
An engine dyno can be more consistent as its possible to control the environment better hence the number of variables reduce and consistency improves.

Individual dynos are also variable so its really pretty pointless doing any comparisons by rolling road vs engine dyno or from dyno to dyno.

As I understand it you can make comparisons on a dyno (with a few percent of error run to run, and power obviously goes down with heatsoak) but losses in the drivetrain are over-estimated, and vary a lot. It's better to use wheel hp or KW like the Australians and Americans do, so part of the problem is the tradition of quoting engine horsepower in this country. However engine horsepower sounds more impressive.

Speed, gearing, tyres, wear, heat and oil all affect actual/estimated drivetrain losses. Another problem is drivetrain inertia, but that can be avoided by holding the engine at different rpms.

[bordseye]

we've been through all the above guys - what we want now is data. dyno bhp figure vs manufacturer claimed number on a standard car. Anyone got any?

[Cabinet Enforcer]

bordseye said:

we've been through all the above guys - what we want now is data. dyno bhp figure vs manufacturer claimed number on a standard car. Anyone got any?

Dyno 220bhp, manufacturer figure 212hp

Other results on the day included a standard focus ST170 which read IIRC about 8hp short of manf figures. So the dyno was probably running a tad optimistic

[a7x88]

That must be the best performing standard ST170 on the planet! I remember being at a rolling road day with a couple and IIRC none made over 160!

[300bhp/ton]

bordseye said:

Bit of a debate going on in the Jaguar area about dyno numbers, Jaguars claimed BHP and the discrepancy between the two. Not easy to solve without an engine dyno. But I did wonder - there must be lots of PHers who have dynoed their car whilst it has still been standard so there must be some useful info about how accurate rolling road dynos are.

If you have dynoed your standard car ( has to be standard since only then do we know the manufacturers claimed number) what was the result and what did the manufacturer claim?

If you'd read my first reply in that thread, you'd know this is pointless. Far too many factors, many of which I highlighted.

You could for instance take the same car to 4 different dyno's and get 4 completely different results. None of which might be similar to the claimed figures.

Or you could dyno the same car on the same dyno back to back but alter the software settings and get massively different results.

The wheel up figure is good and can be for fun or as a tuning tool and comparison to mods. But it isn't really directly comparable to much else.

And flywheel figures are guessed and made up. So completely pointless.

[anonymous-user]

Standard VHPD made 186HP and 130lb/ft which was within a bit of what was quoted. Plenty of dyno data here

[StottyEvo]

RS tuning has a dyno that reads low, I had a prinout for 360 with my car, it made 342hp on RS tunings Dyno. It regularly upsets people. At a PH rolling road day there an XKR-S I believe was ran, it was 510hp from factory and had an exhaust on it (no mapping) it made 550hp.

So I'd say Jaguars HP figures are generally lower than the truth

[xRIEx]

StottyEvo said:

RS tuning has a dyno that reads low, I had a prinout for 360 with my car, it made 342hp on RS tunings Dyno. It regularly upsets people. At a PH rolling road day there an XKR-S I believe was ran, it was 510hp from factory and had an exhaust on it (no mapping) it made 550hp.

So I'd say Jaguars HP figures are generally lower than the truth

XKR-S is advertised as 550PS, XKR as 510PS.

[300bhp/ton]

bordseye said:

we've been through all the above guys - what we want now is data. dyno bhp figure vs manufacturer claimed number on a standard car. Anyone got any?

But you are missing the point. The dyno figure can vary wildly. In an article to demonstrate this. A magazine took a 4th Gen Trans Am to a dyno. Early in the morning with cool ambient temps. Did a power run. Next day they returned mid afternoon when it was really hot out and did another power run. No changes to the car or dyno.

Using what is often referred to as STD figures, this means no correction factor being applied the actual power the car is making there and then. There was a 29rwhp difference. That's huge!!!!

Using SAE correction calculations on both sets results changed he difference to 0.5rwhp.

So you can see the obvious issue here. Unless someone knows if it is a corrected figure or not and how it was corrected, the range of figures you could get are huge.


The next issue is dyno type. An inertia Dynojet will read approx 20-30rwhp higher than an eddy current Mustang dyno for the same car.

So another massive variable.

Just these two figures alone mean you can see a 50rwhp difference from the same car. That all the while will be making the same power under the same conditions.

Edited by 300bhp/ton on Thursday 6th August 15:20

[giblet]

StottyEvo said:

RS tuning has a dyno that reads low, I had a prinout for 360 with my car, it made 342hp on RS tunings Dyno. It regularly upsets people. At a PH rolling road day there an XKR-S I believe was ran, it was 510hp from factory and had an exhaust on it (no mapping) it made 550hp.

So I'd say Jaguars HP figures are generally lower than the truth

Was that the one I organised last year? Explains why my rustbucket gave out such low numbers! Iirc the Jag was a prefacelift XFR

[StottyEvo]

giblet said:

StottyEvo said:

RS tuning has a dyno that reads low, I had a prinout for 360 with my car, it made 342hp on RS tunings Dyno. It regularly upsets people. At a PH rolling road day there an XKR-S I believe was ran, it was 510hp from factory and had an exhaust on it (no mapping) it made 550hp.

So I'd say Jaguars HP figures are generally lower than the truth

Was that the one I organised last year? Explains why my rustbucket gave out such low numbers! Iirc the Jag was a prefacelift XFR

That's the one! As I recall all cars pulled the expected HP or below except the XKR.

If you want a nice Dyno printout I recommend TRL in Halifax, comparatively a 400hp on RS tunings rollers is 430+ on theirs!

[StottyEvo]

xRIEx said:

StottyEvo said:

RS tuning has a dyno that reads low, I had a prinout for 360 with my car, it made 342hp on RS tunings Dyno. It regularly upsets people. At a PH rolling road day there an XKR-S I believe was ran, it was 510hp from factory and had an exhaust on it (no mapping) it made 550hp.

So I'd say Jaguars HP figures are generally lower than the truth

XKR-S is advertised as 550PS, XKR as 510PS.

My mistake, was an XKR

[Sinatra21]

My volvo C30 T5 made 224bhp, factory states 218 although it was run on v power. A few other cars on the day made under power notably EP3 type r's. About 3 hit 180 to 190hp.

[underphil]

in general turbo cars make more power than the official figures, whilst high revving n/a motors tend to fall a bit short

[s m]

I'd rather see them performance tested to see what they actually do on the road rather than seeing dyno figures. Although that in itself brings problems of consistency and repeatability for tests done.
Some cars seem to punch above their quoted weight and power - others don't

Porsche always seem to perform well for their quoted bhp