DF performance v DG

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itchyfeet
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Re: DF performance v DG

Post by itchyfeet »

937carrera wrote:FWIW, these are the results from my top speed model - for an unladen panel van, campers are heavier /slower, though aerodynamic drag is the biggest influence.

DF - 75 mph
DG - 83 mph
DJ - 94 mph

The UK brochures say 73 / 81 / 94

yeh theory is great but what about hills?
1988 DG WBX LPG Tin Top
1989 DJ digijet WBX Holdsworth Villa 3 Pop Top
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937carrera
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Re: DF performance v DG

Post by 937carrera »

Up or down ? :)

Model would have to include new variables for gravity (fixed, at least at sea level) and angle of the hill, no doubt expressed in radians to make it more difficult for me to get my head around. Gearing considerations / change points could I think also become more relevant too, but I think the VW engineers thought of that.

I just developed it out of intellectual curiosity as a way of back calculating what power my hybrid engine had without taking it to a rolling road, then tested against some other cars where data was already known. It seems a half decent fit for an amateur and produces some counter intuitive figures.

Hill climbing ability used to be quoted for VW's. The brochure I have for the Aircooled has the values. I haven't seen any for the WBX and interestingly while the UK brochures continued to have performance data, the German brochures did not. Perhaps related to the fact that in Germany if your new car could not meet the quoted performance figures in the brochure you had rights of rejection. Something like that anyway - or maybe just a pre-internet myth.

At 70 mph around 80% of the power requirement is being used to overcome aero drag and you only need ~ 50HP to maintain that speed. Even a 1600 CT has just about that many :)

I imagine in a DF you may have to work the gearbox a bit more to keep it within the narrower available rev range, but as VW themselves said "throughout the lower engine-speed range it produces the same willing flow of power". The DF will be slower than the DG, but when slogging up a long hill the difference may be less than imagined.

I think you mentioned perhaps being down to 40 mph on a m-way hill before. That only needs ~12 hp on the flat, can be achieved in 3rd, 4th and 5th gear, and in 3rd gear that corresponds to 3500 rpm which is peak power in the DF of 60HP or 65HP in a DG. Expressing it another way there's a further 48HP or 53HP available to overcome gravity.
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itchyfeet
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Re: DF performance v DG

Post by itchyfeet »

you are not convincing me, 8 mph less on the flat probably means 32 mph up a long motorway hill, dangerous.
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937carrera
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Re: DF performance v DG

Post by 937carrera »

That's the counter intuitive bit, it isn't a proportional or fixed difference. Aero drag is the by far the biggest factor in top speeds, you need much less power to propel a vehicle at lower speeds than you would think and velocity is cubed which is what drives the aero drag.

Take a look at the sources I used, I didn't reinvent the wheel, just standing on the shoulders of giants

https://www.pistonheads.com/gassing/top ... 46214&i=20

and

http://www.pugheaven.co.uk/When%20a%20c ... tances.htm

I have come across the Pumaracing site before referred to from pugheaven- he knows his engineering stuff

From that I derived :
Power = (1/2) * density of air kg / m3 * (velocity metres / second ^3) * CdA (CdA being drag coefficient * frontal area)

Then convert Watts to HP and add transmission losses
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
1990 RHD 1.9 Auto Sleeper with DF/DG engine

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itchyfeet
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Re: DF performance v DG

Post by itchyfeet »

Sorry I'm not doing theory I'm doing experience.

my DG tintop when used for camping and loaded up struggled up long motorway hills as said as low as 40 mph, fine on the flat , will do 80 plus but hit a hill and the speed just crashed.
Now the same van is empty and weighing around 1400 KG those same hills can be done very much faster around 65

Anyway I'm just about done with my argument, if you are happy with the performance then thats all that matters.
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1989 DJ digijet WBX Holdsworth Villa 3 Pop Top
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Dubworth
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Re: DF performance v DG

Post by Dubworth »

Just to add some real world experience, I quite often get over to North Wales camping. The return section of the journey features some long steep hill climb sections. Now, whilst I do get in the climbing lane, I rarely drop below 50, admittedly this does mean working the gears, but generally the DF lump has enough to torque to carry on going. This is in a fully laden high top camper. It does, however, tend to be happiest cruising on the flat between 65-70.
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Re: DF performance v DG

Post by silverbullet »

Power is a function of torque and rpm.

More torque developed at higher rpm = higher bhp

DF vans would have had lower gearing (bigger numerical ratio) to compensate for the lack of high speed torque.

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937carrera
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Re: DF performance v DG

Post by 937carrera »

silverbullet wrote:
DF vans would have had lower gearing (bigger numerical ratio) to compensate for the lack of high speed torque.

I think they did for the narrower rev range of the diesel vans, but I'm unsure that they did for the DG/DF, at least according to this table http://vwt3.com/t3-tech/t3-data/t3-gearbox-codes/
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
1990 RHD 1.9 Auto Sleeper with DF/DG engine

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Re: DF performance v DG

Post by itchyfeet »

:rofl

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Re: DF performance v DG

Post by CovKid »

That actually makes me feel sick :oops:
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Re: DF performance v DG

Post by Dubworth »

Be just my luck if I got stuck behind a pensioner in a hyundai i10 doing 20 at the bottom of the climb....
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itchyfeet
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Re: DF performance v DG

Post by itchyfeet »

Dubworth wrote:Be just my luck if I got stuck behind a pensioner in a hyundai i10 doing 20 at the bottom of the climb....

Or a T25 DF :lol:
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937carrera
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Re: DF performance v DG

Post by 937carrera »

Have they discovered roads on Mars now ?

You can see how old & badly maintained the road is by the cracks, but this has to be a fake, it isn't even a T25 in the picture :rofl
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
1990 RHD 1.9 Auto Sleeper with DF/DG engine

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Re: DF performance v DG

Post by itchyfeet »

937carrera wrote:it isn't even a T25 in the picture

be patient its A T25 DF it will be there in a while
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Re: DF performance v DG

Post by Dubworth »

itchyfeet wrote:
Dubworth wrote:Be just my luck if I got stuck behind a pensioner in a hyundai i10 doing 20 at the bottom of the climb....

Or a T25 DF :lol:

Oh that's fine, I can just elephant race them ;)
1985 Sheldon High Top 1.9DF

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